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Light Emitting Diodes (LEDs) is one of the newest ways to light up outdoor areas such as streets, stadiums, airports, military bases, harbors, and high mast towers. The main reason for the high focus is power consumption with better brightness. Even though LEDs have so many benefits, researchers should focus on better managing temperatures as the main reason behind the failure of LEDs is overheating. In this article, modern research trends like using heat pipes filled with nanofluids, ionic winds, spray cooling, use of fins, refrigerants, and oil cooling are focused on and discussed concerning LEDs, solar stills, electric vehicle batteries, and different heat transfer devices to develop a new way to handle the heat from higher-wattage LEDs. There is a research thrust in the field of more than one nanoparticle in the base fluid, and its proportions are not analyzed during LEDs cooling study using a heat pipe. It is necessary to control the heat using a more effective technique. Heat pipe with nanofluid is a more efficient, compact, and cost-effective cooling device o reduce LED failure due to higher heat flux. Hence, it is promising to use nanofluid-filled heat pipes to serve the purpose of the life span enhancement of LEDs. [ABSTRACT FROM AUTHOR]

Spray cooling is an effective heat dissipation technology and is widely used in the heat dissipation of encapsulated structures, but most of the research has only focused on the heat transfer performance itself and has lacked the analysis of surface stress and deformation. In this paper, a thermal stress coupling model was established under spray conditions, and the influence of spray parameters such as the spray height, spray flow, and nozzle inclination on heat transfer, surface stress, and deformation were studied. The result indicated that the lower the surface temperature, the smaller the stress and deformation. What is more, there was an optimal spray height (15 mm) to achieve the best heat transfer, and the surface stress and deformation were also minimal at the same time which the values were 28.97 MPa and 4.24 × 10−3 mm, respectively. The larger the spray flow rate, the better the heat transfer effect and the smaller the surface stress and deformation. When the spray flow rate was 24.480 L/h, the minimum values of surface stress and deformation were 25.42 MPa and 3.89 × 10−3 mm, respectively. The uniformity of surface stress distribution could be effectively improved with the increase in flow rate. Compared to 10 and 15 degree nozzle inclination, when the nozzle was perpendicular to the cooling surface, the surface stress and deformation were minimal. [ABSTRACT FROM AUTHOR]

The continuously increasing energy density of electric motors to match the performance of electric vehicles with internal-combustion-engine vehicles demands an advanced cooling strategy. Direct dripping/spray cooling is one of such potential cooling strategies to replace the conventional cooling method for achieving the desired thermal management of next-generation electric motors. In the present work, the heat transfer characteristics of an electric motor with oil-dripping cooling were experimentally investigated considering overload operating conditions. An experimental set-up comprising a 15 kW electric motor and an oil-dripping cooling system was developed. The experimental data were used to evaluate the maximum temperature, heat transfer coefficient and power consumption under the influence of a dripping hole diameter (2 mm, 3 mm, 4 mm), oil flow rate (8 LPM, 12 LPM, 16 LPM) and overload operating power (8.28 kW, 12.05 kW, 14.21 kW). The symmetrical oil distribution over the electric motor and the superior heat transfer from the electric motor to the oil was achieved when the oil-dripping cooling system was designed with the combination of a 4 mm dripping hole diameter and a 12 LPM oil flow rate. The combination of the 4 mm dripping hole diameter and 12 LPM oil flow rate showed the lowest maximum temperatures of 31.9 °C and 46.3 °C for electric motor under overload operating powers of 8.28 kW and 14.21 kW, respectively. In addition, the highest heat transfer coefficient of 7528.61 W/m2-K and lowest power consumption of 18.07 W were achieved for the oil-dripping cooling system with the combination of a 4 mm dripping hole diameter and 12 LPM oil flow rate. The best combination of the operating parameters is proposed for developing the oil-dripping cooling system that enables superior heat transfer characteristics and, thus, an enhanced thermal management of electric motors under overload conditions. [ABSTRACT FROM AUTHOR]

To investigate the spray cooling characteristics and the impact of spray parameters such as chamber pressure, spray height, and spray tilt angle on heat transfer efficiency, a mathematical model based on the Eulerian–Lagrangian frame was established for an R410A closed-loop spray cooling system. The results revealed that the spray pattern is conical, with the center velocity significantly higher than the edge velocity. The temperature distribution of the cooling surface and liquid film height both exhibit a "W" shape, and the surface temperature is lower where the liquid film is thin. There is an optimal liquid film height of approximately 5 μm, at which the cooling surface temperature is the lowest. The surface temperature increases with an increase in the spray chamber pressure. Considering average cooling surface temperature, the optimal tilt angle is 40° with an average surface temperature of 330.1 K. When considering wall temperature and wall heat transfer coefficient uniformity, however, the optimal tilt angle is 10°, leading to the average surface temperature of 332.6 K. When increasing the optimal spray height to 70 mm, the average surface temperature is 313.4 K. [ABSTRACT FROM AUTHOR]

Infiltration through spray-pyrolysis deposition is a one-step and effective method for enhancing the electrode performance of solid oxide fuel cells (SOFCs) at intermediate temperatures. Among various infiltrated electrodes, Pr 6 O 11 has shown promising electrocatalytic properties for oxygen reduction reaction (ORR), despite its relatively low electrical conductivity. This study focuses on obtaining Pr 6 O 11 -infiltrated cathodes using spray-pyrolysis deposition, with different porous backbone layers that exhibit electronic and/or ionic conductivity: La 0.8 Sr 0.2 MnO 3-δ (LSM), Ce 0.9 Gd 0.1 O 1.95 (CGO), and LSM-CGO composite. The electrochemical properties and the subprocesses involved in the ORR are investigated in symmetrical cells. It is observed that the polarization resistance (R p) decreases when the backbone layer exhibits high ionic conductivity. At 650 °C, the R p values are 0.03, 0.06 and 0.1 Ω cm2 for CGO, LSM-CGO and LSM backbones, respectively. The dependence of R p contributions with pO 2 reveals that the same electrochemical processes are involved regardless of the backbone composition, confirming a less resistive oxide-ion transport from the TPB into the electrolyte with the CGO backbone. In contrast, the CGO backbone exhibits higher series resistance (R s) (44.5 Ω cm), attributed to an increased contribution of ohmic losses compared to the LSM backbone (39.8 Ω cm). This study provides valuable insights into the performance and optimization of Pr 6 O 11 -infiltrated cathodes with different backbone compositions for their implementation in SOFCs. [ABSTRACT FROM AUTHOR]

The processes of interaction of liquid droplets with solid surfaces have become of interest to many researchers. The achievements of world science should be used for the development of technologies for spray cooling, metal hardening, inkjet printing, anti-icing surfaces, fire extinguishing, fuel spraying, etc. Collisions of drops with surfaces significantly affect the conditions and characteristics of heat transfer. One of the main areas of research into the interaction of drops with solid surfaces is the modification of the latter. Changes in the hydrophilic and hydrophobic properties of surfaces give the materials various functional properties—increased heat transfer, resistance to corrosion and biofouling, anti-icing, etc. This review paper describes methods for obtaining hydrophilic and hydrophobic surfaces. The features of the interaction of liquid droplets with such surfaces are considered. The existing and possible applications of modified surfaces are discussed, as well as topical areas of research. [ABSTRACT FROM AUTHOR]

Evaporative cooling is an efficient approach for removing heat from nuclear reactors, solar power plants, solar panels, and energy storage devices, such as lithium-ion batteries and fuel cells. Nanotextured surfaces can provide improved evaporative cooling by increasing the total surface area to enhance the heat transfer rate and reducing the temperatures of local hotspots. In this study, we introduced rhombic dodecahedral zeolitic imidazolate framework-8 (ZIF8), a class of metal-organic frameworks, via impregnation to create nanotextured surfaces. The impregnation time was varied to obtain various thicknesses of ZIF8. We found that the increased surface area of ZIF8 improved convection cooling, which considerably reduced the temperature of the heated substrate. Air, mist (buoyant aerosols), and spray (inertial droplets) were independently used as coolants to compare the cooling performance of noncoated (bare) and ZIF8-coated substrates. Compared to the noncoated substrate, the optimal ZIF8 film yielded temperature reductions of Δ T = 13 °C and 10°C for air and spray cooling, respectively. [ABSTRACT FROM AUTHOR]

Spray cooling technology offers high levels of uniform heat removal with very low fluid volumes and has found recent application in relatively small-scale use cases. Since it is a complex process, research can enable spray cooling to be applied more widely and at larger scales, such as in HVAC, as a means to operate more efficiently. Weather conditions are one of the main parameters that directly affect the effectiveness of spray cooling. This study investigates the spray cooling performance for temperature and humidity conditions in six Australian cities. ANSYS Fluent (2021 R1) software is applied for the numerical simulation. The numerical model is first validated with the available literature before a numerical simulation is conducted to assess each city throughout the year. These include Adelaide, Brisbane, Darwin, Melbourne, Perth, and Sydney. The spray cooling pattern, temperature, and humidity distribution, as well as the evaporation effect on different regions in Australia, is simulated and analysed based on the CFD technique. The results from this study indicate that weather conditions influence spray cooling for all cities, especially in summer. Along the wind tunnel, the temperature significantly drops at the spray cooling area, while the humidity increases. Due to the effect of spray cooling inside the wind tunnel, the temperature at the outlet is still lower than the inlet for all cases. However, the humidity at the outlet is higher than the inlet for all cases. [ABSTRACT FROM AUTHOR]

