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Regeneratively cooled scramjets are successfully used as propulsion devices in hypersonic vehicles. During operation, scramjets experience acceleration. This special process causes a dynamic flow process, and heat transfer in the cooling channel commonly occurs, which may cause hazards and control difficulties for scramjets. A dynamic numerical model with a modified heat transfer coefficient calculation method was established to study the transient flow and heat transfer processes in a cooling channel. The dynamic characteristics of the flow and heat transfer under different conditions were calculated and are discussed, including the changes in the inlet fuel mass flow, heat flux, and pressure working conditions. The results indicate that the stable time of the cooling channel outlet fuel temperature is related to the rate of change in the inlet mass flow and heat flux. The stable time of the outlet fuel temperature under decreasing heat flux working conditions was approximately 12.5 s. These results summarize the dynamic flow and heat transfer characteristics, which are significant for designing cooling channels in scramjets. [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]

Spray cooling is a highly effective method of heat removal that has broad practical applications, including use in modern cooling systems designed for microelectronics and microchips. It is known that spray cooling performance is influenced by a huge number of factors. This experimental research is devoted to the study of the influence of a liquid flow rate in the range of 15.1–24.2 cm3/s, heat flux up to 6.4 MW/m2, and nozzle-to-surface distance on the heat transfer rate in non-boiling mode and the distribution of the local temperature of the heat exchange surface during spray cooling. It is shown that the heat transfer coefficient weakly depends on the heat flux for all studied nozzle-to-surface distances. It is demonstrated that the nozzle-to-surface distance has a significant influence on the heat transfer and the temperature distributionon the heating surface during spray cooling in non-boiling mode. At the same time, there is an optimal distance at which the maximum heat transfer rate and uniformity of the temperature are achieved. Criteria and a ratio for determining the optimal distance from the spray nozzle to the heated surface are proposed. [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.

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]

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]

For the objective of enhancing the heat transfer ability of spray cooling, a single-nozzle open-loop spray cooling experiment platform was established with a data measuring system. Based on the surface heat transfer coefficient obtained from the experiment, combined with the visualization system to observe the distribution of droplets during the spray cooling process, the influence of heating power, medium flow rate, nozzle height and typical additives on heating surface coverage and heat transfer characteristics were investigated. The criterion non-dimensional criteria equations for Nu, Re, Pr and size coefficients were fitted and analyzed in comparison with experimental data. The main conclusions are as follows: considering the temperature distribution characteristics of the heating surface and the shape of the spray cone, the heat transfer performance can be optimized by increasing the coverage rate under high heat flux when the flow rate changes, appropriately reducing the coverage rate under low heat flux, and appropriately reducing the coverage rate when the height changes, which creates complete coverage in the droplet concentration area to improve the surface heat transfer capacity. Furthermore, the heat transfer coefficients were improved by 29.3%, 21.8% and 23.8% with different additives (CTAB, ethanol and CTAB–ethanol mixtures) in the working fluid. Each solution had an optimal concentration and heat transfer deterioration was observed at high concentrations. When using non-dimensional criteria equations for parameter calculations, the data are more accurate after considering the effect of dimensional coefficients. [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]

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]

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]

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]

• 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]

• 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]

An experimental bench was constructed for liquid nitrogen spray cooling in an open loop system, and the combined effect of the spray height, heat sink surface diameter, and spray mass flow on the heat transfer characteristics of liquid nitrogen spray cooling was studied by comparing 50 sets of experimental data. In general, the heat transfer performance decreased with an increase in the spray height and heat sink surface diameter, with the latter's influence being more dominant. In the experiment, when the heat sink surface diameter increased to 24 mm, the influence of the spray height on the heat transfer performance decreased. The maximum critical heat flux and maximum heat transfer coefficient were 191.90 W·cm−2 and 16.67 W·cm−2·K−1. [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]

Nanofiber surface coating is a promising method for the enhancement of heat transfer during spray cooling. In the present work, the drop dynamics as well as local and overall heat transfer during single drop impact onto a heated wall covered with a polyacrylonitrile (PAN) nanofiber mat are investigated to obtain insight into the mechanisms governing the heat transport enhancement. The influence of wall superheat, drop impact velocity, and mat thickness on the hydrodynamics and heat transfer from the heated wall to the fluid is studied. The experiments were conducted inside a temperature-controlled test cell with a pure vapor atmosphere maintained with refrigerant FC − 72 (perfluorohexane). The temperature field at the solid–fluid interface was observed with a high-speed infrared camera, and the heat flux field was derived by solving a three-dimensional transient heat conduction equation within the substrate. The presence of the nanofiber mat on the heater surface suppresses the drop receding phase due to the pinning of the contact line at the end of the spreading phase. Two different heat transfer scenarios are observed depending on the wall superheat and drop impact velocity: scenario (I), in which the liquid drop completely penetrated through the porous nanofiber mat and made contact with the heater surface; and scenario (II) in which the vapor produced inside the pores of the nanofiber mat prevented the liquid drop from touching the heater surface. In scenario (I), an up 450% heat flow enhancement during the sessile drop evaporation stage has been observed. In scenario (II), a 80% heat flow enhancement at this stage has been registered. • Wall heat flux during drop impact on nanomat-covered hot substrates is measured. • The drop receding phase is suppressed in the presence of nanofiber mat. • Heat transfer scenario is determined by impact velocity and wall superheat. • Heat transfer in sessile drop phase is significantly enhanced by using nanomats. • Heat transfer enhancement is high at high impact velocities, low wall superheats. [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]

