Heat Transfer Studies of a Heat Pipe

The present investigation reports a theoretical and experimental study of a wire screen heat pipe, the evaporator section of which is subjected to forced convective heating and the condenser section to natural convective cooling in air. The theoretical study deals with the development of an analytical model based on thermal resistance network approach. The model computes thermal resistances at the external surface of the evaporator and condenser as well as inside the heat pipe. A test rig has been developed to evaluate the thermal performance of the heat pipe. The effects of operating parameters (i.e., tilt angle of the heat pipe and heating fluid inlet temperature at the evaporator) have been experimentally studied. Experimental results have been used to compare the analytical model. The heat transfer coefficients predicted by the model at the external surface of the evaporator and condenser are reasonably in agreement with experimental results.     

Thermal Performance Enhancement of L-Shaped Microgrooved Heat Pipe Containing Water-Based Al2O3 Nanofluids

An experimental study was performed to investigate the thermal performance of an L-shaped grooved heat pipe with cylindrical cross section, which contained 0.5 wt% water-based Al₂O₃ nanofluid as the working fluid. The transient performance of the heat pipe and the effect of cooling water temperature on the heat transfer characteristics of the heat pipe were investigated. The outer diameter and the length of the heat pipe were 6 mm and 220 mm, respectively. Experimental results revealed that the temperature of the cooling water has a significant effect on the thermal resistance of the heat pipe containing nanofluids as its working fluid. By increasing the cooling water temperature from 5°C to 27.5°C, the thermal resistance decreases by approximately 40%. At the same charge volume, test results indicated an average reduction of 30% in thermal resistance of heat pipes with nanofluid as compared with heat pipe containing pure water. For transient conditions, unsteady state time for nanofluids was reduced by approximately 28%, when compared with water as the working fluid.     

Heat pipe with PCM for electronic cooling

This article experimentally investigates the thermal performances of a heat pipe with phase change material for electronic cooling. The adiabatic section of heat pipe is covered by a storage container with phase change material (PCM), which can store and release thermal energy depending upon the heating powers of evaporator and fan speeds of condenser. Experimental investigations are conducted to obtain the system temperature distributions from the charge, discharge and simultaneous charge/discharge performance tests. The parameters in this study include three kinds of PCMs, different filling PCM volumes, fan speeds, and heating powers in the PCM cooling module. The cooling module with tricosane as PCM can save 46% of the fan power consumption compared with the traditional heat pipe.     

热管中冷器与前端冷却模块的匹配优化研究

为解决大功率柴油机铝中冷器在高温下强度下降的问题,针对某型载重车冷却模块,在风洞性能试验台架上对其进行了研究,引入热管中冷器解决目前铝中冷器在超高温下的适用性问题,并对铝中冷器做了2种优化改进方案。试验结果显示:热管中冷器可以很好地降低铝中冷器的进口温度,同时优化后的中冷器也能很好地满足中冷要求。在此基础上,水箱散热性能可提升3.36%～13.08%,冷却模块在满足原发动机的散热要求下可以做得更紧凑。     

Performance improvement of PV/T solar collectors with natural air flow operation

The electrical efficiency of a photovoltaic system drops as its operating temperature rises and PV cooling is necessary. The photovoltaic/thermal (PV/T) system is a relatively recent type of solar collector where a circulating fluid of lower temperature than PV module extracts heat from it, cooling the module to improve its output power while the solar pre-heated fluid provides sensible heat. In the present work, air cooling of a commercial PV module configured as PV/T air solar collector by natural flow is presented, where two low cost modification techniques to enhance heat transfer to air stream in the air channel are studied. The considered methods consist of thin metal sheet suspended at the middle or fins attached to the back wall of the air-channel to improve heat extraction from the module. A numerical model was developed and validated against the experimental data obtained from outdoor test campaigns for both glazed and unglazed PV/T prototype models studied. The validation results show good agreement between predicted values and measured data and thus could be used to study analytically the performance of these PV/T air collectors with respect to several design and operating parameters. The modified systems present better performance than the usual type and will contribute to better performance of integrated PV systems for natural ventilation applications in buildings, both space cooling and heating.     

