暖通方面英文论文

Experimental study on laminar convective heat transfer of microencapsulated phase change material slurry using liquid metal with low melting point as carrying fl uid Sihong Song Weidong Shen Jianli Wang Shengchun Wang Jiafeng Xu Key Laboratory of Special Power Supply Chongqing Communication Institute Chongqing 400035 PR China a r t i c l ei n f o Article history Received 4 November 2013 Received in revised form 19 January 2014 Accepted 21 January 2014 Keywords Microencapsulated phase change material slurry Liquid metal Heat transfer enhancement Laminar fl ow Heat dissipation a b s t r a c t The microencapsulated phase change material slurry using liquid metal with low melting point as carry ing fl uid MEPCM LM slurry is a novel and powerful cooling fl uid applied in thermal management of high power electronic devices The laminar convective heat transfer performances of MEPCM LM slurry in a tube with constant heat fl ux were investigated experimentally The effects of MEPCM volume con centration Re and heat rate on heat transfer characteristics were also studied Results indicate that the Fanning friction factor of MEPCM LM slurry d is in good accord with the theoretic values f 16 Re and the MEPCM LM slurry can be considered as Newton fl uid It is also found that the modifi ed local convective heat transfer coeffi cient h x for MEPCM LM slurry is higher than that for pure gallium Fur thermore the h xincreases with increasing volume concentration and Re The h xincreases with increasing the heat rate before phase change fi nished and the variation trend of h xwith heat rate is contrary after phase change fi nished but the h x is not much infl uenced by the imposed heat fl uxes tested The kind of MEPCM LM slurry has good application future in practice 2014 Elsevier Ltd All rights reserved 1 Introduction In recent years with the rapid developments of the electronic technique high degree of integration and enhanced performance has led to high heat dissipation electronic devices 1 3 Due to a dramatic increase in chip densities and power densities as well as a continuous decrease in physical dimensions of electronic pack ages the heat dissipation in microelectronic packaging is becoming increasingly important 4 The lifetime and reliability of electronic components fall as the operating temperature increases 5 effec tive cooling solutions are critical for the design of electronic devices for preventing thermal breakdown and extending working life of semiconductor components The conventional cooling meth ods such as air cooling and water cooling are rapidly becoming inadequate for dissipating intense heat loads often encountered in new electronic devices 6 Furthermore though some better cooling methods such as thermoelectric cooling 7 jet impinge ment 8 spray 9 microchannel heat sinks 10 heat pipe 11 and thermosyphon cooling 12 upgrade the cooling performance these cooling methods have some disadvantages in machining and practical application Therefore it is absolutely necessary that new feasible ways will be found to solve the problem of highly effi cient thermal management New cooling fl uid with high heat transfer capability such as liquid metal LM and latent functionally thermal fl uid LFTF have been proposed to enhance the cooling effi ciency in recent research In the year of 2002 Liu and Zhou 13 and Liu et al 14 used fi rstly the liquid metals with low melting point and their alloys as cooling fl uid to cool the computer chip and achieved the good cooling effect because of high heat conduction of liquid metal Ma and Liu 15 16 and Ma et al 17 presented an overall review on chip cooling using liquid metals or their alloys as coolant illustrated the principles of several typical pumping methods demonstrated for the fi rst time the a heat driven liquid metal cooling device in which the whole liquid fl ow loop was driven by a MFD pump pow ered by a one stage thermoelectric device directly using waste heat from the hot chip and proposed a novel strategy to thaw quickly the frozen low melting by implanting in advance a wire heater into the liquid metal Li et al 18 investigated a novel method to signifi cantly lower the chip temperature using liquid gallium as the cooling fl uid and obtained very attractive results Deng and Liu 19 20 presented a MEMS based microcooling device usingliquidmetalandnumericallysimulatedthethree http dx doi org 10 1016 j ijheatmasstransfer 2014 01 059 0017 9310 2014 Elsevier Ltd All