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]

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]

In this study, a closed-loop spray cooling system using R134a as the working fluid was established. The heat transfer characteristics and influencing mechanism of transient spray cooling were studied. The transient spray cooling curve under quenching was built accurately. The results show that the vapor film suppressed time, tsup, is the main period that the spray cooling must pass through. The flow rate and the sub-cooling of R134a have little effect on the cooling rate but the critical heat flux, which are mainly affected by chamber pressure. The transient Jacob number decreases with the increases of chamber pressure. As Jacob number decreases, the growth of vapor film is inhibited, then the tsup reduces in consequence. The surface temperature drop point and critical heat flux increases with the rise of chamber pressure. The maximum critical heat flux is 70.08 W/cm² in this experiment. [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]

• Droplet flow and misty flow are observed in long distance spray. • The maximum heat transfer coefficient is obtained in droplet flow stage. • Surface drying appears and the critical heat flux is reached in misty flow stage. • Increasing the spray pressure can increase the critical heat flux by 59.5 %. • Recommended spray operating parameters are obtained. Spray cooling has substantial potential for wide application, given its unique characteristics, including high heat transfer coefficient, absence of contact thermal resistance, and the elimination of cooling hysteresis. In this study, a visualized experimental system featuring a gas-liquid two-phase cyclonic nozzle was constructed to study both the heat transfer and fluid morphology features pertaining to the wall. With the increase of wall superheat, a progressive transition across four distinct stages of heat transfer – liquid film, stream flow, droplet flow, and mist flow can be found. Of these, the droplet flow stage outperforms others in terms of heat transfer performance and is thus advocated as the preferred mode of application. In response to increasing spray pressure, the droplets generated on the wall during the droplet flow stage undergo a reduction in size, yet become more densely distributed. A notable increase in critical heat flux by 59.5 % is observed when spray pressure is elevated from 0.2 to 0.4 MPa. While raising the spray flow rate leaves the heat transfer performance during the liquid film stage largely unchanged, it effects a significant augmentation in critical heat flux. Our experimental results ultimately obtain the optimal operating parameters that facilitate the wall surface's retention in the droplet flow stage. These experimental results are poised to contribute meaningfully to the engineering practices associated with spray cooling applications. [ABSTRACT FROM AUTHOR]

• Synergistic enhancement spray cooling use micropillars and capillary-driven wicking hybrid structures. • The coolant can self-supplied reach the target surface under the capillary force. • The circulation heat transfer enhancement process of wicking-rewetting-vapor evacuation. • HTC and maximum heat flux were improved 79.6% and 80.0%, respectively on the hybrid structure surface. • A maximum reduction of approximately 30.8 °C was obtained for ONB. Spray cooling, which has a fast thermal response and low flow rate consumption, is a common and effective thermal management approach for high power electronic systems. Typically, to improve the heat transfer coefficient (HTC) for spray cooling, the target surface wettability is decreased, promoting nucleation boiling by reducing the surface energy barrier for bubble nucleation. However, the merging of bubbles and the formation of a vapor film have limitations in further increasing the critical heat flux (CHF). Hence, maximizing both the HTC and CHF of spray cooling can be mutually exclusive on the same heated surface. This paper presented a novel design for a vapor-liquid separation structure surface. It consisted of lower micropillars that serve as the vapor separation channel. On top of that, there was an upper multi-layer copper micromesh that wicks and spreads the coolant. Additionally, nano-grass attached to the micromesh to enhance capillary force and increase the nucleation sites simultaneously. Subsequently, we calculated the forward velocity and pressure drop of the droplets passing through the copper mesh to study the effects of droplet velocity and capillary force on heat transfer performance. The results show that the forward velocity of the droplets decreases rapidly as the increase of copper mesh layers and it drops to zero when passing through the third mesh. Therefore, the capillary force generated by the copper mesh is another important factor for the atomized droplets to pass through the copper mesh and reach the target surface. We have proven that all the vapor-liquid separation structure surfaces satisfy the condition ΔP cap > ΔP pore + ΔP local. Hence, the synergistic effects of the micropillar, copper micromesh, and nano-grass enable a circulation process of wicking-rewetting-vapor evacuation, which improves the onset of nucleate boiling (ONB), enhances the HTC, and increases the maximum heat flux dissipated by the system. Enhanced HTC (7.94 W/cm2/K) and maximum heat flux (817 W/cm2) were achieved at 1.50 ml/s on the vapor-liquid separation structure surfaces (#200 2-layer), representing an enhancement of 79.6% and 80.0% compared with that for the flat surface and the temperature of the heated surface at the ONB maximum decreased by approximately 30.8 °C. [ABSTRACT FROM AUTHOR]

