Heat transfer of dilute spray impinging on hot surface (simple model focusing on rebound motion and sensible heat of droplets)

In the present paper, focusing on the effects of the rebound motion and sensible heat of droplets on spray-cooling heat transfer in the high temperature region, a simple model was developed to predict the heat flux distribution of a dilute spray impinging on a hot surface. In the model, the local heat flux was regarded as the sum of the heat flux components by droplets, induced air flow, and radiation. To estimate the heat flux component by droplets, it was assumed that the heat flux upon droplet impact is proportional to the sensible heat which heats up the droplet to the saturation temperature and the proportional factor C is constant. In addition, to take account of the contribution of the heat flux upon impact of rebounded droplets, it was assumed that the flight distance of droplets during rebound motion is distributed uniformly from 0 to L_max (maximum flight distance) . The values of C and L_max determined by experimental data of local heat flux indicate that the assumptions employed in the present model is valid at least as the first order approximation.     

Characterisation of the spray cooling heat transfer involved in a high pressure die casting process

A systematic experimental study was conducted to examine the heat transfer characteristics from the hot die surface to the water spray involved in high pressure die casting processes. Temperature and heat flux measurements were made locally in the spray field using a heater made from die material H-13 steel and with a surface diameter of 10 mm. The spray cooling curve was determined in the nucleate boiling, critical heat flux, as well as the transition boiling regimes. The hydrodynamic parameters of the spray such as droplet diameters, droplet velocities, and volumetric spray flux were also measured at the position in the spray field identical to that of the test piece. Droplet size and velocity distribution were measured using a PDA system. A new empirical correlation was developed to relate the spray cooling heat flux to the spray hydrodynamic parameters such as liquid volumetric flux, droplet size, and droplet velocity in all heat transfer regimes. The agreement between experimental data and predicted results is satisfactorily good.     

Experimental Studies of Nanofluid Droplets in Spray Cooling

Spray cooling is used in cooling of electronic devices to remove large heat fluxes. Heat transfer to droplets impinging on a heated surface and boiling off has been studied. Most work is on a well-controlled system of a single drop falling onto a horizontal heated plate from a fixed height. These have revealed the droplet impingement mechanics to be a function largely of Weber number and excess temperature, and a range of regimes is observed similar to those in pool boiling, with a clearly identifiable critical heat flux. Nanofluids exhibit enhanced boiling heat transfer in pool boiling. The effect of nanoparticles on droplet boil-off was studied in this work. Nanofluid drops were let fall onto a surface at temperature greater than the saturation temperature, and behavior and heat flux were recorded and contrasted to that of a pure fluid. The working fluids used were pure water, ethanol, and dimethyl sulfoxide (DMSO) and ethanol– or DMSO–nanoparticle solutions (the nanoparticles were aluminum, with concentrations of up to 0.1% by weight in DMSO and 3.2% by weight in ethanol). High-speed photographic images of droplet evolution in time were obtained and indicate that there are differences in the behavior of nanofluid droplets as they boil off the surface, compared to pure fluids. Increasing nanoparticle concentration decreases the receding droplet breakup on rebound after impingement and appears to reduce the maximum spreading of a droplet as well. Maximum recoil height is reduced with increasing nanoparticle concentration. Experimental measurements of the heat fluxes associated with the pure and nanofluid droplets did not show significant enhancement, though there was noticeable improvement in the DMSO nanofluids.     

Heat transfer characteristics of spray cooling in a closed loop

A closed loop spray cooling test setup is established for the cooling of high heat flux heat sources. Eight miniature nozzles in a multi-nozzle plate are used to generate a spray array targeting at a 1 × 2 cm² cooling surface. FC-87, FC-72, methanol and water are used as the working fluids. Thermal performance data for the multi-nozzle spray cooling in the confined and closed system are obtained at various operating temperatures, nozzle pressure drops (from 0.69 to 3.10 bar) and heat fluxes. It is exhibited that the spray cooler can reach the critical heat fluxes up to 90 W/cm² with fluorocarbon fluids and 490 W/cm² with methanol. For water, the critical heat flux is higher than 500 W/cm². Air purposely introduced in the spray cooling system with FC-72 fluid has a significant influence on heat transfer characteristics of the spray over the cooling surface.     

