High Heat Flux Applications Research Articles

Variable Area Jet Impingement for Enhanced Junction Temperature Control of High-Power Electronics

Abstract Convective heat transfer by jet impingement cooling offers a suitable solution for high heat flux applications. Compared to techniques that rely on bulk conduction in series with convection, direct liquid impingement reduces the thermal resistance between power device hot spots and the coolant. Although capable of highly efficient cooling, static impingement devices must be designed for the worst-case cooling requirements for a transient power profile. This can result in wasted hydraulic performance. Aircraft, highway vehicles, and heavy machinery fall into this category where a substantial factor of safety is required. This work proposes a method for improving power electronics reliability by limiting temperature fluctuations at reduced coolant pressure requirements during transient power cycling using a variable area jet. Single phase jet impingement cooling is implemented in an active control scheme using a variable diameter iris mechanism as the primary nozzle architecture. In addition to pressure drop and temperature control, the active nozzle structure introduces the ability to create pulsating jet flows to further enhance the heat transfer compared to fixed-geometry nozzles. The key underlying fluid mechanics characteristic of pulsating flows is the effect of disrupting the thermal boundary layer on the electrical device surface. By introducing a variable diameter jet, eddy formation can be fine-tuned for optimal boundary layer disruption. Using the definition of the Strouhal number, vortex shedding created by the non-steady jet flows is directly correlated with the resulting Nusselt number as a function of the iris kinematics. An experimental apparatus for jet impingement thermal-fluid testing is used to evaluate the Nusselt number versus Strouhal number for a parametric study of variable diameter iris configurations. The apparatus utilizes a voice coil actuator to achieve sine and square waveforms, to vary the amplitude of actuation, and to vary the mean of actuation. Finally, power cycling with a single emulated hot spot is performed to estimate the reliability increase as a result of maintaining constant junction temperatures with the active jet impingement scheme.

Evaluation of innovative thermal performance augmentation techniques in heat sinks & heat pipes: Electronics & renewable energy applications

Heat sinks and heat pipes have vast applications including industrial domains requiring high heat flux. Due to numerous uses of these heat transfer devices, there have been great advancement in this area through innovation. This study focuses use of nanofluids and hybrid nanofluids for thermal performance enhancement of heat pipes and heat sinks. There are five different arrangements finless sintered copper wicked heat pipe, water based TiO2 filled heat pipe, TiO2 and SiO2 based hybrid nanofluid filled heat pipe, water cooled cylindrical pin finned heat sink and Fe2O3 and TiO2 hybrid nanofluid cooled heat sink. There are two test rigs one for testing heat pipe configurations and other for heat sinks. All the five elements are tested at constant heat flux ranging from 900 W/m2 to 3600 W/m2, in four equal steps. It’s observed that nanofluids play a very significant role in thermal performance augmentation, the affect is even more significant in heat pipes compared to heat sinks. Interestingly, nanofluids based heat pipes are proved more significant for high heat flux applications, although, nanofluid cooled heat sinks have slight better performance compared to the said but at a higher expense of weight, auxiliaries, space and cost. At 3600 W/m2 nanofluid based heat pipes have 33 % lower thermal resistance compared to water cooled heat sink. Therefore, nanofluid based heat pipes are primarily significant unless use of heat sink deemed dire. Nusselt number of heat sink is calculated at various heat flux and at variable mass flux conditions and found increasing with both parameter.

Investigation on the thermal and hydrodynamic performance of a microchannel heat sink partially filled with porous medium

Microchannel heat sinks with porous media are promising solutions for high heat flux applications. This work presents a numerical study on the evaluation and optimization of microchannel heat sinks partially filled with porous medium. The thermal and hydrodynamic performance of the microchannel heat sinks are first systematically investigated and compared with those of the fully filled and unfilled counterparts. The optimization of the partially filled microchannel heat sinks is then conducted by using the response surface method, focusing on the effects of five parameters, including the height and width of the porous medium at the inlet, the height and width of the porous medium at the outlet, and the Reynolds number, on the thermal and hydrodynamic performance of the microchannel heat sinks. Furthermore, the optimization problem with two objective functions is analyzed by calculating the Pareto optimal set within the considered design parameter space. Depending on the available pumping power, the design and operation parameters could be selected from the Pareto optimal set to achieve the best heat transfer performance.

