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In the case of products made of steel, the presence of an oxide layer, which is formed during the steel production process as a result of high temperature, has a significant impact on the process of heat removal from the surface of the cooled material. For this reason, it is necessary to take into account the presence of the oxide layer in mathematical and numerical models used to simulate the distribution of the temperature field in cooled steel products. These models are based on the boundary conditions identified for given production conditions. This paper presents a comparison of the results of the identification of the boundary condition during water spray cooling of Armco iron with the use of the inverse solution. Numerical calculations were carried out using two models of heat conduction. In the first model, the presence of an oxide layer with different thermophysical properties than the base material (Armco iron) was taken into account. The second model assumed no oxide layer on the cooled Armco iron surface. It was found that the inverse solution obtained in the case of the heat conduction model taking into account the thickness of the oxide layer is correct in time and as a function of temperature. Thus, the boundary condition model obtained as a function of temperature is universal. However, this model requires an additional layer of oxides with different thermophysical properties than the base material to be included in the finite element model (FEM). Based on the conducted uncertainty tests of the inverse solution, it was found that the results of the determined boundary condition in the absence of the oxide layer on the cooled surface are subject to an error higher than 10% in comparison to the maximum reference value of the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Dry ice pulsed sublimation spray cooling (PSSCDI) is an innovative, efficient, energy-saving, and fast cooling method for the thermal management of chip-level electronic components with high thermal load in the aerospace field. Aiming at the heat transfer mechanism of dry ice sublimation spray cooling, a heat transfer model of pulsed dry ice sublimation spray cooling based on flow discretization is proposed in this paper. According to the heat transfer mechanism and the formation mechanism of dry ice reflected by the Joule–Thomson effect, the experimental platform of pulsed dry ice sublimation spray cooling was designed and built. The relationship between spray height, spray flow rate, and spray cone Angle was determined by experiments, and the best spray height was determined. The effects of mass flow rate, pulse period, and duty cycle on heat transfer characteristics of pulsed dry ice sublimation spray were investigated experimentally. The experimental results show that the instantaneous temperature change process of simulated heat source under different working conditions can be divided into three stages: heating, cooling, and constant temperature. The heat flux and heat transfer coefficient of pulsed dry ice sublimation spray cooling under high heat load increase with the increase of the duty ratio and mass flow of dry ice spray, but decrease with the increase of pulse period. However, the heat flux of the duty cycle and heat transfer coefficient of the duty cycle decrease with the increase of duty cycle, which indicates that under the premise of considering dry ice consumption, a lower duty cycle will make pulsed dry ice spray cooling have a higher heat transfer efficiency. When the mass flow rate of dry ice spray is 13 g/s, the pulse period is 0.2 s, the duty cycle is 50 %, the heating power is 40W, the relative optimal cooling heat flux is 48 W/cm2, and the relative optimal heat transfer coefficient is 5400 W/(m2·K). [ABSTRACT FROM AUTHOR]

Industrial growth has led to increased electronic component integration, resulting in higher working temperatures and technological failures. Cryogen spray cooling (CSC) could reach cryogenic temperatures with fast-responding, but its heat transfer limitation due to liquid film deposition is potentially challenging. In this study, CSC cooling is incorporated with either a cold air jet (CAJ) or a sweeping jet (SWJ) to enhance heat and fluid flow. A K-type thermocouple is used for measuring the surface temperature, while the improved Duhamel theorem is used to estimate the heat flux. The spray and film intensities are recorded through the Mie-scattering methodology via high-speed camera. The results demonstrated that the CAJ/SWJ exhibited a significant reduction in surface temperature and film intensity, enhanced heat dissipation and spray efficiency compared to CSC cooling. More precisely, it demonstrated a 26%–36% heat transfer coefficient improvement, along with a 52% increase in the corresponding time of the critical heat flux. Meanwhile, the CAJ/SWJ intensifies heat transfer during film and transition boiling, inspired by the fast liquid film evaporations. In contrast, the SWJ is much preferred due to its wider and stable coolant coverage throughout film evaporation, as seen by its greater heat transfer coefficient. The combined results demonstrated the need to promptly eliminate the deposited film, as it holds the potential to serve as a pivotal resolution for the advancement of thermal management technology. [ABSTRACT FROM AUTHOR]

