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8 Journal of Donghua University(Eng．Ed．)Vo!．23．No．3(2006) Parametric Influence on Thermal Performance of Flat Plate Closed Loop Pulsating Heat Pipes YANG Hong·hai(杨洪海) ，KHANDEKAR Sameerz，GROLL Manfred 1 College of Environmental Science and Engineering．Dongh∞ University．Shanghai 201 620 2 Institute of Nuclear Technology and Energy System，University of Stuttgart，70569 Stuttgart，Germany TIlis paper presents all experimental study on a flat plate closed loop pulsating heat pipes．It consisted of total 40 channels with square cross sectioa (2× 2 nirll2．165 mm long)machined directly on an aluminum plate(180 X 120 X 3 mJ )，which was covered by a transparent plate．1n1c working fluid employed was ethano1．As the results,the influence parameters of thermal performance were investigated，such as filling ratiot hea t load and operational orientations etc． Filling ratio was found to be a critical parameter，and its effect was rather complicated．According to its values the PHP plate could have four distinct working zoues with differem operational characteristics and heat trailsfer performance．The effect of heat load 0n thermal performance was found to be positive，and ．m general， increa sing the heat loa d would improve heat transfer performance．In order to analyze the effect of gravity on thermal performance，three different heat modes and total seven tilt angles were tested and compared． Successful operation at all orientations wi th respect to gravity was also achieved ． Keywords：fZat plate closed loop pulsating heat pipes， parametric infZUeTIces，heat transfer characteristics． Meandering tube pulsating heat pipes(PHPs)，which are very attractive entrants in the family of heat pipes， have alrea dy found successful applications in cooling power and microelectronicsE due to their simple structures，cost effectiveness and excellent thermal performance．They also have a potential for thermal control application for space and avionics． Since its introduction in 1990’s research activity in this area has steady increased，including various experimental studies and attempts of mathematical modelingE2-4]．Yet reliable experimental data for PHPs is very limited， especially in the wake of the ongoing miniaturization process and efforts to combine electronic compo nents and heat spreaders on the chip leve1．A logical next step for further applications of PHPs in microelectronics coo ling is to design integral struc tures , i．e． PHPs as an integral part of thermal spreader／ substrate，having typical dimensions as applicable for multi· chip modules or PCBsL ．Previous studies suggest some intercsting thermal behavior of plate structures Iike inter- channel heat transfer．contact angle hysteresis，capillary effects due to sharp corners etd- ． These and other ope rational aspects need to be further studied and verified ． Experimental Setup Fig．1 is the photography of the experimental setup．It basically consists of vacuum device, electrical power supply，cooling water cryostat，data logging systemt the visualization part，etc． Fig．1 Photograph of the experimental setup The details of the PHP test section are shown schematically in Fig．2．The specimen was made with an aluminum plate(180× 120× 3 alia0)with 40 parallel rectangular channels milled on it directly，forming a looped serpentine structure．Each channel has the dimension of 1 65×2×2 mm3．A po lycarbonate plate with a transparent silicon gasket was covered tightly on the top of the PHP plate for visualization．A copper heater bloc k (100× 30× 15 mm0)and a water。cooled copper block (100× 6O× 12 mm3)were attached on the back face of the PliP plate． The area be tween the heater and the cold plate formed the Received Nov．9，2005 Supported by the German National Science Foundation(DFG)(No．GR一412／33) Correspondence should be addressed to YANG hong-hai，associate prof．，E-mail：yhh@dhu．edu．cn 维普资讯 http://www.cqvip.com Joorrml of Oon,：lm Universi (的 ．Ed．)Yo1．23，No．3(2006) 9 adiabatic section， where a vacuum／filling tube was provided near the center point． Axial temperature distribution was measured by standard Type．K thermo- couples(ID／OD = 0．5／1．5 ram)． which were factory calibrated and programmed with claimed net accuracy of± 0．5"C．The“Ahlborn-Almemo”programmable universal data logging system(TYPE 5590—1，up t0 50 channe1) was employed to collect the ternpe rature data．The data reporting frequency was always 1 Hz．with a resolution of 0．I oC for temperature．