The Study of Inverted V-Type Heat Pipe Cooling Mechanism Based on High Heat Flux Heating Elements

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1 th IHPS, Taipei, Taian, No. 6-9, The Study of Inerted V-Type Heat Pipe Cooling Mechanism Based on High Heat Flux Heating Elements Li Jing, Ji Shengtao, Yao Zemin The Key Lab of Enhanced Heat Transfer and Energy Conseration, Ministry of Education, School of Chemistry and Chemical Engineering, South China Uniersity of Technology, No. 38 WuShan Road, Guangzhou 56, PR China Tel : , ljing@scut.edu.cn ABSTRACT Currently, it is difficult to achiee cooling requirements only depend on the mean of natural conection finned heat sinks hen the cooling object are the high heat flux of heating elements. In order to let highly integrated heating element to maintain in the safe range of orking temperature, a ne structure of inerted V-graity heat pipe radiator is proposed. Comprehensie analysis of thermal resistance of the model, study the distribution of the temperature and elocity field of single air flo in use of the numerical simulation softare (CFD). The results of calculation and simulation sho that, the thermal resistance mainly reflect on the resistance of metal materials and the air conection thermal resistance of heat fins; The total heat capacity of the model is 9.W,hich oerall size is (mm),it can control the temperature of.w high components heating element in 36. ; With the obious cooling effect, and distribution of temperature, the resistance of air flo is large, and the flo structures need to be further optimized. Key ords: high heat flux; graity heat pipe; numerical simulation; heat transfer enhancement. INTRODUCTION As technology adances, electronic components hae rapidly deeloped toard to the direction of high integration and intensie [], so that small size carrier can load more heat; heat flux increase and the temperature ill rise rapidly. The degree of heat dissipation speed directly impact on the operating temperature of electric components and affect its performance and serice life.for example, hen LED chip temperature rise to about, its performance ill be seerely affected[-3]. Therefore high heat flux dissipation of electronic components is the key technologies needed soled hen electronic components toard to high integration. Heat pipe radiator [-6] use the high latent heat of medium to transfer heat hen the medium aporize from liquid to apor, and the heat ill shed from the heat source by means of heat conection flo of air through the radiator fins. Using a appropriate structure, filled into the appropriate orking medium to ensure that the heat pipe ork normally under a certain degree of heat flux, Calculating the mass of orking medium by a reasonable means, using a appropriate structure to ensure that heat pipe ill ork normally under a certain degree of heat flux, On the heating elements in high-poer electronics components, the ay of heat transfer of heat pipe hae gradually been a practical and efficient and ith good deelopment prospects heat dissipation type. In the current heat dissipation field of electronic components, natural conection fin-type radiators [7] are most idely used, but it can not reach the heat dissipation requirement of high-poer electronic components or ill lead to a heay body and consume more materials for a mid-poer electronic components. The article for the shortage of cooling problem of poer electronic components, design the inerted V-type heat pipe radiator, ith a hole caity structure, it not only reduce the poor closed chance beteen heat pipe and substrate, but also ensure the liquid medium can coer the substrate eenly,increase the heat transfer area beteen substrate and medium, reduce the star-up time of heat pipe. This paper focus on the heat dissipation shortage of high-poer electronic components, improing heat transfer and heat dissipation rate, reducing the operating temperature of electronic components as a starting point; propose a ne type of inerted V-shape heat pipe structure, making a theoretical calculation of its mechanism, using the fluid dynamic CFD softare to simulate the three-dimensional parameters of the air flo

