Influence of Ambient Temperature on Radiative and Convective Heat Dissipation Ratio in Polymer Heat Sinks

polyme s A icle In luence o Ambien Tempe a u e on Radia i e and Con ec i e Hea Dissipa ion Ra io in Polyme Hea Sinks Jan Kominek * , Ma in Zacha , Michal Guzej, E ik Ba uli and Pe Ko bacek   Ci a ion: Kominek, J.; Zacha , M.; Guzej, M.; Ba uli, E.; Ko bacek, P. In luence o Ambien Tempe a u e on Radia i e and Con ec i e Hea Dissipa ion Ra io in Polyme Hea Sinks. Polyme s 2021,13, 2286. h ps://doi.o g/10.3390/ polym13142286 Academic Edi o : Ni in Meh a Recei ed: 16 June 2021 Accep ed: 2 July 2021 Published: 12 July 2021 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2021 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). Hea T ans e and Fluid Flow Labo a o y, Facul y o Mechanical Enginee ing, B no Uni e si y o Technology (BUT), Technicka 2896, 616 69 B no, Czech Republic; [email p o ec ed] (M.Z.); [email p o ec ed] (M.G.); [email p o ec ed] (E.B.); Pe [email p o ec ed] (P.K.) *Co espondence: [email p o ec ed] Abs ac : Minia u iza ion o elec onic de ices leads o new hea dissipa ion challenges and adi- ional cooling me hods need o be eplaced by new be e ones. Polyme hea sinks may, hanks o hei unique p ope ies, eplace s anda dly used hea sink ma e ials in ce ain applica ions, especially in applica ions wi h high ambien empe a u e. Polyme s na i ely dispose o high su ace emissi i y in compa ison wi h glossy me als. This high emissi i y allows a la ge amoun o hea o be dissi- pa ed o he ambien wi h he ou h powe o i s absolu e su ace empe a u e. This pape shows he change in adia i e and con ec i e hea ans e om polyme hea sinks used in di e en ambien empe a u es. Fu he mo e, he obse ed polyme hea sinks ha e di e en ly o ien ed g aphi e ille caused by hei molding p ocess di e ences, he e o e hei he mal conduc i i y aniso opies and o e all cooling e iciencies also di e . Fu he mo e, i is also shown ha a high adia i e hea ans e leads o minimizing hese cooling e iciency di e ences be ween hese polyme hea sinks o he same geome y. The measu emen s we e conduc ed a HEATLAB, B no Uni e si y o Technology. Keywo ds: polyme hea sink; he mal managemen ; he mal conduc i i y; adia ion; con ec- ion; composi es 1. In oduc ion Wi h new ad ancemen s in elec onics, especially in hei minia u iza ion, new cooling challenges need o be me so ha he elec onics’ e iciency and se ice li e would no dec ease. Polyme s, due o hei easy ab ica ion, low cos and low weigh , a e al eady ind- ing hei ways in many ac i e cooling applica ions despi e hei low he mal conduc i i y as polyme hea exchange s in cooling elec onics and also in he au omo i e indus y [ 1 – 4 ]. Thei amo phous s uc u e esponsible o hei low he mal conduc i i y p e en s hei usage in passi e cooling applica ions as hea sinks. This disad an age could be elimina ed i he igh ille s wi h high he mal conduc i i y would be inco po a ed in o he base polyme ma ix using mode n p oduc ion echniques, hese composi es could be used in elec onics passi e cooling applica ions whe e hey would b ing wi h hem o he polyme bene i s [ 5 – 11 ]. High he mally conduc i e polyme s could be used as hea sinks especially in applica ions whe e high ambien empe a u e is expec ed. Mode n polyme blends o e high he mal conduc i i y, bu in compa ison wi h adi ionally used aluminum and coppe , hei conduc i i y s ill emains low app oxima ely by en old. Howe