A General Predictive Technique for Heat Transfer during Saturated Film Boiling in Tubes

Abstract

A simple predictive technique for heat transfer during film boiling in tubes is presented. This technique is based on the two-step model and consists of a graphic correlation for nonequilibrium quality and an equation for liquid droplet cooling at high pressures. It has been developed from and verified with data for water, nitrogen, para-hydrogen, R-113, methane, and propane. The range of data includes equilibrium qualities from 0.1 to 2.9, pressures from 1.4 to 215 bar, reduced pressures from 0.01 to 0.97, mass flux from 30 to 3442 kg/m² s, tube diameters from 2.5 to 14.9 mm, heat flux from 0.012 to 2.1 [Macute]W/m², and wall temperatures from 81 to 1112 K. For all 722 data points analyzed, heat transfer coefficients based on actual vapor temperatures are correlated with a root-mean-square error of 15%.     

Analytical Model for Describing Experimental Results on Parameters Influencing Heat Transfer in Film Boiling with Spray Quenching

An analytically solvable mathematical model is developed to estimate heat transfer quantities in the film boiling region of metal quenching with water sprays. The model is based on the hydrodynamic of a single droplet which is separated from the metal by a vapor film. The temperature profile within the droplet is calculated as semi-infinite body because of the short contact time. It is validated with own experimental results and those from the literature. The influence of size and velocity of the droplet, spray flux, surface temperature, temperature of the cooling water and the salinity level are discussed. The droplet size and velocity play a less significant influence on the heat transfer. The heat transfer coefficient is found to increase linearly with the spray flux. The heat flux is proportional to the difference of boiling and water temperature. With the model it is shown, that even for the high impingement densities the droplet covered area is very small.     

Spray evaporative cooling to achieve ultra fast cooling in runout table

Spray evaporative cooling, in lieu of conventional laminar jet impingement cooling, has potential to achieve the anomalously high strip cooling rate of Ultra Fast Cooling – 300 °C/s for a 4 mm thick carbon steel strip – in Runout Table of Hot Strip Mill. In the present study, evaporation time of a single droplet impinging on a hot carbon steel strip surface has been analytically evaluated as a function of droplet diameter from fundamental heat transfer perspective based on the premise that a spray can be considered as a multi-droplet array of liquid at low spray flux density. Droplet evaporation time thus evaluated has been used to estimate strip cooling rate achievable in Runout Table of Hot Strip Mill by spray evaporative cooling. The proposed analytical model predicts that it is indeed possible to achieve the ultra-high cooling rate of Ultra Fast Cooling by spray evaporative cooling by suitable reduction of droplet size. A general analytical expression has also been developed to estimate critical droplet size to achieve Ultra Fast Cooling as a function of steel strip thickness. Predictions of the analytical model have been validated using CFD simulation with a modified Discrete Phase Model.     

Electrocaloric Refrigeration using Multi-Layers of Electrocaloric Material Films and Thermal Switches

The electrocaloric effect in thin films of electrocaloric material has the potential to be used for efficient cooling systems. We numerically calculated the effect of the parameters in electrocaloric refrigeration with multi-layers of electrocaloric material films and thermal switches by changing the contact thermal conductance to improve thermal performance. It was found that the average heat transfer efficiency was 10% and the average heat flux transferred to the cold side of the system was 2.4 × 10⁴ W/m² for the standard conditions of a frequency of 100 Hz and a temperature difference between the hot side and the cold side of the system of 20 K. The average heat flux transferred to the cold side of the system was maximum when the thickness of the electrocaloric material was 70 µm and thickness of the heat storage material 100 µm. The average heat transfer efficiency was maximum at the two layers of the electrocaloric material.     

