Flat Loop Heat Pipe with Bi-Transport Loops for Graphics Card Cooling

A flat loop heat pipe (FLHP) with bi-transport loops is developed for the cooling of graphics card with high heat flux up to 80W/cm². The evaporator and the pipes are made of copper and ultrapure water (electronic resistivity > 18 MΩ-cm) as the working fluid. To give the loop heat pipe (LHP) better performance, the evaporator is made in a flat shape to reduce the contact resistance between the evaporator and the chip. The advances of the LHP with bi-transport loops are discussed. The heat transfer performance is tested with different filling rate in different orientations. The test results show that the LHP can start up easily and can transport large amount of heat stably. The orientation of the condenser above the evaporator gives a better performance, and filling with 13 g of water gives a better performance. Limited by the evaporator temperature lower than 90°C, the LHP can transport 320 W when the evaporator is above the condenser and 380 W when the condenser is above the evaporator.     

Heat Transfer in a Loop Heat Pipe using Diamond-H2O Nanofluid

The main aim of this study is to enhance the thermal performance of loop heat pipe (LHP) charged with nanofluid as the working fluid. Thus, experiments are conducted to investigate heat transfer characteristics of using diamond-H₂O nanofluid with nanoparticle mass concentration ranged from 0% to 3% in a LHP as a working medium for heat input range from 20 W to 60 W. The three-dimensional model, laminar flow and heat transfer governing equations are solved using the finite volume method. The simulations are carried out with three-dimensional model based on the characterization of the working fluid inside the LHP to give an insight into the heat transfer and fluid flow mechanism. The LHP performance is evaluated in terms of temperature distributions and total thermal resistance of LHP. It is inferred that the temperatures obtained at all points in evaporator side of LHP charged with diamond-H₂O nanofluid are lower and reach their steady state faster than LHP charged with pure water. At the constant heat input, test results showed the average decrease of 5.7%?10.8% at nanoparticle mass concentrations ranging from 0.5% to 3% in R_th of LHP as compared with pure water (0%).     

低温环路热管综述

环路热管是以多孔毛细芯抽吸力为动力的相变传热设备,可根据实际应用改变结构形式,能在远距离传热的同时保持良好的均温性,并且可在微重力环境下运行。环路热管工作温区较广,按照其工作温区一般可分为高温环路热管(350 K以上)、常温环路热管(200～350 K)和低温环路热管(200 K以下)。为了满足深空探测的需要,低温环路热管广泛应用于航天设备温控系统中并表现出优异的性能。按照孔隙特征和结构形式将用于环路热管的毛细芯分为四种,简要阐述每种毛细芯制备和特点;综合分析了近年来低温环路热管技术主要理论和实验研究成果,将目前低温环路热管常见的工作温区分成五个部分,分析影响低温环路热管传热性能的因素,包括工质充装量、反重力高度、次蒸发器功率等。最后,提出优化措施以满足未来深空以及地面应用的需求。     

Sorption heat pipe—a new thermal control device for space and ground application

Sorption heat pipe (SHP) combines the enhanced heat and mass transfer in conventional heat pipes with sorption phenomena in the sorbent bed. SHP consists of a sorbent system (adsorber/desorber and evaporator) at one end and a condenser + evaporator at the other end. It can be used as a cooler/heater and be cooled and heated as a heat pipe. SHP is suggested for space and ground application, because it is insensitive to some “g” acceleration. This device can be composed of a loop heat pipe (LHP), or capillary pumped loop (CPL) and a solid sorption cooler. The most essential feature is that LHP and SHP have the same evaporator, but are working alternatively out of phase. SHP can be applied as a cryogenic cooler, or as a fluid storage canister. When it is used for cryogenic thermal control of a spacecraft on the orbit (cold plate for infrared observation of the earth, or space), or efficient electronic components cooling device (lased diode), it is considered as a cooler. When it is applied as a cryogenic storage system, it insures the low pressure of cryogenic fluid inside the sorbent material at room temperature.     