rights reserved Corresponding author Tel 86 023 68759673 fax 86 023 65552461 E mail address songsh0915 S Song International Journal of Heat and Mass Transfer 73 2014 21 28 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage dimensional heat transfer process involved in the cooling chip de signed and tested the cooling system of LED lamp with the liquid GaIn20alloy Both the experiments and theoretical analysis indi cated that liquid metal cooling was a powerful way for heat dissi pation of high power electronic devices USA Nanocooler Company carried through subsequently study and made some progress 13 Ghoshal et al 21 designed a chip cooling equipment using liquid metal with high thermal conductivity which realized 200 W cm2 heat dissipation capability On the other hand many studies have been made in the past on the heat transfer enhancement of LFTF because of its big apparent specifi c heat Goel et al 22 carried out an experimental investigation to evaluate the heat transfer characteristics of microencapsulated phase change material slurry using water as carrying fl uid MEPCM W slurry in a laminar tube fl ow under constant fl ux Yamagishi et al 23 presented a detailed description of MEPCM properties construction of an experimental rig Wang et al 24 and Chen et al 25 26 investigated experi mentally the laminar rheological characteristics and convective heat transfer behaviors of MEPCM W slurries in a horizontal circu lar tube under constant heat fl ux Hu and Zhang 27 and Zhang et al 28 29 investigated numerically the fl ow and heat transfer process of MEPCM W slurry for a hydrodynamically fully devel oped fl ow by using an effective specifi c heat capacity model and internal heat source model introduced a modifi ed expression of lo cal convective heat transfer coeffi cient to enable evaluation for the convective heat transfer of MEPCM slurry and analyzed the vari ous factors infl uencing the heat transfer enhancement It was found that the MEPCM W slurry can be considered as Newtonian fl uid The Nusselt number for the MEPCM W slurry is from 1 5 to 4 times higher than for pure water fl ow while the viscosity is about 5 57 times of that of water The Stefan number and MEPCM con centration are the most important parameters infl uencing the heat transfer enhancement of phase change slurries All research results show that liquid metal and MEPCM W slur ry could enhance the convective heat transfer because of the high heat conductivity of liquid metal and the big apparent specifi c heat of MEPCM slurry respectively However the small specifi c heat of liquid metal and the low heat conductivity of the conventional MEPCM W slurry are disadvantageous to enhance the convective heat transfer Therefore we proposed a novel and powerful way to enhance the convective heat transfer that MEPCM particles are mixed into liquid metal to gain a kind of MEPCM LM slurry with high heat conductivity and big apparent specifi c heat synchro nously 30 and investigated numerically the convective heat transfer enhancement of MEPCM LM slurry in laminar fl ow under constant heat fl ux 31 The results show that the heat transfer ability of MEPCM LM slurry is better than that of MEPCM W slurry and liquid metal The above references show that there is an absence of experi mental research on laminar convective heat transfer of the MPCM LM slurry In order to better understand the mechanisms of the laminar heat transfer of the novel MEPCM LM slurry in this paper an experimental system which was built to study the fl ow and heat transfer characteristics is described The laminar friction and heat transfer characteristics of the MEPCM LM slurry in circu lar tube under constant wall heat fl ux in the hydraulic fully devel oped region are investigated including the effects of volume concentration Re and heat rate on the internal wall temperature and local convective heat transfer coeffi cient The experimental re sults offer important basic for the further practical application of MEPCM LM slurry 2 Physical properties of MEPCM LM slurry 2 1 MEPCM LM slurry The microencapsulated Eicosane C20H42 with melamine formaldehyde shell was prepared by in situ polymerization The melting temperature and fusion heat of analytical grade Eicosane were Tm 36 4 C L 246 J g respectively and the core shell mass ratio was about 0 72 The volume average diameter of MEPCM par ticles was found to be 12lm Because liquid gallium was adopted as carrying fl uid the surface character