During the expe riment。the total PHP specimen was insulated well except the front face for visualization．Thus the heat loss is kept less than 5％ ．Here the value of the heat input loa d is considered nearly equal to the electrical power． Photograph of the plate Fig．2 Schematic of the PH_P test section HP late Ethanol was used as the working fluid．before charging． the p"late was vacuumed until about 6×10一 mbar． fining ration(is defined as the ration of the liquid volume enclosed in the loops to the total loop volume)ranged from dry structure t0 90％ ． In this study the thermal resistance of the dry structure was also measured as reference values．In"addition． three-heat modes including total seven-tilt angles could be tested ：they were bottom heat mode(30。，60。and 90。)， horizontal heat mode(0。)and top heat ilx~e(一30。，一6o’， 一 90。)respectively． Results and Discussion 1 Division of working zone and operational principle According to the different value of the filling ratio， the PHP plate showed different operational characteristics． Four working zones were divided as shown in Table 1 and Fig．3： (1)Working zone I：when the filling ratio was quite low (1ess than 2O％)．the specimen operated in bottom heat mode．but could not operate in horizontal or anti·gravity top heat mode．There was almost no pulsating action．The specimen worked mainly as two-phase gravity assisted thermosyphon rather than PHPs． (2)Working zonesⅡ：when the filling ratio was be tween 25％ and 40％ ，the specimen ope rated both in bo ttom and horizo ntal heat mode。but it still could not operate in anti·gravity top heat mode．That means in this zone pulsating action played role together with gravity action． (3)Working zone 111：when the filling ratio was in the range of 45％ to 75％ ，the specimen operated rather well in all global orientations． Th at means pulsating action predominated in this working zone，and the specim en worked as true PHP． (4)Working zones Ⅳ ：when the filling ratio was greater than 75％ ，the specimen could operate in bo ttom heat mode and occasionally in horizontal heat mode(it was not always guaranteed)。but it could never operate in anti· gravity top heat mode． Th at means pulsating action decreased again even disappeared in this zone． Table 1 Filling ration and possible operation mode Filling Bottom Horizontal Top rati0 heat nl0de heat In0de heat In0de Zone O％ 5％ 2O％ 3O％ 40％ 5O％ 65％ 8O％ 9O％ 100％ Empty plate，heat transfer by conductivity √ √ Single phase liquid, co nvective flow x／ ＼／ I Ⅱ Ⅲ Ⅳ heat transfer througll nature 2 Comparison of thermal performance， analyses of the influence parameters In this paper，thermal performance was evaluated by average evapo rate tempe rature，thermal resistance and the maximum lim itation of heat input load．Here，thermal resistance was defined as ternpe rature difference be tween evapo rator and condenser divided by heat input load． 2．1 Eff酏t offilling ratio Ollthermal performance As abo ve mentioned ， filling ratio was a critical parameter．For different filling ratio，the aluminum P臁 showed different operational characteristics． Hence thermal pe rformance changed greatly with filling ratio as shown in Fig．3．It co uld be found that： (1)In working zone I (filling ratio．5％一20％)，the specim en showed relative smaller thermal resistance and 维普资讯 http://www.cqvip.com 1O Journal of Donghua University(Eng．Ed．)Vo／．23，No．3(2006) ： 一-o-IⅢ(X )W[ ‘。m 。 ‘mde 一 (90 一 ~)50W 2 一 · a) + I 一 【lI】 一 -一 一 三 三 ／／ ／厂’＼／ ／／ ＼ ．， 一 一 ～ ： 三；==一 ---4"SS一‘ 一 1 —● E= —： 0 1．20 1．00 0 80 0．60 20 40 60 80 Filling ratio(％) (a)Bottom heat mode +50w Horizontal mode(0。) (b) ．．-iImw 、 ，一 ’ +I50W I ．／ +2tX)W ，250W ＼'／， _．．1IXIW 一 35oW ：一 ／ 一 一 4cmW -_= 0 0．40 兰 0．20 20 40 60 80 Filling ratio( ) (b)Horizontal mod e + 1I)0w Top heatmode(--90。1 (c) + I50w 一 200w 一 250W ‘ 30(1W ●一 一 一 一 350W =：= especially when the filling ratio was in the between of 50％ and 65％ ． The PHPS showed excellent thermal performance in all heat modes and tilt angles (Fig．4)． Comparing the case for filling ratio 50％ and 65％ ， the former showed a little be tter thermal pe rformance． However．their differences were very small except in the case of bottom heat mod e with lower heat input(1ess than 50W )． (4)In working zone IV (filling ratio> 75％)，the specimen showed the poorest thermal pe rformance．There were two probable reasons．One was that for too higher filling ratio，there was no enough free space for bubbles to expand and to play the pumping action．