An experimental evaluation of the performance of a remote 2U loop thermosyphon

In recent years, the increasing application and diversity of electronic systems have resulted in a significant demand for research and development in electronic cooling. Designing and evaluating thermal modules is crucial, especially for systems generating high heat flux levels. This study proposes a novel remote loop thermosyphons (RLTS) for high heat flux applications and cooling 2U servers, which has been assessed experimentally. Additionally, several critical factors are examined, including the filling ratio (FR), air flow rate, and heat load. The experimental study evaluated air flow rates range from 130 to 170 cubic feet per minute (CFM), and the filling ratios span from 40 % to 70 %. The heat load varied from 100 W to 700 W. Compared to the conventional loop thermosyphon, the remote loop thermosyphon exhibited superior performance and sustained higher heat flux. Specifically, this thermal module can effectively manage 640 W at an inlet air temperature of 28 °C with an airflow rate of 170 CFM. Moreover, compared to other available loop thermosyphons, the proposed thermal module demonstrated enhanced heat flux up to 94 W/cm2. These results suggest that the new remote loop thermosyphon is a promising solution for cooling high heat flux applications, such as 2U servers. Furthermore, among all the investigated FR effects the 50 % FR performed better than the other filling ratios. At this filling ratio, the thermal resistance reached a minimum value of 0.078 K/W. The findings indicated that increasing the heat load decreased thermal resistance to a certain threshold, beyond which thermal resistance started to increase. This increase in thermal resistance was caused by restricted space above the evaporator. The confined space effect in a loop thermosyphon system significantly influences the performance and should be considered as a crucial factor in future design endeavors.

Experimental study on flow boiling heat transfer performance of nanowire-printed substrates with porous-like structures

In this study, ZnO nanowires (NWs) were demonstrated to have considerable potential for heat transfer enhancement in flow boiling. Unlike the conventional NW fabrication method using metal-assisted chemical etching, we use the microcontact printing (µCP) method comprising solution-based growing methodologies to create ZnO NWs on different substrates, including bare Si-, Cu-, and Al-coated surfaces. This ZnO NW fabrication approach successfully overcomes the substrate limitation of typical NWs, such as Si- and Cu-NWs, which can be only formed on its intrinsic material following the high-cost fabrication approaches required by the metal catalyst. Additionally, a high pore-connected structure was observed on the interfacial area of the ZnO NWs, with the nanocore distribution area considerably enhanced by lengthening the NW length, beneficial to bubble nucleation and surface coolant supply during nucleate boiling. The length of the ZnO NW substrates is controllable by varying the growing period during the hydrothermal growth procedure. Further, the ZnO NW substrates reveal a remarkable enhancement in the flow boiling heat transfer regardless of the substrate variation, thereby exhibiting high variability in the substrate selection and potential of being used as the cooling approach for the thermal management of the high heat flux applications. We conducted a series of experiments related to the surface wettability, wicking, and bubble characteristics for different test substrates under saturated flow boiling conditions to explicitly determine the underlying mechanism in the heat transfer enhancement of the ZnO NWs.

Optimizing Energy-Efficient jet impingement cooling using an artificial neural network (ANN) surrogate model for high heat flux Semiconductors

