喷雾冷却发热壁面温度非均匀性实验研究

喷雾冷却是一种新型的高热流密度换热方式,利用可视化手段对喷雾冷却中发热体壁面温度非均匀性进行了实验研究。采用热电偶多点测温及红外成像相结合的方法测量发热体壁面温度,通过分析提出了温度不均匀度现象;结合壁面液膜特性及多普勒激光测速仪所测之液滴参数的空间分布,分析阐述了温度非均匀现象的成因与影响因素。研究结果表明,发热壁面温度的不均匀来自于喷雾特性的变化,通过选择合适的喷雾高度使工质从壁面中心向四周附壁流出,可以克服喷嘴空心回流区影响、增强壁面中心的换热,使发热壁面温度更均匀、同时提高冷却效果。本文突破了将壁面温度分布视为均匀的传统观点,为喷雾冷却的性能评估提供了新的途径,对喷雾冷却特性进一步的科学研究和工程应用具有参考价值。     

Theoretical investigation on the mechanism of surface temperature non-uniformity formation in spray cooling

In this study, the effects of droplet velocity non-uniformity, SMD (Sauter mean diameter) distribution non-uniformity, droplet number non-uniformity, and heating power on the fluid film thickness, fluid film velocity, and surface temperature distribution were investigated, and then the surface temperature non-uniformity correlations in non-boiling regime and nucleate boiling regime were correlated. The results show that: with the decreasing of the spray parameters non-uniformity, the fluid film thickness on the heating surface becomes more uniform, and the fluid film velocity increases, thus the surface temperature non-uniformity decreases. The highest surface temperature appears in the centre of the heating surface, and the lowest is nearby the position where the fluid film appears. The droplet number non-uniformity contributes the largest portion of impact on the surface temperature non-uniformity, followed by the droplet velocity non-uniformity. The effect of droplet SMD non-uniformity is the minimal. Finally, the surface temperature non-uniformity correlations in non-boiling regime and nucleate boiling regime were correlated with a mean absolute error of 20%.     

Spray cooling of enhanced surfaces: Impact of structured surface geometry and spray axis inclination

Experiments were conducted to study the effects of enhanced surfaces and spray inclination angle (the angle between the surface normal and the axis of symmetry of the spray) on heat transfer during spray cooling. The surface enhancements consisted of cubic pin fins, pyramids, and straight fins. These structures were machined on the top surface of heated copper blocks with 2.0 cm² cross-sectional areas. Measurements were also obtained on a heated flat surface to provide baseline data. PF-5060 was used as the working fluid. The spray was produced using a 2 × 2 nozzle array under nominally degassed conditions (chamber pressure of 41.4 kPa) with a volume flux of 0.016 m³/m² s and a nozzle height of 17 mm. The spray temperature was 20.5 °C. For the geometries tested, the straight fins had the largest heat flux enhancement relative to the flat surface, followed by the cubic pin fins and the pyramid surface. Each of these surfaces also indicated an increase in evaporation efficiency at CHF compared to the flat surface. Inclination of the spray axis between 0° and 45° relative to the heater surface normal created a noticeable increase in heat flux compared to the normal position (0° case). A maximum heat flux enhancement of 23% was attained for the flat surface. The straight finned surface had a maximum heat flux enhancement of 75% at an inclination angle of 30° relative to the flat surface in the normal position. However, only a marginal increase (11%) was observed in comparison to the straight finned surface in the normal position (0° case).     

Experimental and theoretical investigation of a nocturnal radiator for space cooling

An aluminium nocturnal radiator, painted with an appropriate white paint, was established on the roof of the Department of Environmental and Natural Resources Management in Agrinio, Western Greece. The lightweight metallic radiator was used for cooling the ambient air below its initial temperature. The dynamic thermal performance of the system during summer months was calculated using an accurate mathematical model, based on the heat transferred from the air circulating inside the radiator to the ambient air. Furthermore, an extensive validation process was carried out. Thus, the experimental air temperature values at the radiator's outlet were compared with the theoretical ones and a very good agreement was achieved. The validation process was extended for two radiator's paints with different emissivities, in order to investigate the impact of different materials’ optical parameters on the system's efficiency.Moreover, the more effective lightweight nocturnal radiator was used to provide space cooling or pre-cooling for the building of the University. Indoor air temperature values of the thermal zone connected with the radiator were compared with those of a similar zone without any cooling system and the results demonstrated a remarkable effectiveness of the system.     

