Fabrication And Performance Study Of Multi-scale Capillary Structure For Loop Heat Pipe

The miniaturization,high integration and high frequency of electronic equipment poses challenges for efficient heat dissipation technology.As a kind of highly efficient passive-phase change heat transfer device,loop heat pipe(LHP)shows great application potential in the field of electronic cooling.At present the main area of application of the LHP is space technology.However,the terrestrial application of the LHP is still in the immature stage.There are many problems to be solved including miniaturization,anti-gravity,highly efficient heat dissipation and reliability.The core of the LHP is the capillary wick which provides capillary pressure to maintain the cycle,and evaporation/boiling takes place in the wick.The structure of the capillary wick related directly to the performance of the LHP.In this paper,multi-scale structure is proposed to solve the conflicts existing in phase change heat transfer process,such as capillary pressure and vapor venting,capillary pressure and viscous force,and finally improved the performance of the LHP.Began with preparation of multi-scale structure,the effects of various length scales on thermal property,wettability,and capillary performance of the wick were studied.Then,a noval LHP with composite multi-scale capillary wick was designed,and the mechnism of heat transfer enhancement of the noval LHP was researched.Furthermore,the structure of capillary wick was re-optimized,effect of vavious parameters include structure and external on the performance of the LHP was sresearched.Finally,the thermal performance of the LHP were highly improved.The paper has carried out from the following aspects:(1)Simple preparation of multi-scale structure: the method of preparation of nano and micro porous were explored using simple loose solid-state sintering technology.Effect of various preparation parameters on the sintered structures were studied such as particle size and shape,pore former concentration.Because of the limited research of the nanometer copper powder sintering technology,this experimental study concentrates on the sintering process of nano powder.Under the sintering process of 600 ? temperature,4.8 ?/min heating rate and 30 min temperature preservation time,we obtained the porous structure which is suitable to the capillary wick of the LHP.Furthermore,in order to improve the capillary pressure,nano-modification for sintered porous structure were carried using the thermal oxidation and chemical oxidation technology,and two types of multi-scale structure are obtained.The first type of multi-scale structure contained nano pore and micro cavity,and the surface of the porous were covered with nano structures.Another one contains two types of pore with the size of a few microns and hundreds of microns,and nanometer microstructure also exists on its surface.The micro topology and thermophysical properties were characterized,and the effect of various characteristic scales on the thermophysical properties parameters were identified.This preparation method of multi-scale structure is not only suitable for the capillary wick of the LHP,but also can offer a reference for the design of heat transfer enhancement surface.(2)Interface phenomenon in multi-scale structures: The dynamic wetting process on multi-scale surfaces were studied by droplet deposition experiment.Two modes of small droplet emission was found,which has been not yet reported regarding the porous surface.We also indicated that the competition among the spreading,infiltration and the capillary wave is the dominate mechanism of small droplets emission.We recognized the three-phase contact line's early inertia movement regularity(Dt/D0=C(t/?)?),and get how different characteristic scales influence the inertia diffusion and penetration.Experiments also show that the surface roughness can enhance the wettability,strengthen fluid spreading.However,the wettability of the porous structure shows in both spreading and penetration.In terms of nano porous structure,improving the surface roughness will reduce penetration greatly.Whereas roughness have little influence on permeability of the micro porous structure.This feature provides guidance for the capillary wick.Capillary pumbing is also one type of interface phenomenon.In this work,infrared technology has been used to study the influence of the preparation parameters on the performance of capillary pumbing,and we demonstrated that micro porous structure achieves better capillary performance.And there is a best match relationship between particle diameter and pore former concentration.(3)The application of the multi-scale composite capillary wick in the LHP: Combining with the feature of the LHP and the rule acquired from the above research,we innovated the evaporator and capillary wick structure design,the micro channel machined on the evaporator substrate for conventional LHP was canceled.We applied the modulated porous wick to the LHP,and as a result,the capillary wick has multi-scale characteristics: the small pore(~ ?m scale),the larger pore(~ 100 ?m scale),vapor channel(~ mm scale).Experiments were performed with water as working fluid at the unfavorable operational orientation i.e.anti-gravity direction.The results show that under the same power input,new type of loop heat pipe(MBE)decreased operating temperature by more than 20 ?compared with the conventional LHP(MWE).Meanwhile,the heat flux is increased about three times as the conventional one.We also proved the multi-scale composite wick improved heat transfer mechanism of the LHP: different pore size effectively solve the conflict among the capillary pressure,viscous resistance,and the flow resistance.By canceling the micro channel machined on the evaporator bottom,evaporation can occur directly on the primary wick adjacent to the evaporator wall which shortens the heat transfer path.Evaporation mainly happens on the primary wick near the side of the evaporator,whereas the second capillary wick is mainly acted as the fluid supply.Vapor-liquid interface away from compensation chamber improved heat flux and weaken heat leak.Composite structure capillary wick effectively solves the conflicts between evaporation enhancement and coefficient and heat leakage reduction,which need different thermal conductivity.(4)Optimization of structure and performance of the LHP: Effect of various structure and external parameters on the thermal performance of the LHP was carried out systematically,including primary wick size,particle size,liquid charge ratio,tilt.Experiments show that,the porous stack height(h),stack width(w)and vapor channel width(p)should be synergized to have best LHP performance.Wider porous stack width increases the surface area for liquid film evaporation,but hinders the vapor release out of the porous stack.In this study,the best geometric parameters are h=1.5 mm,p=1.5 mm,w=3 mm.Experiments also identified the the dominant heat transfer mechanism for the noval LHP,i.e.,it is nucleate boiling heat transfer at small heat loads,and film evaporation heat transfer at moderate or large heat loads.In addition,the charging ratio,particle diameter and running tilt can influence the start-up and steady state performance of the LHP.Under the optimized conditions proposed by this work,the noval LHP achieved the evaporator wall temperature of 63 ? at the heat load of 200 W for the anti-gravity operation,under which the heat flux attained 40 W/cm2,which is 2-6.7 times of those reported in references,and the thermal resistance is only 0.12 K/W.