e , hey could be used in applica ions whe e he hea ans e ia conduc ion inside he ma e ial is no es ic ing he o e all hea ans e om he cooled pa o he ambien , which is ypical in passi e cooling using a hea sink [ 4 ]. Especially in applica ions whe e hea ans e om he hea sink by con ec ion is s ic ly limi ed (enclosed space) o whe e hei na u ally high emissi i y would lead o high hea ans e ia adia ion (applica ions wi h high ambien empe a u e). Beha io o polyme hea sinks en iched wi h g aphi e lakes used in ambien wi h oom empe a u e was s udied in [ 12 ]. These esul s showed especially he impo ance Polyme s 2021,13, 2286. h ps://doi.o g/10.3390/polym13142286 h ps://www.mdpi.com/jou nal/polyme s Polyme s 2021,13, 2286 2 o 12 o he choice o mel en y du ing hei ab ica ion bu did no del e deepe in o he hea sinks’ e iciency i condi ions would be any di e en om he oom empe a u e and i na u al con ec ion would be limi ed due o use in enclosed space as is o en he case in eal applica ions. As polyme hea sinks ha ha e na u ally high emissi i y hea dissipa ion ia adi- a ion canno be igno ed, he hea sinks’ design should be subjec ed o his mode o hea ans e . Impo ance o adia ion on he o e all cooling e iciency was s udied in [ 13 – 16 ]. Huang, L. e al. in [ 14 ] s a e ha compa ing high emissi i y coa ing wi h a low emissi i y one applied o an LED ilamen bulb can educe i s empe a u e by 10%. B a o, R.H. e al. in [ 16 ] showed in hei publica ion ha in case o an a ay o elec onic chips moun ed be ween wo pa allel pla es he he mal adia ion ep esen ed 33% o he o al hea ans e . This pape shows how hea dissipa ion om a polyme hea sink changes when sub- jec ed o di e en ambien empe a u es. Change o ai p ope ies wi h ising empe a u e by ens o deg ees is minimal; he e o e, he hea ans e coe icien by con ec ion s ays almos he same. Hence, using a me al hea sink wi h glossy su ace in an ambien whe e he empe a u e should ise would lead o ise o he en i e cooling sys em by he same ma gin. Howe e , i a hea sink wi h high su ace emissi i y is applied, his inc ease in ambien empe a u e can lead o signi ican ise o hea dissipa ion ia adia ion, he e o e be e cooling e iciency. 2. Ma e ials and Me hods 2.1. Geome y o Tes ed Hea Sinks The es ed hea sinks had dimensions (58 × 40 × 106) mm wi h 9 ins (de ails a e shown in Figu e 1). Fo he s udy’s pu pose he chosen design was simplis ic o allow an easy way o calcula e dissipa ed hea h ough he hea sinks’ ins acco ding o [ 17 ]. Hea sinks we e p oduced om Polyamide 66 as he ma ix ma e ial wi h g aphi e lakes as he main ille ha ensu ed he high he mal conduc i i y. This blend is comme cially a ailable and was p oduced by A ien (P ague, Czech Republic). The he mal conduc i i y o his ma e ial anged om 5 o 20 W/m · K depending on he ille o ien a ion, densi y 1640 kg/m 3 . The hea capaci y is unknown (no essen ial because he da a p esen ed in his pape is o he he mal s eady s a e). All he alues we e aken om he o icial da a shee . Figu e 1. Geome y o es ed hea sinks (dimensions a e in mm). To compa e he in luence o he mel low, wo polyme hea sinks o he same geome- y we e used in he measu emen . The di e ence be ween he wo hea sinks was in he mel en y poin du ing p oduc ion. One hea sink was p oduced wi h he mel en y along he hea sink’s longe base’s edge so he mel would low in pa allel wi h he hea sink’s ins and he o he had he mel en y on he sho e base’s edge pe