THE BOILING OF THE REFRIGERANT R134a IN THE RECTANGULAR MICROCHANNELS OF THE CPU S COOLING SYSTEMS

Transcription

1 TEHNOMUS - New Tenologies and Produts in Maine Manufaturing Tenologies THE BOIING OF THE REFRIGERANT R134a IN THE RECTANGUAR MICROCHANNES OF THE CPU S COOING SYSTEMS iliana PĂTUEANU 1, Tudor PĂTUEANU 1 A Department of Meanial Engineering, Stefan el Mare University of Sueava, 13 University Street, zip ode:709 Sueava, Romania, 1 lilianapatuleanu@yaoo.om, A Department of Computer Siene, Tenial University of Cluj-Napoa, abour Boulevard , zip ode: Cluj-Napoa, patuleanut@yaoo.om Abstrat: Te advaned tenology must ensure a proper ooling of eletroni omponents in order for tose to operate at optimal temperatures. Te eat transfer inside te CPU s ooling system is performed by eat exangers, wi inlude mini and miroannels. Te ooling proess wit refrigeration agents represents an effiient solution. Tis artile aims to study ow te boiling proess of te refrigerant R134a under goes in te retangular miroannels of te ooling systems and, also, te manufaturing of su systems. Furtermore, it is presented a alulation model for eat transfer troug retangular miroannels at te boiling point. Te alulation model used is validated by omparison wit results obtained by oter autors in speialized literature. Keywords: eat transfer, frigerant R134a, boiling, mini and miroannels. 1. Introdution Te proess of vaporization troug mini and miroannels of some refrigeration agents in partiular onditions of flow [1] was studied and onsidered by a ig number of sientists. Yan and in (1998) investigated te boiling proess of te Freon R134a, wi oured in orizontal miniannels wit a ydrauli diameter D = mm, a mass flow rate G = kg/m s, a termal flux q = 5 0 kw/m, a ynolds number = Kodabande and Palm (001) investigated te boiling proess of te Freon R134a, loalized in orizontal irular miniannels wit te diameter of 1,5 mm, a mass flow rate G not measured, for a termal flux q = 8 44 kw/m.. Considerations regarding te ooling of CPUs by use of refrigeration agents Te ooling wit refrigeration agents is part of te ategory of speial ooling systems alled extreme, a metod designed just for some users. Tese ooling systems allow te temperature values of te CPU to be brougt in te negative area. By using tis metod, te operation at maximum effiieny of te omputer may be aieved, avoiding te overeating of te proessor tat may endanger its integrity. Te priniple of operation (Fig. 1) of tese ooling systems is te same wit tat of te refrigerator. In te system, ompression is produed, ondensing, expansion, and finally, te vaporization of te refrigeration agent. Te proessor is in diret ontat wit te vaporizer of te set-up were te refrigeration agent absorbs te eat produed by te proessor wen it evaporates, as it passes from liquid state to gaseous state. Te gas reeived from te vaporizer is ompressed in te ompressor and returns to 3

2 TEHNOMUS - New Tenologies and Produts in Maine Manufaturing Tenologies liquid state in te ondenser of te set-up, by ondensation, tat is, delivering eat to te environment. Tus, te pressurized liquid freon enters te termal expansion valve, were, due to te Joule Tomson effet, te temperature substantially dereases to its minimum value. Figure 1: Te representation of te freon ooling proess, []. Tese ooling systems also inlude te systems apocill and Prometeia. Asetek, from Denmark, as been produing sine 1997 te apocill series of CPU ooling units, Asetek apoill Miro Cooler in Fig.. Te tenology implemented in tese units is alled apor Pase ooling and it was proven to be ten times more effiient tan water ooling and fifty times more effiient tan air ooling, so tat iger frequenies of proessors were aieved. Figure : Cooler Asetek apoill Miro, [3]. Te Prometeia ooling system differs from te apoill system as it is not given as a kit, and onsequently, it is arder to assembly. Te freon ooling systems offer a ig degree of performane, tese being able to ool te proessor below 0 ºC. However, tey also ave a great disadvantage, tat is, teir ig osts. 3. Te alulation of eat transfer at boiling troug retangular miroannels Te alulation model proposed by [1] is applied to te boiling proess of Freon R134a troug miroannels tat are diretly engraved in te silion ips. It is onsidered tat te ooling is performed in order to dissipate te eat flux q = 10 kw/m from a proessing unit of a omputer. Parallel miroannels ave a retangular setion wit a widt a = 00 μm, a eigt b = 00 μm and a lengt = 10 mm. Te eated perimeter is P = b + a + b = m, wile te surfae area is A = a b = m. By assumption, te ontat angle θr is 0 degrees and = 100. Te refrigeration agent R134a as te temperature TB,i = 93,15 K at te entrane of te orizontal miroannels. Te properties of te Freon R134a, from [4, 5] at saturation temperature Tsat = 99,7 K and te pressure of 1 atm are: te dynami visosity of te liquid µl = Ns/m, te dynami visosity of te vapours μv = Ns/m, te density of te liquid ρ = 106 kg/m 3, te density of te vapours ρ = 5,5 kg/m 3, te speifi mass eat p, = 1199 J/kg K, te termal ondutivity of te liquid k = 8, W/mK, te termal ondutivity of te vapours k = 9, W/mK, te superfiial tension of te liquid σ = 14, N/m, te latent eat of vaporization = 17, 10 3 J/kg, te speifi entalpy of te liquid i = 591,47 x 10 3 J/kg, te speifi entalpy of te vapours i = 558,85 x 10 3 J/kg Te loation of te inipient pase of te boiling proess Te loation of te inipient pase of boiling and te radius of te avity is determined by following te steps: For some termal flux, it is determined te temperature of te supereated wall: 33

