Study on Effect of Different Porosity on Thermally Driven Heat Transfer in a Centrifugal Force Field

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1 5th Internatonal Conference on Cvl Engneerng and Transportaton (ICCET 015) Study on Effect of Dfferent Porosty on Thermally Drven Heat Transfer n a Centrfugal Force Feld 1 Xa Je1,a*, Chang Ha-png,b, and Tang Hou-xang3,c Department of Theory Tranng, Flght Drllmaster Tranng Base, Chna College of Energy and Power Engneerng, Nanng Unversty of Aeronautcs and Astronautcs, Chna 3 Department of Basc Course, Automoble Management Insttute, Chna a xx_chna@163.com, bchppe@nuaa.edu.cn, ctanghouxang@163.com Key words: porous medum; porosty; thermally drven Abstract: Based on the exploture of turbne blade super coolng technology, porous medum s nstalled n a new knd of coolng confguraton wth coolng tunnels. Experments and numercal smulatons are carred out to nvestgate the thermally drven heat transfer rules n the new knd of coolng confguraton wth dfferent porosty n a centrfugal force feld. The results of experments and numercal smulatons are consstent bascally. The results of study show that the heat transfer rules of the new coolng confguraton are dentcal under dfferent porosty. The thermally drven heat transfer of the coolng confguraton flled wth porous medum can be enhanced wth ncrease of the rotate speed, heat flux and coolng ar speed. At the same tme, the heat transfer effect can be weakened wth ncrease of the porosty under hgh porosty. Introducton That f ncreasng gas s nlet temperature effectvely and relably s the root of enhancng thermal effcency and thrust-weght rato accordng to gas turbne s thermal crculaton. Turbne blade s coolng desgn wll be the key to solve queston to reach future hgh-powered engne s technology ndex. Focusng on the stuaton, Professor Chang Ha-png and Academcan Guo Zeng-yuan brought up a new coolng technque based on the theory of thermal drve [1]. Slght coolng channels are the core of the technque. Coolng medum s put nto the slght coolng channels. Porous medum s nstalled n channels to enhance slght channels structure ntensty. At the same tme, flowng of the flud n the porous medum exst dsperson effect. It can make molecular groups mxed radally, make radal temperature dstrbuton unform and even, and eventually enhance heat exchange evdently [-4]. Along wth enhancng heat exchange, resstance to flow also augments a lot. But the ncreasng resstance can be overcome wth enough buoyancy. Based on the fundamental study[5-7], n ths paper, experments and numercal smulatons were carred out to nvestgate the thermally drven heat transfer rules n the new knd of coolng confguraton wth dfferent porosty and compare ther heat transfer effect. Expermental Research Expermental Faclty Fg1 s a scheme of expermental apparatus. The whole expermental apparatus contans power system, heatng system, coolng system and measurng system, etc. Coolng ar enters hollow shaft, passes gas collecton case nto expermental model, and streams out of the turntable. There are two layers hole boards n the gas collecton case to make gas enter the expermental model equably to smulate the turbne blade s work state. Fg and Fg3 are photos and sketch map of the testpeces. In order to obtan the closed channels llustrated n Fg3, one body shell, fve nner hulls and two end plates were manufactured. In the closed channels, foamed metals made wth colomony are used n the porous medum area(porosty s 98% and 93%), 015. The authors - Publshed by Atlants Press 683

2 whch s llustrated concretely n Fg. The external dmenson of the testpeces s 50mm*36mm*38mm. Except that the wdth of the porous medum channel s 3mm, the wdth of all the others s 1mm. Contract grant sponsors: Natonal Natural Scence Foundaton of Chna(507607) and the Foundatonal Research of the Natonal Defence (J1400D001) In the experments, the heatng theca posted on the body shell heats the cycle channels, and adustng electrc current can create dfferent heat. At the same tme, coolng ar goes through the nner hulls to form coolng channels, and takes away the heat dscharged by cycle flud. Adustng the ar flux can create dfferent coolng ar speed. The operatonal range of the experments ncludes: heat flux ( w/m ), coolng ar speed ( m/s) and rotate speed ( r/mn). In order to research the thermo-drvng heat transfer effectveness of flud n the closed channels nstalled wth porous medum, the author defnes a parameter (KH) of capacty of thermo-drvng. It characterzes the capacty of the flud n the closedchannels, whch takes heat to the coolng end wth the crcular flow of thermo-drvng. The concrete defnton ust as follows: q KH T max T mn 1.belt pulley 3.turntable skn 5.fexble couplng Where, q s heat flux of the heatng surface, T max and T mn respectvely s the hghest and lowest temperature measured from the surface of the walls along wth the orentaton of the crcular flow of thermo-drvng. Expermental Results and Dscusson Fg4, Fg5 and Fg6 llustrate the dagrams that the Fg. Photos of expermental mode temperature on the heatng surface changes along wth heat flux, rotatng speed and ntake velocty of the coolng ar, here the porosty s 93% n porous area. As llustrated n Fg4, wth the same heat flux densty, the temperature of the heatng end along wth the postve drecton of the Y axs consstently falls, because after absorbng heat, the temperature of flud ncreases and the densty decreases, and the flud affected by centrfugal force moves along wth the negatve drecton of the Y axs. That demonstrates the flow of thermo-drvng takes place n the enclosure after beng flled wth porous medum (If there s no thermo-drvng but only Fg.3 Schematc of expermental model thermo-conducton, the temperature on the heatng surface wll be unformty, and along wth the Y axs wll have no change. The temperature of the heatng end along wth the postve drecton of the Y axs consstently ncreases under rotaton. That s contrary to the stuaton under no rotaton.). Furthermore, the temperature at all the measurng ponts ncrease n varyng degrees along wth ncrease of heat flux. The hgher the heat flux, the greater the temperature dfference between the hghest and lowest measurng ponts. That demonstrates heat flux ncreases and thermo-drvng capacty mproves and then thermo-drvng heat transfer effectveness contnuously rses. Fg5 and Fg6 llustrate the temperature at all the measurng ponts decrease and the temperature dfference between the hghest and lowest measurng ponts decreases. That demonstrates 684 ar nlet ar outlet ar outlet 10 7.turntable 9.expermental model Fg.1 Schematc of expermental apparatus