Thermofluid Characteristics in Microporous Structure With Different Flow Channels for Loop Heat Pipe

The use of two-phase heat transfer devices using capillary action in a microscale porous structure such as a loop heat pipe (LHP) is a promising heat transport technology. This is because they have characteristics of higher heat transfer power and longer heat transport distances with no electrical power compared with conventional heat pipes. The thermal performance of an LHP is governed by the thermofluid behavior in a microscale porous structure called the wick. In this research, high-performance wicks made of polymer have been developed, and their pore distribution and permeability were evaluated. The effects of the vapor channel's shape on the loop's thermal performance have been investigated by calculation and experiment to enhance evaporator performance. A mathematical model of the evaporator considering super heat in the channel, pressure drop across the wick, and two-phase pressure loss on the boundary face between the wick and the evaporator case was newly developed. The experiment was also conducted as a function of the groove shapes. From calculations and test results, it was found that in order to increase the maximum heat transport capability and decrease the operating temperature, the groove should be well distributed.     

Loop Heat Pipes: A Review of Fundamentals,Operation,and Design

The loop heat pipe (LHP) is a passive two-phase heat transport device that is gaining importance as a part of spacecraft thermal control systems and also in applications such as in avionics cooling and submarines. A major advantage of a loop heat pipe is that the porous wick structure is confuned to the evaporator section, and connection between the evaporator and condenser sections is by smooth tubes, thus minimizing pressure drop. A brief overview of loop heat pipes with respect to basic fundamentals, construction details, operating principles, and typical operating characteristics is presented in this paper. Finally, the paper presents the current developments in modeling of thermohydraulics and design methodologies of LHPs.     

Heat transfer with flow and phase change in an evaporator of miniature flat plate capillary pumped loop

An overall two-dimensional numerical model of the miniature flat plate capillary pumped loop (CPL) evaporator is developed to describe the liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The entire evaporator is solved with SIMPLE algorithm as a conjugate problem. The effect of heat conduction of metallic side wall on the performance of miniature flat plate CPL evaporator is analyzed, and side wall effect heat transfer limit is introduced to estimate the performance of evaporator. The shape and location of vapor-liquid interface inside the wick are calculated and the influences of applied heat flux, liquid subcooling, wick material and metallic wall material on the evaporator performance are investigated in detail. The numerical results obtained are useful for the miniature flat plate evaporator performance optimization and design of CPL.     

Loop heat pipe for cooling of high-power electronic components

In this paper, we present a new development of loop heat pipe (LHP) technology in its applications to cooling systems for high-power IGBT elements. An advanced method of LHP evaporator wick manufacturing has been proposed. Following this approach, a 16 mm outer diameter and 280 mm-length LHP evaporator was designed and manufactured. Nickel and titanium particles were used as raw material in LHP evaporator wick fabrication. LHP with a nominal capacity as high as 900 W for steady-state condition and more than 900 W for a periodic mode of operation at a temperature level below 100 °C and a heat transfer distance of 1.5 m was designed through the cooling of a high-power electronic module. An experimental program was developed to execute LHP performance tests and monitor its operability over a span of time. An investigation of the effects of LHP performance of parameters such as evaporator and condenser temperatures and LHP orientation in a gravity field was brought about. As regards the results of this initial series of tests, it was found that LHP spatial orientation within the nominal range of heat loads has no drastic effect on overall LHP functioning, whereas condenser temperature does play an important role, especially in the range of heat load close to critical. A 2D nodal model of the evaporator was developed and provides us with confirmation of the suggestion that when high-power dissipation levels are available, low wick conductivity is well adapted for LHP applications.     

不同加热方式下环路热管蒸发器可视化研究

针对环路热管内部工质相变及流动换热问题,设计了环路热管蒸发器中心通道可视化实验平台,研究了不同加热方式对热管内工质状态和传热特性的影响。结果表明：加热方式直接影响热管10W启动过程,双面加热启动速度最快。相同热载荷时,不同加热方式下环路热管热阻及蒸发器中心通道内液面高度和成核情况存在差异。10W - 40W热载荷时,随着热载荷的增大,三种加热方式的传热热阻均在减小。40W-50W热载荷时,顶部加热方式下的热管性能出现恶化,底部加热传热性能出现停滞,仅双面加热性能稳定并有提高趋势。随着热负荷的增加,蒸发器中心通道内气液界面升高、气泡的产生变得更加剧烈,蒸发器通过吸液芯向储液器的漏热量增加,进而影响环路热管的性能。     

An experimental investigation on capillary pumped loop with the meshes wick

A copper-water capillary pumped loop (CPL) is developed for heat recovery applications in the field of the refrigeration and air conditioning. The multi-layer copper meshes are used as the capillary structure in the CPL. The startup characteristics and heat transfer performance of the CPL are investigated experimentally. The experimental results show that for a range of power applied to the evaporator, the system presented reliable startups and continuous operation. The heat transfer performance of the evaporator will be improved if the charging rate of the working fluid or the heat flux is increased properly. The optimal charging rate could be obtained in the range of 70–76% for the given experimental conditions. The study has demonstrated the proposed CPL configuration is able to perform the heat recovery applications.     