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]

The spray cooling is investigated for a high-temperature steel plate using varying temperature coolant for different nozzle-plate orientations. Before experimentation with very-high mass-flux spray, droplet-train impact assessment indicates liquid film breakup intensification down the inclined plane. The heat transfer analysis suggests a significant cooling performance change with an inclination change at high-temperature coolant. For 50°C coolant usage in horizontal-case, cooling rate, average surface heat flux, critical heat flux, and average heat transfer coefficient are 161°C/s, 2.20 MW/m2, 2.62 MW/m2, and 3.28 kW/m2K, respectively. The 40°C coolant has a low-performance factor, while 50°C has high cooling performance. [ABSTRACT FROM AUTHOR]

Due to the excellent heat transfer performance, spray cooling is widely used for heat removal of high-heat-flux electronic devices. In this study, the influence of spray characteristics on spray cooling heat transfer is tested experimentally, where both water and 4% ethanol-water mixture are used as the working fluid. The Sauter mean diameter and droplet number distributions are measured by the Particle/Droplet Imaging Analysis system, and the fluid volumetric flux distributions are measured by the self-designed Precision Mobile Experiment Bench. The results show that the heat transfer efficiency of the 4% ethanol-water mixture is superior to that of water. Especially, the critical heat flux is about twofold that of water for the Spray A nozzle. For a fixed nozzle, the Sauter mean diameter of the 4% ethanol-water mixture is smaller, while the fluid volumetric flux shows an increase under the same conditions. Small Sauter mean diameter and large fluid volumetric flux can induce more droplets to entrain bubbles. Then the mechanisms of improved spray cooling heat transfer are analyzed based on the spray characteristics measurement results. It is revealed that the Sauter mean diameter and mean fluid volumetric flux exert joint influence on the heat transfer efficiency, and the mean fluid volumetric flux has an obvious effect between different nozzles. Two dimensionless heat flux correlations are proposed for the single-phase and nucleate boiling regimes with mean absolute errors of 15.56% and 14.68%, respectively. Highlights: Sauter mean diameter of 4 vol.% ethanol-water is smaller than water, but the fluid volumetric flux is higher than water. Small Sauter mean diameter and large fluid volumetric flux induce more droplets generating entrained bubbles, which can enhance the heat transfer performance. Fluid volumetric flux is predominant in improving heat transfer performance, especially in the nucleate boiling regime. Spray characteristics have an obvious influence on the critical heat flux. Dimensionless heat flux is correlated with characteristic numbers integrating Sauter mean diameter and fluid volumetric flux. [ABSTRACT FROM AUTHOR]

A simulation study of droplet impingement on a moving film at different incidence angles in spray cooling application was conducted, and the following results were found: the variation of the angle leads to alternating changes in the neck pressure difference, flow field, and maximum shear which directly causes the transition of the coronal morphology from asymmetric to symmetric and finally back to an asymmetric shape, with corresponding changes in the order of appearance of the left and right jets and secondary droplets. After focusing on temperature distribution and heat flux variation, the temperature distribution is found to be related to horizontal and vertical component velocities of the droplet; the angle affects the heat flux variation at the impact point, and the maximum heat flux decreases with the interaction of the liquid film and droplet, which tends to move downstream. Therefore, the morphological characteristics and temperature distribution are asymmetric when a difference is found between the horizontal velocity of the droplet and the film velocity, and the larger the difference, the more evident the asymmetry is. Therefore, the cooling effect is best, and the shear stress is most significant when the liquid film's and droplets' horizontal velocities are equal. • Variations in the angle of incidence lead to alternating changes in left and right crown height, neck pressure difference, flow field, and heat flux distribution. • Analysis of the shear stress of the liquid film after the impact, and reveals many interesting features. • For spray cooling applications, the effect of nozzle angle on cooling efficiency is clearly explained. • The incidence angle and moving liquid film interact together to achieve optimal cooling compared to single factor conditions. [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]