Spray cooling has been considered one of the most promising thermal control methods of high-heat flux devices. Most of the spray cooling research focuses on electronic components as the main application object to achieve higher heat dissipation heat flow in ambient temperature regions for small areas. Water is the most common cooling medium. This paper investigates the application of spray cooling thermal control over large areas. In this study, the heat-transfer characteristics of liquid nitrogen (LN2) for large areas was investigated by conducting experiments. The test surface is 500 mm × 500 mm, which was cooled by a nine-nozzle array. The spray nozzles used in the experiment were conical nozzles with an orifice diameter of 1.6 mm, a spray angle of 120°, and a spray height of 42 mm. Liquid nitrogen was forcefully ejected from nozzles by the high pressure of a liquid storage tank to cool the test surface. According to the cooled surfaces, spray directions, and spray pressures, three groups of experiments were conducted. The results showed that the smooth flat surface has the best heat-transfer performance in three kinds of surface structures, which are macro surface, porous surface, and smooth flat surface. The heat-transfer coefficient varied by ±20% with different spray directions, and the surface heat-transfer coefficient increased linearly with increasing spray pressure. Most of the spray cooling research focuses on heat dissipation in the ambient temperature region for equipment over small areas. The results can benefit thermal control application in various fields. The research in this paper can provide a reference for the application of large-area spray cooling, and the application areas mainly include metal manufacturing processing cooling, aircraft skin infrared radiation characteristics modulation, and laser weapon equipment cooling. [ABSTRACT FROM AUTHOR]

Spray cooling is a heat transfer technology that has already shown its advantages and limitations. There has been increasing interest from academia and industry in combining this technology with nanofluids as coolants, owing to their potential for heat transfer enhancement. Nevertheless, there is a lack of understanding of the physical mechanism leading to this enhancement with the presence of technical problems that prevent the use of nanofluids in spray cooling applications. In this study, we investigate the effect of water-based TiO 2 nanofluids on both spray characteristics and heat transfer using an industrial full-cone pneumatic nozzle. For this purpose, three mass concentrations (0.05 wt.%, 0.1 wt.%, and 0.2 wt.%) were prepared and tested. We monitored the droplet sizes and velocity profiles with a particle dynamics analysis system. Moreover, the temporal temperature decrease of a heated aluminum block from 190 to 65 °C was measured via an infrared camera to calculate the heat transfer rate and heat transfer coefficient. The presence of nanoparticles is shown not to substantially alter the spray characteristics. Moreover, heat transfer is augmented mainly in the boiling regime due to more nucleation sites formed by the deposited nanoparticles. However, in the non-boiling regime, the contribution of adsorbed nanoparticles to the heat transfer enhancement diminishes. Overall, the aluminum block is cooled down 6%, 12%, and 25% faster than the DI water by the nanofluids at 0.05 wt.%, 0.1 wt.%, and 0.2 wt.%, respectively, including boiling and non-boiling regimes. [ABSTRACT FROM AUTHOR]

The thermal management problem of IGBT is becoming more and more prominent, and it is urgent to develop a new cooling technology with high efficiency, stability and flexibility. Based on the main problems and bottlenecks in the current IGBT module cooling, was conducted a detailed analysis and evaluation of the widely used IGBT cooling technologies, as well as spray cooling, jet impingement cooling technology and micro-channel cooling technology, which are the hot spots of the current research. It shows that compared with other cooling technologies, micro-channel cooling technology, especially microchannel flow boiling cooling technology, has the advantages of compact structure, strong heat transfer capacity, high heat transfer coefficient, less working fluid charge, and good temperature uniformity performance, which have great development prospects in the field of thermal management of electronic devices. Thus, in this paper, it was proposed that microchannel flow boiling cooling is an optimal solution to the thermal management of new electronic devices, which provides an important reference for the design and application of IGBT module heat sinks. [ABSTRACT FROM AUTHOR]

Spray cooling has demonstrated advantages in the thermal control of avionic devices. In this study, a spray cooling system is established using liquid nitrogen (LN2) as a working fluid to investigate the effect of heat transfer characteristics on a large heating surface. The temperature response measurement and experimental analysis of the heating surface of a large-area shell structure under the boundary conditions of different heat flux and temperature regions were carried out. The temperature response curves for the outer surface of the shell structure from the initial temperature to the set temperature were obtained under different heat flows and temperature zones. The results show that the thermal response time of the surface spray phase cooling is less than 2 s, the stability time for the cooling temperature of the covering layer is longer than 60 s, and the cooling state can be maintained for 30 min under different working conditions. This study can provide a technical reference for the application of spray systems in the field of aircraft cooling. [ABSTRACT FROM AUTHOR]

With the continuous integration and miniaturization of electronic devices, the heat transfer of the electronic devices continues to surge. This means that thermal management equipment with higher heat flux cooling capacity is required to maintain its normal operation. This paper systematically reviews the progress of spray cooling. In the first part, the thermal dissipation mechanism of spray cooling in the non-boiling regime and boiling regime are summarized, and the correlation formula of heat transfer is summarized. In the second part, the influencing factors of various parameters of the nozzle are summarized, the experimental research and numerical simulation research are summarized separately, and some means and methods to strengthen heat transfer are listed. In the third part, we summarize the current application research of spray cooling in some hot new fields, including electronic technology, aerospace, biomedicine, battery safety, etc. The research prospects and challenges in these fields are highlighted. This research provides a timely and necessary study of spray cooling. [ABSTRACT FROM AUTHOR]

Kingfishers during rain remain dry even standing still on a branch, because of converting normally vertical impacting to horizontal droplets transport, and bioinspired applications with the mixed-wettability surface exhibit the maximum horizontal transport distance by up to 6.2 D 0. [Display omitted] Kingfishers stand on a branch, and raindrops tumble translationally from feathers during raining, enlightening functional surfaces design and liquid transport control. Far-ranging studies on oriented transportation are confined to vertical impacting, which is, to date, in-depth philosophy of horizontal droplets transport on motionless surface deems to be rather serviceable. This study, employed mixed-wettability surface inspired by kingfishers' feather, occurs on directional transportation issues, such as the synergies of wettability-controlled, driving force and transportation capability. Here we conduct both experimental testing and CFD-aided numerical modelling to reproduce the asymmetric bouncing and directional transport phenomena. We found that the anisotropic surface manipulates to convert normally vertical impacting to horizontal droplets transport. Law of the thrown droplet, on the other hand, is predominated by the wettability-controlled surface, while the coexistence of contact angle difference and surface offset location cooperatively dictates the intensity and patterns of the thrown droplet. Of all these factors, the post-optimized surfaces are designed first and then the regime map of transportation pattern is elaborated. Results manifest that the elements induce the maximum horizontal transport distance by up to 6.2 D 0 , and first desorption time is only 7.8 ms. The findings shed light on engineering design principles that can pave the way for novel applications in anti-icing, lubrication, and spray cooling. [ABSTRACT FROM AUTHOR]

Micro-structured surfaces can affect heat transfer mechanisms because of enlarged specific surface areas. However, employing the Leidenfrost effect during liquid nitrogen (LN2) droplet cooling of a heated micro-structured surface possessing a fin with a spacing much smaller than the diameter of the droplet has not yet been explored. In the present work, a direct numerical simulation (DNS) is carried out to investigate heat transfer mechanisms of the LN2 droplet, whose diameter is sufficiently larger than the structured spacing of fin, impinging on a micro-structured surface with variable velocities. For a comparative study, a smooth surface is also employed in numerical simulations. The spreading mechanisms and vaporization behavior of the droplet along with liquid film morphology at various conditions are investigated. Results show that a smaller fin size inhibits LN2 in entering into the grooves between the fins and left the surface untouched by the droplet completely, and eventually, a thinner liquid film is spread out in contrast to the smooth surface. Notably, at a low Weber number, the droplet can be shrunk or even rebounded away from the wall after impinging on the wall. The fastest vaporization behavior for both surfaces, namely smooth and micro-structured, is obtained at a Weber number of 180. Additionally, an effective heat transfer upon the micro-structured surface is observed at a low impinging velocity of the droplet. [ABSTRACT FROM AUTHOR]

As one of the most promising thermal management solutions, spray cooling has the advantages of high heat-transfer coefficient and maintaining a low temperature of the cooling surface. By summarizing the influential factors and practical applications of spray cooling, the current challenges and bottlenecks were indicated so as to prompt its potential applications in the future. Firstly, this paper reviewed the heat-transfer mechanism of spray cooling and found that spray cooling is more advantageous for heat dissipation in high-power electronic devices by comparing it with other cooling techniques. Secondly, the latest experimental studies on spray cooling were reviewed in detail, especially the effects of spray parameters, types of working fluid, surface modification, and environmental parameters on the performance of cooling system. Afterwards, the configuration and design of the spray cooling system, as well as its applications in the actual industry (data centers, hybrid electric vehicles, and so on) were enumerated and summarized. Finally, the scientific challenges and technical bottlenecks encountered in the theoretical research and industrial application of spray cooling technology were discussed, and the direction of future efforts were reasonably speculated. [ABSTRACT FROM AUTHOR]