Spray cooling of hot substrates is a technology that can provide uniform and intense cooling of surfaces of complex geometry. It is applicable to a wide range of surface temperatures associated with different applications, from cryogenic cooling to steel quenching. The performance of spray cooling is determined by surface temperature, spray parameters such as drop diameter, impact velocity, number flux and the thermal properties of the liquid and substrate. These parameters also determine different regimes of drop wall interactions such as drop deposition, nucleate boiling, drop dancing, thermal atomization and film boiling with drop rebound. The present experimental and theoretical study focuses on the transitional boiling regime, which is currently the least studied regime due to its complexity. The residence time of a single drop is modelled, taking into account the development of a thermal boundary layer in the substrate, the expansion of the vapour phase due to boiling in the drop, and the percolation of the vapour channels on the substrate. Moreover, the model allows to predict the heat flux during spray cooling in the transitional boiling regime. The model agrees well with the experimentally measured heat flux of sprays with low number flux. [ABSTRACT FROM AUTHOR]

In response to the restrictions of the international community on the utilization of refrigerants with high GWP, the replacement of high-GWP refrigerants with non-azeotropic refrigerants and reduction in refrigerant charge have gradually become important research topics in academia. In this study, based on the VOF method, a 2D model of a sheet flow pattern was developed by considering the thermophysical properties and multicomponent phase change of a mixture. The falling-film evaporation of a non-azeotropic refrigerant composed of R32 and R134a outside a horizontal tube was numerically simulated. The effects of the spray height (H), tube diameter (d), inlet temperature (T inlet), Reynolds number (Re), heat flux (q), and mass fraction of R32 in the liquid-phase mixture (M R32_liquid) on the heat transfer coefficient (HTC) were studied. The results indicate that with the increase of circumferential angle (Φ), the local HTC exhibits a trend of decreasing, increasing, decreasing, and increasing in sequence. The largest contribution to the increase in the average HTC comes from the impact and fully developed regions. An increase in H , T inlet , and M R32_liquid is beneficial to the expansion of the fully developed region, whereas a decrease in d leads to expansion of the fully developed region. An increase in Re does not affect the extent of the fully developed region. The increase in H , Re , T inlet , and M R32_liquid increases the average HTC , but the effect of T inlet is not significant. Under the conditions of d = 24 mm, H = 13 mm, Re = 4000, M R32_liquid = 0.5, and q = 10 kW/m2, as the inlet temperature increases from 243.15 K to 283.15 K, the average HTC increases by 2.5%. An increase in q does not affect the average HTC , whereas an increase in d does not improve the average HTC. • Numerical model of falling-film evaporation of non-azeotropic refrigerant was developed. • Heat transfer coefficient (HTC) distribution along circumferential angle was concluded. • Largest contribution to HTC comes from impact and fully developed regions. [ABSTRACT FROM AUTHOR]

Spray cooling is an attractive solution to cool high heat flux dissipating surfaces. However, the modelling of its transport process is very challenging due to simultaneous interactions of multiple droplets with the heated wall. In this context, we have performed an experimental investigation of two consecutive droplets impacting on a heated substrate. First droplet (Droplet #1) impacts the hot surface at a certain We number, followed by oncoming droplet (Droplet #2) impacting the same substrate at the same We number, after a designated time (∼ thermal diffusion time scale). Stainless steel foil heater is utilized to provide constant volumetric heat generation, along with a desired initial wall temperature boundary condition. The temperature field of heated SS substrate is altered due to the impact of Droplet #1. Afterwards, Droplet #2 merges with Droplet #1, spreads, recedes, and the combined droplet mass attains a quasi-equilibrium shape. The variation of peak heat flux during impact of Droplet #2 is distinctly lower compared to the initial impact of Droplet #1. Further spreading during impact of Droplet #2 beyond the region of Droplet #1 leads to interaction of liquid with region of higher temperature substrate and results in greater net heat transfer at the peripheral region of the spreading combined droplet mass. The key differences during Droplet #2 impact is delineated compared to Droplet #1 impact on a dry heated substrate. Additionally, the role of We number of Droplet #2 and the initial substrate temperature on the ensuing heat transfer to the droplet is also reported. This study helps in understanding the nuances of such consecutive droplet interaction and its influence on applications such as spray cooling so that better models may eventually be developed, incorporating the experimental details and outcomes. [Display omitted] • Investigation of thermal transport during consecutive impact of two droplets. • Altered thermal boundary condition leads to significantly different variation of heat flux. • Higher heat flux happens at the peripheral region during the impact of second droplet. • Heat transfer rate rises with increase in the Weber number and temperature of the substrate. [ABSTRACT FROM AUTHOR]

