Heat Transfer and Pressure Drop Measurement in Minichannels

HEAT TRANSFER AND PRESSURE DROP MEASUREMENT IN MINICHANNELS Jiří HEJČÍK, Mi osla JÍCHAx Abs ac : This pape desc ibes he sui able echniques o he hea ans e and p essu e d op measu emen in smoo h ubula channels wi h he in e nal diame e less han 5 mm. The i s expe imen al da a a e men ioned and hei e alua ion is p esen ed. 1. INTRODUCTION The e o o achie e g ea e alues o hea ans e coe icien has accompanied design enginee s o hea ing equipmen o almos 100 yea s [1]. P obably he easies way o inc easing i is o dec ease he channel diame e (a leas acco ding o he classical co ela ions). Al hough his me hod seems o be e y simple, i s applica ion is echnologically e y challenging, hus his me hod has only been used in special applica ions (e.g. mili a y and ai c a indus y) e en hough o a long ime. The wide use o i occu s only in he las 20 yea s because o he g owing in e es in highly e icien and compac hea exchange s; mainly because o he need o cool a sh inking and mo e powe ul elec onics. The p oblem o hea ans e in channels o such a small diame e has g own in impo ance and some di e en ca ego ies we e es ablished du ing he las decade. P obably he mos applied classi ica ion was de ined in 2002 by Kandlika [2] who di ided he channels in o 3 ca ego ies. The i s ca ego y is called a con en ional channel and con ains channels wi h hyd aulic diame e s g ea e han 3 mm. Nex ca ego y called minichannels con ains hose wi h hyd aulic diame e s diame e in he ange 200 μm - 3 mm, and inally he las ca ego y is called mic ochannels whe e he channels ange om 10 μm o 200 μm. The minichannel ca ego y is e y in e es ing mainly o mechanical enginee s because i allows designing small and e ec i e hea exchange s which a e no hype sensi i e o he pu i y o he lowing media. Howe e hese hea exchange s seem o be con enien o lo s o applica ions, he ac ha he e a e no cu en ly a ailable eliable p ocedu es o hei design p e en s hei u ilisa ion. The p oblem wi h a minichannel exchange design is a ibu ed o he hea ans e a e p edic ion. Al hough Kandlika , e al. [3] s a ed ha he co ela ions o con en ional channels could be used, he esea ch done on minichannels is no so clea . x Ing. Jiří Hejčík, Ph.D.; Ene gy Ins i u e, Facul y o Mechanical enginee ing, B no Uni e si y o Technology, Technická 2896/2, 616 69 B no; email: hejcik@ me. u b .cz p o . Ing. Mi osla Jícha, CSc.; Ene gy Ins i u e, Facul y o Mechanical enginee ing, B no Uni e si y o Technology, Technická 2896/2, 616 69 B no; email: [email p o ec ed]u b .cz EPJ Web o Con e ences , 0101 (2012) DOI: 10.1051/epjcon /2012250101 © Owned by he au ho s, published by EDP Sciences, 2012 This is an Open Access a icle dis ibu ed unde he e ms o he C ea i e Commons A ibu ion License 2.0, which pe mi s un es ic ed use, dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed. A icle a ailable a h p://www.epj-con e ences.o g o h p://dx.doi.o g/10.1051/epjcon /20122501021 2. TEST RIG A minichannel es ig was buil o p o e he possibili y o he con en ional channel co ela ions applica ion o ubula channels wi h inne diame e in ange o 0.25 – 5 mm. Comp essed ai was used as a es medium. I is s o ed in an ai -p essu e ank and i lows h ough he p essu e educing al e and he hea e o he p essu e/ empe a u e heade , whe e a empe a u e senso is moun ed as well as a p essu e po o p essu e d op measu emen . A e ha he ai goes h ough a es sec ion, whe e a es ed minichannel is moun ed and cooled by wa e o an ice-wa e mix u e. The nex empe a u e/p essu e heade is loca ed a e he es sec ion o acili a e he empe a u e and p essu e measu emen . The comp essed ai mass low measu emen and egula ion is loca ed behind his head and i e mina ing he comp essed ai low pa h. The schema ic iew o he ig is shown in he igu e 1. Figu e 1 Tes ig Thin wall s ainless s eel ubes we e used as es channels in his es ig. They we e inse ed in o he es sec ion and joined o he p essu e/ empe a u e heade s. Hence h ee addi ional empe a u e senso s we e s icked on he minichannel su ace o measu e he wall empe a u e nea he heade s and in he middle o he minichannel. The es sec ion was hen illed up wi h wa e o ice-wa e mix u e, so ha he minichannel was ully subme ged and he es ig was p epa ed o he hea ans e coe icien o p essu e d op measu emen . 