As the demand for high-performance electronic devices and electric vehicles (EVs) increases, the importance of energy-efficient thermal management has become more important. This study presents the optimization of two high-heat flux (865.8 W/cm2) semiconductor thermal management systems targeting an EV inverter. The cooling system employs a jet impingement integrated with microposts. The thermal resistance (Rth.avg) and pumping power (Ppump) were evaluated through computational fluid dynamics (CFD) simulations by varying the jet nozzle parameters, micropost variables, and volume flow rate. To ensure the accuracy of the CFD model, experimental validation was also conducted, demonstrating less than a 5 % error. Then a surrogate model predicting Rth.avg and Ppump was developed using the artificial neural network (ANN) method. The sensitivity analysis subsequently identified the primary influencing factors of the system, and the Pareto optimal fronts were determined via the elitist non-dominated sorting genetic algorithm (NSGA-II). Through optimization, the approach provided multiple optimal designs for a broad spectrum of Ppump values, which was feasible within a brief period (less than 200 s) due to the rapid estimations by the ANN-based surrogate model. The experimentally validated design achieved a 65 % improvement in heat transfer coefficient (∼102.96 kW/(m2·K)) at similar pumping power levels (∼0.42 W) compared to previously reported jet impingement studies. Moreover, the design chosen through the optimization (Case B) projected a 140 % enhancement in heat transfer coefficient (∼150.2 kW/(m2·K)) with only a 63 % rise in Ppump (∼0.65 W). By exploring a more diverse range of multi-jet and micropost configurations, we were able to achieve higher performance. This study highlights the potential of combining jet impingement cooling with microposts as a highly attractive strategy for thermal management in high heat flux and multi-hot spot applications.

Experimental study on turbulent characteristics of dispersed bubbly flow in a narrow rectangular channel by two-phase PIV

Narrow rectangular channels are widely used in high heat flux applications, due to the high heat transfer efficiency and compact structure. However, a rare understanding was known about the two-phase turbulent characteristics. In this study, an experimental investigation of bubble and turbulent characteristics of dispersed bubbly flow in the narrow rectangular channel was carried out by using the high-speed camera and two-phase particle image velocimetry. The bubble size effect was studied independently using the continuous spectrum bubble generator. According to the results, the bubble size distribution conforms to a Weibull distribution, and the wall-peak void fraction distributions were found. Both turbulent enhancement and suppression were observed in current conditions. Large bubbles enhance turbulence intensity, while small bubbles suppress it. The eddy viscosity ratio ϕ characterizes the modification of turbulence intensity by bubbles, with a critical value of ϕ=1.

Process development for laser powder bed fusion of GRCop-42 using a 515 nm laser source

Copper is widely used in high heat flux and electrical applications because of its excellent electrical and thermal conductivity properties. Alloying elements such as chromium or nickel are added to strengthen the material, especially for higher temperatures. Cu4Cr2Nb, also known as GRCop-42, is a dispersion-strengthened copper-chromium-niobium alloy developed by NASA for high-temperature applications with high thermal and mechanical stresses such as rocket engines. Additive manufacturing (AM) enables applications with complex functionalized geometries and is particularly promising in the aerospace industry. In this contribution, a parametric study was performed for GRCop-42 and the AM process laser powder bed fusion (PBF-LB/M) using a green laser source for two-layer thicknesses of 30 and 60 μm. Density, electrical conductivity, hardness, microstructure, and static mechanical properties were analyzed. Various heat treatments ranging from 400 to 1000 °C and 30 min to 4 h were tested to increase the electrical conductivity and hardness. For both layer thicknesses, dense parameter sets could be obtained with resulting relative densities above 99.8%. Hardness and electrical conductivity could be tailored in the range of 103–219 HV2 and 24%–88% International Annealed Copper Standard (IACS) depending on the heat treatment. The highest ultimate tensile strength (UTS) obtained was 493 MPa. An aging temperature of 700 °C for 30 min showed the best combination of room temperature properties such as electrical conductivity of 83.76%IACS, UTS of 481 MPa, elongation at break (A) at 24%, and hardness of 125 HV2.

Exploring heat transfer efficiency in non-boiling spray cooling

Spray cooling today is one of the most effective methods for high heat flux application. Since there are numerous factors influencing the heat transfer in spray cooling, some issues related to the effect of the nozzle-to-surface distance, heat flux and liquid subcooling on the heat transfer efficiency in non-boiling mode remain debated. This study is dedicated to a comprehensive experimental investigation of the impact of various factors, including the distance from the nozzle to the heater (2–35 mm), heat flux density (up to 6.9 MW/m2), liquid flow rate (8.8–25.2 mL/s), and initial liquid temperature (20 and 80 °C) on heat transfer in non-boiling spray cooling using different nozzles with varying spray angles (30–90°). Heat transfer results were obtained based on field temperature measurements using high-speed infrared thermography. It is shown that the nozzle-to-surface distance has a significant effect on the spatial distribution of the temperature field and the heat transfer rate in spray cooling. It is shown that for each nozzle and heating surface there is an optimum distance at which the maximum heat transfer rate at non-boiling spray cooling is observed and relationship for their determination are proposed. The heat transfer coefficient for deeply subcooled liquid (80 K) is practically independent of the heat flux. At low subcooling levels (20 K), the heat transfer coefficient increases with the increase in heat flux associated with the influence of evaporation from the liquid film surface on the total heat transfer. The results obtained are analyzed, compared and generalized with data from the literature, and a correlation is proposed for determining the heat transfer in non-boiling spray cooling.