Mechanisms of film boiling heat transfer of normally impacting spray

The heat transfer mechanisms of horizontally impacting sprays were studied experimentally. An impulse-jet liquid spray system and a solid particle spray system were used. The liquid spray system is capable of producing uniform droplets with the independent variables of droplet size, velocity, liquid flow rate, and air velocity. The horizontally impacting sprays give a lower heat transfer at film boiling than the corresponding vertically impacting spray. The film boiling heat transfer is mainly controlled by the liquid mass flux. At low liquid mass flux and low droplet Weber number, the heat transfer increases with the droplet Weber number. At high droplet Weber number or high liquid mass flux, the heat transfer is not significantly affected by the droplet Weber number.     

A Honeycomb Microchannel Cooling System for Microelectronics Cooling

A honeycomb porous microchannel cooling system for electronics cooling was proposed in this article. The design, fabrication, and test system configuration of the microchannel heat sink were summarized. Preliminary experimental investigation was conducted to understand the characteristics of heat transfer and cooling performance under steady single-phase flow. In the experiments, a brass microchannel heat sink was attached to a test heater with 8 cm² area. The experimental results show that the cooling system is able to remove 18.2 W/cm² of heat flux under 2.4 W pumping power, while the junction wall temperature is 48.3°C at the room temperature of 26°C. Extensive experiments in various operation conditions and parameters for the present cooling system were also conducted. The experimental results show that the present cooling system is able to perform heat dissipation well.     

High heat flux spray cooling with ammonia: Investigation of enhanced surfaces for CHF

A spray cooling study was conducted to investigate the effect of enhanced surfaces on Critical Heat Flux (CHF). Test surfaces involved micro-scale indentations and protrusions, macro (mm) scale pyramidal pin fins, and multi-scale structured surfaces, combining macro and micro-scale structures, along with a smooth surface that served as reference. Tests were conducted in a closed loop system using a vapor atomized spray nozzle with ammonia as the working fluid. Nominal flow rates were 1.6 ml/cm² s of liquid and 13.8 ml/cm² s of vapor, resulting in a pressure drop of 48 kPa. Results indicated that the multi-scale structured surface helped increase maximum heat flux limit by 18% over the reference smooth surface, to 910 W/cm² at nominal flow rate. During the additional CHF testing at higher flow rates, most heaters experienced failures before reaching CHF at heat fluxes above 950 W/cm². However, some enhanced surfaces can achieve CHF values of up to ≈1100 W/cm² with ≈67% spray cooling efficiency based on liquid usage. The results also shed some light on the current understanding of the spray cooling heat transfer mechanisms. Enhanced surfaces are found to be capable of retaining more liquid compared to a smooth surface, and efficiently spread the liquid film via capillary force within the structures. This important advantage delays the occurrence of dry patches at high heat fluxes, and leads to higher CHF. The present work demonstrated ammonia spray cooling as a unique alternative for challenging thermal management tasks that call for high heat flux removal while maintaining a low device temperature with a compact and efficient cooling scheme.     

3-D modeling of the dynamics and heat transfer characteristics of subcooled droplet impact on a surface with film boiling

The impact of a subcooled water and n-heptane droplet on a superheated flat surface is examined in this study based on a three-dimensional model and numerical simulation. The fluid dynamic behavior of the droplet is accounted for by a fixed-grid, finite-volume solution of the incompressible governing equations coupled with the 3-D level-set method. The heat transfer inside each phase and at the solid–vapor/liquid–vapor interface is considered in this model. The vapor flow dynamics and the heat flux across the vapor layer are solved with consideration of the kinetic discontinuity at the liquid–vapor and solid–vapor boundaries in the slip flow regime. The simulated droplet dynamics and the cooling effects of the solid surface are compared with the experimental findings reported in the literatures. The comparisons show a good agreement. Compared to the water droplet, it is found that the impact of the n-heptane droplet yields much less surface temperature drop, and the surface temperature drop mainly occurs during the droplet-spreading stage. The effects of the droplet’s initial temperature are also analyzed using the present model. It shows that the droplet subcooling degree is related closely to the thickness of the vapor layer and the heat flux at the solid surface.     