Effects of spray characteristics on critical heat flux in subcooled water spray cooling

Effects of spray parameters (mean droplet size, droplet flux, and droplet velocity) on critical heat flux (CHF) were studied while these parameters were systematically varied. The effect of each parameter was studied while keeping the other two nearly constant. The mean droplet velocity (V) had the most dominant effect on CHF and the heat transfer coefficient at CHF (h_c), followed by the mean droplet flux (N). The Sauter mean diameter (d₃₂) did not appear to have an effect on CHF. By increasing V, CHF and h_c were increased. This trend was observed when all other spray parameters were kept within narrow ranges and even when relaxed to wider ranges, indicating the dominant effect of V. The effect of N, although not so much as V, was also found to be significant. Increasing N resulted in an increase in CHF and h_c when other parameters are kept in narrow ranges. A dilute spray with large droplet velocities appears to be more effective in increasing CHF than a denser spray with lower velocities for a given N. The mass flow rate was not a controlling parameter of CHF.     

Single nozzle spray cooling heat transfer mechanisms

An investigation into single nozzle spray cooling heat transfer mechanisms with varying amounts of dissolved gas was performed using two powerful techniques. Time and space resolved heat transfer distributions produced by a single nozzle were measured using an array of individually controlled microheaters, while visualization and measurements of the liquid-solid contact area and the three-phase contact line length were made using a total internal reflectance technique. The presence of dissolved gas increased the effective subcooling of the liquid, and shifted the spray cooling curves to higher wall temperatures, but CHF was also increased. The phase-change heat transfer contribution was found to correlate directly with the contact line length for the experimental conditions tested.     

燃气轮机进气喷雾蒸发式冷却与Li-Br吸收式制冷冷却增能对比分析

燃气轮机进气冷却技术是一种能够有效提高高温环境下燃气轮机发电机组输出功率的重要手段。介绍了几种主要的进气冷却技术方法,并分析了进气冷却技术的作用机理。着重对比分析了华菱涟钢能源中心所采用的两种进气冷却技术对燃气轮机输出功率的影响规律,并阐明了两种进气冷却技术各自的优缺点。根据运行参数以及相关数据分析的对比研究结果,为燃气轮机进气冷却技术在实际工程上的应用提供一定的参考。     

Numerical analysis of temperature non-uniformity and cooling capacity for capillary ceiling radiant cooling panel

Capillary ceiling radiant cooling panel is a high temperature cooling system, which could pose low energy consumption to meet thermal comfort requirements. A computational fluid dynamics (CFD) simulation study on heat transfer of chilled water flow in the capillary of ceiling radiant cooling panel was performed to attain surface temperature distributions and cooling capacities. Six influencing factors included chilled water inlet parameters, conditions of gypsum plaster and capillary mats structural parameters were considered to obtain the complicated relationships between capillary radiant panel conditions and heat transfer performance. The index of temperature non-uniformity coefficient was proposed to evaluate temperature profiles of ceiling panel surface. The results of the simulation were compared with the values depicted in ASHRAE Handbook and good agreement had been achieved. The average difference between simulation results and the values reported by ASHRAE handbook was within the region of 15%. The research results showed that temperature non-uniformity coefficient was negatively correlated with temperature of chilled inlet water (linear correlation), water velocity (correlation coefficient R = −0.85), and pipe diameter (correlation coefficient R = −0.93), but positively and linearly correlated with tube spacing. Cooling capacity was found to have negative linear correlation with temperature of chilled inlet water, covering thickness and tube spacing.     