In recent years, global automotive industries are going through a significant revolution from traditional internal combustion engine vehicles (ICEVs) to electric vehicles (EVs) for CO2 emission reduction. Very similarly, the aviation industry is developing towards more electric aircraft (MEA) in response to the reduction in global CO2 emission. To promote this technology revolution and performance advancement, plenty of electronic devices with high heat flux are implemented on board automobiles and aircraft. To cope with the thermal challenges of electronics, in addition to developing wide bandgap (WBG) semiconductors with satisfactory electric and thermal performance, providing proper thermal management solutions may be a much more cost-effective way at present. This paper provides an overview of the thermal management technologies for electronics used in automobiles and aircraft. Meanwhile, the active methods include forced air cooling, indirect contact cold plate cooling, direct contact baseplate cooling, jet impingement, spray cooling, and so on. The passive methods include the use of various heat pipes and PCMs. The features, thermal performance, and development tendency of these active and passive thermal management technologies are reviewed in detail. Moreover, the environmental influences introduced by vibrations, shock, acceleration, and so on, on the thermal performance and reliability of the TMS are specially emphasized and discussed in detail, which are usually neglected in normal operating conditions. Eventually, the possible future directions are discussed, aiming to serve as a reference guide for engineers and promote the advancement of the next-generation electronics TMS in automobile and aircraft applications. [ABSTRACT FROM AUTHOR]

The production of electricity from photovoltaic panels has experienced significant developments. To manage the energy flows introduced into the electricity grid, it is necessary to estimate the productivity of PV panels under the climatic conditions. In this study, a photovoltaic panel is modelled from thermal and electrical points of view to evaluate electrical performance and identify the temperature distribution in the layers. The analysis performed is time dependent and the problem is solved using the finite difference technique. A methodology is introduced to estimate the cloudiness of the sky, which affects radiative heat exchange. The calculation method is validated using experimental data recorded in a laboratory of the University of Calabria. Temperature and electrical power are predicted with RMSE of 1.5–2.0 °C and NRMSE of 1.2–2.1%, respectively. The evaluation of the temperature profile inside the panel is essential to understand how heat is dissipated. The results show that the top surface (glass) is almost always colder than the back of the panel, despite being exposed to radiation. In addition, the upper surface dissipates more heat power than the lower one. Cooling systems, such as spray cooling, work better if they are installed on the back of the panel. [ABSTRACT FROM AUTHOR]

This study introduces a methodology to compute spray cooling in the nucleate boiling regime. It examines the cooling of a horizontal circular tube, although the approach is not restricted to this geometry. While numerous investigations have considered nucleate boiling in the past, these are usually restricted to spray impingement onto a flat plate and not onto more complex surfaces. Furthermore, most prior studies are highly empirical in nature. The present study circumvents both of these restrictions. On the one hand, with more complex surfaces additional factors must be considered, in particular the influence of the local impingement angle on the number flux of impinging drops. On the other hand, the heat transfer for single droplets is first computed using existing theoretical solutions and then the principle of superposition is used to compute the global heat transfer from the surface over which the spray impinges. Accompanying this computation, a drop size distribution is specified in order to also investigate the influence of not only one representative drop size, but also the variance of drop size distribution on the heat transfer, an influence seldom reported on. Furthermore, the probability of drop-drop interaction on the surface has been estimated and the heat transfer correspondingly adjusted. This approach of computing total net heat transfer on a surface is first validated using available measurements in which the heat transfer in the nucleate boiling regime from a flat plate were used for comparison. The heat transfer from a circular tube was then computed as a function of mass flow rate, drop size distribution (expectation and variance) and wall temperature. Conditions for maximum heat transfer, corresponding to the critical heat flux, were identified. In all cases the net heat transfer was limited by the onset of drop-drop interaction on the substrate, i.e. the net coverage of liquid on the surface. The results of these parametric studies indicate clearly how the mass flow rate can be adjusted to maximize heat transfer while avoiding excess fluid consumption. Furthermore, the Sauter mean diameter is shown to be an appropriate drop size to use, if no further information is available concerning the size distribution. • A non-empirical methodology to computing heat transfer in the nucleate boiling regime. • Spray cooling computation taking into account explicitly the size distribution of drops. • Insight into spray conditions, which maximizes heat transfer while minimizing liquid consumption. • Consideration of highly varying number density of drops when cooling a surface of complex geometry. [ABSTRACT FROM AUTHOR]

Spray cooling is an efficient cooling technology with high heat dissipation, and has been extensively utilized in various industry areas. The interaction mechanism between the spray droplet and environmental space with high pressure and temperature is extremely complex, especially with non-condensable gas. In this paper, a pseudopotential multicomponent multiphase lattice Boltzmann model for droplet migration with non-condensable gas in high temperature and pressure was proposed. The mechanism of transfer characteristics for droplet in steam were studied. It is found that the mass transfer of droplet is controlled by density and condensation, droplet growth is mainly impacted by density at initial stage, and in later period, the growth is greatly weakened by the condensation suppression of non-condensable gas. The calculated results show that when Fo > 0.4, mass transfer is significantly decreased. Heat transfer is also affected by non-condensable gas, and 2% gas declines the droplet heat transfer rate by about 10%. It is also found that the mass transfer rate is approximately linearly associated with the ratio of non-condensable gas content inside and outside the droplet. The results can be used for development of spray cooling model, and have positive guiding significance for optimizing spray cooling system. • A heat and mass transfer model of droplet and steam with non-condensable is established. • The mechanism of transfer characteristics for droplet in steam were studied. • The mass transfer of droplet is controlled by density and condensation in different stage. • The mass transfer rate is approximately linearly associated with the ratio of non-condensable gas content. [ABSTRACT FROM AUTHOR]

Cooling systems often rely on numerical calculations with precise heat transfer boundary conditions (HTBC), usually expressed as average values. The value of the average HTBC depends on the area in which they are determined. The paper presents a new approach showing the influence of the choice of the method of supplying liquid onto the surface on the non-uniformity of energy transfer from the cooled surface, which is important in the selection of the determination domain on the HTBC. The tests were carried out for two full-cone nozzles with different spray angles and two materials with different thermal properties. The local and average heat fluxes were determined for various distances of the nozzles from the surface and various liquid supply pressures. Cooling capacity, unevenness of energy transfer from the surface and quenching characteristics were evaluated and assessed using cooling curves. The study concludes that adopting HTBC based on average heat flux variation does not cause a significant error in the assessment of energy transfer during cooling with a nozzle with spray angle of 60° for EN 1.4845 steel. However, for Inconel and a nozzle with spray angle of 45°, the noticeable non-uniformity requires the application of a local HTBC. • Unevenness the spray cooling process. • Possibilities of using the average heat transfer boundary condition. • Recommendations for the use of local heat transfer boundary condition. [ABSTRACT FROM AUTHOR]

• Nano-droplets impinging on cylindrical wall is investigated. • Four modes of droplets impacting the cylindrical wall are reported. • Effects of droplet size, initial velocity, impact position and wall wettability on droplet dynamics are analyzed. • Physical mechanism of droplet impacting the cylindrical wall is revealed. The impact of droplets on solid walls is the primary behavior in spray cooling, seawater desalination, self-cleaning, and other technological processes. Based on the molecular dynamics (MD) method, the morphological evolution characteristics of nanodroplets impinging on the copper cylinder wall are studied. The effects of droplet size, initial velocity, impact position, and wall wettability on nanodroplet dynamics are analyzed. The results indicate that a single nanodroplet impacting on a nanocylinder exhibits four distinct impact modes: spread-partially wrapped-deposition (SPD), spread-completely wrapped-deposition (SCD), spread-completely wrapped-splitting-deposition (SCSD), and spreading-partially wrapped-splitting-deposition (SPSD). The trend of the influence of initial velocity and droplet size on the impact mode is similar. As the initial velocity and droplet size increase, the droplet's kinetic energy and inertia force increase, successively exhibiting the SPD, SCD, and SCSD modes. The trend of the influence of impact position and wall wettability on the droplet impact mode is similar. When the impact eccentricity distance is small, and the wall is highly hydrophilic, it exhibits the SCD mode. With increasing impact eccentricity and enhanced wall hydrophobicity, the adhesion force between the droplet and the cylinder wall decreases, making the droplet less likely to be captured and more prone to splitting, exhibiting the SPSD mode. The change in droplet centroid height and mean square displacement could predict whether the droplet undergoes a splitting mode, and its influence is most significant at the impact position. Constrained by the cylindrical wall, the droplet impacts radially and spreads primarily in that direction. With the enhancement of wall hydrophilicity, the migration region of the three-phase contact line expands, and the droplet spreading transforms into synchronous development along the radial and axial directions. The research results can enrich the dynamic mechanism of micro-nano scale droplet impact and provide potential guidance for optimizing relevant technologies. [ABSTRACT FROM AUTHOR]

