无沸腾喷雾冷却中流量和喷头高度对换热性能的影响

采用薄膜电阻加热器进行了喷头进口压力，喷头类型，喷头高度对换热系数影响的实验研究。研究了冷却介质的质量流量对换热性能的影响，并测量了同一喷头在不同喷头高度下的换热系数大小。实验发现当喷雾面积近似等于实验用薄膜加热器面积时冷却能力达到最大。根据以上实验结果可以最优化喷雾冷却性能。     

Spray impingement cooling with single- and multiple-nozzle arrays. Part I: Heat transfer data using FC-72

With electronic packages becoming more dense and powerful, traditional methods of thermal energy removal are reaching their limits. One method of direct contact cooling capable of removing high heat fluxes while still being compact in size is spray impingement cooling, but its heat transfer behavior is not understood well enough to enable systematic, practical system design. This work presents the results of a large parametric study of spray cooling using a number of different nozzle patterns. It was found that nozzles that use the fluid most efficiently to remove thermal energy were limited by low peak heat fluxes and that the highest peak heat fluxes were obtained when phase change was avoided. Multiple nozzle arrays allowed for higher peak heat fluxes but used fluid inefficiently due to interactions between neighboring sprays. In general, the geometric pattern of the nozzle arrays had little effect on overall heat transfer performance.     

Heat transfer modeling and simulations for electronic cooling systems embedded with phase changing materials

Electronic systems used in communication devices and mission critical systems such as directed‐energy weapons and spacecraft call for requirements to dissipate high heat flux in limited spaces. This study attempts to model the heat transfer in an electronic module, dissipating heat to a coolant in a microchannel as well as to ambient by natural convection and radiation. These results have been used to compare the effectiveness of a new cooling system, embedded with phase changing materials within the heat spreaders. The heat transfer model takes care of the heat stored in the phase changing material, heat transferred by natural convection and radiation effects, in addition to the convection heat transfer in the microchannel cooling passage. Hence, the developed model can successfully take into account the effect of the variation in operating atmospheric conditions which are significant in systems employed in harsh thermal environments. Simulations have been performed for a wide range of heats and appropriate coolant flow rates are being set for a prescribed safe operating temperature of the processor. Simulations for cooling systems with phase changing materials embedded within the available heat spreader shows that it can provide stable operation of electronic systems during peak heat loads as well as in extreme ambient conditions.     

侧风条件下带消能导流装置的直接空冷岛冬季冻结风险模拟研究

哈密某电厂在其空冷岛中使用了一种新型消能导流装置来抵抗侧风影响、稳定机组背压。然而,该电厂冬季空冷单元散热管束冻结的情况依然存在。为了探究该装置对空冷岛冬季防冻的影响,利用Fluent软件对该电厂冬季大风时空冷岛的流动传热特性及各空冷单元的冻结风险进行了模拟研究。研究表明：空冷岛“消能导流装置”整体上对空冷单元的防冻起负面作用;在冬季大风条件下该装置迎风侧空冷单元的散热量平均超出警戒值27%以上,最高达到50%;消能导流装置主要通过提升轴流风机空气流速来增加对应空冷单元的换热量,该装置迎风侧空冷单元轴流风机的轴向空气流速甚至能达到与环境侧风相同的水平,这导致对应空冷单元换热量激增,更容易出现冻结事故;大风条件下该装置在空冷岛下方形成的高压区域分布并不均匀,临近主厂房与相邻空冷岛一侧的高压区域压力更高、面积更大,这些区域空冷单元的冻结风险更高。     

喷雾冷却在单相区中换热特性的数值模拟研究

喷雾冷却是一种高热流密度的冷却方式,按传热机理的不同可以分为单相区和两相区。针对单相区,建立三维传热模型,利用CFD仿真技术对喷雾冷却过程进行数值模拟,并研究了喷雾的高度、流量密度等参数对喷雾冷却传热性能的影响规律。结果显示:当传热稳定时,传热面温度由圆心沿径向逐渐增高,并且随喷雾流量密度的增大,温度分布的均匀性变差;保持喷雾作用面积不变,增大喷雾流量或减小喷雾高度,喷雾的传热能力显著增强;数值模拟结果与实验数据吻合,验证了该模型的可靠性,可为喷雾冷却系统的设计、优化提供依据。     

Fundamental Research on Convective Heat Transfer in Electronic Cooling Technology

During the past six years comprehensive research programs have been conducted at the Beijing PolytechnicUniversity to provide a better understanding of heat transfer characteristics of existing and condidate cool-ing techniques for electronic and microelectronic devices.This paper provides a review and summary of theprograms with emphasis on direct liquid cooling.Included in this review are the heat transfer investigationsrelated to the following cooling modes:liquid free,mixed and forced convection,liquid jet impingement,flowingliquid film cooling,pool boiling,spray cooling,foreign gas jet impingement in liquid pool,and forced convectionair-cooling.     