Numerical investigation of thermal runaway propagation in a Li-ion battery module using the heat pipe cooling system

It is a promising cooling strategy to use the heat pipe for the Li-ion battery module, which can maintain the temperature of the battery module properly and prevent high temperature, triggering the thermal runaway among adjacent batteries. In this study, the thermal runaway model is simulated through the internal short circuit, which couples with Volume of Fluid (VOF) model of the heat pipe cooling and solves in ANSYS FLUENT to realize the heat and mass transfer between batteries and heat pipes. A user-defined function (UDF) code including mass source and energy source is used to calculate the heat and mass transfer in VOF model during the thermal runaway process. Numerical simulations are adopted to probe thermal runaway processes of a single battery under different operation conditions and the thermal runaway propagation from a battery to adjacent batteries. It is concluded that the heat pipe cooling system cannot prevent the thermal runaway of a single battery, but it can prevent the thermal runaway propagation from a battery to adjacent batteries.     

Heat pipe cooling technology for desktop PC CPU

As Pentium-IV, a high-performance PC CPU, emerges, CPU’s thermal design power shows sharp increase. Since conventional cooling method, aluminum extruded heat sink has disadvantages like poor cooling performance, acoustic noise increase and weight rise, cooling module in the form of remote heat exchanger using heat pipe is developed. Especially, using system fan exhausting heat inside to cool CPU reduces acoustic noise with lowered quantities of fan and makes it possible to reduce manufacturing unit price. This paper proposes cooling using heat pipe, recommends Pentium-IV CPU cooling module on change of fan revolution speed, and looks for possibility of reducing acoustic noise.     

A Feasibility Study About Using SiO2 Nanofluid Screen Mesh Wick Heat Pipe for Cooling of High-Power LEDs

Effective and timely heat removal from high-power light-emitting diodes (LEDs) is crucial to their performance and lifetime. The strategy of using a screen mesh wick heat pipe with SiO₂ nanofluid as the working fluid for LED heat dissipation is comprehensively evaluated. An experimental system is set up to study the heat transfer performance of the heat pipe. The obtained experimental results give optimal conditions/parameters for the heat pipe: 60% charging ratio, 30° incline angle, and 1wt% concentration of the nanofluid. Compared with a heat pipe using the secondary distilled water as the working fluid, the thermal resistance of the heat pipe using the SiO₂ nanofluid as the working fluid is generally reduced by around 35–40% for the investigated heat load range of 1–60 W. Based on an equivalent heat conductivity of the SiO₂ nanofluid heat pipe derived from the experimental results, an Icepak modeling effort for the cooling system of a 60-W LED lamp is then expended. The numerical results show that the temperature of the LED lamp remains low and quite uniform across the LED chip region, indicating the technical feasibility of using this class of heat pipes for cooling of high-power LEDs.     

Parametrical analysis of the design and performance of a solar heat pipe thermoelectric generator unit

This paper describes a solar heat pipe thermoelectric generator (SHP-TEG) unit comprising an evacuated double-skin glass tube, a finned heat pipe and a TEG module. The system takes the advantage of heat pipe to convert the absorbed solar irradiation to a high heat flux to meet the TEG operating requirement. An analytical model of the SHP-TEG unit is presented for the condition of constant solar irradiation, which may lead to different performance characteristics and optimal design parameters compared with the condition of constant temperature difference usually dealt with in other studies. The analytical model presents the complex influence of basic parameters such as solar irradiation, cooling water temperature, thermoelement length and cross-section area and number of thermoelements, etc. on the maximum power output and conversion efficiency of the SHP-TEG. Simulation based on the analytical model has been carried out to study the performance and design optimization of the SHP-TEG.     

