Boiling Delay Phenomenon in a Thermosyphon Heat Sink and Its Effect on Device Performance

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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.     

Novel adsorption refrigerators with separate type two phase closed thermosyphon designs

Two adsorption refrigerators with separate type two‐phase closed thermosyphon heating processes are designed to improve the heat transfer performance and to reduce the system's moving parts number and system's size. One adsorption refrigerator uses the design of three‐effect separate type two‐phase closed thermosyphon heating/cooling; the other one uses the design of separated separate type two‐phase closed thermosyphon heating and force convection cooling. The results show that the coefficient of performance (COP) and specific of cooling power (SCP) can be improved because of the pressure difference mass recovery process. The system with the separate type two‐phase closed thermosyphon heating and force convection cooling design is more attractive because fewer moving parts are used and the system's size can be reduced for building. Moreover, the thermal transfer of the adsorption beds is designed as separate type heat pipe, which can avoid the problem of corrosion from heat or sink sources, such as heavy‐duty exhaust gases or seawater. Under the nominal working condition, the evaporating temperature, COP and SCP can reach ?21 °C, 0.26 and 474 W kg^?1. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermal intensification of heat transfer characteristics on the plate‐fin heat sink with piezoelectric fan

This study applied the computational fluid dynamic (CFD) code, ANSYS Fluent for simulating the effect a piezoelectric fan installed inside the rectangular channel by numerical simulation method for transient flow field and investigating the influence of each parameter. To remove the disorganized form of energy from the electronic components, the reversible piezoelectric effect is employed to energize the piezoelectric fan. To observe the variation of fan characteristics and to predict the convective heat transfer coefficient, CFD code ANSYS Fluent 15.0 is used. The numerical simulation parameters included are Nusselt number, number of fins (n = 12 and 14), and counter‐shift (inward and outward‐phase), and distance between the upper portion of the fan tip to the front part of the low thermal reservoir. Numerical analysis was carried out to evaluate the effect of thermal flow fields on the heat sink and piezoelectric fan employed in a flow domain. the results showed that by varying the height from channel bottom to the center of piezoelectric fan improves the performance of the piezoelectric fan, piezoelectric fan swinging in a transient phenomena and also simultaneously influences fluid flow behavior on the heat source surface, the fan vibration at counter‐phase has a better rate of heat transfer than vibration in in‐phase.     

A performance comparison between an air‐source and a ground‐source reversible heat pump

In this study, the performance of a reversible ground‐source heat pump coupled to a municipality water reticulation system, is compared experimentally and with simulations to a conventional air‐source heat pump for space cooling and heating. A typical municipality water reticulation system comprises hundreds of kilometres of pipes designed in loops that will ensure adequate circulation of water. This results in a substantial heat exchanger with great potential. Indirect heat transfer occurs between the refrigerant and ground via the municipality water reticulation system that acts as the water‐to‐ground heat exchanger. The experimental and simulated comparisons of the ground‐source system to the air‐source system are conducted in both the cooling and the heating cycles. Climatalogical statistics are used to calculate the capacities and coefficients of performance of the ground‐source and air‐source heat pumps. Results obtained from measurements and simulations indicate that the utilization of municipality water reticulation systems as a heat source/sink is a viable method of optimizing energy usage in the air conditioning industry, especially when used in the heating mode. Copyright © 2001 John Wiley & Sons, Ltd.     

数据中心服务器水冷散热器的数值模拟与实验验证

水冷散热器在数据中心服务器CPU芯片冷却技术中发挥着重要的作用。如何获得高性能的散热效率成为了该领域关注的重点。针对一种翅柱式水冷散热器,用数值模拟的方法,通过改变翅柱的结构参数来优化散热器的散热性能以及流动特性。在相同的翅柱间距下,改变翅柱的直径和高度,在不同的入口流量下,研究其温度,努塞尔数,压降,摩擦系数,分析比较其综合系数对散热性能的影响,并对结果进行了实验验证。结果表明翅柱高度3.9mm,直径为0.9mm的散热器其综合系数最大     

Influence of fan setting height on the cooling performance of a plate‐fin‐type heat sink with microprocessor

The cooling performance of a plate‐fin‐type heat sink equipped with a cooling fan was investigated experimentally. The heat sink was 80 mm long, 43 mm wide, and 24 mm in height (including the 4‐mm‐thick base). The cooling fan was 40 × 40 × 15 mm and was set to direct the air flow vertically in the downstream half of the heat sink. We focused on the influence of the height (which varied from 5 to 20 mm) that the fan was set at, on the heat transfer coefficient of the heat sink. The maximum value of the heat transfer coefficient was achieved at a setting height of 5 mm. At this height, the volumetric heat transfer coefficient was 1.8 times as high as that in a parallel flow under the same fan power. This result indicates that the cooling performance of heat sinks with a cooling fan can be improved by using this kind of compact structure. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 512–520, 2001     