• The high temperature cylindrical surface cooled by spray was simulated. • The axial tilt spray can effectively solve the hot spot problem and improve the cooling uniformity. • The temperature and wall heat flux distribution of the cooling area under different spray angles were analyzed. • Only when the spray boundary is tangent to the cylindrical surface (H = 70 mm), the circumferential tilt Angle has a positive effect on the wall heat flux. In the hot-rolled seamless steel tube production process, quenching is a crucial process. Spray cooling technology can achieve uniform and high-intensity cooling during the steel tube production process, which is expected to improve production efficiency and product quality. This study performs numerical simulations to explore the effects of spray cooling on a heated cylindrical surface. The influence of spray angles is focused on the cooling efficiency, while the spray height is also found to be another crucial variable. In the simulation, the axial tilt angle and circumferential tilt angle vary between 0 ∼ 30°, and the spray height changes within 50 ∼ 150 mm. A high-temperature steel tube is selected as the test object. The detailed analysis is discussed on the variations in the cooling area and wall heat flux under different spray angles. The results indicate that the axial tilt angle can effectively solve the hot spot. With an increase in axial tilt angle and spray height, the cooling area expands gradually. Meanwhile, the wall heat flux decreases with an increasing axial tilt angle and spray height. However, when the spray height is too high (H > 100 mm), the excessive axial tilt angle will have a negative influence on the wetting region. Although the circumferential tilt angle greatly changes the shape of the cooling area, it has a smaller effect on the size. It is worth noting that the spray boundary is just tangent to the cylindrical surface at H = 70 mm. Only at this moment does the circumferential inclined spraying exhibit a positive influence on the wall heat flux. [ABSTRACT FROM AUTHOR]

• A new model established to simulate the entire electrospray cooling process. • MPCMS can significantly improve cooling characteristics at low wall temperatures. • Increasing applied voltage can improve heat transfer characteristics. • Cooling performance is the best when inlet temperature is close to phase change peak. Electrospray cooling (ESC) technology is increasingly emphasized due to its efficient heat removal capacity, minor coolant consumption and accurate thermal control. Previous articles had focused on either experimental exploration on ESC performance or numerical methods on solitary electrospray such as Volume of Fluid [1] , Boundary Element Method [2] and Lagrangian model [3]. However, there is no comprehensive model characterizing the whole ESC process. Moreover, there was barely investigation on ESC enhancement using phase change materials (PCM). Herein, this study has bridged this gap by developing a new comprehensive model to explore the ESC enhancement with microencapsulated phase change material slurry (MPCMS), and conducted experiment to verify it. The influences of applied voltage, mass concentration and inlet temperature of MPCMS on the ESC were discussed. Results revealed that MPCMS offered superior cooling performance during melting process due to latent heat absorption compared to deionized water. Nevertheless, as wall temperatures increased, the deposition, higher viscosity and lower thermal conductivity of MPCMS would compromise cooling efficiency. Moreover, higher applied voltage enhanced cooling performance by increasing droplet velocity, and reducing the possibility of droplet rebound. When the inlet temperature approached the peak melting temperature, the convective heat transfer coefficient reached its maximum. [ABSTRACT FROM AUTHOR]

Background: In contrast to traditional impact-based testing, the Dynamic Tensile Tear Test (DT3) has shown great potential to characterise high-grade pipeline steels as it mimics the in-field pipeline conditions. However, material characterisation using the DT3 method has only been performed at room temperature and lower-shelf characterisation has not yet been investigated.This study investigates a solution to perform low-temperature characterisation and analyse dynamic brittle fracture behaviour using the DT3 setup.A large-surface spray cooling system using liquid nitrogen was constructed and implemented onto the DT3 system. Cooling performance and temperature uniformity were assessed using thermocouples across a large surface area up to 412 cm2 (2 × 206 cm2). Numerical validation was performed through Finite Element (FE) analysis using the Modified Bai-Wierzbicki (MBW) material model. A combined stress–strain criterion was used to take into account cleavage failure.Temperatures down to -125 °C were reached using the spray cooling system and a fracture tests was performed at -80 °C. The obtained data and resulting fracture surface indicated clear brittle fracture behaviour. An average crack velocity of 152 m/s was measured, which is in the same order of magnitude associated with crack velocities observed in large-scale testing.The constructed spray cooling system proved to be capable of cooling a large volume down to cryogenic temperatures while achieving acceptable temperature uniformity. Lower-shelf characterisation of X70 grade pipeline steel was achieved using the DT3 method and a good correlation was obtained between numerical data and experimental observations.Objective: In contrast to traditional impact-based testing, the Dynamic Tensile Tear Test (DT3) has shown great potential to characterise high-grade pipeline steels as it mimics the in-field pipeline conditions. However, material characterisation using the DT3 method has only been performed at room temperature and lower-shelf characterisation has not yet been investigated.This study investigates a solution to perform low-temperature characterisation and analyse dynamic brittle fracture behaviour using the DT3 setup.A large-surface spray cooling system using liquid nitrogen was constructed and implemented onto the DT3 system. Cooling performance and temperature uniformity were assessed using thermocouples across a large surface area up to 412 cm2 (2 × 206 cm2). Numerical validation was performed through Finite Element (FE) analysis using the Modified Bai-Wierzbicki (MBW) material model. A combined stress–strain criterion was used to take into account cleavage failure.Temperatures down to -125 °C were reached using the spray cooling system and a fracture tests was performed at -80 °C. The obtained data and resulting fracture surface indicated clear brittle fracture behaviour. An average crack velocity of 152 m/s was measured, which is in the same order of magnitude associated with crack velocities observed in large-scale testing.The constructed spray cooling system proved to be capable of cooling a large volume down to cryogenic temperatures while achieving acceptable temperature uniformity. Lower-shelf characterisation of X70 grade pipeline steel was achieved using the DT3 method and a good correlation was obtained between numerical data and experimental observations.Methods: In contrast to traditional impact-based testing, the Dynamic Tensile Tear Test (DT3) has shown great potential to characterise high-grade pipeline steels as it mimics the in-field pipeline conditions. However, material characterisation using the DT3 method has only been performed at room temperature and lower-shelf characterisation has not yet been investigated.This study investigates a solution to perform low-temperature characterisation and analyse dynamic brittle fracture behaviour using the DT3 setup.A large-surface spray cooling system using liquid nitrogen was constructed and implemented onto the DT3 system. Cooling performance and temperature uniformity were assessed using thermocouples across a large surface area up to 412 cm2 (2 × 206 cm2). Numerical validation was performed through Finite Element (FE) analysis using the Modified Bai-Wierzbicki (MBW) material model. A combined stress–strain criterion was used to take into account cleavage failure.Temperatures down to -125 °C were reached using the spray cooling system and a fracture tests was performed at -80 °C. The obtained data and resulting fracture surface indicated clear brittle fracture behaviour. An average crack velocity of 152 m/s was measured, which is in the same order of magnitude associated with crack velocities observed in large-scale testing.The constructed spray cooling system proved to be capable of cooling a large volume down to cryogenic temperatures while achieving acceptable temperature uniformity. Lower-shelf characterisation of X70 grade pipeline steel was achieved using the DT3 method and a good correlation was obtained between numerical data and experimental observations.Results: In contrast to traditional impact-based testing, the Dynamic Tensile Tear Test (DT3) has shown great potential to characterise high-grade pipeline steels as it mimics the in-field pipeline conditions. However, material characterisation using the DT3 method has only been performed at room temperature and lower-shelf characterisation has not yet been investigated.This study investigates a solution to perform low-temperature characterisation and analyse dynamic brittle fracture behaviour using the DT3 setup.A large-surface spray cooling system using liquid nitrogen was constructed and implemented onto the DT3 system. Cooling performance and temperature uniformity were assessed using thermocouples across a large surface area up to 412 cm2 (2 × 206 cm2). Numerical validation was performed through Finite Element (FE) analysis using the Modified Bai-Wierzbicki (MBW) material model. A combined stress–strain criterion was used to take into account cleavage failure.Temperatures down to -125 °C were reached using the spray cooling system and a fracture tests was performed at -80 °C. The obtained data and resulting fracture surface indicated clear brittle fracture behaviour. An average crack velocity of 152 m/s was measured, which is in the same order of magnitude associated with crack velocities observed in large-scale testing.The constructed spray cooling system proved to be capable of cooling a large volume down to cryogenic temperatures while achieving acceptable temperature uniformity. Lower-shelf characterisation of X70 grade pipeline steel was achieved using the DT3 method and a good correlation was obtained between numerical data and experimental observations.Conclusions: In contrast to traditional impact-based testing, the Dynamic Tensile Tear Test (DT3) has shown great potential to characterise high-grade pipeline steels as it mimics the in-field pipeline conditions. However, material characterisation using the DT3 method has only been performed at room temperature and lower-shelf characterisation has not yet been investigated.This study investigates a solution to perform low-temperature characterisation and analyse dynamic brittle fracture behaviour using the DT3 setup.A large-surface spray cooling system using liquid nitrogen was constructed and implemented onto the DT3 system. Cooling performance and temperature uniformity were assessed using thermocouples across a large surface area up to 412 cm2 (2 × 206 cm2). Numerical validation was performed through Finite Element (FE) analysis using the Modified Bai-Wierzbicki (MBW) material model. A combined stress–strain criterion was used to take into account cleavage failure.Temperatures down to -125 °C were reached using the spray cooling system and a fracture tests was performed at -80 °C. The obtained data and resulting fracture surface indicated clear brittle fracture behaviour. An average crack velocity of 152 m/s was measured, which is in the same order of magnitude associated with crack velocities observed in large-scale testing.The constructed spray cooling system proved to be capable of cooling a large volume down to cryogenic temperatures while achieving acceptable temperature uniformity. Lower-shelf characterisation of X70 grade pipeline steel was achieved using the DT3 method and a good correlation was obtained between numerical data and experimental observations. [ABSTRACT FROM AUTHOR]