Visualization of film wavelike characteristics and measurement of film thickness in spray cooling

An experimental investigation was performed to study the heat transfer in an eight-nozzle spray cooling system with de-ionized water as the working fluid. Visualization of the liquid-solid contact area and the flow near the heated surface was made using a microscopic lens system in conjunction with an advanced high-speed camera. The film thickness and film wavelike characteristics under liquid volume flow rates ranged from 2.78×10 -6 m 3 /s to 1.39×10 -5 m 3 /s and surface temperatures between 22℃ and 78.2℃ were examined respectively. The development process of the liquid film on the heated surface was observed. The local mean film thickness, the film wavelike characteristics and the behavior of the bubbles appeared in the liquid film were captured using an image processing technique. It is discovered that there exists a climax of local mean film thickness during the starting process of spray cooling. When the liquid film reaches the dynamic stable state, the dimensionless mean film thickness decreases with the increase of the liquid volume flow rate, and increases with the increase of surface temperature generally. Besides, the volume flow rate has a more significant impact on the wavelength and amplitude of the liquid film compared to the surface temperature.     

吊顶辐射冷却塔供冷系统试验研究与性能分析

通过供冷试验及TRNSYS软件模拟研究了吊顶辐射冷却塔供冷系统的运行效果,分析了气象参数、部件结构等因素对系统供冷效果的影响,并与常规供冷系统全年的运行能耗进行对比。试验与理论分析结果表明,吊顶辐射冷却塔供冷系统的有效供冷量能够满足用户供冷需要,供冷房间温度稳定且分布均匀。冷却塔供冷效果与大气湿球温度、建筑内部负荷、热交换器结构等因素有关。辐射顶板末端与冷却塔供冷匹配性高,系统全年运行时数增加,应用于需全年供冷的建筑节能效果显著。     

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.     

The effects of micro-structured surfaces on multi-nozzle spray cooling

Experiments were conducted to investigate heat transfer characteristics of spray cooling with eight nozzles for micro-structured surfaces included cubic pin fins and straight pin fins of different sizes. Liquid volume flow rate ranged from 2.46 × 10⁻² m³/s/m² to 3.91 × 10⁻² m³/s/m² and the corresponded inlet pressures changed from 0.28 MPa to 0.6 MPa by keeping the inlet water temperature between 20.4 °C and 24.31 °C. And the input power of heat block varied from 180 W to 1080 W. The results show that the heat transfer performances of straight fins2 and straight fins3 are the best in single phase zone, but the cubic pin fins is better in two phase zone. Notably, the critical point between single phase zone and two phase zone shifts to left with the increasing of liquid volume flow rate. Moreover, with the liquid volume flow rate increasing, the heat transfer coefficient increases as well, but straight fins1 and polished surface are not sensitive to this change. For a deeper analysis of the heat transfer enhancement, a dimensionless number (DM) is created to characterize heat transfer performance of different microstructures in single phase heat transfer. We verified the dimensionless number using experimental results in this study and previous literature. Furthermore, the micro-structured surfaces have negligible effects on temperature distribution except for cubic pin fins.     

Effect of cooling surface temperature difference on the performance of high-temperature PEMFCs

Internal temperature distribution of the high-temperature proton exchange membrane fuel cell (HT-PEMFC) is affected by the cooling temperature, heat generation and reactant gas flow. Reasonable temperature control is helpful to improve the fuel cell performance and durability. In this work, a three-dimensional model that couples the reactant flows, species transport, heat transfer, charge transfer, and electrochemical reaction, was developed to simulate the HT-PEMFC operation. A solid mechanics model was established to analyze the stress distribution of the fuel cell. The polarization curves, distributions of temperature, membrane proton conductivity, current density and stress are investigated for different cooling surface temperature. Furthermore, the effect of assembly temperature on the stress of phosphoric acid-doped polybenzimidazole (PBI) membrane is discussed. Results reveals that the peak power density and uniformity of current density decrease with the increase of cooling surface temperature difference. The peak power density decreases by 9.14% when the temperature difference increases from 0 K to 40 K. The cooling surface temperature difference of less than 10 K and the voltage range in 0.5–0.7 V can achieve better current density uniformity and smaller current density change rate. In addition, the membrane in fuel cell has the highest stress, and increasing the assembly temperature is helpful to reduce the membrane stress. When the assembly temperature increases from 293.15 K to 343.15 K, the max and min compressive stresses in membrane in-plane decrease from 39.436 MPa to 31.416 MPa–24.934 MPa and 17.369 MPa at the temperature difference of 30 K, which decreases by 36.77% and 44.7%, respectively.     