Experimental research on cooling performances of radial expanding-channeled heat sinks applied for multiple heat sources

Intrinsic two-phase flow instability using multiple parallel boiling channels hinders the actual implementation of flow boiling cooling techniques in high heat flux and multiple heat source applications. Recently, we proposed a radial expanding-channeled heat sink (ECHS) to realize a high two-phase flow stability under extreme conditions. In the present work, the cooling performances of two heat sinks connected in series or in parallel were investigated with the heat load adjusted individually. The results indicated that when two ECHS were connected in parallel, the inlet flowrate was evenly distributed into the two heat sinks, and their heat transfer performance was identical. At a sudden change in the heat load of one path, the flowrate of the other heat sink may change by 10% to 30% without inducing flow instability or heat deterioration. The thermal resistance for both heat sinks was lower than 0.025 °C/W, and no instability or dry-out was observed as long as the initial flowrate was larger than 0.28 kg/min. With the proposed ECHS, multiple heat sources such as central processing unit, server, rack, etc. can be connected in series or in parallel to construct an integrated cooling system for large-scale data center applications.

Converging Jet Impingement for Enhanced Liquid Cooling

Abstract Jet impingement cooling is an advanced thermal management technique for high heat flux applications. Standard configurations include single, axisymmetric jets with orifice, slot, or pipe nozzles. This choice in nozzle shape, number of jets, and jet inclination greatly influences the turbulence generated by fluid entrainment due to differences in initial velocity profiles and location of secondary stagnation points. Regarding high power electronics with integrated jet impingement schemes, turbulence, and heat transfer rates must be optimized to meet the extreme cooling requirements. In this study, the heat transfer rates of dual inclined converging jets are investigated experimentally. Emphasis is placed on the comparison of different jet schemes with respect to geometrical parameters including nozzle pitch, incline angle, and nozzle-to-target plate spacing. A parametric experimental investigation is performed as a point of comparison using a modular, additively manufactured jet setup. Computational simulations are used to evaluate the effect of jet configuration on stagnation pressure associated with maximum heat transfer rates, and an empirical Nusselt number model is provided. The results show the effects of convergence height on jet behavior and the associated impacts on heat transfer.

Enhancing the performance of composite phase change materials using novel triply periodic minimal surface structures

Organic phase change materials (PCMs) are widely used in thermal management applications owing to their high latent heats of fusion and low cost. Unfortunately, their poor thermal conductivities severely hamper the thermal transport process. Hence, PCMs are often hybridized with high thermal conductivity materials that promote phase change for time-sensitive applications and potentially increase the power density of thermal energy storage (TES) devices. Optimized thermally conductive networks with high-latent-heat PCM matrices are critical in the thermal management of high heat flux applications. With advancements in additive manufacturing, it has become easier to create complex topologies, such as triply periodic minimal surfaces (TPMS), that can further augment the performance of composite PCMs. TPMS surfaces have shown great promise in improving thermal transport performance but investigations into their thermal properties remain limited. In this study, the thermal performances of PCM infused TPMS structures of wall thicknesses varying from 0.4 mm to 0.8 mm were systematically investigated. The TPMS structures were designed using level-set equations, and each system of structures was fabricated by selective laser melting, a metal additive manufacturing technique. Numerical investigations were performed to understand the phase change dynamics of PCM in TPMS cells and experiments were performed to determine their thermal properties and phase change performance for thermal energy storage applications. Our simulations revealed that for structures with uniform wall thickness of 0.4 mm, the novel Neovius structure resulted in the highest effective thermal conductivity enhancement. For a given thickness, it is thus hypothesized that more complex TPMS topologies have higher surface areas and volumes which enhance their thermal conductivities. However, the complex shape of the Neovius structure could not be further uniformly thickened without self-intersecting geometries. Ultimately, a balance of both the surface area and thickness of the TPMS cell determines the thermal conductivity enhancement. Overall, the 0.6 mm thick IWP cells had the highest thermal conductivity of 13.84 W/m K, which was consistently verified in experiments. This is likely due to the fact that it has a larger surface area than the P structure but is also thicker than the 0.4 mm Neovius structure. To further demonstrate the potential application of our PCM-based TPMS design, low temperature thermal cycling tests were performed. Our results showed the IWP cell structure promoted a more uniform temperature distribution as compared to the P cell. Phase change in the 0.6 mm thick IWP cell structure was completed 1.4 times faster than the 0.8 mm thick P cell and 1.8 times faster than the 0.6 mm thick P cell. The purpose of this study is to provide useful guidelines for the selection of TPMS structures for various applications such as battery packs and spacecraft and to open doors for the co-optimization of materials and device geometry for enhanced energy and power density.