Optimization of the design parameters of Parallel-Plain Fin heat sink module cooling phenomenon based on the Taguchi method

This paper presents an effective method for predicting and optimizing the cooling performance of Parallel-Plain Fin (PPF) heat sink module based on the Taguchi method. The numerical simulative analyses of PPF heat sink module have been constructed to understand the affecting situation of its related modeling parameters. The design parameters evaluated are the outline design of the heat sink module and the wind capacity of fan, and the highest temperature (or thermal resistance) of this module is considered as the performance characteristics. Taguchi method for design of experiment (DOE) and the analysis of variance (ANOVA) are employed to efficiently seek the combination of optimized design parameters. From the numerical simulative analyses, the optimum design parameters to obtain the lowest value of the highest temperature (or thermal resistance) are found, which is the target of this research.     

An in situ thermal response test for borehole heat exchangers of the ground-coupled heat pump system

Thermal response tests (TRTs) are crucial for the estimation of the ground thermal properties and thermal performance of the borehole heat exchanger (BHE) of the ground-coupled heat pump (GCHP) system. In this article, a TRT apparatus was designed and built to measure the temperature response of inlet and outlet sections of BHE in the test borehole, the apparatus can effectively operate under both constant heating flux modes and heat injection and extraction modes with a constant inlet temperature. A TRT for a project of GCHP located in the Jiangsu province of China was carried out by the experimental apparatus. Based on the experimental data, the heat transfer performances of BHE under heating and cooling modes were evaluated, and the ground thermal properties, which include the ground thermal conductivity, ground volumetric specific heat, borehole thermal resistance and effective soil thermal resistance, were determined by the line source model. The results indicate that the experimental device and analysis model proposed in this article can be effectively applied to estimate the ground thermal properties and thermal performance of BHE. During the process of thermal response of ground, the fluid temperatures vary acutely at the start-stage of 8 h, and then tend to be a steady state after 40 h. The test data during the start-stage should be discarded for improving the estimation accuracy of ground thermal properties. At the same time, the effective soil thermal resistance increases continuously with time and a steady-state value would be reached after the start-time, and this steady-state thermal resistance can be used to evaluate the required length of BHE. In addition, the heat transfer rate of the BHE under different operating conditions can be used for the further evaluation on long-term operation performance of GCHPs.     

The review of thermal management technology for large-scale lithium-ion battery energy storage system

大容量锂离子电池储能系统对完善传统电网和高效利用新能源都具有非常重要的作用。为了实现大容量锂离子电池储能系统的高倍率化、长寿命化以及高安全性,高性能电池热管理系统的研发刻不容缓。本文总结了温度对锂离子电池性能的影响规律,综述了空冷、液冷、热管冷却、相变冷却这4种典型热管理技术的研究概况,分析了热管理技术在锂离子电池储能系统中的应用与研究状况。随着锂离子电池储能系统工作倍率的提高,产热量随之增大,对热管理系统的要求也越来越高。下一步的研究工作应围绕空冷系统优化、基于新型冷却介质的液冷系统、经济型热管及多目标优化设计这4方面展开。     

Improving Photovoltaic Module Efficiency Using Water Cooling

An effective way of improving efficiency and reducing the rate of thermal degradation of a photovoltaic (PV) module is by reducing the operating temperature of its surface. This can be achieved by cooling the module and reducing the heat stored inside the PV cells during operation. In this paper, long-term performance modeling of a proposed solar-water pumping system is carried out. The system, which is used for irrigation purposes, consists of a PV module cooled by water, a submersible water pump, and a water storage tank. Cooling of the PV panel is achieved by introducing water trickling configuration on the upper surface of the panel. An experimental rig is developed to investigate and evaluate PV module performance with the proposed cooling technique. The experimental results indicated that due to the heat loss by convection between water and the PV panel's upper surface, an increase of about 15% in system output is achieved at peak radiation conditions. Long-term performance of the system is estimated by integrating test results in a commercial transient simulation package using site radiation and ambient temperature data. The simulation results of the system's annual performance indicated that an increase of 5% in delivered energy from the PV module can be achieved during dry and warm seasons.     

