Internatinal Jurnal f Innvatin and Applied Studies ISSN 2028-9324 Vl. 9 N. 3 Nv. 2014, pp. 1043-1047 2014 Innvative Space f Scientific Research Jurnals http://www.ijias.issr-jurnals.rg/ NATURAL CONVECTION HEAT TRANSFER FROM A HEAT SINK WITH FINS OF DIFFERENT CONFIGURATION Murtadha Ahmed and Abdul Jabbar N. Khalifa Mechanical engineering department, Alnahrain University, P.O. Bx 64040, Baghdad, Iraq Cpyright 2014 ISSR Jurnals. This is an pen access article distributed under the Creative Cmmns Attributin License, which permits unrestricted use, distributin, and reprductin in any medium, prvided the riginal wrk is prperly cited. ABSTRACT: Experiments were carried ut n natural cnvectin heat transfer frm square pin fin heat sinks subject t the influence f its gemetry and heat flux. A ttal f 50 fins were blted int the upper surface f the base plate. The area f the base plate is 250mm by 250mm. The base plate and the fins were made f aluminum.over the tested range f Rayleigh number, 12.45 10 6 Ra 58.59 10 6, it was fund that the slid pin fin heat sink perfrmance fr upward rientatins depended n the Rayleigh number and generally shws higher heat transfer cefficients than thse f the perfrated/hllw pin fin nes. Fr all tested hllw/perfrated pin fin heat sinks, hwever, the heat transfer perfrmance fr heat sinks with hllw/perfrated pin fins was better than that f slid pins. The temperature difference between the base plate and surrunding air f these heat sinks was less than that f slid pin ne. KEYWORDS: Natural cnvectin; Square slid/perfrated pin fin; Heat sinks 1 INTRODUCTION Natural cnvectin heat transfer frm a heat sink has been the subject f a large number f experimental and theretical investigatins. Heat sinks with extended surfaces have been widely used in varius engineering applicatins especially in air cnditining as well as in electrnic devices, electrical and internal cmbustin, slar energy applicatins, cling f nuclear reactr fuel elements, imprving heat transfer in radiatrs fr air cnditining and in air cled heat exchangers. In mst f the electrnics cling applicatins, the final heat transfer medium is air. Heat transfer frm a finned heat sink depends n many parameters; temperature difference between the heat sink and the fluid, shape and size f the fins, number f fins, the type f flw and the fluid, thermal cnductivity and spacing between the fins. Natural cnvectin heat transfer frm slid and perfrated pin fin heat sinks can be fund frm experimental and theretical wrks. An experimental study was cnducted n natural cnvectin heat transfer frm square pin fin and plate fin heat sinks subject t the influence f rientatin by Huang et al. [1]. Over a tested range f 1.8 10 6 < Ra < 4.8 10 6, it was reprted that the dwnward facing rientatin yielded the lwest heat transfer cefficient fr bth pin fin and plate fin gemetry. The finning factr Ф (which represents the ttal surface area, A t divided by the base plate area, A bp ) fr the perfrated pin fin was greater than frm the slid pin fin.the heat transfer cefficient increases with decreasing the finning factr. In additin, the advantage f the pin fins escalates with the increase f the Rayleigh number due t the mre pen ends fr air ventilatin. Elshafei [2] studied the natural cnvectin heat transfer frm circular pin fin heat sinks subject t the influence f its gemetry, heat flux and rientatin. Their results shwed that the slid pin fin heat sink perfrmance fr upward and sideward rientatins was a cmpetitive nature, depending n Rayleigh number (3.8 10 6 Ra 1.65 10 7 ) and generally shws higher heat transfer cefficients than thse f the perfrated/hllw pin fin. The heat transfer perfrmance fr heat sinks with hllw/perfrated pin fins was better than that f slid pins. The temperature difference between the base plate and surrunding air f these heat sinks was less than that f slid pin. Zgrafs and Sunderland [3] investigated the heat transfer perfrmance f inline and staggered pin fin arrays in natural cnvectin with different inclinatin angle, and cncluded that the inline arrays generally yielded higher heat transfer rates than the staggered nes. In additin, their Crrespnding Authr: Murtadha Ahmed 1043