The lightweight, precision and high integration of modern electronic devices lead to a sharp increase in local heat flux, which poses a severe challenge to the traditional cooling technology. There is an urgent need to find a cooling method with high heat dissipation capacity and high efficiency to meet the requirements of today's thermal management systems. Based on the Joule Thomson effect of carbon dioxide, a dry-ice spray cooling experimental system was established, and two types of array nozzles were designed and processed. The changing laws of cooling characteristics of tapered array nozzle and Laval array nozzle under different mass flow rates and different spray heights were discussed. The experimental results show that the heat transfer effect is the best when the spray mass flow rate is 36.29 g s−1 and the spray height is 2.5 cm, the corresponding critical heat flux is 452.03 W cm−2, the heat transfer coefficient is 36939.21 W m−2 °C−1, and the surface temperature of the simulated heat source is 37.64 °C. For Laval array nozzles and tapered array nozzles, the cooling performance of the former is better under the same experimental conditions, so the production of Laval array nozzles should be given priority in the selection of array nozzles with different parameters. [ABSTRACT FROM AUTHOR]

Spray cooling is a promising and effective cooling method, which may be used to solve the heat dissipation problem of high-power electronic components. In this paper, the influences of flow rate, sub-cooling degree, and refrigerant charge on the spray cooling heat transfer performance with R134a as the working fluid were experimentally studied. Results showed that the enhancement effect of the increase of sub-cooling degree on the heat transfer performance is very limited in this experiment. When the sub-cooling degree rises from 5 to 8 K, the heat transfer coefficient increases by 2.6% on average at lower heat flux (45.93–72.55 W/cm2). The increase of refrigerant charge can effectively improve the heat transfer performance, especially at higher heat flux (84.02–105.25 W/cm2). However, the Jacobian number decreases with the increase of the refrigerant charge, which hinders the improvement of heat transfer performance at higher heat flux. Therefore, there is an optimal refrigerant charge of 1.05 kg in this experiment. In addition, based on the experimental data, a Nusselt number dimensionless correlation of R134a solution with higher prediction accuracy was proposed. [ABSTRACT FROM AUTHOR]

In the present paper, experimental and numerical studies for the interaction studies between different liquid droplets and hot metal surfaces were carried out. The obtained results were interpreted using graphs and figures. Droplet impact, spreading, breakup and Leidenfrost phenomenon were observed with the help of a video camera at 100 fps. The interaction studies were done at different surface temperatures and different droplet‐impinging velocities. Three metals and three liquids were selected for the present study. Copper, aluminium and steel were selected as metals with varying roughness factors maintained at different temperatures. At the same time, in liquid entities, methanol, ethanol and kerosene were selected along with water in some portions. The selections of the metals were based on the thermal diffusivity property as it influences the thermal interaction between the droplet and the hot metal surface. The temperature range of the metal surface was kept from 150°C to 425°C, and the impinging velocity range of droplets was in the field of 2.4–4.9 m/s. For computational studies, the Ansys Fluent R3‐2019 version was used. Numerical analysis for the interaction process was carried out using the volume of fluid (VOF) method, solving Navier–Stokes and energy equations in a 2D‐geometry transient simulation. Parameters like surface temperature, heat flux and spreading ratio w.r.t. change in Weber number were investigated and presented in the present paper. Obtained results are useful for better understanding of droplet physics while interacting with hot surfaces which further helps in development of new methods for effective spray system designs. Results show that for a lower value of Weber number, the droplets spread less as compare to high Weber number. Also, spreading ratio of droplets depends only on the surface roughness. [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.