Spray cooling has been widely employed in many applications due to its high flux removal ability. A previous study has been conducted to reveal the large-scale spray cooling performance of an industrial used single nozzle. Continuously, influence of multiple-nozzle distribution has also been numerically investigated in present work. The mean heat flux and its standard deviation and uniformity are used to qualify the cooling performance. A flat wall with 1.6 m in length and 1.0 m in width has been taken as the research object. Effects of nozzle number, distance and offset have been parametrically compared. It is found that increasing nozzle number could promote mean heat flux, improve the uniformity of cooling patterns and enhance heat transfer performance. A best nozzle number of 10 could be obtained by an equation fitting. Decreasing nozzle distance turns out to be detrimental to heat transfer. The reason comes from the collisions and interactions of two too adjacent nozzles. Based on choices in real practice, two types of arrays i. e. perpendicular and skew array have been discussed and compared. It is concluded that the skew array could obtain higher heat flux with more uniform distribution. [ABSTRACT FROM AUTHOR]

In the current work, by altering the vapour bubble formation and their coalescence rate, the Leidenfrost effect is tried to eliminate or minimize by dissolving carbon dioxide in the water spray. The heat transfer analysis indicates that the heat removal rate steps up with the increment in carbon dioxide concentration up to 8 gm/litre soda concentration in water and a further rise in carbon dioxide concentration, quenching rate declines. The droplet impact mapping and the film boiling analysis corroborate the aforesaid results. The evaporation experiment also supports above findings and indicates unalteration of boiling point. The spray cooling result shows 37% enhancement in critical heat flux (CHF from 1.2 MW/m2 to 1.67 MW/m2) at 8 gm/litre soda concentration in water. The comparison of the critical heat flux with the results reported in the literature clearly asserts the suitability of the current methodology for the fast cooling operation. • Heat flux increases with rise in soda concentration up to 8 gm/litre in water. • The highest CHF of 1.67 MW/m2 is obtained in case of 8 gm/litre soda concentration in water. • The droplet impact mapping regime corroborates the finding obtained in case of spray cooling. • The comparison confirms the appropriatness of the current process for fast quenching. [ABSTRACT FROM AUTHOR]

The effects of variation of spray characteristics (mass flux, inlet pressure, flow rate, nozzle tip to target distance and plate thickness) on heat flux were systematically investigated. The round spray released from a full cone internally mix atomizing nozzle was impinged with a wide range of air and water pressures on stationary hot steel surface of definite dimension. The effect of each parameter was examined while keeping others nearly fixed. Four different plate thicknesses i.e., 4 mm, 6 mm, 8 mm and 10 mm were tested and effect of plate thickness on heat transfer was determined. Surface heat flux at each experimental condition was computed from the transient temperature history measured by K-type thermocouples embedded at bottom surface of the plate. The mass impingement density was measured by the help of a simple mechanical patternator. The maximum surface heat flux of 4895.525 kW/m2 was achieved at an inlet water pressure of 4 bar, air pressure of 3 bar and nozzle height of 120 mm for an initial temperature of 850 °C of the 4 mm steel plate.

Highlights • The effects of the volumetric flux and frequency on intermittent spray cooling characteristics are determined. • The Strouhal number is introduced to account for impact frequency. • Correlations for single-phase, nucleate boiling and CHF are developed. Abstract Intermittent spray cooling is an active flow control technique to enhance spray cooling performance, increase the efficiency of coolant usage, and reduce pumping power consumption. However, the cooling characteristics and mechanism of intermittent sprays have not been investigated sufficiently, especially in the nucleate boiling region and critical heat flux region. Methods that model intermittent spray cooling are insufficient and require expensive and laborious spray dynamic characteristics measurements for each operating condition. In this study, experiments were performed to investigate the effect of frequency and volumetric flux on the heat transfer characteristics of intermittent sprays at both the single- and two-phase regimes. The results demonstrate that both the volumetric flux and frequency affect the heat transfer. The commutative impact of two consecutive injection cycles would enhance the drop impact and spray-wall interference, corresponding to the results of the boiling curve and critical heat flux (CHF). However, this impact becomes more remarkable with increasing frequency and is suppressed gradually as the volumetric flux increases. Based on these results, the mechanism and model on heat transfer in an intermittent spray are proposed. The Strouhal number, representing the relative effects between the impact frequency and volumetric flux, is introduced to account for intermittent features, based upon which correlations of single phase, nucleate boiling, and CHF are developed. [ABSTRACT FROM AUTHOR]