3 rotatng speed (centrfugal acceleraton) or ntake velocty of the coolng ar ncreases, thermo-drvng heat transfer effectveness also rses n=900rpm 5390W/m 1100W/m 1600W/m 33700W/m 48500W/m q=1600w/m 300rpm 540rpm 780rpm 100rpm 1380rpm Temperature(k) Temperature(K) Y Dstance(mm) Y Dstance(mm) Fg.4 Curves of temperature varyng wth heat fluxes Fg.5 Curves of temperature varyng wth rotatng speeds Temperature(K) 36 q=1600w/m n=660rpm Y Dstance(mm) 3.1m/s 37.4m/s 48.1m/s 64.1m/s 96.m/s Fg.6 Curves of temperature varyng wth coolng ar speeds q=1600w/m ε =0.93% ε =0.98% n=900rpm ε =0.93% ε =0.98% q=1600w/m n=660rpm q(w/m ) Fg.7 Curves of KH varyng wth heat fluxes under two knds of porosty ε =93% ε =98% n(rpm) v(m/s) Fg.8 Curves of KH varyng wth rotatng speeds Fg.9 Curves of KH varyng wth coolng ar speeds under two knds of porosty under two knds of porosty The paper researches the thermo-drvng heat transfer n the enclosure wth porous medum whose porosty s 93%, meanwhle, t makes a comparson between the results and those porosty s 98%. The 685

4 research demonstrates the enclosure has the same temperature dstrbuton rule under two knds of porosty. So, the temperature dstrbuton chart when the porosty s 98% s not gven n ths paper. Fg7, Fg8 and Fg9 respectvely demonstrate the dagrams of the comparson of the parameters of thermo-drvng heat transfer capacty between two knds of porosty when heat flux, rotatng speed and coolng ar speed change. The fgures llustrate that the parameter of thermo-drvng heat transfer capacty ncreases and thermo-drvng capacty mproves along wth ncrease of heat flux, rotatng speed and coolng ar speed under two knds of porosty. Although the heat transfer rule s the same, the enclosure wth 93% porosty has much better thermo-drvng heat transfer effectveness than the other. The reason s as follows. In porous medum area, sold medum s heat conducton coeffcent s hgher than the flud n ths examnaton. So, sold medum s heat conducton and convecton heat transfer between sold medum and flud have very mportant acton n porous medum area. The contact area between sold medum and flud ncreases along wth decrease of porosty. Consequently, t strengthens convecton heat transfer n porous medum area and mproves the enclosure s thermo-drvng heat transfer capacty and effectveness. Numercal Research Mathematcal Model In order to fnd out the flud s flow and heat transfer character, flow and temperature feld n the enclosure wth porous medum and the flud s thermo-drvng heat transfer capacty under dfferent porosty, the paper carres out numercal smulaton. The paper sets up mathematcal model accordng to expermental model s sze n numercal smulaton. Basc equatons are as follows. no porous medum u 0 x u u x P x x g u E P T k u x x x wth porous medum u 0 x 1 u u u x E P x P x x x k eff T x 1 ek e uk R x eff 1 u g ek e uk R x u eff s gyral angle velocty; s porosty; k eff s effectve heat conducton coeffcent(k eff =k f +(1-)k s, k f s flud heat conducton coeffcent, k s s sold heat conducton coeffcent); E s flud nner enerry; s the nfltraton rate of porous medum. The boundary condtons accordng to expermental parameters are as follows: Heatng surface s supposed changeless heat flux; coolng surface carres through heat transfer wth lqud n the enclosure and coolng ar, so t s supposed couplng surface; the nlet and outlet surface of the coolng ar channel are supposed velocty nlet and pressure outlet; all the other surfaces are supposed as perfect adabatc walls. In the numercal smulaton we adopt Gambt to buld grd and Fluent s Coupled Implct Steady Solver. Because there s shearng stran n flud feld under rotaton, we adopt RNG k- model and Two-Layer Model near wall. 686