渐变孔隙率对泡沫铝矩形通道热沉流动和换热性能的影响

风冷是最普遍的电子器件散热方式。对填充渐变孔隙率泡沫铝的热沉的流动与换热性能进行CFD模拟,考察了孔隙率分别沿流动方向和高度方向发生梯度变化对热沉性能的影响。结果表明:与均匀孔隙率热沉相比,孔隙率沿高度方向渐变的热沉压力损失减小,最佳的孔隙率渐变方式为沿高度方向由0.963 0递减至0.700 0,与当量孔隙率0.831 5的均匀孔隙率热沉相比,明显提高了泡沫铝热沉的综合性能。当进口速度高于3.0 m/s后,渐变孔隙率热沉的综合性能甚至优于高孔隙率(0.963 0)的均匀孔隙率热沉。     

复合式土壤源热泵系统设计

设计了一套复合式土壤源热泵性能实验台,采用冷却塔作辅助冷源、太阳能热水器作辅助热源,能够实现冬季供暖、夏季供冷、一年四季提供60~80℃生活热水的功能。给出了系统中热泵机组、空调末端设备、冷却塔、水泵、太阳能热水器等主要部件的选型计算方法,经搭建好的土壤源热泵性能实验台运行测试,制冷和制热效果良好,同时也解决了土壤热平衡问题。     

Design and evaluation methods for a water cooling panel system for decay heat removal from a high-temperature gas-cooled reactor

An experiment was performed to simulate a water cooling panel system for decay heat removal from a high-temperature gas-cooled reactor (HTGR) and to investigate the performance of decay heat removal and the temperature distribution for components of the system. The experimental apparatus is composed of a pressure vessel 1 m in diameter and 3 m in height, containing heaters with a maximum heating rate of 100 kW which simulates the decay heat of the reactor core and cooling panels surrounding the pressure vessel. The analytical code THANPACST2 was applied to analyze the experimental data and to investigate the validity of the analytical method and model proposed. Under conditions using helium gas at a pressure of 0.73 MPa and temperature of 210°C in the pressure vessel, temperatures of the pressure vessel were well estimated to within differences of −29 to +37°C compared to the experimental data. The analyses indicate that the heat removed by the cooling panel is 11.4% less than the experimental value and the heat transferred by thermal radiation is 74.4% of the total heating value. It was also found that the lower head of the pressure vessel is effectively cooled by natural convection through the flow channels at the upper and the lower edges of the skirt-type support of the pressure vessel. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(3): 159–175, 1997     

新型多孔铜微通道热沉散热性能实验研究

新型多孔铜微通道散热技术采用多孔铜微通道结构,增加热沉与冷却工质的接触面积,提高热沉的散热性能。利用单室金属-气体共晶定向凝固工艺,通过控制冷却速度、过热度、气压等工艺参数,从而制备优质的多孔铜材料。根据多孔铜微通道热沉散热原理,搭建散热性能测试平台,研究冷却工质流量、多孔铜材料的孔径和孔隙率、入口截面斜率角对多孔铜微通道热沉散热性能的影响规律。结果表明:增加冷却工质流量有利于提高多孔铜微通道热沉的散热性能;在恒定体积流量下,减小孔径有利于提高多孔铜微通道热沉的散热性能;当多孔铜孔隙率为30.8%时,多孔铜微通道热沉散热性能最佳;入口截面斜率角对多孔铜微通道热沉散热性能的影响较小。     

复合热沉环路热管的热点散热性能模拟研究

针对芯片功耗与集成度提高而导致的局部热点问题,设计了一种用于芯片散热的复合热沉环路热管系统。建立了环路热管蒸发段模型,通过数值模拟的方法,证明了复合热沉环路热管系统能够降低热点温度,提高散热表面的温度均匀程度,且散热效果与热点的分布位置有关。当热点的热流密度为160W/cm2,热沉横向、纵向导热率分别为1500W/(m?K)、24W/(m?K)时,热点温度为88.88°C,相比于无热沉时降低了5.96°C。研究了不同热沉导热率下的热沉厚度对热点温度的影响,结果表明：若导热率各项同性,热点温度随热沉厚度的增加而降低,之后趋向不变;若为各项异性,存在最优的热沉厚度,使热点温度最低。     

Exergy analysis of lithium bromide/water absorption systems

Exergy analysis of a single-effect lithium bromide/water absorption system for cooling and heating applications is presented in this paper. Exergy loss, enthalpy, entropy, temperature, mass flow rate and heat rate in each component of the system are evaluated. From the results obtained it can be concluded that the condenser and evaporator heat loads and exergy losses are less than those of the generator and absorber. This is due to the heat of mixing in the solution, which is not present in pure fluids. Furthermore, a simulation program is written and used for the determination of the coefficient of performance (COP) and exergetic efficiency of the absorption system under different operating conditions. The results show that the cooling and heating COP of the system increase slightly when increasing the heat source temperature. However, the exergetic efficiency of the system decreases when increasing the heat source temperature for both cooling and heating applications.     