A smart and sustainable energy approach by performing multi-objective optimization in a minichannel heat sink for waste heat recovery applications

Minichannel heat sink is widely used in waste heat recovery systems for their compactness and ability to recover heat effectively from high heat flux applications. However, the heat recovery efficiency is constrained by the flow configurations resulting in flow maldistribution. Numerous neural network combined evolutionary algorithms have been used to reduce pressure drop and flow maldistribution factors in the literature. But it is very challenging to assign appropriate weights to these parameters with no physical significance between them for optimization studies. To overcome this, TOPSIS-based optimization studies have been used in the current work to reduce the flow maldistribution factor (ϕ) and increase the Nusselt number (Nu) with ribs and inclined structures. Four Minichannel designs are studied to assess the channel heat recovery efficiency from small-scale incinerators using water and Graphene oxide (GO) nanofluid for three different volume fractions of GO-0.02%, GO-0.07%, and GO-0.12%. The motive is to determine an optimal nanofluid volume fraction and a suitable Minichannel configuration for the given heat flux. The TOPSIS method handles five criteria, including the combination of weightage for the maldistribution factor and Nusselt number. For criteria I ((ϕ)min: (Nu)max = 0.0:1.0) maximum weightage is given to heat transfer, the ribbed channel has gained a higher performance score for GO-0.07% nanofluid volume fraction. For criteria V ((ϕ)min: (Nu)max = 1.0:0.0) maximum weightage is given to maldistribution reduction, the ribbed inclined channel has gained with significantly higher performance score for all the studied nanofluid volume fractions. Further, the study is extended to determine the heat recovery efficiency, and it is found that with the increase in mass flow rate and nanofluid volume fraction, the heat recovery efficiency increases significantly. In particular, the maximum heat recovery efficiency of 66% was obtained for ribbed Minichannel using GO-0.12% nanofluid.

Thermal-hydraulic performance and multi-objective optimization using ANN and GA in microchannels with double delta-winglet vortex generators

This study presents a multi-objective optimization approach to design microchannels with double delta-winglet vortex generators for obtaining the best hydrothermal performance under high heat flux. Grey relational analysis, neural networks, and Non-dominated Sorting Genetic Algorithm (NSGA-II) are utilized in the optimization process. Back-propagation neural network improved by genetic algorithm (GA-BP) is found to have superior generalization and prediction abilities for Nusselt number (Nu) and friction factor (f) compared to Back-propagation neural network (BPNN). The design parameters include the angle between vortex generator and bottom plane (θ1), the angle between the baseline of vortex generator and water incoming flow direction (θ2), the distance from the apex of the vortex generator to the baseline (h), and inlet velocity (vin). NSGA-II is employed to obtain the optimal design point (θ1 = 90°, θ2 = 45°, h = 0.3 mm, vin = 0.8 m/s). The hydrothermal performance under this design point is significantly improved, as compared to smooth microchannel, with a 128.1% increase in Nu, a 72.0% increase in f, and a performance evaluation criterion (PEC) of 1.89. Furthermore, flow and temperature fields, entropy production, local Nu, local f and axial vorticity are analyzed. The results highlight the potential of double delta-winglet vortex generators to improve overall thermal performance in high heat flux applications.