Heat transfer characteristics of the evaporator section using small helical coiled pipes in a looped heat pipe

An experimental study was carried out for the heat transfer characteristics and the flow patterns of the evaporator section using small diameter coiled pipes in a looped heat pipe (LHP). Two coiled pipes: the glass pipe and the stainless steel pipes were used as evaporator section in the LHP, respectively. Flow and heat transfer characteristics in the coiled tubes of the evaporator section were investigated under the different filling ratios and heat fluxes. The experimental results show that the combined effect of the evaporation of the thin liquid film, the disturbance caused by pulsation and the secondary flow enhanced greatly the heat transfer and the critical heat flux of the evaporator section. In final, two dimensionless empirical correlations were proposed for predicting the heat transfer coefficients of the evaporator section before and after dryout occurs.     

Parametric analysis of loop heat pipe operation: a literature review

Loop heat pipes (LHPs) are heat transfer devices whose operating principle is based on the evaporation/condensation of a working fluid, and which use the capillary pumping forces to ensure the fluid circulation. Their major advantages as compared to heat pipes are an ability to operate against gravity and a greater maximum heat transport capability. In this paper, a literature review is carried out in order to investigate how various parameters affect the LHP operational characteristics. This review is based on the most recent published experimental and theoretical studies. After a reminder of the LHP operating principle and thermodynamic cycle, their operating limits are described. The LHP thermal resistance and maximum heat transfer capability are affected by the choice of the working fluid, the fill charge ratio, the porous wick geometry and thermal properties, the sink and ambient temperature levels, the design of the evaporator and compensation chamber, the elevation and tilt, the presence of non-condensable gases, the pressure drops of the fluid along the loop. The overall objective for this paper is to point the state-of-the-art for the related technology for future design and applications, where the constraints related to the LHPs are detailed and discussed.     

A mathematical model to analyze heat and mass transfer characteristics of a CPL evaporator porous wick

A capillary pumped loop (CPL), because of its high power thermal transport character, has been developed as an attractive system for the thermal discharge of electronic chips with high power loads, especially on spacecrafts. A working fluid having relatively larger heat of evaporation, methanol for example, may transfer significant heat flux. In this paper a new mathematic model is constructed, in which the most important character is the treatment or the unsaturated region of the evaporator porous wick. Numerical simulation of heat and mass transfer in the evaporator porous wick is carried out with a new three‐layer model. The importance of unsaturated layer to simulate the present problem is clear. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 209–218, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20066     

Modeling and analysis of startup of a loop heat pipe

The startup behavior of a loop heat pipe (LHP) is one of the key aspects in evaluating its working performance. A mathematical model of the startup process of a LHP is established based on the node network method in this work. A parametric analysis on the startup characteristics of the LHP is conducted, where the effects of startup heat load, thermal capacity of the evaporator and compensation chamber, heat sink temperature, ambient temperature, heat leak from the evaporator to the compensation chamber and cooling on the compensation chamber on the startup characteristics of the LHP are evaluated, which contributes to the improvement of the LHP startup performance.     

Operation characteristics of a small-scale capillary pumped loop

This paper presents an experimental investigation of a capillary driven two-phase loop, configured on a capillary pumped loop (CPL) mode and designed for the thermal management of up to 50 W. Tests were performed with an internally grooved evaporator with a hollow polyethylene porous structure using acetone and anhydrous ammonia as the working fluids. For a range of power applied to the evaporator, the system presented reliable start-ups and continuous operation for several hours for each working fluid. As compared to acetone, anhydrous ammonia system showed a better heat transport capability. The capillary evaporator depriming and re-priming was also experimentally investigated. The results showed that the proposed CPL configuration is able to handle the porous structure dryout and promote its rewetting without major external interference.     

