Miniature loop heat pipes for electronics cooling

Miniature loop heat pipes for electronics cooling

V.G.Pastukhov *,Yu.F.Maidanik,C.V.Vershinin,M.A.Korukov

Institute of Thermal Physics,Pervomaiskaya St.91,Ekaterinburg 620219,Russia

Received 27December 2002;accepted 28December 2002

Abstract

The paper is devoted to the development of miniature loop heat pipes (mLHPs)with a nominal capacity of 25–30W and a heat-transfer distance up to 250mm intended for cooling electronics components and CPU of mobile PC.It gives the results of investigating several prototypes of mLHPs incorporated into remote heat exchanger (RHE)systems in different conditions.It has been established that in the nominal range of heat loads orientation does not practically affect the mLHPs operating characteristics.Under air cooling the total thermal resistance of such a system is 1.7–4.0°C/W and depends strongly on the cooling conditions and the radiator efficiency.In this case the mLHP Õs own thermal resistance is in the limits from 0.3to 1.2°C/W,and the maximum capacity reaches 80–120B T .The obtained results make it possible to regard mLHPs as quite promising devices for RHE systems providing thermal regimes for electronics components and personal computers.

Ó2003Elsevier Science Ltd.All rights reserved.

Keywords:Miniature loop heat pipe;CPU;Remote heat exchanger;Thermal resistance

1.Introduction

Loop heat pipes (LHPs)are highly efficient heat-transfer devices with a considerable potential for development and application in various fields.At present LHPs are successfully employed in space engineering [1].Usually these devices have a cylindrical evaporator from 12to 28mm in diameter.The shape and the size of the condenser may be quite different depending on the means and conditions of its cooling.The length of the vapor and the liquid lines connecting the evap-orator and the condenser can reach 10m and more,and their diameter is,as a rule,in the range from 3to 8mm.The absence of a wick in these lines makes them easy to bend giving them the

*

Corresponding author.Tel.:+7-3432-678-791;fax:+7-3432-678-799.E-mail address:pastukhov@itp.uran.ru (V.G.Pastukhov).

1359-4311/03/$-see front matter Ó2003Elsevier Science Ltd.All rights reserved.

doi:10.1016/S1359-4311(03)00046-2

Applied Thermal Engineering 23(2003)1125–1135

/locate/apthermeng

1126V.G.Pastukhov et al./Applied Thermal Engineering23(2003)1125–1135

required shape.Another important advantage of LHPs is their low sensitivity to the change of orientation in1-g conditions.

In order to expand thefield of LHPs application,for instance,for cooling electronics and personal computers,it is necessary to miniaturize these devices.This task is quite complicated as a decrease in the evaporator diameter below8mm leads to limitations connected with the pecu-liarities of the principle of LHP operation.At the same time to obtain the aims mentioned above it is necessary to use LHPs with an evaporator diameter of no more than6mm and a diameter of the vapor and the liquid line of1.5–2.0mm.For thefirst time the possibility of creating such devices was demonstrated in the paper[2].

In PC the demand for heat-transfer devices with the characteristic dimensions mentioned above isfirst of all connected with the cooling of the central processing unit(CPU).The tendency for a rise in the CPU capacity resulted at present in increasing heat release to50–100W for desktop PC and to20–35W in notebook PC[3,4].In the latter case the problem of cooling is complicated by the limitedness of available space and the increase of the total heat release from the adjacent units (DRAM,HDD.etc.).The problem of the CPU cooling consists in restricting its temperature to 80–90°C at a heat-sink temperature of35–45°C.Until recently the solution of this problem was connected with the use of heat pipes(HP)as the most efficient heat-transfer devices.It should be noted that an HP itself cannot provide cooling.It is,as a rule,only part of an integrated system, which ensures heat collection and sink and may be equipped,for instance,with a fan.Therefore the most objective parameter characterizing the system efficiency is the total thermal resistance (R tot)determined by the ratio of the temperature difference between the heat source and the heat sink to the value of the dissipated capacity.Under air-cooling of an HP condenser the sink temperature is equated to the ambient temperature.A comparison of this parameter shows that the best values of0.2–0.25°C/W belong to thinflat heat pipes[5,6]and‘‘Vapor Chambers’’[3]. These devices are intended for heat removal in the vicinity of a source and mainly play the role of heat-flow transformers.The capacity dissipated with their help can reach50–160W.For RHE systems there are developments with the use of ordinary HPs(of cylindrical,elliptic or rectangular section)with a cross-section area from7to27mm2and a heat-transfer length up to150mm [3,4,7].The maximum capacity of such systems at a horizontal position reaches10–27W,and the total thermal resistance is from1.8°C/W[3]to3.5°C/W[4].At a vertical orientation,when the evaporator is above,their capacity decreases approximately two times.With the aim of increasing the capacity several HPs are combined into a bundle.There is also an RHE with an oscillating HP with six turns of a pipe3mm in diameter[8].Under air-cooling at a speed of2m/s it can transfer up to50W at the thermal resistance of about1.3°C/W.

In complete evaluation of the qualities of one or another cooling system important parameters are also:mass,size,simplicity of positioning in the object of application and cost.The design features of LHPs provide a good possibility of their application of heat removal in any convenient place of the object and without limitations on the arrangement of its parts in space.

ponents of thermal resistance in an RHE system with an mLHP

It is common practice to evaluate the efficiency of RHE systems by the value of the total thermal resistance R tot,which includes the whole sequence of resistances from the heat source to