The low flow rate laminar water jet is the most economical and efficient cooling methodology; however, the low heat removal capacity makes this process inappropriate for fast cooling operations. In the current work, the heat transfer rate has been increased by enlarging the stagnation zone length. This was attained by reducing the viscosity, surface tension and altering other thermo-physical properties of the coolants. The thermal analysis reveals that the maximum critical heat flux is achieved in case of 240 ppm Cetyltrimethylammonium bromide + water i.e., 1.7 MW/m2, which is 1.39 times higher than pure water. This is attributed to the significant reduction in the surface tension and viscosity. Use of additives like sodium carbonate and surfactant enhance the stagnation zone length. Furthermore, due to foaming nature of the coolant (500 ppm Ethanol + water), the heat transfer rate declines and a critical heat flux of 1.37 MW/m2 is observed. The visual observations are consistent with the results obtained for each coolant. Based on the high heat removal capacity, the low mass flux laminar jet cooling methodology with additives provide better cooling than that of the high-pressure laminar jet and high mass flux spray cooling. [ABSTRACT FROM AUTHOR]

In this study, the effects of surface roughness (Ra) on single-phase and two-phase cooling performances of HFE-7100 and water under different coolants, pressures, and subcoolings were studied. The eight surfaces were unidirectionally roughened. A general trend was that the heat transfer increased with increasing Ra. Owing to the larger contact angle and fewer nucleate sites, the smoother surface had poorer heat transfer performance. Visualization was performed to reveal secondary nucleation on the smooth surface. Finally, correlations were developed to predict the heat transfer performance for different values of Ra. [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]

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]

• Different scale surfaces are investigated in closed loop two-phase spray cooling. • Fluid distribution on the macro- surface has an important effect on heat transfer. • CHF are improved by 116 % at surface temperature 80 °C on multi-scale surface. • Maximum evaporation efficiency reached 60 % at the chamber pressure of 60 kPa. • Enhancement ratio varies with the surface temperature on structured surfaces. Dielectric liquid spray cooling is a promising way to dissipate heat of high-power electronic devices. Surface modification is a most cost-effective method to enhance spray cooling. Inspired by leaf veins, this paper designs and fabricates macro-scale, micro- and nano- scale, and multi-scale structured surfaces for dielectric liquid spray cooling. The cooling characteristics are tested on a two-phase spray cooling system using HFE-7100. The results reveal that the heat transfer is enhanced on all the structured surfaces. Two bionic leaf vein structures, reticulated veins and parallel veins, are designed for macro-scale structured surfaces. The results show that the former one is superior to the other thanks to its better liquid distribution. For the micro- and nano- scale structured surfaces, due to the larger surface area and higher thermal conductivity, the graphene coating outperforms the carbon nanotube coating in heat transfer. Multi-scale structured surfaces, featured with leaf veins and micro- and nano- coatings, further enhance heat transfer. The heat flux increases by 116 % compared with that of the smooth surface. The evaporation efficiency reaches 60 % at the surface temperature of 80 °C. Furthermore, the effect of surface temperature on the enhancement ratio of heat transfer is analyzed, revealing various enhancement mechanisms of different scaled structured surfaces. [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]

An experimental study on multi-nozzle spray cooling of a downward-facing heater surface has been carried out in the SPAYCOR facility at KTH, to provide data assessing the feasibility of spray cooling for in-vessel melt retention (IVR) in light water reactors. To help understand the characteristics and influential factors of the liquid film formed on the heater surface in spray, a numerical study on the dynamics of an isothermal liquid film on the heater surface has also been performed by adopting the OpenFOAM platform, and Eulerian and Lagrangian methods for liquid film and droplets, respectively. The present study is an extension of the previous modeling from hydrodynamics to thermal-hydraulics of the spray cooling problem, via adding heat flux of the heater and two convective heat transfer models between the heater wall and the liquid film. Moreover, droplets-film interaction model is modified. The SPAYCOR experiment is simulated by the numerical models, and the simulation results show a good agreement between the numerical and experimental data, in particular when the modified droplets-film interaction model is applied. After the validation of the numerical models against the SPAYCOR experiment, the numerical models are employed to investigate influential factors on heat transfer, such as mass flux, nozzle-to-surface distance, and nozzle matrix layout. The results indicate that heat transfer is enhanced by increasing mass flux and decreasing nozzle-to-surface distance, and the change of nozzle matrix from inline to staggered layout has little impact on heat removal capacity or temperature distribution of the multi-nozzle spray cooling. [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]