The heat transfer and fluid flow of a partially heated microchannel heat sink

Numerical modeling of the conjugate heat transfer in microchannel heat sink is presented. As the most of the cooling applications deals with the partial heated sections, the influence of the heating position on the thermal and hydrodynamic behavior is analyzed. The laminar fluid flow regime and the water as a working fluid are considered. It is observed that partial heating together with variable viscosity has a strong influence on thermal and hydrodynamic characteristics of the micro-heat sink.     

Current status of ground source heat pumps and underground thermal energy storage in Europe

Geothermal Heat Pumps, or Ground Coupled Heat Pumps (GCHP), are systems combining a heat pump with a ground heat exchanger (closed loop systems), or fed by ground water from a well (open loop systems). They use the earth as a heat source when operating in heating mode, with a fluid (usually water or a water–antifreeze mixture) as the medium that transfers the heat from the earth to the evaporator of the heat pump, thus utilising geothermal energy. In cooling mode, they use the earth as a heat sink. With Borehole Heat Exchangers (BHE), geothermal heat pumps can offer both heating and cooling at virtually any location, with great flexibility to meet any demands. More than 20 years of R&D focusing on BHE in Europe has resulted in a well-established concept of sustainability for this technology, as well as sound design and installation criteria. Recent developments are the Thermal Response Test, which allows in-situ-determination of ground thermal properties for design purposes, and thermally enhanced grouting materials to reduce borehole thermal resistance. For cooling purposes, but also for the storage of solar or waste heat, the concept of underground thermal energy storage (UTES) could prove successful. Systems can be either open (aquifer storage) or can use BHE (borehole storage). Whereas cold storage is already established on the market, heat storage, and, in particular, high temperature heat storage (> 50 °C) is still in the demonstration phase. Despite the fact that geothermal heat pumps have been in use for over 50 years now (the first were in the USA), market penetration of this technology is still in its infancy, with fossil fuels dominating the space heating market and air-to-air heat pumps that of space cooling. In Germany, Switzerland, Austria, Sweden, Denmark, Norway, France and the USA, large numbers of geothermal heat pumps are already operational, and installation guidelines, quality control and contractor certification are now major issues of debate.     

Effect of Newtonian heating/cooling on free convection in an annular permeable region in the presence of heat source/sink

The influence of Newtonian heating/cooling in the presence of heat source/sink has been investigated on laminar free convective flow in a vertical annular permeable region. The mathematical model for the problem has been considered as a boundary value problem consisting of two simultaneous ordinary differential equations. The boundary value problem has been transformed to nondimensional form. This has given rise to a number of parameters representing both geometrical and physical features of the problem. Closed‐form analytical solutions of the governing equations have been obtained for two different cases of internal heat generation/absorption. To assess the effects of governing parameters on the fluid velocity and temperature, a number of profiles of these field variables have been presented. The efficacy of the distinct processes on the field variables has been discussed extensively. The main outcome obtained in this study is that the velocity as well as temperature is enhanced in the case of the Newtonian heating while the opposite behavior occurs in the Newtonian cooling for both cases of source and sink. Furthermore, the influence of the governing parameters has been shown on the skin friction, volume flow rate, and the Nusselt number.     

Effect of Convection Heat Transfer on Performance of Waste Heat Thermoelectric Generator

Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.     

基于预测的复合地源热泵系统控制方法实现

在冬冷夏热且夏季冷负荷远大于冬季热负荷的地区常采用带有冷却塔的复合式地源热泵系统,其控制策略存在极大的优化空间。文章提出了直接比较冷却塔和与土壤换热器相连的板式换热器的出口温度的控制方法,并通过人工神经网络预测板式换热器机组侧的出口水温来实现此控制方法。通过FLUENT软件建立复合式地源热泵系统动态数值模型,获取建立神经网络的数据,采用3层BP网络,建立了多个预测板式换热器机组侧出口温度的模型。研究结果表明,采用神经网络可以准确实现此预测,绝对误差不超过0.4℃。     

High heat flux thermal management through liquid metal driven with electromagnetic induction pump

In this paper, a novel liquid metal-based minichannel heat dissipation method was developed for cooling electric devices with high heat flux. A high-performance electromagnetic induction pump driven by rotating permanent magnets is designed to achieve a pressure head of 160 kPa and a flow rate of 3.24 L/min, which could enable the liquid metal to remove the waste heat quickly. The liquid metal-based minichannel thermal management system was established and tested experimentally to investigate the pumping capacity and cooling performance. The results show that the liquid metal cooling system can dissipate heat flux up to 242 W/cm² with keeping the temperature rise of the heat source below 50°C. It could remarkably enhance the cooling performance by increasing the rotating speed of permanent magnets. Moreover, thermal contact resistance has a critical importance for the heat dissipation capacity. The liquid metal thermal grease is introduced to efficiently reduce the thermal contact resistance (a decrease of about 7.77 × 10⁻³ °C/W). This paper provides a powerful cooling strategy for thermal management of electric devices with large heat power and high heat flux.