pendicula o he hea sink’s ins. En y poin s o bo h measu ed hea sinks a e shown in Figu e 2. Polyme s 2021,13, 2286 3 o 12 Figu e 2. Mel en ance along sinks’ edge ( ed), di ec ion o mel low depic ed by g een a ow. Le —edge o longe base’s side (mel low pa allel o he ins), igh —edge o sho e base’s side (mel low pe pendicula o he ins). Fu he mo e, an aluminum hea sink o he same geome y was also compa ed wi h he wo polyme hea sinks. The aluminum hea sink had he mal conduc i i y 130 W/m · K and a low su ace emissi i y. One o he measu ed polyme hea sinks wi h high emissi i y and he aluminum hea sink wi h low emissi i y a e shown in Figu e 3. Figu e 3. Measu ed polyme (le ) and aluminum ( igh ) hea sinks side by side. The esul s om he cooling e iciency we e used o demons a e he impo ance o he su ace emissi i y in applica ion wi h highe ambien empe a u es. 2.2. Measu emen Desc ip ion The measu ed polyme hea sinks we e si ua ed inside a measu emen box, which was placed inside a he mos a ic chambe Binde MK 720 (manu ac u e Binde , H adec K alo e, Czech Republic) du ing he conduc ed measu emen s. The box wi h one o he measu ed hea sinks inside he he mos a ic chambe is shown in Figu e 4. The box’s holde s ouched he hea sinks only in ou poin s o ensu e minimal hea conduc ion away om he hea sinks and in o he box i sel . The he mos a ic chambe se ed as he sou ce o he desi ed ambien empe a u e and he box p o ec ed he hea sinks om any unwan ed o ced con ec ion c ea ed by he chambe ’s ans. The box’s on doo also se ed as a placemen o he FLIR E5 he mog aphic came a. The hea leading o he hea sink was gene a ed wi h a esis ance hea e . The con ac su ace a ea be ween he hea sink and he hea e was 35 × 40 mm. The placemen o he hea e on he hea sink’s base is shown in Figu e 5. A he mally conduc i e pas e wi h a he mal conduc i i y o 4.2 W/m · K was applied be ween he hea sou ce and he hea sink base. This pas e ensu ed a good he mal con ac , which was he same o e e y expe imen . Polyme s 2021,13, 2286 4 o 12 Figu e 4. Hea sink placemen inside he measu emen box (le ), measu emen box inside he he mos a ic chambe wi h a ached he mal imaging came a (middle), de ail o he measu emen box ( igh ). Figu e 5. Hea e placed in he middle o he hea sink’s base (dimensions a e in mm). The hea ou pu o he hea e was se o 15 W wi h a p ecision o ± 0.1 W. I was u he con olled wi h a LabView p og am o ensu e he same amoun o hea was gene a ed ega dless o he hea e ’s esis ance change caused by inc ease o i s empe a u e. E e y measu emen was epea ed wi h de aching and ea aching he hea e . The measu emen s’ esul s we e compa ed o elimina e he possibili y o he bad con ac be ween he hea e and he hea sink leading o he measu emen e o . Bo h measu ed polyme hea sinks we e measu ed in wo posi ions as in [ 12 ]. The posi ions we e named ‘de aul ’ and ‘upside-down’. This led o change o he ille o ien a- ion in ela ion o g a i y and he e o e buoyancy in case o he polyme hea sink wi h he mel en y poin pa allel o he ins, which as was shown in [ 12 ] led o di e en cooling e iciencies. Change o he mel low di ec ion in ega d o g a i y is shown in Figu e 6. Polyme s 2021,13, 2286 5 o 12 Figu e 6. O ien a ion o he mel en y o bo h measu ed hea sinks in wo di e en measu emen posi ions, le —de aul posi ion, igh —‘upside-down’. Mel en ance along he ed edge, mel low in he di ec ion o he g een a ow. ( A ) hea sink wi h mel en ance along