Highlights • The impact of geometrical surface modifications on the thermal performance of the spray cooling system was studied. • The maximum heat transfer enhancement was 80%, for surface modified with four circular and radial grooves. • A significant effect of nozzle differential pressure has been found on spray cooling thermal performance. Abstract An experimental investigation was conducted to study the impact of geometrical surface modification on the thermal performance of a spray cooling system. All experiments were performed using a closed loop spray cooling system. Deionized water was used as the working fluid. Three different modified surfaces were examined and compared with a plain copper surface under the same operating conditions. The first surface (M1) was modified with four circular grooves each having a width and depth of 0.5 mm and a pitch of 1.5 mm. The second and third surfaces (M2) and (M3) were modified with four circular grooves each overlaid with four and eight radial grooves, respectively. Each radial groove had width and depth of 0.5 mm. All surfaces were tested at three nozzle differential pressures: 80 kPa, 140 kPa, and 185 kPa. The nozzle-to-surface distance, coolant inlet temperature, surface temperature, and chamber pressure were maintained at 10 mm, ∼22 °C, <100 °C, and atmospheric pressure, respectively. The results indicated that the nozzle differential pressure had a significant effect on the spray cooling thermal performance of all surfaces. Furthermore, surface (M3) had the highest heat transfer enhancement ratio at all operating conditions, followed by surfaces (M2), and (M1), where the maximum heat transfer enhancements were 80%, 36.3%, and 28.7%, respectively. Thus, signifying that using surfaces modified with a combination of circular and radial grooves can enhance spray cooling heat transfer performance. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • High mass flux spray cooling on an inclined plate enhances CHF from 1.21 – 1.46 MW/m2. • The surface and cross- sectional temperature distribution asserts uniform cooling. • The SEM and EDS analysis clearly asserts unaltered surface morphology. Abstract The main obligation in the successful implementation of ultra-fast cooling in a manufacturing process are: (1) significant minimization of film boiling effect, (2) achievement of unaltered surface morphology and (3) uniform cooling on the surface of the plate. The literature does not reveal any methodology, which depicts simultaneously enhancement, unaltered surface morphology and uniform cooling. Therefore, in the current work, an attempt has been made to develop an appropriate cooling process depicting all the aforesaid requirements. The cooling on an inclined condition of the plate mitigates the aforesaid requirements. The result reveals that the heat removal rate enhances (CHF from 1.21 MW/m2 to 1.46 MW/m2) due to augmentation of sweeping rate of vapor film from the hot surface as inclination angle increases from 0° to 30°. Further increase of the inclination from 30° to 60° decreases the cooling rate due to reduction of droplet velocity, residence time and the replacement rate of the vapor and liquid layers. The optimum inclination of the plate to achieve the maximum average surface heat flux is 30°. In addition, the temperature distribution on the surface and across the thickness of the heat treated plate confirms uniform cooling. The SEM image and the EDS of the current heat treated metal is compared with the SEM images and EDS data of the metals cooled by potential coolants such as surfactant added water, NaCl added water and MgSO 4 added water and the comparison clearly asserts unaltered surface morphology for the current case. The variation of the hardness on the surface and across the thickness clearly asserts the excellent shock absorbing characteristics. [ABSTRACT FROM AUTHOR]

Abstract Spray cooling technology as a high heat flux cooling method is widely used to meet the increasingly cooling requirements, and the enhancement of spray cooling has gradually become a hot spot. In this study, three surfactants of AOS, CTAB and Tween20 with different concentrations were added into pure water to investigate their effects on spray cooling at low temperature range (≤100 °C) in a closed system. The three kinds of surfactants obviously improved the heat transfer performance, which had different optimum concentrations. The lowest mean surface temperature was obtained by CTAB with concentration of 200 ppm. The surface temperature non-uniformity of different surfactants spray was not improved, while the heat transfer deteriorated at high concentrations. In further study, the inclined mode method was used to alleviate the influence of bubble accumulation at high concentrations. The results showed that the inclined spray could significantly enhance the heat transfer capacity of the spray with surfactants and produce the optimum concentration point backward shift phenomenon. Moreover, the inclined spray mode showed a better promotion effect on spray cooling of surfactants with large surface tension. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Experiments were conducted to investigate the performance of the spray cooling heat transfer. Pure water and the mixture of water and ethanol (the volume fraction of ethanol ranges from 0.05% to 10%) were used as working fluids. The performance of pure water and the mixture were compared using the identical heater and solid-cone spray nozzle. The maximum heat flux in the experiment was up to 170 W/cm 2 . The experimental results indicated that adding ethanol to water is one of the most effective methods to enhance the heat dissipation and to control the device temperature simultaneously. The volume fraction of ethanol in water is the main factor affecting the spray cooling performance. The optimal volume fraction of ethanol for the improvement of the heat transfer performance is 4%. [ABSTRACT FROM AUTHOR]