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]

The electrospray (ES) cooling technique, which is a combination of spray cooling and high-voltage electrostatic technology, has attracted considerable interest because of its simple unit structure and low power consumption. To overcome the low flow rate and high boiling point problems of current working fluids, a mixture of R141b and ethanol was used as the working fluid to experimentally investigate the break-up modes at a high flow rate. Furthermore, the effects of break-up modes and ethanol concentration on spray characteristics, cooling performance, and impingement characteristics were examined. The experimental results indicated that ES at a high flow rate exhibited four break-up modes: dropwise, varicose, whipping, and ramified. As the spray mode transitioned from the dropwise to the ramified mode, the droplet size decreased, and the droplet splash phenomenon on the hot surface was suppressed. The cooling performance was considerably enhanced with the evolution of the break-up mode, and the critical heat flux (CHF) was increased by 27.3 % when the break-up mode was transformed into the ramified mode from the dropwise mode. Under certain conditions, an ES cooling efficiency of 92.6 % was achieved. Furthermore, the volume fraction of ethanol (corresponding to the best cooling performance) increased with the wall temperature. In this study, considering the actual requirements of the chip for the cooling capacity, the optimal volume fraction of ethanol in the whole temperature region was determined to be 30 %. These findings can provide theoretical guidance for thermal management of high-power electronic devices. [ABSTRACT FROM AUTHOR]

• Gravity affected the velocities of isolated films on the vertical smooth surface. • Heat transfer between the vertical and upward surfaces presented no measurable difference. • The Bo of the upward surface was smaller than that of the vertical surface for HFE-7100. • The data map of liquid film flow in spray cooling was further updated. In this study, an experimental investigation was conducted on the dynamics and heat-transfer characteristics of liquid film flow for upward- and vertical-facing spray cooling. The heat-transfer performances at different inlet pressures, nozzle-to-surface distances, and heat flux inputs between the two orientations were compared, and the morphologies, wetted areas, and velocities of the isolated liquid films were quantitatively captured. The morphologies of the HFE-7100 liquid films and their distributions on the vertical and upward surfaces exhibited no significant differences. In most cases, the heat-transfer performance of the upward surface was slightly better than that of the vertical surface. Gravity affected the velocity of the isolated liquid films on a smooth vertical surface, whereas its effect was significantly suppressed on an unpolished surface. The Bond number of the isolated liquid film flow on the upward surface was verified to be naturally smaller than that on the vertical surface. Dimensionless groups, including the Bo, We , and Re numbers of the liquid film flow in spray cooling, were obtained to further account for the effect of gravity. The reason why the dimensionless groups presented such small orders was explained. The data map of liquid film flow (dimensionless groups) during spray cooling was further enriched. [ABSTRACT FROM AUTHOR]

[Display omitted] • A structured synthesis method has been created to enable the controlled synthesis of CuO-NPs. • The synthesized CuO-NPs showed good crystallization with a particle size of 64.50 nm. • The synthesized CuO-NPs exhibited good stability with a zeta potential of −39.84 mV. • The surfactant added CuO nanofluids enhances the cooling rates and surface heat flux. In this study, surfactant-added CuO nanofluid, is employed to enhance spray cooling efficiency. Initially, CuO nanoparticles (CuO-NPs) are synthesized using the sonochemical method. During the nanoparticle characterization process, we examined the crystallinity, functional groups, particle size, elemental composition, and morphological characteristics of the synthesized CuO-NPs. Subsequently, CuO nanofluid is prepared, and its stability and thermophysical properties are determined. The heat transfer efficiency of the prepared nanofluid is evaluated based on parameters such as cooling rate and heat flux, which are determined through inverse heat conduction analysis. The zetasizer estimated the average nanoparticle size and zeta potential to be 64.50 nm and −39.84 mV, respectively. The XRD exhibits the monoclinic shape of CuO crystals, as claimed in JCPDS file No. 01-080-1916. FTIR spectroscopy identified the Cu O bond at a wavelength of 597.30 cm−1. The SEM analysis shows the agglomerations of spherical nanoparticles. The CuO phase was identified by the presence of Cu and O elemental peaks in the EDS spectra. Furthermore, the experimental results showed that the use of surfactant-added CuO nanofluid spray significantly increased the critical heat flux (CHF) and the cooling rate by 1.31 times and 1.16 times, respectively, compared to pure water. [ABSTRACT FROM AUTHOR]