Enhanced boiling heat transfer in mesochannels

Heat transfer characteristics are studied for a hybrid boiling case that combine features of spray cooling and flow boiling. In such a hybrid system, a liquid is atomized and the surrounding vapor is entrained into the droplet cone to provide an initial quality for enhanced boiling. An in-house experimental setup was developed to obtain surface temperature and heat flux measurements in a series of converged mesochannels for hybrid boiling. To compare the heat transfer performance of this hybrid technique, a flow boiling module was also developed using the same series of converged mesochannels. The inlet and exit hydraulic diameter of the mesochannels was 1.55 and 1.17 mm, respectively. The heat flux was in the range of 15–45 kW/m² and the estimated mass flux varied from 45 kg/m²s at the channel inlet to 110 kg/m²s at the channel outlet. Moreover, a model was presented to predict surface temperatures and heat transfer coefficients for flow boiling and hybrid boiling in mesochannels. This model was developed based on Chen’s formulation (1966) [21] but with two essential modifications. First, the laminar entry length effect was taken into consideration for heat transfer coefficient calculation. Second, the boiling enhancement factor was calculated based on the fluid properties. The model was compared to the experimental data and several other correlations for both cases. This model shows good agreement with the experimental data (mean deviations on the order of 12–16%).     

Design and Testing of a Graphite Foam-Based Supercooler for High-Heat-Flux Cooling in Optoelectronic Packages

The feasibility of using graphite foam as a heat sink and heat spreader in optoelectronic packages is assessed. A “supercooler” is designed, fabricated, and tested to verify its cooling capability under high heat flux conditions in a typical optoelectronic package. The supercooler uses graphite foam as a primary heat transfer material. Water is soaked into the graphite foam, and under evacuated pressure, boiling is initiated under the heating region to provide enhanced cooling. Experiments were conducted for a heat flux of up to 400 W/cm² deposited over a heating area of 0.5 mm × 5 mm. Two-dimensional transient temperature distributions were recorded using a high-speed infrared camera. Data were obtained for steady heating, and for periodic heating with frequency up to 8 Hz. Results show that the supercooler is very efficient in dissipating heat away from the heating region. The average cooling rate during the cooling period exceeds 170 K/s.     

Super-high heat flux removal using sintered metal porous media

Introduction Recently there have been a demand for the technique to efficiently and steadily cool down extremely high heat flux of over 10 MW/m2, to fulfill not only the needs for plasma facing components in nuclear fusion reactors, but also the needs associated with sophisticated computers or downsizing of such devices as high-density laser equipment and power devices. However, existing cooling techniques in such high heat loading environment are basically based on high speed and highly subc…     

燃气内燃机和吸附制冷机组成的冷热电三联供系统

对一种微型楼宇冷热电三联供系统进行了技术经济分析．该系统由小型燃气发动机和热水驱动的吸附制冷机组成．为了提高系统的热电输出比，系统设置了一电热泵．分析了该系统在不同负荷率（PLR）下的一次能源利用效率（PER），确定了高效运行的参数范围；比较分析了该系统在不同热电输出比、热水输出比条件下的节能性；并通过一实例对系统的经济性进行了分析．研究表明，该系统具有宽广的热电输出比、较高的总能利用率和经济可行性，适合小型商业场所和家庭使用，图5表2参9     

Multi-criteria thermoeconomic and thermodynamic assessments of the desalination-integrated two-phase liquid-immersion data center cooling system

The waste heat management of the data center cooling systems has a significant share in the energy-efficient operations of the data centers. In this study, a new hybrid desalination-data center cooling system is proposed to reduce the cost drawback of the waste heat in the data center cooling operations. A two-phase liquid-immersion cooling unit is selected as the data center cooling method with the cooling load range of 0.7 to 1.5 kW. It is a promising solution thanks to the high heat flux removal performance but there is still a lack of research about waste heat management. The waste heat of the immersion cooling system is used to heat up the feed side of the desalination module. A direct contact membrane distillation system as preferred as the desalination module with the membrane area range of 5 to 75 cm². The proposed hybrid system is investigated according to the thermodynamic, economic, and thermoeconomic aspects. The thermoeconomic assessment is done concerning the unique exergy-cost matrix of the original design. The maximum thermal and exergy efficiencies are found as 64.5% and 53.7%, respectively. The daily distilled water rate can reach 6.13 kg at the highest cooling load and membrane area. Compared to the stand-alone data center cooling operation, the hybrid system has higher capital and operation costs. The payback period is found 3.72 years that means the proposed system is economically feasible for real applications. Also, the levelized product cost of the hybrid design is calculated in the range of 2.69 to 5.33 SGD/h. In the multiobjective optimization study, the best trade-off point is decided at the cooling load of 1.1 kW whilst the membrane area varies between 5.12 and 5.19 cm².     