The focus of this study is on the heat flux during cooling of a hot substrate by an impacting monodisperse drop train. Such a drop train is used for achieving high heat flux densities over small surfaces. The temperature inside the substrate is measured with high temporal and spatial resolution using thermocouples. Furthermore, a new analytical model has been developed to describe and predict the transient cooling and temperature of a semi-infinite body by the impacting drop train, assuming a single point heat sink on the surface. The experimental data agree very well with the theoretical predictions for various drop train frequencies. • Transient cooling of a hot substrate by an impacting drop train. • Analytical solution of the 2-D temperature field inside the substrate. • Short residence times of rebounding drops enable super positioning of single impacts. [ABSTRACT FROM AUTHOR]

• Jet impingement is compared against other cooling methods for electronic devices. • Role of wettability engineering in enhancing heat flux removal is analyzed. • Experiments with various cooling fluids on nonuniform surfaces are summarized. • Effects of wettability and surface science on jet impingement cooling are discussed. • Industrial retrofits and methods to make more durable surfaces are reviewed. The rising requirements of computing capability with an ever-declining packaging volume has triggered a global surge in the thermal management industry of electronic devices over the past several years. It is of utmost importance to maintain the electronic chip temperature below a specified limit, as overheating may lead to performance degradation, chip failure, and even operational hazards in extreme situations. Numerous attempts to meet the high cooling requirements of the heat-dissipating chips have been made using various devices and techniques, like heat pipes, pool boiling, microchannel heat sinks, spray cooling, jet impingement (JI), etc. Among these, the JI technique has several advantages in the efficient removal of heat in comparison to other approaches. Certain parameters play significant roles in heat removal by jet impingement. For example, coolant (working liquid) properties, flow arrangement (single-phase/multi-phase, laminar/turbulent flow), and target surface thermal and chemical properties are important actors. For both direct and indirect types of JI cooling, the surface properties of the target substrate are among the most essential and controllable features for enhancing heat removal. This paper emphasizes the importance of wettability engineering of the target substrate and presents a critical and comprehensive review of several possible surface engineering techniques that have been adopted by researchers to enhance heat transfer rates from electronic devices via JI cooling. A futuristic outlook on why and how the JI technology can be used in industry is also presented. Challenges abound and in view of this realization, intense R&D efforts are required in this arena. [ABSTRACT FROM AUTHOR]

A comparative study of liquid jet and spray is explored over plain and pillared copper surfaces with coolant flow (water) ranging over 3000 < Re < 10500, and heat fluxes up to the critical heat flux limit. Liquid jets and sprays are hydrodynamically characterized using high-speed videography and the phase-Doppler particle analyzer system. The interaction of liquid jets and sprays with the respective metallic substrates is experimentally investigated and strong liquid splattering is observed over the pillared surfaces at higher coolant flow rates. The resulting splattering mass fraction is measured over a range of coolant flow rates and radial positions of the nozzle. Subsequently, the effect of splattering on heat transfer performance is compared for various heat flux conditions by measuring the steady-state surface temperature. Heat transfer coefficient is found to be a strong function of coolant flow rate, operating heat flux conditions, and splattered mass fraction. Significant differences are not seen in these quantities between the liquid jet and spray for a plain surface. However, spray cooling clearly shows superior heat transfer enhancement over a liquid jet for a pillared surface where the critical heat flux limit is significantly delayed. The apparent advantages of heat transfer performance enhancement with patterning of surfaces are not always guaranteed, and the design consideration must address the coolant interaction with it to ensure a net advantage, which may get diminished due to liquid splattering induced by the interaction of the coolant with the patterned surface. [ABSTRACT FROM AUTHOR]

• The effect of varying surface wettability was probed on spray cooling heat transfer. • Surfaces with high wettability had low wall temperature in the nonboiling regime. • Early incipience of boiling on hydrophobic surfaces led to greater heat transfer. • Evaporation fraction increased as the contact angle decreased. • Modified heat transfer correlations were derived for both single- and two-phase regimes. This study conducted a spray cooling experiment to investigate the effects of varying surface wettability on the heat transfer performance of modified copper surfaces in both single- and two-phase regimes. The contact angles of the test surfaces ranged from 0° to 150°. The spray cooling experiment was performed using deionized water. The results indicated that surface wettability influenced heat transfer performance during spray cooling. In the nonboiling regime, the surface with higher hydrophilicity exhibited higher heat transfer performance owing to the larger wetting area available for heat transfer. The surface temperature of the superhydrophobic surface reached saturation at a relatively low heat flux; therefore, nucleation occurred rapidly, and the increase in the heat transfer coefficient intensified as the heat flux increased. Consequently, the heat transfer performance of the hydrophobic surface exceeded that of the plain surface in the two-phase regime. Evaporation fraction values were derived for the various test surfaces. Finally, a dimensionless surface wettability number, defined as the ratio of the droplet contact diameter to the droplet height, was used to establish heat transfer prediction models considering surface wettability in the single-phase and two-phase regimes. The heat transfer performance in the single-phase and two-phase regimes could be predicted within error bands of 18 % and 33.3 %, respectively. [ABSTRACT FROM AUTHOR]