Characterizing effect of particle size on flow boiling in sintered porous-microchannels

Flow boiling in porous microchannels draws extensive attention recently owing to its great potential in high heat flux applications. Subcooled flow boiling of deionized and degassed water was carried out to characterize transport performance. Porous microchannels were sintered from six-sized dendritic-like copper particles: 10, 30, 50, 90, 120 and 150 μm with the same layer thickness of 400 μm. The dimensions of 23 parallel porous microchannels are 600 μm × 1200 μm × 2800 μm (width × depth × length). The inlet subcooling degree and mass flux was maintained at 40 K and 142 kg/m2·s, respectively. Experimental results show that the particle size has great effects on the heat transfer coefficient (HTC) and Critical heat flux (CHF) of flow boiling in porous microchannels. Medium particle size (90 μm or 120 μm) remains higher performance in HTC and CHF. In heat fluxes between 60∼160 W/cm2, average HTCs of both PM-90 and PM-120 could reach almost 120 kW/(m2·K) or so. The visualization study reveals that the underlying heat transfer mechanism in porous microchannels is dominated by the nucleate boiling in low heat flux and then, by the thin film evaporation from moderate to high heat flux. Moreover, two boiling crisis phenomena have been observed: (a) in microchannels sintered from optimum size particles, the rewetting-dryout cycle has been well established and sustained without being interrupted by occasionally explosive boiling events, indicating capillary limits; (b) in non-optimum samples, high wall superheat in the rear part usually leads to explosive boiling persistently, which interrupts periodic oscillation modes with heat flux exceeding to moderate level and results in CHF conditions. An optimal ratio of bottom wall thickness and particle diameter, δ/d, is found in the range of 3∼5 for the flow boiling in sintered porous microchannels.

A novel 3-D pulsating heat pipe module for high heat-flux applications

This study proposed a novel pulsating heat pipes (PHP) for high-flux thermal module. According to the open literature, the maximum value of the heat flux is 23.7 W∙cm−2 handled by PHP, which is far behind industry requirements. Therefore, the present study proposes a novel 3D PHP suitable for high heat-flux applications. The PHP were experimentally tested for different working fluids, including methanol and water. Furthermore, the experiments were performed to analyze the influence of various operating parameters, such as filling ratio (25–60%), heat load (100–800 W), and air flow rate 100–250 cubic feet per minute (CFM). The results showed that this PHP can sustain a heat flux up to 70 W∙cm−2 with the maximum case temperature under 80 °C, which is approximately three times higher than the available literature. Furthermore, the performance of the PHP module was compared against the commercial module with wicked heat pipe. The PHP exhibited about 7% higher heat dissipation compared to the commercial module. Since the proposed module uses simple copper tubes in its construction over the wicked structure used in commercial module, the novel 3D PHP module provides additional economic and mass production capability.

Experimental and numerical studies on the spray cooling of deionized water in non-boiling regime

High-efficiency spray cooling technology has received increase of interest for high heatflux applications. Experimental and numerical investigations on spray heat transfer in single-phase deionized water are reported in this paper. The 3D numerical model is established based on Discrete Phase Model and Lagrange Wall Film model, comprehensively considering the exchange of mass, momentum and energy between the air (continuous phase) and water droplets (discrete phase), and the heat transfer in liquid film at the heated surface. PDA is adopted to measure Sauter Mean Diameter (SMD) of near-wall droplets. The influences of mass flow rates, nozzle diameters and spray heights on spray heat transfer characteristics are discussed. Results indicate that increasing mass flow rate can reduce SMD due to high We number and improve heat transfer coefficient. Liquid-film thickness is high at either low or high mass flow rate, but has minor influence on cooling performance. The optimal nozzle diameter is 0.6 mm rather than the smaller or higher diameter because the spray area is similar to the heated area. Similar reason is for the optimal spray height of 20 mm.