As data centers increasingly become the backbone of the digital age, managing their substantial energy consumption and mitigating heat generation are paramount. This paper focuses on direct liquid cooling as a transformative technology for enhancing energy efficiency and operational safety in high-density computing environments. Significant advancements and persisting challenges in this field have been identified by analyzing various direct liquid cooling methodologies including immersion, spray/jet, and microchannel cooling. Comparative analysis of these systems reveals their potential to substantially lower thermal resistances and improve energy utilization. Despite these advancements, the research highlights persistent challenges such as integration complexities and scalability issues. Recommendations for future research directions to improve the efficiency and ability of direct liquid cooling applications in data centers were concluded, emphasizing the need for user-friendly and cost-effective solutions, comprehensive guidelines for cooling method selection, and hybrid cooling systems. It also recommends advanced energy management strategies such as real-time power adjustment that dynamically matches energy supply with computational demand to optimize efficiency. These contributions underscore the importance of advancing data center cooling technologies to meet future demands. • A comprehensive review of data center direct liquid cooling advances. • The ranges of heat flux, PUE, and cost are compared for different technologies. • Comprehensive guidelines for cooling method selection are presented. • The main limitation of direct liquid cooling systems lies in complex integration. • Coupling direct liquid cooling with tactics like waste heat recovery is promising. [ABSTRACT FROM AUTHOR]

In the present work, spray cooling experiments of a hot steel plate were carried out with three different nozzles in order to provide accurate experimental data for the modellers. Special attention was paid to for both the measurement of the surface temperatures and the characterization of the sprays. Firstly, the surface temperatures were measured using K-type thermocouple wires welded directly to the surface of the plate in a separate contact. This technique provides an accurate measurement of the surface temperature during the cooling. Secondly, the spray characteristics of each nozzle were also thoroughly investigated. It was found that the droplet size and velocity distributions of each nozzle followed a log-normal law. The corresponding Sauter mean diameter (SMD) and mean velocity ranged from 170 to 230 µm and from 5.6 m s−1 to 22.4 m s−1, respectively. Spray cooling was started after heating the plate between 500°C and 600°C using a radiant panel. Cooling rates were very high and the time to reach ambient temperature varied from 4 s to 1 min depending on the nozzle used. Heat Flux (q˙′′\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${\dot{q}}^{{\prime}{\prime}}$$\end{document}) and Heat Transfer Coefficient (HTC) were calculated from the temperature data. It was found that high levels of critical heat flux (CHF), around 9 MW m−2, were achieved for two of the three nozzles studied, including the one with the lowest flow rate of only 1.6 L min−1. Finally, the results obtained in this study could be used to validate numerical codes such as FDS and FireFOAM, which are commonly used in fire safety engineering. [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]