Dynamic test method for determining the thermal performances of heat pipes

With a view toward shortening the necessary time to examine the thermal performances of heat pipes, a novel dynamic test method is originated and compared to the conventional steady-states test. A set of dynamic parameters of thermal performances of heat pipes is ideated from the observed transient phenomenon. Bending angles, fill ratios, and shapes of heat pipes are investigated in order to study their influences on the thermal performances of heat pipes for both steady-state and dynamic tests. A model based on the investigated dynamic test is established to explain the experimental results. Experimental results demonstrate that deformation of heat pipes would damage the thermal performances of heat pipes most significantly. Larger fill ratios would increase the operation limitations but also lead to less sensitive temperature responses of heat pipes. The parameters and the influences of factors between the steady-state test and the dynamic test are found to be remarkably analogous. As a consequence, the dynamic test can be adopted instead of the steady-state test to determine the thermal performance of heat pipes when high efficiency is of prior concern.     

中深层地热Ｕ型井地埋管换热器的试验研究

地热能作为一种非碳基能源,具有储量丰富、清洁可再生等特点,开发利用地热能有助于碳达峰的实现。在中深层地源热泵领域,我国主要以单井同轴管为主,而相对高效的中深层地热U型井地埋管案例屈指可数。为了了解中深层地热U型井地埋管换热性能及井下换热参数变化,完成了新型的Ｕ型井地埋管换热器工程,并在此基础上进行了实验研究。首先,开展了地温测量,确定了研究区的地层温度,根据热储的物性条件选取了水平井段及对接位置;其次,分析空载循环试验工况下循环水的流量及井下温度的变化情况,研究了负载工况下供回水温度、流量、换热量、不同井段对换热的贡献率、井下温度的动态变化、U型井的恢复能力等因素。实验结果表明,中深层Ｕ型井地埋管换热器井底温度会随运行时间增长而降低,流量大且回水温度较低的情况下,换热器的换热量比较高,最高为1336.8kW;回水井对换热量的增加有限,每百米增加0.12℃,实际工程中可以考虑减小口径,降低建设费用。U型井地埋管换热器的地温恢复能力较强,停止运行24h左右井底温度与初始温度差为-13℃。研究结果有助于研究人员对中深层Ｕ型井地埋管换热器有更进一步的认识,从而推动中深层地热能的健康可持续发展。     

Performance of serpentine channel based Li-ion battery thermal management system: An experimental investigation

Thermal management of large-scale Li-ion battery packs is of great significance to their safety and life cycle, which would impact their applicability in electric vehicles. Of the many strategies developed for this purpose, indirect liquid cooling has already demonstrated quite high potentials in thermal regulation of such battery systems. In this study, a compact lightweight serpentine wavy channel configuration was chosen to construct an indirect liquid cooling system for a battery module of cylindrical Li-ion cells. The serpentine channel has a number of six internal minichannels. Experimental test data were used to conduct a comprehensive thermal analysis to examine the highest temperature, the maximum temperature difference, and the heat accumulation percentages, and so forth within the battery pack. Results have revealed the ability of the cooling system to maintain the module temperature within appropriate working conditions for electric vehicle applications for most cycling tests including two driving cycles. Furthermore, the analysis insights raised by this study could be useful in understanding the cooling performance of the liquid-based thermal management systems for electric vehicles.     