of initial MEPCM was chan ged by electroless copper plating for obtaining compatible metal Nomenclature cvolume concentration of the copperized MEPCM cmmass concentration of the copperized MEPCM cp specifi c heat capacity J kg 1K 1 dparticle diameter m tube diameter m fFanning friction factor h modifi edconvectiveheattransfer coeffi cient W m 2K 1 Icurrent A kthermal conductivity W m 1K 1 Llatent heat of fusion kJ kg 1 length of test tube m m mass fl ow rate NuNusselt number PrPrandtl number DPpressure drop Pa q heat fl ux density w m 2 Qheat rate W Qv volume fl ux of slurry m3s 1 r0inner radius of the test tube m ReReynolds number Ttemperature C uvelocity m s 1 Uvoltage drop V Vvolume of slurry m3 xmass ration at axial position m Greek symbols qdensity kg m 3 uvolume ratio dthickness of Cu plating layer ldynamic viscosity mPa s cshear rate s 1 hdimensionless inner wall temperature stime s Subscripts b bulk fl uid slurry b0slurry without phase change Cucopper plating layer fliquid gallium iinlet initialno copperized mmelting point oexternal wall surface outlet pparticle wwall 22S Song et al International Journal of Heat and Mass Transfer 73 2014 21 28 character with liquid gallium The copperized MEPCM was mixed into liquid gallium by being mixed round mechanically and being librated ultrasonically then the even and steady MEPCM LM slurry was obtained for experimental study in this paper 2 2 Physical properties 2 2 1 Theoretic calculation The density and specifi c heat of the copperized MEPCM are cal culated using mass and energy conservation respectively The physical properties of the initial MEPCM in Ref 30 are adopted The volume ratio of initial MEPCM in copperized MEPCMuCuis calculated as follows uCu 1 dp initial dp initial 2d 3 1 where dp initialis the diameter of the initial MEPCM and d is the thickness of Cu plating layer The density of the copperized MEPCMqpcan be calculated as follows qp qCuuCu qp initial 1 uCu 2 whereqCuis the density of Cu Then the density of MEPCM LM slurry can be expressed as follows qb qf 1 c cqp 3 where c is the volume concentration of the copperized MEPCM and qfis the density of liquid gallium The specifi c heat of the copperized MEPCM cp pis calculated as follows cp p cp CuxCu cp p initial 1 xCu 4 where cp Cu cp p initial are the specifi c heat of Cu and initial MEPCM respectively and xCuis the mass ration of Cu plating layer xCu uCu qCu qp The specifi c heat of MEPCM LM slurry at non phase change state cpb0can be written as cpb0 cp pcm cp f 1 cm 5 where cp f is the specifi c heat of liquid gallium and cmis the mass concentration of the copperized MEPCM cm c qp qb The thermal conductivity of the copperized MEPCM kpis given by 24 1 kpdp 1 kp initialdp initial dp dp initial kCudpdp initial 6 where dpis the diameter of the copperized MEPCM dp dp initial 2d kCu kp initialare the thermalconductivity of the Cu and initial MEPCM respectively Then the thermal conductivity of MEPCM LM slurry can be evaluated using Maxwell s relation 24 as follows kb kf 2 kp kf 2c kp kf 1 2 kp kf c kp kf 1 7 where kfis the thermal conductivity of liquid gallium 2 2 2 Experimental test The phase transition point and latent heat of the MEPCM LM slurry were measured by NETZSCH STA409PC with heating rate of 5 K min from 20 C to 110 C In Fig 1 it begins to melt at 36 6 C and ends at 44 2 C for different volume concentration of MEPCM The fusion heat is 4 979 J g 6 307 J g 7 817 J g for c 10 c 15 and c 20 slurry respectively It is found that fu sion heat of slurry increases with increasing volume concentration of the MEPCM while phase change temperature range is same for different volume concentration slurry The dynamic viscosity of MEPCM LM slurry has important ef fects on laminar convective heat transfer of slurry and is relative to volume concentration and temperature of slurry The dynamic viscosities of MEPCM LM slurry with c 10 c 15 and c 20 were measured by viscosity meter NXS 11A The relation be tween viscosities of three samples and shear rates at 40 C and 50 C is shown in Fig 2 It is shown in Fig 2 that the MEPCM LM slurry can be considered as a Newtonian fl uid because the dynamic viscosity values are approximately constant as the shear rates change The dynamic viscosity increases with increasing volume concentration For the same concentration the dynamic viscosity reduces as the temperature increases The correlative physical properties of the MEPCM LM slurry with different concentration based