• Enhanced surfaces such as straight fin surface and cubic fin surface with relatively large heating surface of 4 cm2 were designed. • A parametric study is conducted to evaluate effects of flow rate, spray distance and enhanced surfaces on spray cooling. • Visualization was presented to elucidate enhanced mechanism of spray cooling. • Compared to flat surface, HTCs on both straight fin surface cubic fin surface are significantly enhanced up to nearly 60%. • At a spray distance of 25 mm, heat flux about 250 W/cm2 is achieved on cubic fin surface. Spray cooling as an effective cooling technique is widely used in practice. Compared to small heating surfaces, the significantly increase of surface would pose a great challenge to the significant enhancement of spray cooling. Specially, single nozzle spray cooling suffers from non-uniform liquid thin film on a relatively large heating surface, severely inhibiting the formation of efficient thin film evaporation. In this study, two enhanced surfaces such as straight fin surface and cubic fin surface with a relatively large heating surface of 4 cm2 were designed for single nozzle spray cooling. A parametric study was conducted with aims to evaluate the effects of flow rate, spray distance and enhanced surfaces on the formation of thin liquid film and then the performance of spray cooling. Visualization was presented to observe the development of liquid film to elucidate the enhanced mechanism of spray cooling. In this work, spraying height is varying from 20 to 40 mm. Coolant flow rate is ranging from 70 to 100 ml/min. The experimental results show the distribution of liquid film plays an important role in spray cooling, especially in the single-phase regime. In two-phase regime, nucleate boiling is significantly boosted on the engineered surfaced. For instance, the cubic fin surface leads to an enhancement of HTC of nearly 24 % compared to that of flat surface. [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 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]

Spray cooling is an effective heat transfer technique capable of handling high heat fluxes and temperatures. The study of droplet impact offers a simplified approach to understanding the fundamentals of spray cooling and elucidates the effects of droplet diameter and velocity. Experiments on a single-droplet impact on a thin liquid film with boiling bubbles are carried out in this study. Two-phase interface evolution is observed using high-speed video, and the heater surface temperature variation at the impact point following an impact is recorded and analyzed, which can be divided into the response stage, the recovery stage, the steady stage, and the slow recovery stage. Furthermore, a cooling factor based on the change in heater surface temperature is introduced to evaluate the cooling effect of droplet impact. Results show that the steady stage plays a significant role in the heater surface cooling process. Moreover, the maximum cooling factor increases appreciably with increasing droplet diameter and impact velocity. [ABSTRACT FROM AUTHOR]

Experiments have been performed to study the effect of enhanced surfaces including micro-finned, micro-/nano-coated (coating thickness of 100 µm and 200 µm) and hybrid surface on heat transfer coefficient and critical heat flux using spray cooling. Commercial HFE-649 coolant obtained from 3 M Corporation has been used to spray over copper heater of 10 mm2 cross-sectional area using a full cone pressure swirl nozzle with 0.51-mm orifice diameter. The pressure swirl nozzle squeezed the incoming fluid and dispersed into spray droplets in the volumetric flux range between 1.0 and 4.5 cm3/cm2s. Results have been obtained for various surfaces at optimum distance of nozzle-to-surface and comparison is done with the plain surface. Findings revealed that heat transfer performance improved considerably with hybrid surface, which showed at least 360% enhancement in heat transfer coefficient and 72.9% in critical heat flux in contrast to the plain surface. An improvement of 7.1% in critical heat flux is observed for hybrid surface in comparison with other enhanced surfaces. Furthermore, experiments were conducted to examine the effect of inclination angle (0°, 15°, 30°, and 45°) on heat transfer performance. The heat transfer coefficient, critical heat flux, and cooling efficiency all improved dramatically at 15°C inclination angle. In comparison with horizontal spray, an improvement of 18.9% and 20.1% in heat transfer coefficient and critical heat flux, respectively, have been observed. However, heat transfer performance degraded with the further increase of inclination angle. Other experimental parameters remain constant, such as the volumetric flux, spray height, and spray pressure. [ABSTRACT FROM AUTHOR]