• The transient liquid film deposition of cryogen spray cooling was investigated. • The CAJ enhanced the CAJ-to-spray mixing and mitigated the film thickness. • The CAJ-to-spray interaction induced CRVP that caused asymmetric film coverage. • The CRVP permitted fast evaporation with dominant CAJ at the spray-periphery while permitting fast evaporation with dominant spray at the spray-core. • The experimental film intensity and simulated film thickness were linearly correlated and significantly influenced the surface temperature. Cryogen spray cooling (CSC) has become a promising auxiliary-cooling technique for laser dermatology, which recently enhanced by cold air jet (CSC-CAJ). However, the CAJ-to-spray interactions could worsen the CSC performance and associated liquid film behavior. In this study, the transient behavior of liquid film deposition and surface heat transfer of CSC-CAJ was investigated. A high-speed camera was used to capture the film intensity through Mie-scattering technique. Fast-response TFTC thermocouple was used to measure the transient surface temperature. The large eddy simulation (LES) was used to extract the vortical structure, while the discrete-phase model (DPM) and Eulerian wall film (EWF) were coupled for the spray process and the transient film modeling. The results showed that the CAJ significantly enhanced CAJ-to-spray mixing and mitigated the film thickness (i.e., 32 % reduction). The CSC-CAJ had asymmetric film coverage due to induced counter-rotating vortex-pairs (CRVP) from the CAJ-to-spray interaction, which laterally clustered the spray. It permitted fast evaporation with dominant CAJ at the spray-periphery while permitting fast evaporation with dominant spray at the spray-core. Consequently, the CSC-CAJ showed 75 % temperature increment at the windward side and 15 % temperature reduction with 60 % heat transfer coefficient enhancement at the impingement point. The experimental film intensity and simulated film thickness were linearly correlated and significantly influenced the surface temperature. The combined results affirmed the significant impact of the CAJ on the transient film deposition, which potentially helps promote advanced cooling technology. [ABSTRACT FROM AUTHOR]

• The heat transfer enhancement and surface temperature non-uniformity are numerically studied. • The radial groove surface has a stronger heat transfer capability than the straight groove surface. • There is an optimal groove width and optimal groove number on the radial groove surface. • A novel enhanced surface achieves maximum heat flux with a small heat transfer area. • The design of the enhanced surfaces should focus on the flow and distribution of the liquid film more. A numerical model of spray cooling for enhanced surfaces in the non-boiling regime is created with deionized water as cooling medium. This model is validated by contrasting the simulated data with the corresponding experimental data, and the relative deviation is within 12 %. Compared with a smooth surface, the area of the enhanced surface is expanded, the cooling performance is significantly improved, but the surface temperature distribution is less uniform. Compared to the straight groove surface with the same area, the heat flux of the radial groove surface is higher because its surface structure is more conducive to the spreading and leaving of the liquid film. Studies of the effects of the structure parameters (groove width and groove number) of the radial groove surface on the heat transfer show that there is an optimal groove width (0.2 mm) and groove number (45) for the strongest cooling capacity, and both the velocity and thickness of the liquid film are significant factors in cooling performance. The faster the liquid film velocity, the thinner the liquid film and, thus, the better the heat transfer. Motivated by these findings, three novel enhanced surfaces are machined to increase liquid film fluidity and heat transfer area even further. The cooling performance of the three novel enhanced surfaces is further improved, and the slope groove surface acquires the strongest cooling capacity due to the acceleration of liquid film flow by gravity, although its heat transfer area is not the largest. Research on all enhanced surfaces indicates that thinner liquid film and faster liquid film flow should be the primary design goals of the enhanced surface, followed by an increase in heat transfer area. [ABSTRACT FROM AUTHOR]

This paper presents an evaluation of the residual bond behaviour between rebar and recycled aggregate concrete (RAC) using granite natural coarse aggregates and different recycled coarse aggregate (RCA) replacement levels (δ = 0%, 30%, 50%, and 100%) for both pull-out and cubic specimens. After high-temperature exposure (T = 200, 400, and 600 ℃) followed by water spray cooling, the results revealed a consistent decline in bond strength and compression strength with increasing temperature. Compared to ambient conditions, bond strength and compression strength decreased by 65.2% ∼ 77.8% and 53.2% ∼ 58.4%, respectively, after exposure to 600 ℃. The reduction rate of residual bond strength changed significantly at about 400 ℃. There was a linear relationship between RAC compression strength and rebar–RAC bond strength after high temperature exposure followed by water spray cooling. With increasing RCA replacement levels, the relative bond strength gradually improved at the same temperature. Based on the regression analysis, empirical formula for the rebar–RAC bond strength and bond–slip relationship models were proposed, all taking into account RCA replacement levels and temperature. • HTWSC is high-temperature exposure followed by water spray cooling. • Compressive strength of RAC with 100% RCA replacement level is approximately 70% of that of NAC after HTWSC. • Relative bond strength increases with an increase in RCA replacement level after HTWSC. • Prediction model for bond strength between rebar and RAC after HTWSC is developed. • Bond–slip relation between rebar and RAC after HTWSC is suggested. [ABSTRACT FROM AUTHOR]

In the nuclear energy utilization, the passive containment cooling system (PCCS) holds paramount importance, with its heat transfer performance standing as a critical aspect of nuclear safety. This study simulated the flow and heat transfer behaviors of PCCS with equal height difference by APROS software under a broad range of steady state operating conditions. At first, the flow pattern of natural circulation is categorized by phase state of flow through operational parameters. A criterion model of boundary of phase change is proposed with prediction deviation of less than ±25 %. Furthermore, the influence of various operation parameters such as temperature, non-condensable gas content, and pressure on the heat transfer behaviors of natural circulation was also revealed. A new heat transfer coefficient correlation for both single-phase and two-phase conditions is proposed with prediction deviation of ±15 %. Finally, the thermal stratification phenomenon in the containment vessel with non-condensable gas was investigated, indicating that local heat transfer characteristics in the upper region surpass those in the lower section on account of deposition of the non-condensable gas at bottom of the vessel. [ABSTRACT FROM AUTHOR]

We propose the high-performance evaporative spray cooling strategy by combining the hierarchical flake formation and the intermittent spraying. A simple, scalable, and cost-effective method was introduced to fabricate hierarchical flakes with multi-scale wicks where the size of the pores varies along the direction normal to the wick plane, which provides substantially improved wicking capability for capillary pumping. In addition, the intermittent spray methods allowed for more efficient evaporative cooling by enabling the reduction in spraying volume. Through the thermal management experiments, the hierarchical flake heat exchanger could provide up to ∼32.1% reduced amount of sprayed water and ∼ 1.8 times higher energy efficiency compared to the untreated aluminum heat exchanger during intermittent spraying. Using the quantitative analysis on the single-droplet evaporation and water retention ability, we systematically demonstrate the effects of the combination of hierarchical flake and intermittent spray scheme on the efficient thermal management. We believe that our outcomes of this work show the potential of the spray cooling method for the heat exchangers and present the scalable evaporative surfaces applicable to various evaporative cooling applications. • Hierarchical flake fin is fabricated via a scalable dipping method. • Wicking capability is quantified by a single-drop evaporation test. • Water retention ability is systematically evaluated during intermittent spraying. • Water savings and energy efficiency are demonstrated by a heat exchanger test. • Hierarchical flake with intermittent spray facilitates efficient thermal management. [ABSTRACT FROM AUTHOR]