Multiple source electrospraying enables higher droplet mass flux and greater cooling coverage area in comparison with its single source counterpart. The local convective heat flux to a multiple source electrospray array has been experimentally investigated using thin foil thermography. The designed array had a packing density of 115 N n cm − 2 and was operated in the cone-jet regime of spraying with ethanol as the working fluid. The electrospray array heat transfer performance is explored for varied flow rates ( Q  = 2–4 μL min − 1 N n − 1 ) and electric fields ( E d  = 4–8 kV cm − 1 ). Cooling performance was shown to be dependent on both parameters. An 89% and 64% increase in the peak and average heat flux respectively was measured for the increasing electric field strength. This was due to increasing droplet mass flux, resulting in increasing contact line length density on the heated substrate, from narrowing of the spray plume, reduced residence time and droplet evaporation en-route to the target surface. This narrowing spray plume, at higher driving electric fields, also resulted in a more defined radial convective heat flux profile. The performance of the array was compared with that of a single source under similar experimental conditions. The array achieved more uniform cooling over a larger area than its single source counterpart. Comparing both devices for similar total working fluid flow rate highlight that the array delays the onset of pool cooling. This enables higher total working fluid flow rates in the evaporative cooling regime and subsequently higher convected thermal energy transport. [ABSTRACT FROM AUTHOR]

The transient heat transfer performance of spray cooling with R21 is studied experimentally. Three variation patterns of surface temperature depending on the initial surface temperature are observed. There is only one rapid temperature drop at lower initial surface temperature and two drops with a shoulder transition at higher initial surface temperature. Further increasing the initial surface temperature will deteriorate heat transfer. Based on the pool boiling heat transfer curve and one-dimensional transient heat conduction model, the temperature variation patterns and processes are discussed which relate to nucleate boiling, transition boiling and film boiling, respectively. The experimental results reveal that the deterioration temperature decreases with increase of the heating flux and nozzle-to-heater distance, while increases with the spray flow rate. Based on the experimental results a region map of temperature variation patterns in transient spray cooling is given. [ABSTRACT FROM AUTHOR]

Cryogen spray cooling (CSC) is commonly applied in laser dermatology to protect the epidermis from thermal damage. Many efforts have attempted to improve the cooling capacity of CSC, among which the use of expansion-chambered nozzles is an effectively simple method with considerable potential. This study examined the influences of the expansion-chambered nozzle structure, including the ratios of inlet nozzle diameter to discharge nozzle diameter and of chamber diameter to discharge nozzle diameter on R134a and R404A spray cooling. Fifteen transparent expansion-chambered nozzles with the expansion chamber aspect ratio of 1.0, chamber diameter to discharge nozzle diameter ratios of 5.0–10.0, and inlet nozzle diameter to discharge nozzle diameter ratios of 0.6–1.4 were tested. The internal flow pattern inside the expansion chamber, external spray pattern, and surface heat transfer characteristics of cryogen spray using different nozzles, including the straight-tube nozzle, were investigated. The structure of the expansion chamber was found to have an important effect on the spray patterns and cooling characteristics. The spray radius obviously decreased when the expansion-chambered nozzles were used, and the spray pattern became narrower as the ratio of chamber diameter to discharge nozzle diameter increased. By contrast, the increase in ratio of two nozzle diameters enlarged the spray radius. Surface temperature and heat flux with different nozzles and cryogens showed a similarity, and correlations of surface temperature and heat flux were proposed. The introduction of expansion-chambered nozzles could effectively improve the spray cooling capacity. The minimum average surface temperature during the fully developed spray period could be reached for both R134a spray and R404A spray by an expansion-chambered nozzle with a chamber diameter to discharge nozzle diameter ratio of roughly 5.0 and an inlet nozzle diameter to discharge nozzle diameter ratio of roughly 0.6. [ABSTRACT FROM AUTHOR]