NATURAL CONVECTION HEAT TRANSFER FROM A HEAT SINK WITH FINS OF DIFFERENT CONFIGURATION investigatin shwed little influence f inclinatin when the inclinatin angle was less than 30 frm the vertical. Sparrw and Vemuri [4] studied the fin rientatin n natural cnvectin/radiatin heat transfer frm pin fin arrays. Their results revealed that the upward facing rientatin yielded the highest heat transfer rates, fllwed by the sideward facing and the dwnward facing nes. Dialameh, et al. [5] made a numerical study t predict natural cnvectin frm an array f aluminum hrizntal rectangular thick fins f (3 mm< t < 7 mm) with shrt lengths (L< 50 mm) attached n a hrizntal base plate. The results shwed that natural cnvectin heat transfer cefficient increases with increasing temperature differences and fin spacing and decreases with fin length, and the fin thickness and fin height des nt affect the value f average heat transfer cefficient cnsiderably. Kbus and Oshi [6] carried ut a theretical and experimental study n the perfrmance f pin fin heat sinks. A theretical mdel was prpsed t predict the influence f varius gemetrical, thermal and flw parameters n the effective thermal resistance f heat sinks. Subsequently, Kbus and Oshi [7] investigated the effect f thermal radiatin n the heat transfer f pin fin heat sinks and presented an verall heat transfer cefficient that was the sum f an effective radiatin and a cnvective heat transfer cefficient. Fisher and Trrance [8] presented the analytical slutins relevant t the limits f free cnvectin fr pin fin cling. They suggested that the design f pin fin heat sink culd be ptimized by prperly chsing the pin fin diameter and the heat sink prsity. Fr cnventinal heat sinks, the minimum thermal resistance was abut tw times greater than that in an ideal limit accrding t the mdel f in viscid flw with idealized lcal heat transfer. The abve literature indicates that many studies have been carried ut fr different types f fin arrays, but still there is lack f knwledge f the natural cnvectin heat transfer frm a surface with hllw /perfrated pin fins. The main bjectives f the present experimental wrk are t determine the effects f gemetric parameters and base t ambient temperature difference n the natural cnvectin heat transfer frm square slid and perfrated pin fins heat sink. Anther bjective is t crrelate the experimental data f verall Nusselt number with the average mdified Rayleigh number fr such cnfiguratins. 2 EXPERIMENTAL SETUP AND PROCEDURE 2.1 PIN FIN ASSEMBLY A ttal f 50 fins are used in this experimental investigatin. Twenty fins have tw hles, ne f the hles is hrizntal and the ther is vertical. The fins are blted int the upper surface f the base plate, the dimensins f the base plate is 250mm 250mm. Thermal paste was used t minimize the thermal cntact resistance between the pin fins and the base plate; the pin fins were fixed at equal spacing between the span wise directins f 18.125 mm. The base plate was made f the same material as the fins because f the cnsideratins f cnductivity machinability, and cst. The base plate and the fins were made f aluminum because high thermal cnductivity, lw radiative emissivity and gd cntact between them were assured. The experimental rig is shwn in Fig. 1. 2.2 HEATING SYSTEM During experiments, the heat sink base plate was heated by an attached electric heater, with an identical size as the base plate, which culd supply a specific heat flux. The vltage and current f the electric input t the heaters were cntrlled by an AC pwer supply unit. The supplied pwer was calculated using the measured vltage and current supplied t the heaters. The supplied vltage and current int the heaters were measured by a digital clamp meter (UT200A). The presence f thin layer f high thermal cnductivity mica ensured that gd thermal cntact existed between the heater and the heat sink base. The bttm surfaces f the base plate as well as the heater blck were insulated by tw layers f insulatin, firebrick and glass wl blanket. The whle assembly, base, heater with assciated thermal insulatin, was lcated in a well-fitted pen-tpped wden bx f 20mm thick as shwn in Fig. 1. The upper edges f the wden bx and the tp surface f the laterally-placed thermal insulatin were flushed with the upper surface f the heat sink base in which the pin fins prtruded perpendicularly. The pwer supplied t the heater was cntrlled by a variac transfrmer t btain cnstant heat flux alng the base plate, and was measured by in line multi-meter (an ammeter and vltmeter). A ttal f 12 K-type thermcuples were apprpriately distributed amng the base plate t measure its average temperature, T bp.these thermcuples were pressed int fine hles and glued in psitin with epxy s as t ensure gd thermal cntact. All thermcuples were pre-calibrated with an accuracy f ±0.1 ºC. The temperatures at varius lcatins n the surface f the base plate were measured by calibrated K- type thermcuples cnnected t a 12-channel temperature recrder (Mdel BTM-4208SD). The whle assembly was lcated in an envirnmental chamber inside the labratry. Heat sinks f slid and perfrated pin fins as shwn in Fig. 2 were tested in the enclsed rm at nearly fixed temperature with the pwer inputs ranging frm 150 W t 1400 W. Steady state cnditins were indicated by nticing the repeatable readings ISSN : 2028-9324 Vl. 9 N. 3, Nv. 2014 1044