Another one was that the capillary action of sharp angled edges decreased greatly as the filling ratio was too high．Both of them hindered the fluid flow． (5)In the case of bottom heat mod e there existed two optimum filling ratio，they were 15％ and 40％ ，showing relative be tter thermal pe rformance． The former corresponded to the maximum thermosyphon action of the structure：the latter was probable the mutual results of thermosphon action and pulsating action． (6)In the case of horizontal mod e for lower heat load (1ess than 150 W )．40％ was an optimum—filling ratio；but for higher heat load(more than 200 W)．50％ filling ratio showed better thermal performance． It was tentatively analyzed as follows：with the increase of filling ratio，on one hand，the capillary action of sharp corners decreased gradually，but on the contrary，pulsating action increased at first and achieved maximum at 50％ filling ratio，and then decreased again．An other fact was that higher heat input generally enhanced the pulsating action．So there should be an optimum—filling ratio and it changed with heat input and heat mod e． (7)Increase of heat load mitigated the difference of thermal pe rformance caused by the different filling ratio (Fig．4(a))． As abo ve analysis，effect of filling ratio on thermal pe rformance was complicated，it is be cause thermosyphon action，pulsating action，capillary action of sharp corners， which decided the thermal performance were themselves greatly depe nded on the filling ratio． 2．2 Effect of heat input on thermal performance Fig．4 depicts the effect of heat input on the thermal resistance．It was found that increasing heat load markedly improves the thermal performance．Till abo ut 200 W input powers the performance improvement was quite drastic while thereafter it was mild．This tendency was the same for all filling ratios and inclination angles． In genera1．the heat input was the “pump’’for the thermo-fluidic action． Thus increasing the “pumping po wer” increased the pe rformance．At low filling ratios 如 ∞ 舳 ∞ ∞ 如 ∞ ● O O O O O l(芝 一0u写 一s3二duI矗LI 如 ∞ 舳 ∞ ∞ 如 ∞ n n l(芝 一03u皇∽【s0三一 上| LI卜 l(≥～ 一03udlSl∞u_l 维普资讯 http://www.cqvip.com J~ rm l of Doo0／~a University(Eng．Ed．)Vo1．23，No．3(2006) 11 (Zone I—Thermosyphon mode and Zone II—Transition zone)，increasing the heat input made the liquid layer in the counter—current flow thinner． The effective fluid velocity also increased thus enhancing the local wall heat transfer coefficient． In Zo ne 111 operation， bubble pumping action got enhanced due to rapid bubble growth． Also，after a certain heat input，the flow changed from purely capillary slug flow to churn or semi—annular and sometimes to fully annular flow in individual channels．This greatly enhanced the heat transfer coe fficient． Thus ， increasing heat load enhanced the pe rformance till a certain type of dry—out occurs．In Zo ne I，the dry—out occurred as a combination of counter．current flow limitation and insufficient fluid inventory． starving some channels completely In other zones，no dry．out could be observed． Experiments were terminated for safety reasons beyond 120℃ loeal temperatures． 1 20 Heatload(v (a)Bottom heat mode (b)． Horizontal mode(0。) —．-} =27 { —．。l =舯％ FR=∞％ 、 —．。l =6s％ - { ： == ． ． 0 1．oo 0 8O 0．6O 0．40 O 2O O oo 0 2oo 3oo 400 5oo Heatload(v (b)Horizontal mode (c)、 T。p 。 m。 。(一90。 1+rR 。。 、 I二：．!==：：： ＼ 、 i 、 、 、 ： ．．．⋯ O 2oo 3oo 400 500 Heat1oad rW3 (c)Top heat mode Fig．4 Effect of hea t flux on the thermal resistance of the tested structure Fig．5 records the maximum heat input load achievable in three different heat modes for reaching the average eva19orator temperature 100℃ ．it was found that in the case of bottom heat mode，when filling ratio was in a wide range from 10％ to 70％ ，the PHP spe cimen worked rather well and pe rmitted higher heat loads in the case of horizontal and anti—gravity top heat mod es， only when filling ratio was in a relative narrow range from 45％ to 70％ ，the PHP specimen showed higher heat transfer pe rformance． ≥ 一 勺 旦 工 量 E 矗 — — ．_一 Bottom heat mode(90。) — — Horizontal heat mode(0。) — — ， Top heat mode (-90。) Filling ratio Fig．5 Maximum permit heat load(Te~100"C) 2．3 Eff t of heat mode and tilt angle on thermal performance Heat mode evidently affected the operation performance of the aluminum P胁 ．In the case of bo ttom heat mode， the PHP plate ope rated at all filling ratios~in horizontal heat mod e，PHP plate could only ope rate in working zones 11 andⅢ (filling ratio：25％一75％)；in anti—gravity top heat mod e．