R-134a spray dynamics and impingement cooling in the non-boiling regime

R-134a spray as it impinges on the flat endplate of a circle is studied experimentally. In order to optimize R-134a spray cooling efficiency, a detailed characterization and understanding of liquid spray formation is essentially needed. An optical image system was used to quantify the spray flow structure. LDV measurements were used to characterize the local velocity /and velocity fluctuation distribution from a commercial available nozzle in both axial and radial directions. The radial velocity are found to be the largest at the outer edges of the spray, and they continuously decrease across the spray toward the center axis; while the corresponding axial velocity is the maximum there. Moreover, spray heat transfer in non-boiling regime was shown to be dependent on the velocity of the impinging spray in terms of Weber number and other related parameters which are in good agreement with those of previous studies.     

Evaporative heat transfer characteristics of a water spray on micro-structured silicon surfaces

Experiments were performed to evaluate the evaporative heat transfer characteristics of spray cooling of water on plain and micro-structured silicon surfaces at very low spray mass fluxes. The textured surface is made of an array of square micro-studs. It was found that the Bond number of the microstructures is the primary factor responsible for the heat transfer enhancement of evaporative spray cooling on micro-structured silicon surface in the present study. A qualitative study of evaporation of a single water droplet on plain and textured silicon surface shows that the capillary force within the microstructures is effective in spreading the deposited liquid film, thus increasing the evaporation rates. Four distinct heat transfer regimes, which are the flooded, thin film, partial dryout, and dryout regimes, were identified for evaporative spray cooling on micro-structured silicon surfaces. The microstructures provided better cooling performance in the thin film and partial dryout regime and higher liquid film breakup heat flux, because more water was retained on the heat transfer surface due to the capillary force. Heat transfer coefficient and temperature stability deteriorated greatly once the liquid film breakup occurred. The liquid film breakup heat flux increases with the Bond number. Effects of surface material, system orientation and spray mass flux were also addressed in this study.     

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.     

Experimental and theoretical investigation of surface temperature non-uniformity of spray cooling

In this study, a test system for spray cooling, in which the heating surface temperatures were simultaneously measured by thermocouples and an infrared imager, was set up. A mathematical model of spray cooling heat transfer characteristics was presented based on the fundamentals of dynamics and heat transfer. The temperature distribution on the heating surface was investigated by the experimental and theoretical methods, the surface temperature non-uniformity and its influencing factors were analyzed. The predictions by the model coincided with the experimental results well, and a comparison was demonstrated with a deviation below 10%. It can be concluded that the surface temperature non-uniformity is influenced by the spray characteristics, nozzle-to-surface distance, inlet pressure, heat flux, spray angle and the system pressure. In the case of the same heat flux, the surface temperature non-uniformity can be reduced by the small spray angle, low system pressure, low nozzle-to-surface distance, and the high inlet pressure.     

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.     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Monodisperse Spray Cooling of Small Surface Areas at High Heat Flux

Spray cooling is an effective method to remove high heat fluxes from electronic components. To understand the physical mechanisms, this work studies heat transfer rates from single and dual nozzle distilled water sprays on a small heated surface (1.3 mm × 2 mm). Thermal ink jet atomizers generate small droplets, 33 μm diameter, at known frequencies, leading to controlled spray conditions with a monodisperse stream of droplets interacting with the hot surface. Of particular interest in this work is the dissipated heat flux and its relation to the liquid film thickness, the surface superheat, and the cooling mass flow rate. Experimental results show the heat flux scales to the cooling mass flow rate. In comparison to published spreading–splashing correlations, these experiments indicate that the drops impinge on the liquid film and spread without generating splashing, leading to high-efficiency stable heat transfer. Surface temperatures range from 120 to 140°C. In addition, the liquid film thickness is investigated in relation to the heater superheat and a stable thin film is seen at superheats beyond 20°C. The efficiency of the spray system is inversely related to the film thickness and may be due to ejection of liquid from the surface due to bursting of vapor bubbles.     