he longe base’s edge (pa allel o ins)—de aul posi ion, ( B ) hea sink wi h mel en ance along he longe base’s edge (pa allel o ins)—upside-down posi ion, ( C ) hea sink wi h mel en ance along he sho e base’s edge (pe pendicula o ins)—de aul posi ion, ( D ) hea sink wi h mel en ance along he sho e base’s edge (pe pendicula o ins)—upside-down posi ion. 3. Measu emen Unce ain y Analysis To ensu e maximal measu emen p ecision and o minimize all he possible e o s he ollowing measu es we e aken. The used he mocouple was calib a ed o di e en ambien empe a u es beyond i s s anda d measu emen p ecision o ± 2.2 ◦ C. I s new maximum de ia ion was se o only ±0.2 ◦C. When a s eady s a e o he measu emen s was achie ed a he mal came a FLIR E5 wi h measu emen p ecision o ± 2 ◦ C and wi h esolu ion o 120 × 90 pixels was used o u he analyze he empe a u e ield on he hea sinks’ ins. To u he inc ease i s p ecision ano he ype K he mocouple calib a ed he same way as he p e ious one was placed in he back wall inside an a ea which was seen in all he measu emen s. As his wall was dyed wi h a g aphi e sp ay o known emissi i y he he mal image y esul s could be u he adjus ed by compa ing i s alues inside he a ea whe e he he mocouple was si ua ed. This a ea is shown in Figu e 7. To ensu e he ambien empe a u e was as close o i s desi ed alue, wo esis ance he mome e s PT100 we e placed a he bo om o he measu emen box which se ed as he inle once he ai low induced by he na u al con ec ion was s abilized. These he mome e s measu ed wi h he measu emen e o o ±0.2 ◦C. Polyme s 2021,13, 2286 6 o 12 Figu e 7. The mog am wi h an indica ed a ea whe e he he mocouple o he e i ica ion o he co ec measu ed empe a u e ia he mal image y was placed. 4. Resul s All he con igu a ions we e measu ed mul iple imes (as desc ibed in Sec ion 2.2) wi h good epea abili y. The da a p esen ed was gene a ed by a e aging he esul s o each measu ed con igu a ion. 4.1. Tempe a u e Compa ison Bo h compa ed polyme hea sinks we e measu ed wi h 15 W he mal inpu powe o he hea e in wo posi ions as desc ibed in Sec ion 2.2. The ambien empe a u e was main ained a 50 ◦C. The empe a u es o he hea e a e shown in Table 1. Table 1. Hea e empe a u e in all polyme hea sinks conduc ed measu emen s o ambien empe - a u e 50 ◦C. The le e s in pa en heses e e o posi ions acco ding o Figu e 6. De aul Posi ion (◦C) Upside-Down (◦C) Posi ion Di e ence (◦C) Mel en ance along he longe base’s edge (pa allel o ins) 99.0 (A) 97.1 (B) 1.9 (A)-(B) Mel en ance along he sho e base’s edge (pe pendicula o ins) 100.3 (C) 100.5 (D) 0.2 (D)-(C) Mel En y Di e ence (◦C) 1.5 (C)-(A) 3.4 (D)-(B) These esul s can be compa ed wi h he esul s om measu emen o ambien em- pe a u e 20 ◦C. These esul s a e shown in Table 2. By compa ing hese esul s, i can be concluded ha wi h highe ambien empe a u e he impo ance o he mel en y poin in o he mold diminishes and he same s ands o he impo ance o he hea sink’s posi ion i he mel en y poin is pa allel o ins. Going om ambien empe a u e 20 ◦ C o 50 ◦ C dec eased he hea e ’s empe a u e be ween he bes and he wo s hea sinks’ pe o mances by 49.3% om 6.7 ◦ C o only 3.4 ◦ C. In case o e iciency change o he hea sink wi h mel en e ing along he base’s longe edge he hea e ’s empe a u e di e ence d opped by 25% om 2.8 ◦C o 1.9 ◦C. Polyme s 2021,13, 2286 7 o 12 Table 2. Hea e empe a u e in all polyme hea sinks conduc ed measu emen s o ambien empe - a u e 20 ◦C [12]. The le e s in pa en heses e e o posi ions acco ding o Figu e 6. De aul Posi ion (◦C) Upside-Down (◦C) Posi ion Di e ence (◦C) Mel en ance along he longe base’s edge (pa allel o ins) 72.3 (A) 69.5 (B) 2.8 (A)-(B) Mel en ance along he sho e base’s edge (pe pendicula o ins) 76.1 (C) 76.2 (D) 0.1 (D)-(C) Mel En y Di e ence (◦C) 3.9 (C)-(A) 6.7 (D)-(B) 4.2. Con ec ion-Radia ion Hea T ans e Compa ison F om he p e iously shown esul s i is isible ha he di e ence be ween cooling e iciencies o he measu ed hea sinks is signi ican ly lowe o he ambien empe a u e 50 ◦ C compa ed o he esul s o he ambien empe a u e 20 ◦ C. This phenomenon shows ha he means o hea dissipa ion om he hea sinks is signi ican ly di e en . The a io be ween he dissipa ion ia con ec ion and adia ion mus change i he hea sinks ha e an o e all highe empe a u e as he adia ion hea ans e ises wi h he ou h powe o he su ace’s absolu e empe a u e. To p o e his change in hea ans e means om he hea sinks o he ambien he same app oach o he e iciency compa ison as in [ 12 ] was used. The equa ions used in his app oach a e o be ound in [ 17 ] and i analyses he amoun o dissipa ed hea ia hea sink’s ins. Fi s ly, he empe a u e ield on he hea sinks’ ins is measu ed wi h he mal image y and he empe a u e di e ence be ween he hea sinks’ ins empe a u e and ambien is calcula ed. ∆Tij =T in,ij −Tambien . (1) The index i e e s o he in numbe and j o he su ace Sj . To calcula e hea ans e by con ec ion Rayleigh numbe is de e mined. Raij = g·βai ·∆Tij ·L3 j νai ·αai , (2) whe e g is g a i a ional accele a ion, βai is he mal expansion coe icien o he ambien ai , νai is kinema ic iscosi y o he ambien ai , αai is he mal di usi i y o he ambien ai and Lj is cha ac e is ic dimension o he su ace Sj ( L1 , L2 , L3 — in leng h, L4 , L5 —su ace a ea di ided by i s pe ime e ). The ai p ope ies used o he calcula ion a e de e mined om so called ilm empe a u e on he ins. T ilm,ij =T in,ij +Tambien 2. (3) Nex , con ec i e hea ans e is calcula ed as a unc ion o Rayleigh numbe , ambien ai ’s he mal conduc i i y and cha ac e is ic dimension. hcon ,ij = jRaij,Lj,kai , (4) whe e kai is ambien ai ’s he mal conduc i i y a he ilm empe a u e. The adia i e hea ans e was calcula ed using he known su ace emissi i y and S e an–Bol zmann cons an . h ad,ij =εσT in,ij +Tambien ·T2 in,ij +T2 ambien , (5) whe e he su ace emissi i y εwas se o 0.95 and σis S e an–Bol zmann cons an . Polyme s 2021,13, 2286 8 o 12 Fo he ins’ su ace ha adia ed hea be ween hemsel es a iew ac o was calcu- la ed. Figu e 8shows he su aces whe e he iew ac o was conside ed (blue) and whe e i was no ( ed). The ou e su aces wi hou aking iew ac o in o accoun o med oge he an a ea o 18,963 mm 2 and he in e nal su aces whe e he iew ac o was conside ed c ea ed oge he an a ea o 23,331 mm 2 , which is 55.2% o he en i e su ace. This means ha less han a hal o he hea sink’s su ace can adia e hea away eely. Figu e 8. Hea sinks’ su aces wi hou aking iew ac o in o accoun ( ed) and wi h aking iew ac o in o accoun (blue). The inal indi idual hea lows om ins Qiwe e calcula ed: Qi=∑ j Qij,Qij =hcon ,ij +h∗ ad,ij·Aj·∆Tij, (6) whe e h is hea ans e coe icien , he index i e e s o he in numbe and j e e s o indi idual su ace a eas o ins Aj. Table 3shows calcula ed hea lows om ins o he hea sink made wi h he mel low pa allel o i s ins o he measu ed hea sinks in bo h 20 ◦ C and 50 ◦ C ambien empe a