In this article, a low-loss RF coaxial through-silicon vias (TSVs) (C-TSVs) configuration in the silicon interposer with the embedded cooling cavity is proposed. For the single C-TSV array passing through the embedded cooling cavity, the transmission losses with different coolants were compared. The insertion loss immersed in FC770 is 0.14 dB at 40 GHz, which is closed to the typical C-TSV interposer (0.2 dB). It is much smaller than the deionized water (6.49 dB) one. The optimal spacing of multiple C-TSV arrays filled with FC770 coolant is $600~\mu \text{m}$ (vertical transmission of 0.16 dB at 40 GHz). When the cooling cavity is embedded in the interposer with FC770 coolant, the maximum surface temperature of the power devices can be less than 56.8 °C at 250 W/cm2 heat flux. Compared with the solid silicon interposer, the maximum von Mises thermal stress of the C-TSV interposer with the embedded cooling cavity can be reduced by 44.8%. [ABSTRACT FROM AUTHOR]

The spray cooling is investigated for very high mass‐flux coolants at different temperatures. First, the intended spray is characterized for its role in the cooling enhancement, which reveals that droplet detachment frequency, droplet residence time, liquid film, and vapor film wavelength influence heat‐transfer performance for high‐temperature very‐high mass‐flux spray. After that, the furnace heated 6‐mm thick steel plate at 1030°C is quenched using spray possessing average impingement density (231 kg/m2s) and coolant temperatures (30°C, 40°C, and 50°C). The heat transfer performance is analyzed using inverse heat conduction analysis, wherein temperature‐dependent steel thermophysical properties are considered. The heat transfer performance assessment through parameters such as cooling rate, surface heat flux, and heat transfer coefficient curves shows improvement in cooling performance. For 50°C coolant temperature, we observed the maximum improvement in cooling rate, 141°C/s, and critical heat flux, 3.64 MW/m2. The efficiency analysis reveals that coolant at the highest temperature is the most efficient. Also, an approximate cost assessment suggests enhanced coolant temperature as an economical alternative for heat transfer enhancement than additives‐based coolant. [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]

The complexity of roller quenching equipment restricts research on the heat transfer mechanism of ultrathick plates, and the cooling law requires further clarification. In this study, the distributions of temperature gradient, average cooling speed, and heat loss during quenching are analyzed using a 220 mm thick steel plate, and inverse heat conduction is used to determine the surface heat transfer conditions. An ultrathick steel plate is subjected to high‐pressure and low‐pressure water in turn for strong and lasting cooling. Due to the different flow regions on the upper and lower surfaces, the flow from the lower nozzle is slightly higher than that from the upper nozzle, thereby resulting in a good cooling symmetry between the upper and lower surfaces. The results provide an important basis for the optimization of roller quenching technology for ultrathick steel plates. [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]

Experiments were performed to investigate the transient cooling characteristics of intermittent spray cooling (ISC) in single-phase and nucleate-boiling regimes. The consumption of residual liquid in the non-injection period was confirmed via observations of the dynamic behavior through visualization experiments. The time variation of the surface heat transfer is relatively small in the single-phase regime and minimal in the initial nucleate-boiling regime. Further, ISC was considered as a process where the transient mass flow rate was specified by the characteristic time. This idea was successfully evaluated in the prediction of the critical heat flux in cryogen spray cooling. Abbreviations: CHF: critical heat flux; CSC: cryogen spray cooling; DC: duty cycle; ISC: intermittent spray cooling; SFSM: sequential function specification method [ABSTRACT FROM AUTHOR]

An inverse analysis technique based on the conjugate gradient method (CGM) and the discrepancy principle is employed to estimate the time-wise variation of the unknown cooling heat flux in the spray cooling of a hot surface. In contrast to previous studies, the heat conduction equation of the cooled surface is formulated using a dual-phase-lag (DPL) model. In addition, no assumptions are made regarding the functional form of the cooling heat flux. The simulation data required to conduct the inverse analysis are generated by adding random errors to the calculated exact temperatures at the boundaries and interior of the hot body. The validity of the inverse solutions is demonstrated numerically by means of two illustrative examples. Moreover, the sensitivity of the estimation results to the measurement error and measurement location is systematically explored. Overall, the results show that the proposed method provides a robust and accurate approach for estimating the unknown time-dependent cooling heat flux in typical industrial spray cooling applications. [ABSTRACT FROM AUTHOR]