the PHP plate co uld only ope rate in working zoneⅢ (filling ratio：45％一75％)． Fig．6 quantitatively analyzes the effect of tilt angle on thermal performance． It was found：in the case of vertical bottom heat mode(+90。)，the aluminum PHPs always showed the minimum thermal resistance． which increased as tilt angle changed to horizontal(0。) and then to anti—gravity top heat mode (一90。)． This tendency was as the same as for all filling ratios and heat inputs．The results indicated that the gravity vector really affected the fluid flow and heat transfer characteristics of 一 9O 一 60 —3O 0 30 60 90 Tilt angle(。) (a)Heat input 150W ≥^ ＼p — uI|g∞ 暑E LI卜 加 ∞ 舳 ∞ ∞ 加 ∞ ● ● 0 0 0 0 0 ≥^ ＼p— u g∞ aJ JJ LI卜 ≥^、p一0u LI曼∽ aJ jLlJuL1 LIJ ≥^ ＼p一0u LI≈ 3J互三uc_l_ 维普资讯 http://www.cqvip.com Joorrm／of D哪 』a University(Eng．Ed．)Vo／．23，No．3(2006) Tilt angle(。) (b)Heatinput 250W I I — p 三 芒 ＆ 兰 V- (c) "lop heat mode(--90。) L 1 f - ．，v I - 一 ～ I ／ h_ ' ，。。 150 2001：∞ 300 350 ∞ f HeatinputinW att I—Ave rag~T- (c)Top heat mode Fig 7 Variation of average evaporator temperature with time Fig．6 Effect of heat mode and tilt angle 。“ h。 m 。p。 m “ 。 Conclusion PHPs：in the bottom heat mode，gravity played positive action on the fluid flow thus enhanced heat transfer performance；but in horizontal and anti·gravity top heat mod，it played no action or negative action on the fluid flow thus hindered heat transfer． Fig．7 depicts the typical tempe rature-time history for the average evaporator temperature at filling ratio about 5o％ under three different operating conditions． The fluctuations in the evaporator were the minimum f0r the bottom heat mode．The amplitude increased as the inclination angle changed to the horizontal and then to the top heat mode．OveralI smoother operation was obtained at higher heat loads．Unacceptable variations were recorded at relatively low heat input(1ess than 50 W)in top heat m ooe． ． ■ p 三 芒 ＆ 量 p 三 芒 ＆ 三 (a) Bottom heat mode 90。 I ，～ } ， 、．．- r、、 厂 一—— f ∞ IOO 150 2oo 25o 30o 3∞ 40o 450 f H∞I_m Llin蝴 J — A~fage Time(s) (a)Bottom heat mode 00 【b) Horizontal mode(0‘) 一 I ． ．^ ^ ^‘ 山 } I j ／ 5o 1oo 15O 2oo 250 300 350 400 HeatmpL—m w |Il 『--A坩 9e Time(s) (b)Horizontal mode Experimental study was pe rformed on an alum inum flat plate closed loop pulsating heat pipes，thus the effect of filling ratio，heat Ioad and operational orientation on thermal performance were quantitatively analyzed．Here are the brief summaries： (1) Filling ratio was a critical parameter。and its effect on thermal pe rformance was complicated．According t0 its value，four working zones were divided ．Each one showed the different ope rational characteristics and heat transfer pe rformance．Both the ca pillary action of sharp angles，gravity assisted thermosyphon action and pu lsating action changed with the variation of filling ratio，thus there exist some optimum filling ratios leading to the optimum performance， which also depended on other parameters,e．g．heat load and operational orientations， etc． (2)Heat input Ioad was coilsidered as the source of pumping action，and it played the positive role on the operation of PHPS．In general·increasing the heat input would enhance the fluid flow and heat transfer performance．However，there shou ld exist perform ance lim itations to avoid the occurrence of dry·out， which depended on the mechanism of P唧 s． (3)The gravity vector really affected the fluid flow and heat transfer performance．It played positive action in the case of bottom heat m(x~e (+ 0 。)，and played no action or negative action when the PHPs operated in ttle horizontal(O。)and anti·gravity top heat mode(一90 )． Therefore，the best thermaI pe rformance always occurred in verticaI bottom heat mode (+ 9O。)，then decreases gradually as the tilt angle decreases from +90。to 一90。． References [1] [2] Akachi H ．and Pohi~ck F．，Preprints 10 th Int CD ．，Stuttgart，Germany，1997，3：8—12 GrollM ． and Khandekar S，， Proc． 3rd hit Heat Pj Conf．otl 维普资讯 http://www.cqvip.com Joorrm／of￡)【 ghl^a University(En9．Ed．)Vo1．23，No．3(2006) 13 Transport Phenomena in Muhiphase跏 fP， ，Baranow Sandomierski，Pbland，2002：35—43 [3] Khandekar S．，Groll M．and Luckchoura V．，Electronics Cooling·2003·9(2)：38—41 [4] Groll M．and Khandekar S．，Proc．3rd，}lf． l，．On Etwrgy apulF~tdromnent，Shangh~，China，2003．1：723—730 [5] Lee W．，Jung H．，Kim J．，Ct a1．，Proc．11th lnt．Heat Plpe Conf．。Tokyo，Japan-1999=355—360 [6] Khandekar S．，Schneider M．， Schiller P．， et a1．， Microscale Thermophysical Engi,,eeri,,g·2002，6(1) 维普资讯 http://www.cqvip.com