Spray cooling systems usually use liquids to extract large amounts of heat through phase-change vaporization processes. However, in several applications, cooling requirements are transient and an efficient thermal management implies a proper control of the liquid film deposited on the surface. This represents a challenge to the optimization of thermal management systems and raises the question if there are other approaches able to perform cooling without a liquid film. This is why the present work explores sublimation as the phase-change cooling process using dry ice (CO 2 ) particle sprays. By the Joule-Thomson effect through a sudden expansion, liquid carbon dioxide can be converted into dry ice particles. The experimental results obtained with dry ice sprays for transient cooling show how shorter injection durations (<0.5 s) produce more uniform decays in temperature distribution, while longer pulses (>0.5 s) lead to higher heterogeneities on the impact area with potential use for hotspot cooling. The cooling heat flux generated by spray impact produces a maximum around the aforementioned temporal threshold of 0.5 s, although the maximum performance is close to the 1 s injection duration. Finally, the order of magnitude of the measured spatial average energy removed from the surface is compatible with cooling requirements found, for example, in mold cooling processes. Therefore, this evidences its potential application as an additional thermal management strategy to reduce cycle times and improve the industrial production of molding parts. [ABSTRACT FROM AUTHOR]

Methods to avoid the dynamic Leidenfrost effect are of great importance for high heat flux spray cooling which needs an efficient contact of liquid and superheated surface. In this paper, a novel method of increasing dynamic Leidenfrost temperature is proposed through addition of high-alcohol surfactant (HAS). Effects of 1-Octanol and 2-ethyl-hexanol surfactants on dynamic Leidenfrost temperature and spreading dynamic of droplets impacting on superheated surface were investigated for the first time using high-speed photography. Empirical correlations of dynamic Leidenfrost temperature and maximum spreading factor were obtained based on our experimental data. A possible explanation for the dynamic Leidenfrost temperature increase caused by HAS was proposed based on bubble bursting and bubble coalescence. The results show that dynamic Leidenfrost temperature is significantly increased by addition of HAS because of surface tension reduction. The empirical correlations of dynamic Leidenfrost temperature and maximum spreading factor show good agreement with maximum error of 5% and 10% respectively. These empirical correlations could provide reference value for the future research. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the spray cooling heat transfer characteristics using CFD method. The two phase mathematical model is established based on Euler-Lagrange approach. In this model, unsteady-state flow conditions are simulated. Heat and mass transfer between the vapor and the water droplet are computed. The averaged error of numerical results is 10% compared to experimental results in our previous papers. It is concluded that the heat flux and its distribution on heated surface are influenced by the heated surface temperature, mass flux, nozzle to surface distance and the number of nozzles. Furthermore, the heat flux distribution do not change obviously with the increasing of heated surface temperature, and the heat flux increases with the increasing of mass flux. Moreover, there is an optimal nozzle to surface distance while the spray impingement zone circumscribes in heated surface, and more nozzles lead to higher heat flux and better heat flux distribution on heated surface. Considering the disadvantage of manufacture difficulty and cost increase for more nozzles, there is an optimal nozzle number in certain space and heat dissipation requirement. The optimal number of nozzles is 8 in this article. [ABSTRACT FROM AUTHOR]

In this work the single-nozzle spray cooling on a large scale industry-used drum wall has been simulated by a verified numerical model. For a certain spray nozzle, the effects of four parameters, i. e. different spray pressures, different spray heights, different water temperatures, and different wall temperatures, on heat transfer have been analyzed. It is found that the mean heat flux distributions show concentric elliptical circles. Increasing spray pressures will enhance the cooling performance. Decreasing spray heights will improve the heat flux in direct spray areas other than whole simulated drum wall. As expected, reducing water temperature or advancing wall temperature will raise the average wall flux. Both relationships are exponential. The influencing degrees of the four parameters have been compared through Taguchi orthogonal experimental method and the result is: wall temperature > spray pressure > water temperature > spray height. The wall temperature, spray pressure, and water temperature show dominant effects except for the spray height. [ABSTRACT FROM AUTHOR]

Surface heat flux is an important parameter in various industrial applications. It is often estimated based on measured temperature by solving inverse heat conduction problems (IHCPs). In the present work, a filter solution to solve 1D single-layer IHCPs is applied to calculate the surface heat flux for 2D multi-layer mediums. An optimal comparison criterion is implemented for 2D IHCPs to optimize the key regularization parameters. Afterward, the 2D filter solution is used for heat flux estimation with thin-film thermocouple (TFTC) and fine thermocouple (FTC) measurements during cryogen spray cooling. The accuracy of the estimated heat fluxes is tested with the measured temperature response to cryogen spray cooling. A small error (maximum value of 1.0740 °C) is observed between the temperature simulated based on estimated heat fluxes and the measured temperature. The maximum heat flux obtained by the 2D filter solution is 13.6% higher than that obtained by 1D method for TFTC measurement. This finding indicates that lateral heat transfer cannot be disregarded, especially when the heat conductivity coefficient of the material is large. [ABSTRACT FROM AUTHOR]