新能源汽车动力电池冷却系统热仿真及优化

动力电池的温度控制是新能源汽车发展中的一个难题,而电池冷却系统在动力电池的温度控制过程中起着相当重要的作用。利用Solidworks软件对电池包进行建模,利用ICEM CFD软件对电池包模型进行网格划分等前处理。利用Fluent软件并采用控制变量法分别对冷却管道截面宽度、冷却液质量流量和冷却液进口温度等3个对电池包散热性能影响较大的参数进行仿真计算和对比分析。根据仿真结果选择可优化电池包散热性能的参数,并在原方案基础上提出了一种新的冷却管道分布方案。经过仿真计算发现,该方案可有效降低电池在使用过程中的最高温度和温差,提高了电池冷却系统的散热性能。     

高饱和蒸发压力氨喷雾冷却沸腾传热特性

利用冷却工质的相变蒸发带走大量热量的喷雾相变冷却技术成为大功率电子元件散热需求的最佳途径.建立了双喷嘴阵列氨喷雾相变冷却实验系统,研究了饱和蒸发压力以及进口流量对氨喷雾相变冷却传热特性的影响规律.实验结果表明:在氨喷雾相变冷却过程中,维持较高的饱和蒸发压力有利于传热系数提高,过热度降低;流量对传热特性影响较大,低流量时...     

Heat Flux Correlation for Spray Cooling in the Nonboiling Regime

An experimental facility was developed to investigate the nonboiling heat transfer performance of water spray cooling. The effects of mass flux and wall temperature on heat transfer coefficient and heat flux were experimentally studied. It was found that heat transfer coefficient increased with the increasing of mass flux and wall temperature. Generalized correlations were developed for the Nusselt number related to wall temperature and the average Nusselt number as a function of the spray Reynolds number and the nondimensional temperature with an absolute error of 4% and 15% when the Reynolds number is more than 440. Compared with the data of Oliphant et al., it was observed that the usage field of the correlations could be extended to Reynolds number greater than 240.     

Numerical simulation of a closed wet cooling tower with novel design

A closed wet cooling tower with novel design was proposed and numerically investigated. The studied cooling tower consists of two main parts: one heat and mass transfer unit (HMTU) and one heat transfer unit (HTU). In the HMTU, copper tubes are arranged as heat transfer tubes while plastic tubes are collocated to enlarge the mass transfer area between the spray water and the airflow. In the HTU, only copper tubes are adopted as heat transfer tubes. Heat and mass transfer process takes place among the process water, airflow and spray water in the HMTU, while in the HTU only heat transfer between the process water and the spray water is observed. A transient one dimensional distributed-parameter model was adopted to evaluate the cooling tower performance under different operating conditions. Determination of heat and mass transfer coefficients, as well as the influence of Lewis number on the cooling tower performance, was presented.     

海底数据中心散热优化系统的设计与仿真研究

随着电子信息存储设备高能耗密度化的发展趋势,海底数据中心通常需要耗费大量的能源,且其仍存在运行效率较低以及设备能耗较高等问题。研究并设计了一种海底数据中心散热优化装置及其检测系统。首先建立相关模型并分析计算电能利用效率PUE(power usage effectiveness)值,客观地评价海底数据中心散热状况,接着参考工程实例对比不同散热结构并进行仿真实验验证,最后结合散热检测软件平台所监测到的相关数据,印证了散热优化系统能够有效提高海底数据中心的散热效率。     

Challenges in Cooling Design of CPU Packages for High-Performance Servers

Cooling technologies that address high-density and asymmetric heat dissipation in CPU packages of high-performance servers are discussed. Thermal management schemes and the development of associated technologies are reviewed from a viewpoint of industrial application. Particular attention is directed to heat conduction in the package and heat removal from the package/heat sink module. Power dissipation and package cooling characteristics of high-performance microprocessors are analyzed. The development of a new metallic thermal interface technology is introduced, where thermal and mechanical performance of an indium-silver alloy in the chip/heat spreader assembly was studied. The paper also reports on research on other thermal management materials, such as diamond composite heat-spreading materials. Some actual package designs are described to illustrate the enhanced heat spreading capability of heat pipes and vapor chambers.     

Cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film

The cooling characteristics of an impinging spray jet which forms an ellipsoidal liquid film were experimentally investigated in order to estimate the cooling performance of a rotating roll in a hot mill system. The following four conclusions were reached in the study. (1) In the case of a single spray jet, the local heat transfer coefficient at the center position depends on the forced convective heat transfer by the impinging jet. However, the average heat transfer coefficient is proportional to the flow rate density of the cooling water, and it does not depend on the distance between the nozzle and heated surface. (2) In the case of a double spray jet, liquid film interference occurs. The local heat transfer coefficient at the center position is greater, and the cooling performance increases with the increasing flow rate density of the cooling water. (3) The cooling performance of a multispray jet is proportional to the flow rate density of the cooling water. It does not depend on the nozzle construction, distance, or specifications. Also, there is no relation to the liquid film interference. (4) When the optimum specifications of the spray nozzle are used, thermal analysis of a rotating roll shows that the temperature at a depth of 1.3 mm from the surface is below 130 °C. © 2000 Scripta Technica, Heat Trans Asian Res, 29(4): 280–299, 2000