on theoretic calculation and experimental test are listed in Table 1 3 Flow and heat transfer experiment 3 1 Experimental system The experimental system is shown in Fig 3 a heat transfer test section was designed to measure the convective heat transfer and pressure drop of MEPCM LM slurry and an entrance section was Fig 1 DSC for MEPCM LM slurry with different volume concentration Fig 2 The viscosities of MEPCM LM slurry vs shear rates S Song et al International Journal of Heat and Mass Transfer 73 2014 21 2823 designed to ensure fully developed laminar fl ow in test section The MEPCM LM slurry fl owed from the steel reservoir in water bath to entrance section and test section by peristaltic pump then fl owed back to reservoir after being cooled in the heat exchanger in water bath with refrigeration The fl ow velocity can be controlled by adjusting the rotate speed of pump The water temperature of water bath was controlled at 35 0 5 C to ensure all MEPCM par ticles have no phase change in the reservoir and entrance of the test section considering melting temperature of 36 6 C In the slurry reservoir a stirrer was installed to keep MEPCM LM slurry homogeneous In order to ensure all MEPCM particles in heat ex changer to fi nish the solidifi cation process and gallium to keep li quid state the temperature of the cooling water in water bath with refrigeration was controlled at 30 0 5 C considering gallium s solidifi cation temperature of 29 7 C The entrance and test section were two circular copper tubes of about 4 mm inside diameter and 1 mm wall thickness and were 600 mm and 300 mm length respectively The test section was heated by insulated nichrome wire coiled around the outside of the test tube and the total wire resistance was 9 355X The heat rate of wire was manually controlled by the transformer and determined by voltage drop and current across the wire Test sec tion was fi tted with Tefl on fl ange for isolating electrically and ther mally from the adjoining entrance section To minimize the heat loss to the ambient environment the entrance and test section were thermally insulated by rock wool insulation material with a thickness of 20 mm All connected tubes were circular silicone tubes of about 4 mm inside diameter and 1 mm wall thickness Eight T type thermocouples were fi xed on the external surface of test tube by thermal conducted and electrical insulated silica gel at position s of 0 01 0 05 0 09 0 13 0 17 0 21 0 25 0 29 m from the beginning of the test tube respectively The temperatures of inlet of the entrance section and outlets of the test section and the heat exchanger were measured by three T type thermocouples inserted in the tube All temperature data were recorded directly to the computer through a data acquisition system Fluke2620A Pressure drop was measured across the test section using an elec tronic differential transducer with an accuracy of better than 0 01 kPa The fl ow fl ux of slurry was calculated by cubage method using stopwatch and measuring cylinder at acylic fl ow 3 2 Data reduction and error analysis The Fanning friction factor can be calculated by measured pres sure drop based on the following expression f DP 1 2qu 2 2r0 4L p2Dpr5 0 Q2 mL 8 where r0and L are inner radius and length of test tube respectively u and Qm are velocity and volume fl ux of slurry respectively The wall heat fl ux can be calculated as follows qw UI 2pr0L 9 where U and I are the voltage drop and current across the wire respectively The inner wall surface temperature Tw xwas derived from external wall surface temperature Two xby one dimension thermal resistance model The result was given as Tw x Two x ln rw r0 2pkw UI L 10 where rw kware external diameter and thermal conductivity of cop per test tube respectively For singe phase fl uid in circular tube with constant heat fl ux the fl uid temperature can be written as follows Tb x Tb i Tb o Tb i L x 11 where Tb i Tb o are temperature of fl uid at inlet and outlet of test sec tion respectively x is the axial position Then the traditional local Nusselt number can be expressed as follows Nux qw Tw x Tb x 2r0 kb 12 The internal fl ow and convective heat transfer of LFTF are af fectedby temperaturedifference whichpresents stronglynonlinear because the apparent specifi c heat is strongly temperature depen dent in phase change temperature range thus it maybe occurs that convective heat transfer ability is enhanced with decreasing the conventionalconvectiveheat