• The high temperature cylindrical surface cooled by spray was simulated. • The effect of spray spacing on cooling was simulated, and the best spray spacing range was obtained. • The temperature and wall heat flux distribution of the cooling area under different nozzle array forms were analyzed. • It is pointed out that the unidirectional inclined array spray can obtain better cooling effect. Spray cooling technology is an efficient cooling method. In the industrial application, the large area cooling requires the multi-nozzle spray design. Reasonable selection of nozzle array form is particularly important. Therefore, the simulation is applied in this study to investigate the effect of the nozzle array on the spray cooling of steel pipes after the model is validated. A steel pipe with a diameter of 140 mm, a wall thickness of 20 mm, and a length of 350 mm is employed as the research object, with an initial temperature of 900 °C. The scheme has four options for double spray spacing: OLR = −15 %, 0 %, 15 %, and 30 %. The results show that when the OLR grows, the cooling area gradually diminishes, and the cooling area's average temperature and average surface heat flux fall and subsequently increase. When OLR=30 %, the best cooling zone shape is obtained. The minimum average temperature, 231 °C, is obtained when OLR = 15 %. After comprehensive analysis, the ideal spray spacing range is found at OLR = 15 %∼30 %. In order to study the nozzle array, spray nozzles are arranged in multiple rows according to OLR = 30 %. This study also explores the strategies of unidirectional inclined spray and opposite inclined spray for lessening the area of the hot spot. There are six nozzle array arrangements in total. By comparing the results, it is discovered that the inclined spray increases the cooling area and accelerates the surface cooling of steel pipes. Under the side-by-side unidirectional inclined spray and the staggered unidirectional inclined spray, the cooling areas are 146,868 mm2 and 142,914 mm2 respectively, the average temperatures are 157.09 °C and 154.28 °C respectively. It is indicated that side-by-side inclined spray and staggered inclined spray are the better nozzle array arrangement to obtain the maximum cooling efficiency. [ABSTRACT FROM AUTHOR]

Spray cooling experiments on the hot metallic surfaces with different initial temperatures were performed. This paper adopts a self-developing program which is based on the inverse heat transfer algorithm to solve the interfacial heat transfer coefficient and heat flux. The temperature-dependent interfacial heat transfer mechanism of water-air spray cooling is explored according to the wetting layer evolution taken by a high speed camera and the surface cooling curves attained by the inverse heat transfer algorithm. Film boiling, transition boiling, and nucleate boiling stages can be noticed during spray cooling process of hot metallic surface. When the cooled surface's temperature drops to approximately 369-424 °C. the cooling process transfers into the transition boiling stage from the film boiling stage. The wetting regime begins to appear on the cooled surface, the interfacial heat transfer coefficient and heat flux begin to increase significantly. When the cooled surface's temperature drops to approximately 217-280°C, the cooling process transfers into the nucleate boiling stage. The cooled surface was covered by a liquid film, and the heat flux begins to decrease significantly. [ABSTRACT FROM AUTHOR]

Multirow film cooling is an advanced cooling method to provide effective surface cooling for gas turbine components exposed to high temperature. In this paper, the performance of multirow film cooling under different configurations is investigated numerically. The geometrical parameters, including the hole arrangement, hole orientation, and trenched hole, are considered. The numerical setup is validated before the performances of different cooling configurations are compared. The flow and heat transfer characteristics of the various cooling configurations are analyzed, and the different row-to-row interactions are revealed. For the configurations with standard holes, the staggered arrangement of simple angle holes achieves better cooling efficiency than the counterpart with inline configuration, while the inline arrangement with opposite compound angle holes provides the best cooling efficiency. For the configurations with trenched holes, the trench promotes the coolant uniform mixing, and the coolant stays more close to the effusion plate. Thus, the multirow cooling efficiency with trenched holes is better than the cooling configurations with standard holes. [ABSTRACT FROM AUTHOR]

The paper presents the method and results of determination of heat transfer coefficient for air-atomized water spray cooling with consideration of infrastructural factors of industrial cooling conveyor, such as effect of accelerated air. The established values of heat transfer coefficient were implemented into a numerical model of cooling line, with special definition of sprayers and the movement of the part subjected to quenching. After quantitative validation on selected samples, the obtained coefficients were used for the solution of the technological problem by means of localized cooling rate enhancement, which forms a case study confirming reliability of the established water spray heat transfer functions and suitability of the determined models for design of thermomechanical controlled processing of complex-geometry parts. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of the microwave heating coupled with atomization feeding were investigated using ethanol as the spray medium on a pressure swirl nozzle. The effects of spray height, flow rate and temperature on the sauter mean diameter of atomized droplets were examined. The results showed that the droplet sauter mean diameter was 12-130 μm, which increased with the spray height and decreased with the flow rate and temperature of spray medium. Through the fitting of the experimental data, the dimensionless correlation of the droplet sauter mean diameter which was based on orifice diameter, Reynolds and Ohnesorge numbers was obtained. The calculated results were basically consistent with the experimental data within 15% error. The heat transfer characteristics of atomized droplets on high temperature surface of SiC bed heated by microwave were then investigated. The effects of spray flow rate, spray height, and spray temperature on the heat transfer characteristics were examined. The power of spray heat transfer decreased with the temperature and increased with the spray flow rate and spray height. The dimensionless correlation to describe the heat transfer characteristics of atomized droplets on the high temperature SiC surface under the microwave heating was obtained which included thermophysical properties of spray medium, spray parameters, and temperatures of the high temperature bed surface and spray medium, with the error of ±20%. These correlations can be used to predict the sauter mean diameter of the atomized droplets and the power of spray heat transfer in the microwave heating process. [ABSTRACT FROM AUTHOR]

• Experimental study on spray cooling with microencapsulated phase change suspension. • Latent heat of suspension improves spray cooling by up to 33.9 % relative to water. • Excessive suspension concentration results in the deterioration of spray cooling. • Optimal spray rate or height exists when liquid-film exactly covers heat surface. • Optimal inlet temperature is 10 °C lower than melting peak temperature of particle. This paper reveals the spray cooling heat transfer mechanism with the microencapsulated phase change suspension as the working fluid based on an experimental investigation. The heat transfer performances between the spray cooling and jet impinging with suspension are compared. A series of key parameters affecting the heat transfer are analyzed, including suspension concentration, nozzle diameter, spray height, inlet temperature, and spray inclined angle. It concludes that the spray cooling of suspension has a stronger heat transfer capability relative to the jet impinging, but the pressure loss is correspondingly larger. Compared with the water, the spray cooling heat transfer of phase change suspension is superior owing to the latent heat ability of particle core, and the maximum improvement in the current tests is 33.9 %. However, excessive suspension concentration can result in the deterioration of heat transfer because of the larger viscosity. There is an optimal volumetric flow rate and an optimal spray height for the spray cooling, where a suitable spray cone angle is present so that the heating surface may be exactly covered by the liquid-film. The latent heat absorption characteristic of phase change core leads to the existence of an optimal spray inlet temperature, which is about 10 °C lower than the melting peak temperature of the particle core. For the cases of incomplete-coverage, the decrease of the spray inclined angle may enlarge the liquid-film covering area and thence improve the heat transfer. A theoretical analysis indicates that the suspension in current tests has a high complete-melting rate (σ ≥ 93.7 %) when using in the spray cooling. The results may provide the beneficial support for the potential application of spray with phase change suspension for the cooling of electronic devices. [ABSTRACT FROM AUTHOR]

• Three sequential processes occur, including coalescence, spreading, retraction. • Spreading diameter and time are normalized by impact and stationary droplets. • Effects of Weber number, droplet volume ratio, surface wettability are clarified. • Maximum spreading coefficient elevates at larger Weber number or drop volume ratio. • An accurate theoretical model of the maximum spreading diameter is proposed. In numerous applications related to droplet impact, impacting droplets often interact with solid surfaces with adhesive droplets rather than dry surfaces, but the former is significantly less studied than the latter. Here, for the fundamental process of droplet impact on a stationary droplet on the surface, we investigate droplet dynamics and maximum spreading under various conditions. The spreading diameter and time are reasonably nondimensionalized as the spreading coefficient and dimensionless time by considering both the impacting and stationary droplets, and the complex influence of diameters of two droplets is reduced to the effect of a single droplet volume ratio. The proposed dimensionless framework provides a method to simplify influencing factors and facilitates the study of droplet dynamics under different Weber numbers, surface wettabilities, and droplet volume ratios. The maximum spreading coefficient increases with the increase of the Weber number or the droplet volume ratio, while it is nearly independent of surface wettability. A theoretical model of the maximum spreading diameter is established through energy analysis. By considering the coupled effects of various factors and modifying the viscous dissipation, the model has the merits of high prediction accuracy and broad applicability. Our findings can not only advance the basic understanding of the ubiquitous droplet impact phenomena, but also provide useful guidance for applications such as spraying cooling and inkjet printing. [Display omitted] [ABSTRACT FROM AUTHOR]

This work presents the experimental investigation of droplet boiling characteristics on superhydrophilic surfaces. These surfaces are prepared on a copper substrate, and the boiling characteristics are studied and compared with the bare copper surface. Two types of superhydrophilic surfaces exhibiting needle-like (CuO-1) and sheet-like (CuO-2) nanostructures are prepared by the chemical etching oxidation technique. The hierarchical surface structures are characterized by carrying out SEM and XRD analysis. We employ high-speed imaging along with temperature measurement to characterize the heat transfer phenomenon. The surface temperature varies from 100 °C to 190 °C, and the Weber number is maintained constant at 2.78, corresponding to the gentle droplet impact regime. The effectiveness in the heat transfer performance of the different surfaces is characterized by measuring the critical heat flux (CHF) and the Leidenfrost point (LFP), and it is correlated with the surface properties viz. roughness, wettability, and porosity. The superhydrophilic surface alteration reduces the contact angle by about 99.7% compared to the bare surface. This reduction in contact angle corresponds to an increase in Critical Heat Flux (CHF) by 69.95% for CuO-1 and 47.17% for CuO-2, respectively, compared to bare copper. It is also seen that the LFP is delayed on the superhydrophilic surfaces, with an increase in the LFP temperature by 16.67% and 14.28% for CuO-1 and CuO-2 coatings, respectively. The effect of structures on the atomization and formation of secondary droplets is also presented. We report the contact coefficient (k w) for superhydrophilic surfaces for the first time, which is helpful in the theoretical prediction of CHF. These results show that significant improvement in heat transfer can be obtained in spray cooling and hot spot cooling applications without resorting to sophisticated microfabrication techniques. • Significant enhancement in droplet boiling by cost-effective wet etching technique. • 70% increase in CHF and 17% increase in the Leidenfrost temperature. • Atomization is strongly dependent on surface microstructure. • Contact coefficient is reported for the first time for superhydrophilic surfaces. [ABSTRACT FROM AUTHOR]