Thermal management of solar cells using a nano‐coated heat pipe plate: an indoor experimental study

With temperature increasing, the photovoltaic efficiency of solar cells is reduced significantly. Such an efficiency loss may offset the efficiency improvement because of the development of the photovoltaic technology. This paper provides a novel approach for efficiency loss recovery of solar cells. Specifically, a nano‐coated heat pipe plate was integrated with the solar panel to remove heat from the hotspots on solar cells. This study concerns the indoor experiments of a commercial solar cell thermally managed with a heat pipe plate. The temperature rise and non‐uniformity on the solar panel were quantified in different light irradiances. With thermal management by the heat pipe plate, the solar panel shows a temperature‐rise reduction of 47–50%. This implies that half of the efficiency loss of the solar cell can be recovered. In addition, the temperature variation within the solar panel is reduced to 1.0–2.5 °C, which is beneficial in prolonging the longevity of the solar cell. In the experiments, the heat pipe plate can provide a cooling flux of 380 W/m² with light irradiance below 1000 W/m². By incorporating the heat pipe plate with a water jacket, the heat removal flux could be improved to 600 W/m², leading to a solar cell temperature of a few degrees higher than the ambient. Copyright © 2016 John Wiley & Sons, Ltd.     

Condensation of steam in silicon microchannels

A visualization study was performed on condensation of steam in microchannels etched in a 〈100〉 silicon wafer that was bonded by a thin Pyrex glass plate from the top. The microchannels had a trapezoidal cross section with a hydraulic diameter of 75 μm. Saturated steam flowed through these parallel microchannels, whose walls were cooled by natural convection of air at room temperature. The absolute pressure of saturated steam at the inlet ranged from 127.5 kPa to 225.5 kPa, and the outlet was at atmospheric pressure at approximately 101.3 kPa with the outlet temperature of the condensate ranging from 42.8 °C to 90 °C. Stable droplet condensation was observed near the inlet of the microchannel. When the condensation process progressed along the microchannels, droplets accumulated on the wall. As the vapor core entrained and pushed the droplets, it became an intermittent flow of vapor and condensate at downstream of the microchannels. The traditional annual flow, wavy flow and dispersed flow observed during condensation in macrochannels were not observed in the microchannels. Based on a modified classical droplet condensation theory, it is predicted that the droplet condensation heat flux increases as the diameter of the microchannel is decreased. It is also predicted that the droplet condensation heat flux of saturated steam at 225.5 kPa can reach as high as 1200 W/cm² at ΔT=10 °C in a microchannel having a hydraulic diameter of 75 μm.     

The measurement of heat transfer from hot surfaces to non-wetting droplets

An experimental method to measure the heat transfer between a hot surface and a non-wetting droplet is reported in this paper. By means of transient, high resolution, infrared microscopy, surface temperature measurements with spatial and temporal resolutions of ～100 μm and ～4 ms, respectively, are obtained, by observing a thin metallic layer from beneath through an infrared-transparent substrate. Data from the infrared camera is generated at each time-step in the form of a set of temperatures, at closely-spaced locations on the surface of the infrared transparent plate. Subsequent solution of the transient thermal conduction equation within the substrate permits all thermal quantities (heat flux, energy, etc.) to be determined. As a typical result, the heat transferred by a 1.5 mm droplet is measured to be 0.19 J, with the heat flux peaking at 3.5 MW/m² during the 10 ms it spends in the vicinity of the surface, and with a peak transient surface temperature reduction of 47 °C. Error analysis indicates that the uncertainty in this measurement of heat transfer is about 15%.