Factors Influencing the Temperature Distribution of 200 W Light Emitting Diode Module Used in the Spotlight

The junction temperature of LED (light emitting diode) has a significant impact on its performance and lifetime. In this paper, a simplified model based on the finite element analysis is developed to simulate the temperature distribution of the 200 W LED module using software ANSYS. The model contains LED package, the heat pipe radiator, as well as TIM (thermal interface material) between the LED package and radiator. The temperature distribution of the simulation agrees with that of the experimental measurement. Thickness of TIM affects the heat dissipation significantly, the chips temperature and the maximum temperature difference of chips increases sharply with TIM thickness increasing. Substituting aluminum fins with copper fins cannot improve the heat dissipation performance of heat pipe radiator, and the air velocity of heat pipe radiator plays a key role in the heat dissipation. Thermal conductivity of package submount directly affects the chip temperature and the uniformity of temperature distribution of package submount.     

Experimental investigation on mini‐channel cooling–based thermal management for Li‐ion battery module under different cooling schemes

The performance of Li‐ion battery depends on the temperature. Active liquid cooling system can keep the battery temperature within an optimal range, but the system itself consumes energy. This paper reported the experimental work on the thermal performance of liquid cooling system for the battery module under different cooling schemes. It was hoped that energy consumption could be reduced as much as possible. Meanwhile, liquid cooling system could provide effective cooling for the battery module. Two identical real battery modules including 18 cylindrical cells (with and without cooling system) were manufactured for the validity of comparison. The 2 battery modules discharged at the discharge rates of 1C and 1.5C. Charge and discharge cycle test was also carried out. The results indicated that a simple hysteresis control cooling scheme could reduce the energy consumption effectively. The energy consumption was saved by 83.2% and 49% at the discharge rates of 1C and 1.5C, respectively. Meanwhile, the temperature of battery module was still kept within the optimal range. The maximum temperature appeared on different cells in the battery module during the process of charge and discharge. Thus, the temperature dynamic comparison mechanism was very necessary.     

Thermal/structural analysis of radiators for heavy-duty trucks

A thermal/structural coupling approach is applied to analyze thermal performance and predict the thermal stress of a radiator for heavy-duty transportation cooling systems. Bench test and field test data show that non-uniform temperature gradient and dynamic pressure loads may induce large thermal stress on the radiator. A finite element analysis (FEA) tool is used to predict the strains and displacement of radiator based on the solid wall temperature, wall-based fluid film heat transfer coefficient and pressure drop. These are obtained from a computational fluid dynamics (CFD) simulation. A 3D simulation of turbulent flow and coupled heat transfer between the working fluids poses a major difficulty because the range of length scales involved in heavy-duty radiators varies from few millimeters of the fin pitch and/or tube cross-section to several meters for the overall size of the radiator. It is very computational expensive, if not impossible, to directly simulate the turbulent heat transfer between fins and the thermal boundary layer in each tube. In order to overcome the computational difficulties, a dual porous zone (DPZ) method is applied, in which fins in the air side and turbulators in the water side are treated as porous region. The parameters involved in the DPZ method are tuned based on experimental data in prior. A distinguished advantage of the porous medium method is its effectiveness of modeling wide-range characteristic scale problems. A parametric study of the impact of flow rate on the heat transfer coefficient is presented. The FEA results predict the maximum value of stress/strain and target locations for possible structural failure and the results obtained are consistent with experimental observations. The results demonstrate that the coupling thermal/structural analysis is a powerful tool applied to heavy-duty cooling product design to improve the radiator thermal performance, durability and reliability under rigid working environment.     

Heat dissipation design for lithium-ion batteries

A two-dimensional, transient heat-transfer model for different methods of heat dissipation is used to simulate the temperature distribution in lithium-ion batteries. The experimental and simulation results show that cooling by natural convection is not an effective means for removing heat from the battery system. It is found that forced convection cooling can mitigate temperature rise in the battery. Nevertheless, a non-uniform distribution of temperature on the surface of the battery is inevitable and this makes thermal management difficult.As a better means of suppressing increases in temperature, a heat pipe has been used to effect heat dissipation. The connection between the heat pipe and the battery wall pays an important role in heat dissipation. Inserting the heat pipe in to an aluminum fin appears to be suitable for reducing the rise in temperature and maintaining a uniform temperature distribution on the surface of the battery.