5 Numercal Results and Analyss Fg10 s temperature dstrbutng of the expermental model. The temperature on the heatng surface along wth the postve drecton of the Y axs falls, whch s consstent wth expermental result. In order to verfy numercal result, the paper compares numercal heatng surface temperature wth expermental result. Fg11 s the comparson of numercal wth expermental result under the same condtons(q=5390w/m, n=900rpm, ). Fg11 llustrates that numercal and expermental result s change trand s the same. Numercal result s fve degrees hgher than expermental result, because the absolute adabatc condton s not realzed n expermentaton. 315 computaton experment Temperature(k) Y Dstance(mm) Fg.10 Temperature dstrbutng of the expermental model Temperature(K) Y Dstance(mm) ε =0.96 ε =0.93 ε =0.9 ε =0.86 ε =0.83 ε = Fg.11The comparson of numercal wth expermental result ε Fg.1 Curves of temperature varyng wth porosty Fg.13 Curves of KH varyng wth porosty The research [9] ndcates that porous medum can strengthen heat transfer, but the flow drag ncreases more when the porosty s less than 0.8. So the desgn of the new coolng heat transfer confguraton takes bg porosty porous medum nto account. It can strengthen heat transfer, but the flow drag does not ncrease much. Therefore, the paper adopts bg porosty porous medum n numercal smulaton( 80%). Fg1 and Fg13 llustrate the dagrams that the temperature on the heatng surface and the parameters of thermo-drvng heat transfer capacty change along wth porosty under the condtons(q=5390w/m, n=900rpm, ). Fg1 llustrates heatng surface temperature and the temperature dfference between the hghest and lowest measurng ponts decreases along wth the decrease of porosty. That demonstrates porosty decreases, thermo-drvng heat transfer capacty rses. Fg13 also proves that. Fg1 also llustrates the declne extent of heatng surface temperature and the temperature dfference between the hghest and lowest measurng ponts decreases along wth the decrease of porosty. Fg13 llustrates the ncrease extent of the 687

6 parameter of thermo-drvng heat transfer capacty decreases along wth the decrease of porosty. These analyses demonstrate thermo-drvng heat transfer capacty ncreases but ts ncrease extent decreases along wth the decrease of porosty. The reason s as follows: when the porosty decreases, the addton of contact area of sold medum and flud strengthens convecton heat transfer, but the ncrease of flow drag weakens convecton heat transfer. It leads to the decrease of ncrease extent of thermo-drvng heat transfer capacty fnally. Concluson (1) The flow of thermo-drvng takes place n the new coolng confguraton after beng flled wth porous medum. () The new coolng confguraton has the same heat transfer rule under dfferent porosty: when rotatng speed, heat flux and ntake velocty of the coolng ar ncreases, ts thermo-drvng heat transfer capacty also rses. (3) The new coolng confguraton s thermo-drvng heat transfer capacty ncreases but ts ncrease extent decreases along wth the decrease of porosty. References [1] Chang H, Guo Z. A new coolng technque used n turbne blade. Inventon patent No. ZL01108, [] M.Varahasamy,R.M.Fand.Heat Transfer by Forced Convecton n Ppes Packed wth Meda Whose Matrces Are Composed of Spheres.Int. [J].Heat Mass Transfer,1996, 39(18): 3931~3847. [3] Hsu.C.T.,Cheng P.. Closure Scheme of the Macroscopc Energy Equaton for Convecton Heat Transfer n Porous Meda.Int. [J].Commu.Heat Transfer, 1988, 15:689~703 [4] Morrson H L et al.[j]. Appl. Phys., 1949,0:107~109 [5] Mao Junku, Chang Hapng, Fang L. ExpermentalInvestgaton of Flow and Heat Transfer About thethermal Drvng of R1 n Rotatng Small-Sze CrcularEnclosures[J].Journal of Aerospace Power,003,18(4): 498~501. [6] Mao Junku, Chang Hapng, Fang L. Investgaton of Thermally Drven n Centrfugal Feld wth Lqud Crystal [J]. Journal of Engneerng Thermophyscs,004,5(): 9~31. [7]Yang Mn, Chang Hapng, Mao Junku. ExpermentalInvestgaton of Thermally Drvng Heat TransferCharacterstcs of HO and R1 n a Centrfugal Force Feld [J].Journal of Engneerng Thermophyscs,004,5(): 30~304. [8] Fluent Users Gude,1998 [9] Renken K.J,Poulkakous D. Experment and Analyss of Forced Convecton Heat Transport n a Packed Bed of Sphere, Int [J].Heat Mass Transfer 1998,31: 1399~