3. HEAT TRANSFER COEFFICIENT MEASUREMENT The equi ed ai p essu e le el and mass low a e was p ese by he p essu e- educing al e and he needle al e. Nex , he hea e was u ned on and he sys em hea ed up o a couple o minu es. A e ha he p essu e le el and mass low a e was uned up o he equi ed alue and he measu emen s a ed. Tempe a u es, p essu e d op and mass low a e we e he s o ed da a collec ed e e y second o app oxima elly 10 minu es. Then he new mass low a e was se and he p ocedu e epea ed. The hea ans e coe icien , h, is hen calcula ed om he s o ed da a using he equa ion o cons an wall empe a u e hea ans e (1), which could be used due o he negligible he mal esis ance o channel walls. ݄ൌെ௠ሶή௖೛ గή஽ή௅ ή݈݊൬்ೢି்್ǡೀೆ೅ ்ೢି்್ǡ಺ಿ ൰ [Wm-2K-1] (1) whe e ݉ሶ [kg/s] - ai mass low a e, cp [J/kg/K] – cons an p essu e speci ic hea o ai , D [m] – channel diame e , L [m] – channel leng h, Tw [K] – wall empe a u e, Tb,IN, Tb,OUT [K] – comp essed ai inle /ou le empe a u e o/ om es sec ion EPJ Web o Con e ences 01021-p.2 As can be seen in equa ion 1, he hea ans e coe icien depends on he empe a u e di e ence be ween ai and he wall. I means ha he exac empe a u e measu emen is c ucial o he used me hod, which could be a p oblem in he case o small empe a u e di e ences, he e o e he inle empe a u e o comp essed ai has o be as high as possible o dec ease he unce ain y o he hea ans e coe icien measu emen . 4. PRESSURE DROP MEASUREMENT The e was no hea e used o he p essu e d op measu emen . Only he equi ed ai mass low a e (Reynolds numbe ) was se and he p essu e d op ead. The p oblem was ha p essu e d op measu ed was no he one o he minichannel bu he one o he comp essed ai lowpa h om one p essu e/ empe e u e heade o he nex one. I means i also includes some local p essu e d ops. Due o his ano he p essu e d op measu emen wi h a sho ened minichannel had o be done o elimina e he local p essu e d op e ec . The minichannel ic ion ac o was hen calcula ed om he di e ence o he measu ed p essu e d ops (long and sho minichannel) and he leng h o he cu ed pa o he minichannel. 5. RESULTS Fi s hea ans e coe icien measu emen wi h 4.8 mm in e nal diame e smoo h ubula channel was pe o med. The channel wall hickness was 0.2 mm and he channel leng h was 538 mm. This “con en ional” size channel was made o AISI 304 s ainless s eel and ins alled o he es ig o p o e hea ans e coe icien measu emen s. The comp essed ai gauge p essu e was se o 2 ba a he inle side o he channel. The ai was hea ed up o app ox. 80 °C and he es sec ion was illed up wi h an ice-wa e mix u e. The ai low a e a ied in be ween 5 and 55 Nl/min o each he Reynolds numbe alues om 1500 o 15000. Al hough he cons an wall empe a u e condi ion was supposed du ing he p elimina y s age o expe imen s, he i s measu emen showed ha we we e no able o mee ha condi ion. The al e na i e me hod (equa ion 2) based on he loga i hmic mean empe a u e di e ence was used o calcula e he hea ans e coe icien ins ead o equa ion (1). ݄ൌ௠ሶή௖೛ గή஽ή௅ ή݈݊൬்್ǡೀೆ೅ି்ೢǡೀೆ೅ ்್ǡ಺ಿି்ೢǡ಺ಿ ൰ή ்್ǡ಺ಿି்್ǡೀೆ೅ ൫்್ǡೀೆ೅ି்ೢǡೀೆ೅൯ି൫்್ǡ಺ಿି்ೢǡ಺ಿ൯ [Wm-2K-1] (2) Resul s ob ained using equa ion (2) we e ans o med o he non-dimensional Nussel numbe and compa ed wi h Gnielinski’s co ela ion which is commonly used o in ube hea ans e coe icien calcula ion, see igu e 2. The p essu e d op measu emen has no been inished ye . EFM11 01021-p.3 Figu e 2 Measu ed da a 6. DISCUSION Measu ed da a a e in a good ag eemen wi h he Gnielinski’s co ela ion o he Reynolds Numbe be ween 7000 and 15000 ( igu e 2), whe e he ela i e di e ence be ween measu ed and calcula ed da a is lowe han 8 %. The e is a big di e ence in measu ed and calcula ed alues o he Reynolds numbe s lowe han 4000 hough, whe e he ela i e di e ence almos eaches 40 %. I is p obably caused by low ansi ion om lamina o u bulen low, bu he e is no physical e idence o i . The selec ed me hod and measu emen echniques seems o be sui able o he hea ans e coe icien measu emen , as could be seen om he measu ed and calcula ed da a compa ison in igu e 2. Bu he e a e s ill some hings o imp o e, mainly in he ield o da a p ocessing, whe e some di e en echniques e.g. Wilson plo me hod should be used o he hea ans e coe icien calcula ion. 7. ACKNOWLEDGEMENT The au ho s would like o g ea ly acknowledge he inancial suppo ecei ed om he Czech Science Founda ion (unde he p ojec No. P101/11/P538) and om he Facul y o Mechanical Enginee ing, B no Uni e si y o Technology (p ojec No. FSI-S-11-6). 8. REFERENCES [1] S einke, Ma k E.; Kandlika , Sa ish G.: Single-phase hea ans e enhancemen echniques in mic ochannel and minichannel lows. In . P oceedings o he second in e na ional con e ence on mic ochannels and minichannels, June 17-19, 2004. Roches e , USA : ASME, 2004. pp. 141-148. ICMM2004-2328. [2] Kandlika , Sa ish G.: Fundamen al issues ela ed o low boiling in minichannels and mic ochannels. Expe imen al The mal and Fluid Science. 2002, Vol. 26, Issue 2-4, pp. 389-407. ISSN 08941777. [3] Kandlika , Sa ish G., e al.: Hea ans e and luid low in minichannels and mic ochannels. 1s ed. Ams e dam, Ne he lands : Else ie , 2006. 450 p. ISBN 978- 0-08-044527-4. EPJ Web o Con e ences 01021-p.4