u es and di e ences be ween hem. Table 3. Calcula ed hea ans e s om ins o he polyme hea sink wi h mel low pa allel o i s ins o bo h 20 ◦ C and 50 ◦ C ambien empe a u es. The le e s in pa en heses e e o posi ions acco ding o Figu e 6. Hea Sink Mel En e ing Pa allelly o Fins (De aul Posi ion) (A) Mel En e ing Pa allelly o Fins (Upside-Down Posi ion) (B) Ambien empe a u e (◦C) 20 50 20 50 Con ec i e hea low (W) 7.7 6.8 7.6 6.9 Radia i e hea low (W) 3.2 3.8 3.1 3.9 To al hea low (W) 10.9 10.5 10.8 10.7 Addi ionally, Table 4shows calcula ed hea lows om ins o he hea sink made wi h he mel low pe pendicula o i s ins o he measu ed hea sinks in bo h 20 ◦ C and 50 ◦ C ambien empe a u es and di e ences be ween hem. Polyme s 2021,13, 2286 9 o 12 Table 4. Calcula ed hea ans e s om ins o he polyme hea sink wi h mel low pe pendicula o i s ins o bo h 20 ◦ C and 50 ◦ C ambien empe a u es. The le e s in pa en heses e e o posi ions acco ding o Figu e 6. Hea Sink Mel En e ing Pe pendicula ly o Fins (De aul Posi ion) (C) Mel En e ing Pe pendicula ly o Fins (Upside-Down Posi ion) (D) Ambien empe a u e (◦C) 20 50 20 50 Con ec i e hea low (W) 7.2 6.1 7.2 6.0 Radia i e hea low (W) 2.9 3.4 3.0 3.4 To al hea low (W) 10.1 9.5 10.2 9.4 F om hese esul s i is ob ious ha hea ans e by adia ion g ows signi ican ly wi h highe ambien empe a u e. As he same amoun o powe is gene a ed inside he hea e and he adia i e hea ans e g ows, he hea ans e ia con ec ion mus d op. Calcula ed hea ans e om hea sinks’ ins also shows d op in he o e all hea ans e and especially o he case o he hea sink p oduced wi h mel low pe pendicula o i s ins. As he hea sink’s base is he wa mes pa , he amoun o hea adia ed away o he ambien ises he e he mos , hence he o al amoun o hea ans e ed away om ins d ops. 4.3. Con ec ion-Radia ion Hea T ans e Compa ison The same ype o measu emen was done o an aluminum hea sink o he same geome y. The hea e was a ached o he aluminum hea sink wi h inpu powe o 15 W and he sou ce’s empe a u e was measu ed in bo h condi ions wi h ambien empe a u e 20 ◦ C and 50 ◦ C. Table 5shows how he hea e ‘s empe a u e changed be ween he wo measu emen condi ions o he aluminum hea sink and bo h polyme hea sinks. Table 5. Hea e ‘s empe a u e change wi h change o he ambien empe a u e on di e en measu ed hea sinks. The le e s in pa en heses e e o posi ions acco ding o Figu e 6. The da a o he ambien empe a u e 20 ◦C a e om [12]. Hea Sink Hea e Tempe a u e o Ambien Tempe a u e 20 ◦C (◦C) Hea e Tempe a u e o Ambien Tempe a u e 50 ◦C (◦C) Di e ence (◦C) Aluminum 65.7 95.6 29.9 Polyme made wi h mel low along ins (B) 69.5 97.1 27.6 Polyme made wi h mel low pe pendicula o ins (D) 76.1 100.3 24.2 In case o he aluminum hea sink, he hea e ’s empe a u e ose by 29.9 ◦ C wi h he ambien empe a u e change o 30 ◦ C. In case o he polyme hea sink p oduced wi h he mel low pa allel o i s ins and posi ioned in he mos sui able posi ion o induced na u al con ec ion he hea e ’s empe a u e inc emen o he same ambien empe a u e change was 27.6 ◦ C. This p o es ha he amoun o hea adia ed away hanks o i s high emissi i y ises wi h inc easing he ambien empe a u e. This phenomenon is especially isible in case o he hea sink p oduced wi h mel low pe pendicula o i s ins whe e he hea e ’s empe a u e ose only by 24.2 ◦ C. This is caused by hea sink’s unideal hea conduc i i y o i s ins; he e o e, i s o e hea ed base adia es away e en highe amoun o hea .