The heat transfer characteristics of two hydrophilic surfaces, SM-QS3100 and ZCP-0010, were investigated in single-phase and two-phase zones in comparison to a smooth surface concerning spray cooling. The results showed that in the single-phase zone, the SM-QS3100 and ZCP-0010 hydrophilic surfaces could not enhance heat transfer, and the ZCP-0010 surface with better hydrophilicity had the worst heat transfer characteristic. Compared with the smooth surface, the heat transfer coefficients of the two surfaces decreased by 11.5% and 28.8% at a heat flux of 58.0 W/cm2. Within the two-phase zone, both surfaces positively influenced heat transfer, with the ZCP-0010 surface, being more hydrophilic, exhibiting a more notable enhancing effect. The heat transfer coefficients exhibited 12% and 11.5% increments at a heat flux of 151.7 W/cm2. However, the contribution of hydrophilic surfaces to heat transfer tended to be consistent with the heat flux increasing. Although the temperature homogeneity of each surface became worse, the temperature uniformity on the hydrophilic surfaces was better in both the single-phase and two-phase zones. [ABSTRACT FROM AUTHOR]

• The effect of flow rate and electric field intensity on electro-spray morphology is investigated. • The formation and transformation mechanism of each electro-spray jet mode are clarified. • The enhanced heat transfer strategy of electro-spray phase change cooling technology at elevated flow rates is proposed. • At a flow rate of Q = 800 ml/h and an applied voltage of V = 6 KV, the critical heat flux ( CHF ) can reach 100.8 W/cm2. This study has enhanced the application prospects of electro-spray in the cooling field. The advancement of semiconductor and microelectronics technologies has underscored the increasing importance of efficient heat management in electronic devices. To overcome the shortcoming of low flow rates of electro-spray, the present study is motivated by using visualization and heat transfer performance experiments to reveal the morphology evolution and heat transfer characteristics of electro-sprays with high flow rates. The results indicate that the morphology of electro-sprays is closely related to the electric field intensity and the flow rate, and can be classified into six modes: dripping mode, spindle mode, cone jet mode, multi-cone jet mode, simple jet mode, and oscillating jet mode. Furthermore, the formation mechanisms and transition rules of these modes are discussed in detail. In addition, the effects of the flow rate, voltage, and spray morphology on the heat transfer performance of electro-sprays was investigated, and strategies for enhancing heat transfer were subsequently proposes. At a flow rate is 800 ml/h and an applied voltage is 6 KV, the critical heat flux ( CHF ) can reach 100.8 W/cm2, and the maximum heat transfer coefficient ( h max ) can reach 14416.55 W/m2K. This study not only enhances the comprehension of high flow rate electro-spray technology, but serves as a vital reference for the design and application of electro-spray cooling devices. [ABSTRACT FROM AUTHOR]

There is an urgent need to quantify the heat transfer features of spray cooling involving high-temperature transition and film boiling regions. To meet this need, this study proposes integrating inverse heat-transfer computation with sparse regression for estimating the cooling heat flux of high-temperature spray cooling. A series of physical and numerical experiments was performed to verify and validate the proposed algorithm. The preliminary results indicate that the first-order Tikhonov regularization combined with the conjugate gradient algorithm is an efficient way to solve nonlinear inverse heat-conduction problems. Sparse regression is helpful to formulate time-varying boundary heat fluxes by filtering out appropriate functions from a library of candidate functions. The reconstructed boiling curves show that the high-temperature spray cooling experienced five heat transfer regions: the onset of spray cooling, the film boiling region, the transition boiling, the nucleate boiling region, and the single-phase liquid cooling region. Particularly, the film boiling dominates the high-temperature spray cooling, which accounts for, in our case, >80% of the cooling time. [ABSTRACT FROM AUTHOR]

• The combined heat transfer enhancement method is proposed for electrospray cooling. • The CHF is increased by 63 % with cubic-pin-fin surface as compared to the flat one. • The surface modification dominates heat transfer enhancement at 51 °C < T s < 119 °C. • The superior CHF of 7.93 W/cm2 is achieved for cubic-pin-fin surface at T s = 145 °C. • The superior heat transfer performance is achieved with an area ratio of 0.79. Electrospray (ES) cooling is promising to achieve the high-heat-flux thermal dissipation with a low fluid consumption when using structured surfaces as heat sinks of micro-size and high-power electronics. Nevertheless, available research is still limited in the fundamental of combined heat transfer enhancement owing to the macro-structured surface and electric field. In this study, four macro-structured surfaces machined with cubic pin fins, conical pin fins, wavy fins and straight fins are compared to one flat surface under electric field to shed lights on the combined heat transfer enhancement mechanism during ethanol ES cooling. The experimental results demonstrate that both electric field and surface modification can elevate the critical heat flux (CHF) and heat transfer coefficient. A CHF of 7.93 W/cm2 is achieved by the cubic-pin-fin surface with an applied voltage of 4.5 kV at T s = 145 °C, corresponding to a 63 % enhancement over the flat surface. The CHFs at first increase and then decline with the increased area ratio (f), and the superior heat transfer performance is achieved when f = 0.79. In the low temperature region (30 °C < T s < 51 °C), the enhanced convection and bubble formations caused by electric field are deemed as the dominated heat transfer mechanism. The increase in nucleation sites triggered by macro-structured surfaces governs the heat transfer in the mid-temperature region (51 °C < T s < 119 °C). Whereas, the heat transfer enhancement due to macro-structured surfaces declines in the high temperature region (T s > 119 °C), possibly as a consequence of the large thermal resistance induced by the Leidenfrost phenomenon. [ABSTRACT FROM AUTHOR]

• Application of MCrAlY coating to increase heat transfer during water spray cooling. • The use of the coating intensifies heat transfer compared to the uncoated surface. • The thickness of the coating affects the intensity of the heat removal. The main purpose of the paper was to determine the influence of NiCoCrAlY coating application and its thickness on the heat transfer during water spray cooling under surface boiling conditions. The scope of the research included: sensor preparation, measurements of the sensor temperature during cooling, and numerical calculations to determine the intensity of sensor cooling. The intensity of heat removal was determined by the average values of the heat flux on the cooled surfaces. The inverse method for the heat conduction equation was used to determine the heat transfer boundary conditions. Temperature measurements during cooling were carried out for an initial sensor temperature of 500°C, 700°C and 900°C and a liquid pressure 0.1 MPa, 0.2 MPa. Due to this, it was possible to select a coating thickness that contributed to the greatest extent to the increase in the intensity of heat dissipation from the hot surface. The highest values of the heat flux were obtained during the cooling of the sensor with a 134.5 μm coating. [ABSTRACT FROM AUTHOR]

Impinging droplets on a hot surface is a significant phenomenon in many industrial applications, such as spray cooling. Previous studies reported various morphological behaviours affecting the droplet's wetting, heat transfer, and evaporation characteristics. However, there is an ambiguity in understanding the influence of the surrounding medium on the resulting boiling regimes. The present work aims to study this aspect through an experimental investigation using high-speed photography and infrared thermography. During the experiments, the degassed FC-72 liquid droplets have impinged onto a heated chromium-plated calcium fluoride (C a F 2) surface with a constant impact velocity. Three ambient conditions, such as saturated vapour (heated vapour), air at liquid saturation temperature (heated air), and air at room temperature (ambient air), are used. The surface temperature is varied in each ambience until the complete droplet dewetting, i.e., the Leidenfrost phenomenon has appeared. In this study, the stages of boiling are perceived as film evaporation, excessive and nominal bubbly boiling, transition, and Leidenfrost. With different surrounding media, the onset of a regime and its boiling morphology are distinct. During the film evaporation regime, the droplet heat transfer is high for the impact in ambient air, and the contact line evaporation is dominant in the vapour medium. In contrast, heat transfer is improved in a vapour medium during the bubbly boiling process, reaching up to 30% enhancement compared to the droplets in air surroundings. It is attributed to increased nucleation sites and excessive bubbling compared to air media. Correlations from the literature are also revisited to understand the onset of bubbly boiling and the droplet breakup at the Leidenfrost point. An overall comparison of droplet heat transfer during the boiling regimes in surrounding media is discussed in this paper. • The effect of surrounding media on boiling regimes is experimentally investigated. • Boiling regimes are categorised into film evaporation, bubbly boiling, transition, and Leidenfrost point. • In each regime, the droplet's spreading and heat transfer are analysed using shadow photography and infrared thermometry. • The significance of contact line evaporation in various surroundings during the film evaporation regime is presented. • Correlations related to the onset of bubbly boiling and droplet breakup at Leidenfrost point are revisited. [ABSTRACT FROM AUTHOR]