Thermochemical cooling for large thermal load applications

The requirement to reject heat within a small envelope has stunted the development and deployment of high heat flux applications. Thermochemical cooling is a new innovative approach, inspired by engine cooling in aircraft. In aircraft, sensible heat is often rejected to the fuel, without altering its chemical composition, and dissipated through the wings. However, sensible heat absorption is limited to 431 kJ/kg for JP8. Using endothermic reactions, JP8 can absorb as much as 11,688 kJ/kg of heat when converted to hydrogen, carbon monoxide, and other gases. A common refrigerant, such as R134A has an enthalpy of vaporization of 209 kJ/kg. However, after evaluating multiple fuels and reforming literature, it was determined that optimum lower temperature performance could be achieved with a methanol water mixture. This endothermic reaction is a strong candidate for heat absorption. Preliminary test data demonstrated heat absorption as low as 300 °C, with peak absorption at 400 °C. At its peak (R = 400 °C), the reactor is capable of absorbing 3411 kJ/kg. Future efforts will evaluate the use of JP8, which has a larger endothermic reaction potential.

Performance of spray cooling with vertical surface orientation: An experimental investigation

• Vertical surface spray cooling system is built and investigated. • The surface temperature is controlled below 120°C with high input power of 650 W. • The maximum critical heat flux of 596 W/cm 2 is achieved. • The interplay of convection and evaporation heat transfer is studied. • Nusselt number correlation in the single-phase regime is proposed. Spray cooling is a promising thermal management solution for high heat flux applications. Surface orientation is a key parameter of spray cooling performance while vertical surface orientation has been proved to have excellent heat transfer performance. However, key spray parameters regarding the vertical surface spray cooling system have not been comprehensively studied. In this work, a vertical surface spray cooling system is designed and established. The effects of spray volumetric flux, nozzle-to-surface distance, and coolant inlet temperature on the heat transfer characteristics of spray cooling are experimentally investigated. The system shows a magnificent heat dissipation capability as high as a critical heat flux of 596 W/cm 2 at a low spray volumetric flux (0.83 × 10 -2 m 3 ·s -1 ·m -2 ). Nozzle-to-surface distance, which has no obvious effect on the two-phase regime, is observed to play a significant role in the single-phase regime. Interestingly, the heat transfer coefficient can be enhanced by increasing the coolant inlet temperature while it will not necessarily lower the surface temperature. Furtherly, an experimental dimensionless correlation of Nusselt number for the single-phase regime is proposed, demonstrating a higher contribution of coolant flow properties to the spray cooling system performance with vertical surface orientation. This work reveals the underlying mechanism of how the key spray parameters impact the performance of the spray cooling system with vertical surface orientation and is expected to facilitate high-heat-flux heat dissipation and provide guidance for the research on spray cooling.

Computational investigation of single and multi-jet array impingement boiling

Jet impingement boiling has been studied extensively and has been identified as one of the most promising thermal management techniques for high heat flux applications. Unfortunately, only a few numerical studies have been reported in literature and they are mostly limited to single jets. In the present study, both submerged single round jets and confined multi-jet arrays are investigated numerically, using the Eulerian multiphase framework with the Rensselaer Polytechnic Institute (RPI) boiling model to predict heat transfer. The numerical results of the single jet case correlate well with reported experimental data and previously reported numerical results. The numerical results of the multi-jet array correlate well with experimental data reported in the literature, proving that the RPI boiling model can successfully predict the heat transfer of jet array boiling. The effect of conjugate heat transfer in jet impingement boiling heat transfer is also investigated for both single and multiple jet cases. The single-jet results agree with previous reported numerical studies. To improve numerical convergence, especially for higher heat fluxes, use was made of a hydrostatic pressure gradient at the outlet. This allowed for significant improvement in the convergence of the continuity equation. Finally, parametric analyses were conducted for both single and multi-jet arrays in the fully developed nucleate boiling regimes. Parameters included jet-to-surface spacing, Reynolds number and subcooling. The results for the single jet correlate well with the observations of experiments reported in the literature. The results for the multi-jet array showed less sensitivity to changes in jet velocity at low jet-to-surface spacing than the single-jet case. Both single and multi-jet cases showed that reducing the subcooling resulted in an onset of nucleate boiling at lower heat fluxes and that the boiling curve shifted to the left in the nucleate boiling regime.