Experimental study of the effects of structured surface geometry on water spray cooling performance in non-boiling regime

Experiments were conducted to study the effects of enhanced surfaces on heat transfer performance during water spray cooling in non-boiling regime. The surface enhancement is straight fin. The structures were machined on the top surface of heated copper blocks with a cross-sectional area of 10 mm×10 mm. The spray was performed using Unijet full cone nozzles with a volumetric flux of 0.044–0.053 m³/(m²·s) and a nozzle height of 17 mm. It is found that the heat transfer is obviously enhanced for straight fin surfaces relative to the flat surface. However, the increment decreases as the fin height increases. For flat surface and enhanced surfaces with a fin height of 0.1 mm and 0.2 mm, as the coolant flux increases, the heat flux increases as well. However, for finned surface with a height of 0.4 mm, the heat flux is not sensitive to the coolant volumetric flux. Changed film thickness and the form of water/surface interaction due to an enhanced surface structure (different fin height) are the main reasons for changing of the local heat transfer coefficient.     

A comparison of temperature distribution in PEMFC with single-phase water cooling and two-phase HFE-7100 cooling methods by numerical study

Thermal management has been considered as one of the critical issues in proton exchange membrane fuel cell (PEMFC). Key roles of thermal management system are maintaining optimal operating temperature of PEMFC and diminishing temperature difference over a single fuel cell and stack. Severe temperature difference causes degradation of performance and deterioration of durability, so understanding temperature distribution inside a single fuel cell and stack is crucial. In this paper, two-phase HFE-7100 cooling method is suggested for PEMFC thermal management and investigated regarding temperature change inside a fuel cell. Also, the results are compared to single-phase water cooling method. Numerical study of temperature distribution inside a single PEMFC is conducted under various conditions for the two different cooling methods. Fuel cell model considering mass transfer, electrochemical reaction and heat transfer is developed.The result indicates that two-phase HFE-7100 cooling method has an advantage in temperature maintenance and temperature uniformity than single-phase water cooling method, especially in high current density region. It is also revealed that the cell temperature is less dependent on system load change with two-phase cooling method. It indicates that the fuel cell system with two-phase cooling method has high thermal stability. In addition, the effect of coolant flow rate and coolant inlet pressure in two-phase HFE-7100 cooling method are discussed. As a result, two-phase cooling method showed reliable cooling performance even with low coolant flow rate and the system temperature increased as coolant pressure rose.     

夏季降温的太阳房实验分析

对建造在南宁市西郊的一座被动式太阳房在夏季运行时的实验数据进行了分析，其屋顶集热表面在夏季夜间能产生一定的致冷效果。所制取的冷空气不但在夜间可以对该太阳房起到降温的作用，并且可以储藏起来以供白天降温使用。     

Experimental investigation of a solar desiccant cooling installation

Desiccant cooling is a technique based on evaporative cooling and air dehumidification using desiccant regenerated by thermal energy. It is particularly interesting when it is driven by waste or solar heat making this technique environmentally friendly.In this paper, an experimental investigation is carried on a desiccant air handling unit powered by vacuum-tube solar collectors. First, the components are studied under various operating conditions. Then overall performance of the installation is evaluated over a day for a moderately humid climate with regeneration solely by solar energy. In these conditions the overall efficiency of the solar installation is 0.55 while the thermodynamic coefficient of performance is 0.45 and the performance indicator based on the electrical consumption is 4.5. Finally, the impact of outside and regeneration conditions on the performance indicators is studied.     