Conjugate numerical analysis of flow and heat transfer with phase change in a miniature flat plate CPL evaporator

Two-dimensional numerical model for the global evaporator of miniature flat plate capillary pumped loop (CPL) is developed to describe heat and mass transfer with phase change in the porous wick, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The governing equations for different zones are solved as a conjugate problem. The side wall effect heat transfer limit is introduced to estimate the heat transport capability of evaporator. The influences of liquid subcooling, wick material, metallic wall material and non-uniform heat flux on the evaporator performance are discussed in detail.     

Simulation of flow and heat transfer with evaporation in a porous wick of a CPL evaporator on pore scale by lattice Boltzmann method

A three-dimensional transient double-population thermal lattice Boltzmann BGK scheme for the global evaporator of a CPL is proposed to describe the heat and mass transfer in the porous wick, the cover plate and the vapor groove. The lattice BGK D3Q6 model, developed on passive-scalar approach, is proposed to represent the energy distribution function. The D3G15 incompressible lattice BGK model is chosen to represent the pressure distribution function to calculate the pressure and velocity fields. The random porous structure is reconstructed by the quartet structure generation set (QSGS) and is coupled to the lattice Boltzmann method (LBM) by some special boundary schemes for pore scale simulation. The numerical results of different working conditions and different working fluids are presented, which can improve the understandings of heat and mass transport mechanisms in a CPL evaporator and provide guidance for the evaporator design in a CPL system.     

Heat transfer performance of a double-loop separate-type heat pipe: Measurement results

An experimental study is presented for the heat transfer performance of a rectangular double-loop natural circulation system, in which the condensers are made of double tubes with water-steam as the working fluid. Detailed temperature measurements of the core fluid and the wall are made, from which overall heat transfer coefficients for the evaporator, condensers, and entire system are obtained. Parametric studies of the liquid charge level, fluid properties, and heating or cooling conditions on the heat transfer performance are presented and correlation equations are given. The results show that the overall heat transfer coefficients for the evaporator, condensers, and entire loop are all increasing with decreasing liquid charge level. Overhead phenomena at low liquid charge level and thermal oscillation at some situations are also observed and discussed.     

Effect of Surface Tension Variation of the Working Fluid on the Performance of a Closed Loop Pulsating Heat Pipe

In this article, the effect of surface tension variation of the working fluid on the thermal performance of a pulsating heat pipe (PHP) is presented. A two-turn closed loop PHP is fabricated with a combination of copper and quartz tubes having 2 mm inner diameter. Three filling ratios are considered in the present study. A common surfactant namely sodium dodecyl sulfate is used to vary the surface tension of the distilled water which is the base working fluid tested in the PHP. Visualization as well as heat transfer studies is performed in the PHP. Thermal resistances of working fluids with different surface tension are estimated for each filling ratio. Further, the effect of surfactant concentration on the hydrodynamics of the PHP is also discussed. Addition of surfactant induces formation of foam after the filling process and in some phase of PHP operation. It changes the flow regime boundaries and lowers the evaporator temperature under specific operating conditions. A flow regime map is also constructed for the present PHP. The working fluid with a lower surface tension gives a minimum evaporator temperature in the vertical orientation and thus, lowest thermal resistance has been obtained. However, surface tension does not influence the performance in the horizontal position appreciably.     

Numerical simulation of transient heat and mass transfer in a cylindrical evaporator of a loop heat pipe

The paper investigates the transient processes of heat and mass transfer in a cylindrical evaporator of a loop heat pipe (LHP) during the device start-up. One of the most “arduous” prestart situations, which is characterized by the absence of a liquid in the evaporator central core and filled vapor removal channels, has been considered. With such liquid distribution a successful start-up of an LHP becomes possible only after formation of the vapor phase in the vapor removal channels and their liberation from the liquid. The aim of the investigations is to determine conditions that ensure the boiling-up of a working fluid in vapor removal channels. The problem was solved by a numerical method. Simulation of start-up regimes has been performed for different heat loads and different structural materials of the evaporator. Copper, titanium and nickel wick have been examined. Calculations have been made for three different working fluids; water, ammonia and acetone. Account has been taken of the conditions of heat exchange between the compensation chamber and surrounding medium.