Murtadha Ahmed and Abdul Jabbar N. Khalifa f each thermcuple at different lcatins f the heat sink. Each test run tk nearly 2-hurs t reach equilibrium when the pwer is turned n. Fig. 1. Heat sink assembly 3 RESULTS AND DISCUSSION Fr the sake f cmparisn, the thermal perfrmance f heat sink is measured by detecting the variatin f the temperature difference between the base plate and the surrunding air ( T) with the heat input rate (Q). Figure 2 indicates the variatin f T with Q fr tw tested mdules (slid and perfrated heat sink) arrangements. Generally, the results indicate that T increases with increasing heat input rate (Q). In additin, the temperature difference ( T) fr the perfrated heat sink is less than fr the slid heat sink fr each heat input. Fig. 2. Temperature difference variatins T with heat input rate, Q in fr slid and perfrated heat sink ISSN : 2028-9324 Vl. 9 N. 3, Nv. 2014 1045
NATURAL CONVECTION HEAT TRANSFER FROM A HEAT SINK WITH FINS OF DIFFERENT CONFIGURATION The variatin between the temperature difference ( T) and the heat transfer cefficient (h) is shwn in Fig. 3. Generally, the results indicate that the heat transfer cefficients fr tw test samples increase with the temperature difference between the base plate and the surrunding, this increase in bth temperature difference and heat transfer cefficient is due t the increase in the heat input gradually. A cmparisn between the slid and perfrated heat sink shws that the heat transfer cefficient fr slid pin fin is greater than frm perfrated pin fin. Fig. 3. Temperature difference variatins T with average heat transfer cefficient h fr slid and perfrated heat sink 4 CONCLUSIONS In this study, natural cnvectin heat transfer frm square slid and perfrated pin fins heat sink at different heat flux values was investigated experimentally. The effect f gemetric parameters and base t ambient temperature difference n the heat transfer perfrmance f fin arrays was discussed. Based n the preceding discussins, the fllwing cnclusins may be drawn: At the same heat input rate, the temperature difference between the base plate and surrunding air was fund t be less fr hllw/perfrated pin fin heat sink than that fr slid pin ne. The heat transfer cefficient fr slid pin fin is greater than frm perfrated pin fin. Temperature difference between the fin base and fin tip becmes larger, by adding perfratin Perfrated fins have higher fin effectiveness than slid fin and it rises remarkably by adding mre perfratins. ISSN : 2028-9324 Vl. 9 N. 3, Nv. 2014 1046
Murtadha Ahmed and Abdul Jabbar N. Khalifa REFERENCES [1] R.T. Huang, W.j. Sheu and C.C. Wang Orientatin effect n natural cnvective perfrmance f square pin fin heat sinks" Inter Internatinal Jurnal f heat and mass transfer, Natinal Tsing Hua University,2368 2376, Octber2007. [2] E.A.M. Elshafei Natural cnvectin heat transfer frm a heat sink with hllw/perfrated circular pin fins Energy, Mansura University, April 2010. [3] A.I. Zgrafs and J.S. Sunderland Natural cnvectin frm pin fin arrays Exper Therm Fluid, 1990. [4] E. Sparrw and S. Vemuri Orientatin effect n natural cnvectin/radiatin heat transfer frm pin-fin arrays Internatinal Jurnal f heat and mass transfer, 1986. [5] L. Dialameh, M. Yaghubi and O. Abuali, "Natural cnvectin frm an array f hrizntal rectangular thick fins with shrt length Applied Thermal Engineering. 28 December, 2008. [6] C.J. Kbus, T. Oshi Develpment f a theretical mdel fr predicting the thermal perfrmance characteristics f a vertical pin fin array heat sink under cmbined frced and natural cnvectin with impinging flw Internatinal Jurnal f heat and mass transfer, 48(6):1053 63, 2005. [7] C.J. Kbus, T. Oshi the thermal perfrmance characteristics f staggered vertical fin array heat sinks under cmbined mde radiatin and mixed cnvectin with impinging flw Internatinal Jurnal f heat and mass transfer, 48(13):2684 96, 2005. [8] T.S. Fisher and K.E. Trrance Free cnvectin limits fr pin-fin cling, Heat and mass transfer 120 (3) 633 640, (1998) ISSN : 2028-9324 Vl. 9 N. 3, Nv. 2014 1047