The unpredictable spray characterization caused by different manufacturing standards still limits the implementation of spray cooling. To obtain a predictable spray, a droplet generator was in-house designed and fabricated, which can generate predictable monodisperse droplets or multiple droplet trains at a certain droplet generation frequency (f G). Based on the measurement of the droplet flux (N), the controlled droplet trains are classified into the controlled sparse spray and dense spray. In the experiments, deionized water was used as the liquid coolant, and heat transfer characteristics were analyzed in the controlled sparse spray and dense spray. The experimental results show at the same volumetric flow rate the heat transfer curve of dense spray is always above the heat transfer curve of sparse spray. In the cooling area of controlled dense spray, the average Nusselt number N u ‾ L was correlated as l o g N u ‾ L ∝ − (f G / N) 1 / 2 . This study would benefit the application of controlled spray cooling technology in high-heat-flux thermal management. [ABSTRACT FROM AUTHOR]

This paper examines the performance of a heat sink that combines an evaporator and an expansion device into a single component within a compact vapor compression refrigeration system designed for applications with high heat fluxes. The liquid refrigerant at high pressure expands through orifices, generating a spray that impacts a copper heater. The heated surface is enhanced with laser-micromachined micropillars featuring a square cross-section to improve the thermal conductance of the boiling liquid film. Qualitative evaluations of the enhanced surfaces using interferometry analyses determined the heat transfer surface area and assessed the effectiveness of the fabrication technique. The experiments involved using R-134a, a two-orifice expansion device, and enhanced surfaces with pillar edge sizes ranging from 200 to 600 μ m. This analysis encompasses the cooling system's thermodynamic performance, including compressor power and coefficient of performance, as well as steady-state heat transfer parameters such as convection coefficient and critical heat flux. The system achieved a cooling capacity of 230 W (35.9 W/cm 2), with the heater surface temperature staying below 22.9 °C and the highest average heat transfer coefficient reaching as high as 55 kW/(m 2 -K). This represents a 129% increase in thermal conductance compared to mirror polished surfaces, leading to a significant reduction in the temperature of the heated surface. Additionally, the area-corrected heat transfer coefficient is computed to verify the thermal resistance reduction unrelated to the surface area enhancement, and a 60% reduction is found. Furthermore, although this heat transfer enhancement shows minor to no influence on the thermodynamic performance of the refrigeration system, the heater cooling efficiency, defined as the ratio of the enthalpy rise delivered by the heat source to its ideal maximum, increased by 13%. The findings are supported by high-speed video sequences illustrating the boiling heat transfer features. • Compact refrigerator combines evaporator and expansion device in new spray heat sink. • Laser-micromachined surfaces provide two-fold enhancement during flow boiling. • Two-orifice expansion device produced cooling capacity of 230 W (35.9 W/cm 2). • Maximum recorded average heat transfer coefficient of 55 kW/(m 2 K). • The thermal conductance increased by 129% compared to a polished surface. [ABSTRACT FROM AUTHOR]

• Solid-liquid phase change can significantly increase the Nu in the single-phase region. • The higher the height of the cubic fins, the stronger the heat transfer enhancement. • The staggered arrangement of cylinder fins has the best heat transfer characteristics. • Micro-structured surface can lift the Leidenfrost point of boiling. • Empirical corrections with smooth and cubic fins surfaces are obtained. Spray cooling is receiving increased attention due to the highly efficient and uniform heat transfer. However, previous investigations seldomly focused on its enhancement with the combination of phase change materials (PCMs) and micro-finned surface. Herein, in this article, we enhance the spray cooling with NPCMS and micro fins to improve the heat capacity of coolant, heat transfer area and nucleate sites of contact boiling. Results show that two peaks of heat transfer coefficient are observed in sequence as heatflux increases, separately derived from latent heat absorption of melting of core PCM and nucleate boiling of carrying fluid. Compared to the smooth surface, the convective heat transfer coefficients under three cubic fin heights (0.2 mm, 0.3 mm and 0.4 mm) are improved maximally by 22.3 %, 17.6 % and 11.8 % in phase change of NPCMS, while the maximum improvements of 45.8 %, 39.8 % and 39.8 % are obtained in the nucleate boiling. The staggered arrangement of cylinder fins can induce stronger capillary force, which is responsible for the best enhancement of nucleate boiling. Also, the shortest survival time of droplet on the staggered cylinder fins are observed, confirming that capillary force is the reason of increasing heatflux and lifting Leidenfrost point of boiling. [ABSTRACT FROM AUTHOR]

This study examines spray cooling of horizontal, circular tubes when the surface temperature exceeds the Leidenfrost point, and the individual drop impacts are in the film boiling regime. Although film boiling is not an optimal condition for heat transfer, it is encountered in transient cooling processes or when high temperatures persist along the tube. The goal of the study is to apply existing models for drop impact and heat transfer to surfaces more complex than flat plates and to determine the overall steady state heat removal rate as an essential parameter for heat exchanger design. The spray characteristics are prescribed by a local number flux and drop size, assuming a random distribution in space and a uniform velocity of all drops. The angle of individual drop impingement on the cylindrical tube is accounted for through the maximum spreading diameter of the drop on the surface and the normal component of impact velocity. Furthermore, drop-drop interaction on the heated surface is accounted for at high local number flux values through an effective coverage area coefficient. The results are expressed in the form of a local heat transfer coefficient. These results confirm that the most influential parameters regarding heat transfer are the liquid mass flow rate in the nozzle and the drop diameter; however, they also indicate that the non-normal impact of drops over portions of the cylindrical tube leads to a highly reduced number flux; hence heat transfer. This oblique impact angle arises for one from the local spray angle, but more drastically from the curved, cylindrical surface; the latter leading to a dramatic sharp decrease in heat transfer. This is important information when considering the use of multiple spray nozzles, either circumferentially or longitudinally along the tube, since it dictates the optimal spacing between neighbouring nozzles. [Display omitted] • A methodology for predicting spray cooling in the film boiling regime is introduced. • The predication of heat transfer is based on theoretical analysis; only one empirical factor is introduced. • The local impingement angle of drops has been considered in the hydrodynamics of impact and in the influence on the local number flux. • Parametric studies indicate the influence of droplet size, substrate temperature, and mass flow rate on the heat transfer. • This study lays the basis for simulation of practical spray cooling scenarios involving multiple nozzles and/or nozzle arrays. [ABSTRACT FROM AUTHOR]

• The Eulerian-Lagrangian approach is utilized for conducting numerical investigations on the aerothermal process of water injection pre-compressor cooling in an S-shaped intake. • Based on the SAE standard, the transient distortion descriptors of the research cross-sections are quantitatively analyzed, which helps reveal the characteristics of various distortions in the flow field and their spatial-temporal variation processes. • The POD method is applied to identify the modal structures and frequency-energy characteristics of the pressure, velocity, and temperature fields at specific cross-sections. Water injection pre-compressor cooling technology can efficiently increase the engine thrust and expand the flight envelope of the aircraft. Changes in the curvature of the intake and the evaporative cooling of the spray medium naturally distort the total pressure, total temperature, and characteristics of the swirl of the flow before it enters the engine. This study conducts numerical research on the aerothermal process of the WIPCC in an S-shaped intake using the Eulerian-Lagrangian method. The spatial evolution of critical variables along the flow direction and specific cross-sections were obtained by analyzing the statistical time-averaged results of multiphase flow. The transient distortion descriptors were quantified and analyzed following the SAE standard, which revealed the characteristics of various distortions and their spatial-temporal changes. Furthermore, the variables reflecting the transient flow structure were decomposed using the POD method to obtain the coherent structure of each mode and its frequency and energy characteristics. The results showed that aerodynamic breakup was the main cause of droplet size reduction and led to a decrease of 74.4% and 63.8% in Sauter droplet size under two conditions, respectively. The downstream mixing of flows in the S-duct mitigated the distortions caused by the intake's changing curvature and droplet evaporation. After the mixing of flows under specific conditions, the total pressure distortion coefficient decreased from 0.30 to 0.12, the swirl intensity decreased from 4°-6° to 2°-4°, and the high total temperature extent decreased from 0.65 to 0.35. The POD mode of the variables exhibited broadband characteristics with multiple spikes and energy dispersion. The main modes of the drop coefficients in total pressure and temperature corresponded to large- and small-scale structures, respectively, with Strouhal number ranges of 0.079-0.394 and 0.362-1.260. These results reveal the effects of WIPCC on the aerothermal processes in the S-shaped intake and the mechanism of generation and evolution of distortions. Moreover, the results provide reasonable boundary conditions for numerical investigations of wet compression under intake distortions. [ABSTRACT FROM AUTHOR]