2 . HEAT TRANSFER MECHANISM AND MODELING OF HEAT PIPE. Heat transfer mechanism of heat pipe Figure is a operating principle diagram of heat pipe radiator hich structure consist of eaporator section, adiabatic section and condenser section; orking medium absorbs the heat in the eaporator to generate heat and apor, apor moe from the eaporator to the condenser section under the small pressure in the heat pipe, the apor ill release the latent heat hen it condenses in the condenser section, then heat is transferred to all and fins of condenser section, heat shed from the face of the fins by air conection on faces of the fins, cold liquid flo back to the eaporator by driing forces and lastly complete a mass transfer and heat transfer loop, hence reach the purpose of enhance heat transfer by the cyclical moement of flo. Qh Eaporator Adiabatic Section Qc Condenser Section Fig.Working schematic of heat pipe. Structure of conerted V-type heat pipe radiator Basing on the analysis of thermal resistance and the research of heat transfer performance of the heat pipe, breaking the model of numerous pipes embedding in the metal block [8], e designed a kind of inerted V-flo radiator, hich has an integrated structure. As figure shos. The caity of apor flo and the substrate of heat radiator are as a hole; the internal orking mediums completely coer the substrate and contact the substrate directly, Increased the contact area of orking fluid ith the substrate in directly, reducing the thermal resistance beteen apor section and metal block. In the inerted V-type heat pipe, the channel structure hich angle beteen apor flo and all is less than 8º; According to the cooperatie field theory [9], this structure enhances the impact beteen apor and all of condenser section, improe the heat transfer coefficient of heat pipe, enhance heat transfer and dissipation so that increase the heat transfer effect. Fig. Inerted V-type heat pipe radiator Focus on the heat dissipation mechanism and performances of.w heat pipe, as figure shos, the substrate size is 5 9 5(mm); liquid storage chamber size is (mm); the height of the flo caity is 5mm;all thickness is.5mm,length is 5mm, height range; it require that heat source temperature should be controlled under 5,ambient temperature NUMERICAL CALCULATIONS OF CONVERTED V-TYPE HEAT PIPE 3. Thermal resistance of conerted V-type heat pipe According to Neton la of cooling, the total capacity of heat pipe radiator can be expressed as: Q α A T () Where: Q is the total cooling capacity of heat pipe radiator, W; α is the total heat transfer coefficient of heat pipe radiator, W/m² s; A is the heat transfer area of conection beteen outside all and air, m²; Enables T as heat source temperature and enironmental temperature difference alue, i.e. T t s - t a. To enhance heat transfer performance of heat pipe radiator mainly proceeds from increasing the oerall heat transfer coefficient, as sho (): Q α A T A T A R () Q In equation ():R is total thermal resistance of

3 heat pipe radiator, /W; as it is shoing in equation (), the heat transfer area is nearly the same; the method to Improe radiator total heat transfer coefficient is to reduce thermal resistance R. The resistance of inerted V-type heat pipe radiator consist of the thermal resistance of substrate R j, the thermal resistance beteen substrate of eaporation section and the orking medium R bc, the thermal resistance of medium eaporation R b, transmission resistance of orking medium R gs, condensation resistance of orking medium R, transmission resistance beteen internal all of condensation section and orking mediumr c, the thermal resistance of outer all R, thermal resistance of conection beteen outer all and surfaces of fins of condensation section and air R a. Because of temperature drop of orking medium from eaporation to condensation is small, R b, R gs, R can be ignored. Then the total thermal resistance of heat pipe radiator can be expressed as: R R j R bc R lc R R a (3) La of Fourier is the basic la of heat conduction hile heat transfer of conection can be described by cooling la of Neton, hence conductie resistance can be expressed as: b R and thermal resistance can be expressed κ A as : R,so e can achiee: ha δ j R κ A h A j j bc bc δ h A κ A h A lc lc Select aluminum alloy as the heat pipe material. κ A j is the thermal conductiity and area of j hbc aluminum substrate, Abc are heat transfer coefficient of conection beteen substrate and orking medium and heat transfer area, h lc A lc are the heat transfer coefficient of conection beteen internal all of condenser section and orking medium and heat transfer area; a a () κ A is heat conductie coefficient of condenser all and heat conductie area, h a A a are heat transfer coefficient of conection beteen outer all and fins of condenser section and air and heat transfer area. Among them,the unit of heat conductie coefficient is /(m ), the unit of heat transfer coefficient is W/(m ), the unit of area is m. According to size of the radiator, e can kno that A j A bc, A A a, A lc A. 3. Calculation of heat transfer capability Quasi-natural conection heat transfer correlation number (, Pr) b Nu A Gr (5) hl Nu (6) λ 3 gl β tρ Gr (7) µ c p µ Pr (8) λ ρ ρ β (9) ρ t Where: Nu, Gr, Pr are Nusselt number, Grashof number, Prandtl number; β is the expansion coefficient of air, - ; h is the heat transfer coefficient, W / (m ); λ is the thermal conductiity coefficient of aluminum, W / (m ); µ is the dynamic iscosity of air at aerage temperature, Pa s; ρ is the density of the air at aerage temperature, kg/m 3 ; Cp is heat capacity at constant pressure, kj / (kg ), it can be ieed as a constant hen the temperature has a little change; ρ ρ are the density at ambient temperature ta and fins surfaces temperature t, tt -t a. According to empirical correlation of natural conection, e can derie the heat transfer coefficients of all of radiator:.677pr h (.95 Pr) L Gr λ () equialent eigenalue of L is.8m, conductie coefficient of air λ.68w/(m );alue of Pr is.7; heat transfer area of internal all of eaporation section A bc.33m,the apor temperature in the pipe is nearly the same, the thermal resistance can be ignored, thickness of condenser section is.5m, the difference beteen apor saturation temperature and out all temperature t.3,ambient temperature