Effects of titania nanoparticles on heat transfer performance of spray cooling with full cone nozzle

Spray cooling using aqueous titania nanofluids was studied. The temperatures of a testing plate under various spraying conditions were first measured; an inverse heat conduction technique was then applied to convert these measured temperatures into heat transfer coefficients (HTCs). It was found that the HTC increased logarithmically with the volume flux, but was decreased with the increase of the nanoparticle fraction. A correlation analysis was performed to quantify the HTC reduction caused by the increase of nanoparticles, and reconfirmed that the major cause for the HTC reduction was the difference in the impact (or impingement) behavior between solid nanoparticles and fluid droplets. A comparison study of the present findings with the previous published results was also performed and indicated that all results compared were consistent to each other based on the similar spray cooling conditions with different nanofluids or nozzles. The effects by using aquatic titania nanofluids instead of aquatic alumina nanofluids and by using full-cone nozzle instead of solid jet nozzle were specifically assessed and the associated rationales for the differences in these effects were given.     

The Nukiyama curve in water spray cooling: Its derivation from temperature–time histories and its dependence on the quantities that characterize drop impact

Heat transfer from hot aluminium walls to cold water sprays was investigated. The method used was the transient two-side symmetric cooling of a planar aluminium target, previously heated to temperatures of up to 750 K, by twin sprays issuing from full-cone swirl spray nozzles of various gauge. The target’s mid-plane temperature was recorded during the cooling transient by thin-foil K thermocouples and a high-frequency data acquisition system. In order to determine the wall temperature T_w, the wall heat flux and the heat transfer (Nukiyama) curve, two different approaches were used: the first was based on the solution of an inverse heat conduction problem, the second on a suitable parameterization of the Nukiyama curve and on the solution of a minimum problem. Relevant heat transfer quantities, such as the critical heat flux and the single-phase heat transfer coefficient, were obtained from each heat transfer curve. Their dependence on the main parameters characterizing the spray impact phenomenon (mass flow rate, drop velocity and drop diameter) was investigated on the basis of a preliminary hydrodynamic characterization study, and suitable correlations were proposed.     

Experimental investigations of spray generated by a pressure swirl atomizer

This paper has focused on the droplets behavior of kerosene RP-3 spray produced by a pressure swirl atomizers in terms of spray pattern, droplet size spatial distribution, mean droplet size, and distribution index with variations of pressure differential. The analyses have been carried out experimentally with the aid of optical diagnostic methods. The spray pattern, such as spray cone angle and fuel spatial distribution, has been measured by the technique of planar laser induced fluorescence of kerosene. A method for correction of fuel distribution measurement error induced by laser attenuation in spray is proposed and validated. The droplet size spatial distribution in central axis plane of the spray has been measured by a planar droplet sizing method which combining laser induced fluorescence and Mie scattering. The spray pattern in axial center plane and cross-sectional plane perpendicular to axis of the atomizer indicate that the droplets in spray concentrate around the outer periphery and in a narrow annular zone at the near-field of fuel injector exit, and then disperse to produce a solid spray at downstream of the spray. The analyses of droplet size spatial distribution, Sauter mean diameter, and distribution index with pressure differential clearly show the presence of droplets collision and its adverse effects on droplet size uniformity. The spray outline, droplet mass spatial distribution, and droplet size spatial distribution, droplets dispersion and collision in the process of atomization provide a great insight into the processes of atomization and spray development, which are key information for fuel injector design and quality control. The visualizations of spray pattern and droplet size spatial distribution with variations of pressure differential for pressure swirl atomizer are key issues in swirl cup or internally staged airblast fuel injectors because pressure swirl atomizer provides primary atomization or pilot spray which affects the quality of air/fuel mixing in lean-burn combustion. Moreover, a well-defined and complete database regarding the isothermal hollow cone spray is provided for validation of spray model.