4 t a 5, β.3 -. aerage temperature is (55)/37.5,the property of air at aerage temperature are: c p.7 kj/(kg ), µ Pa s, λ.68w/(m ), ρ.36kg/m3.heat transfer coefficient of aluminum alloy κ 37 W/(m ); heat transfer coefficient of conection of air h7.7w/(m ). The boiling heat transfer coefficient of Internal caity is appropriate the same as that of apor condense, hence heat transfer coefficient of surface can be expressed as ; h bc 3 gρl λl r.3 µ l H s ( t t ) () Where: H is the height of internal caity, its unit is m; λ l is the thermal conductiity of liquid orking medium, W/m ;ρ l is the density of liquid orking medium, kg/m³,µ l is iscosity of the liquid orking medium, Pa s, r is the latent heat of aporization of orking medium,j kg - ; t s is the saturation temperature of apor; t is the all temperature,. Pressure of internal caity is 9.5kPa, H.5mm, parameter of orking medium: λ l.5w/m, ρ l 75.6 kg/m 3, µ l Pa s,r7.76kj/kg Heat transfer per unit area is.355 W/cm. Heat transfer capability is much higher than that released from heat source; the radiator can better meet the requirement of heat dissipation. Using the radiator to dissipate heat from a.w heat source, the steady temperature of heat source is: Q. t t a C α A Making a analysis of the calculation process, e can kno that the total thermal resistance of heat pipe is mainly consist of thermal resistance of metal materials and thermal resistance of conection beteen face of radiator and air, and the thermal resistance of orking medium in the pipe can be ignored.. NUMERICAL SIMULATION OF SINGLE-CHANNEL In this paper, the temperature of conerted V-type radiator and air elocity distribution can be analysis by using CFD softare. To simplify the model, e take one part of it as the model, and gie it an analysis; as shon in figure 3, to adjust the parameter of air flo and so on e made an analysis of performance of heat transfer and air flo of the ne kind heat pipe radiator T h bc W / m C otal thermal resistance of radiator:.5 R C/W Total heat transfer coefficient: α 7.6W / m C A R Total heat transferred: Q α A T W Fig3. Single channel of air flo

5 According to the temperature hen the poer of heat source is.w, the internal all temperature of heat pipe is set at 35 heat source, using the second-order upind discretization scheme and the simple pressure-elocity coupling algorithm to simulate the flo of a single heat pipe. The ambient temperature is 5. As shon in figure, it is a single flo channel and its temperature of the cross-section hen z75mm, e can see that: surface temperature of heat pipe is higher than other parts, heat is transferred from surface of heat pipe to fins. As shon obiously in the figure, the temperature decrease from the surface of heat pipe to the end of fins, and has an obious temperature gradient. As the size of fins increase from the bottom up, temperature of short fins are nearly the same as surface temperature of heat pipe, the temperature begin to change gradually hen the size of fins reach a certain degree, the temperature difference beteen both end is about. hich has a good match ith alue of theoretical calculation, hich further confirmed the high heat transfer performance of the ne kind heat pipe radiator. dissipation. Because of the size limit of flo channel of ne kind heat pipe radiator, the air flo resistance inside is large, the air flo is lagging behind beteen fins in the same all, affecting the heat transfer performance seriously, a further structure optimize should be made to enhance heat transfer of heat pipe radiator. Fig5.Cloud and ector of elocity at the section of z75mm(m/s) 5. CONCLUSIONS ()Through theoretical analysis, the thermal resistance of heat pipe radiator mainly appeared in the thermal resistance of metal material, and the conection heat resistance of the surface of heat pipe all and the fin, the thermal resistance of the eaporation and condensation of the orking fluid, and the heat transfer resistance of the steam and the pipe all can be ignored. Fig. Temperature distribution of single air flo channel and the section of z75mm( ) Figure 5 shos the elocity cloud and elocity ector of air flo at the interface hen z75mm; as the space of air flo is small, the resistance of flo is large; hen elocity of air flo is small and the height increase, flo resistance decrease, elocity of mid-part increase obiously, and the elocity reach.8m/s at outlet of the top. Air flo into heat pipe radiator from both ends by buoyancy of itself and pressure difference of beteen inner and outside, to fluxes ill be mixed in middle of flo channel, then it flo in a ertical direction and the heat of all can be transferred aay so that reach the purpose of heat ()Inerted V-type heat pipe radiator ith high heat dissipation, the maximum theoretical heat transfer of unit area is.355 W/cm, can control the temperature of poer-type heat source in the loer alue; hen the flux density is.w, the heat source temperature can be controlled at 36.. (3)According to the numerical simulation analysis, the effect of the inerted V-type heat pipe radiator is obious, the temperature distribution, but the air flo resistance is higher, so the flo structures need to be further optimized. REFERENCES [] Daoping LIU. The deelopment trend of thermal problems. Energy Research and Information.996,(3):

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