3 TEHNOMUS - New Tenologies and Produts in Maine Manufaturing Tenologies T T q k (1) '' Sat, ONB 8,8 Sat / It is alulated te ydrauli diameter: 4A () 6 D a b m PW Te Nusselt number for a retangular miroannel wit aspet ratio a/ b1 by Table 3.3 from [1], as te values Nufl,3 = 3,556 and Nufl,4 = 3,599. Te loal eat transfer oeffiient is alulated as follows: Nuk D W / m K Te subooling temperature will be: (3) q TSub,ONB TSat, ONB K (4) Te temperature TB at te onset of nuleate boiling is: B,ONB Sat Sub, ONB T T T K (5) Te radius r,rit of te first avity tat will nuleate is found, by [1] as: r k sin T m (10).q r Sat, ONB, rit '' 3.. Te temperature variation of te supereated wall and subooled liquid Te temperature variation of te supereated wall, represented in Fig. 3 in red, and te subooled liquid, represented in Fig. 3 in blue, wit respet te avity radius of te ells for q = 30 kw/m, is omputed using relations from [1] as: 1,1r q sin r TSat TSat/ONB_la_r K k sinr r (11) q TSub,ONB TSat, ONB K (1) Te flow speed, by using te ynolds number is: m / s (6) D Te mass flow rate is: m A kg / s (7) Te mass flux is: G m A kg / m s (8) We are interested in determining te onset of nuleate boiling (ONB). Te inipient boiling loation, by [1], is found at distane z from te entrane end: m z T,ONB T, m qp p, B B i (9) Figure 3: A omparison between te variation T Sat/ONB - r and te variation T Sub /ONB - r Te pressure loss inside te setion Te entrane lengt, for te developed laminar flow is alulated wit te relation: 0,05 D mm (13) Beause = 10 mm >, te assumption tat te flux is ompletely developed is valid. 34

4 TEHNOMUS - New Tenologies and Produts in Maine Manufaturing Tenologies Te plae were te boiling starts to our, oexisting in te two pases, is found at distane z1: m z1 T,z T, m qp p, B B i (14) Te total pressure loss is alulated as a sum of te monopasi pressure loss until z and te bipasi pressure in te two pases from z to. p p p f,1 p p f, tp pa pg pe Pa (15) were: - te omponents p, pg, pe an be negleted beause te pressure loss is alulated in orizontal miroannels: - te omponent Δpf,1-p is alulated as: f,1 p D f Umz Um p K (16) were: - te average speed Um = ; - K( ) is te Hagebna fator alulated for retangular annels wit te relation: K 0, , 197 3, ,591 8,9089, Te Poiseuille number (Po = f) for a retangular annel is given by [1] as: f 4(1 1,3553 1, ,701 0,9564 0, 537 ) Tis yields: (18) f z p f,1 p K Pa D (19) Te eated surfae area in te region of te two pases is:, tp A b a b z m (0) Te mass entalpy (speifi): qa, tp TP IN / GA i i J kg (1) Te termodynami quality xe: x i i TP e () In te alulations tere is used te average quality x = xmed situated between 0 and xe, xmed = 0,0047. Te ynolds number for te vaporous part: GxD (3) Te vapour frition fator is: f f (4) Te pressure loss for te vaporous part: dpf dz fg x D Pa / m (5) Te ynolds number for te liquid part is: G 1 x D (6) Te liquid frition fator is: f f (7) dpf fg (1 x) dz D Pa / m Te Martinelli parameter will be: X dpf dpf / dz dz (8) (9) Assuming bot pases are laminar, a situation were is reommended, in [1] ap. 5, 35

5 TEHNOMUS - New Tenologies and Produts in Maine Manufaturing Tenologies te value of onstant C = 5, for wi te amplifiation fator ɸ beomes: 319D C 1 e 1 1 (30) X X Te omponent pf,tp is alulated as: dp dp p F F f, tp / dz dz Pa m (31) Te pressure loss is: a e p G x Pa (3) Te total pressure loss will be: f,1 p f, tp p p p Pa (33) 3.4. Te termal transfer oeffiient on bot pases Te onvetion number Co used in orrelating te data of eat transfer wen boiling is: 1 / / 0,8 0,5 (34) Co x x For numbers O 100, te eat transfer oeffiient for te liquid pase is alulated as: Nuk D (35) O W / m K Te relation proposed in [1] for te eat transfer oeffiient on bot pases is: 0, 0,8 0,7 TP TP, NBD O 0, Fl Co x Bo F (36) Figure 4: Te variation of te eat transfer oeffiient and te onvetion number of title x, for Freon R134a. 4. alidating te results Te liquid flow rate and te eat transfer in miroannels as great importane in a diversity of areas of interest, inluding in miniature termal systems, in ompat eat exangers or in tose designed for ooling eletronis omponents. X. Tu and P. S. Hrnjak, [6], arried out experiments in order to determine te flow rate and pressure drop in retangular miroannels wit D = 69,5 304,7 µm. Tey studied te eat transfer at te vaporization of te refrigerant R134a and also performed experiments wit a termal flow rate of 10, 15 and 0 kw/m, as well as wit a mass flow rate of 87, 13 and 165 kg/m s. Tey analysed te effet of te vapour title x on te eat transfer oeffiient TP (Fig. 5.a), wile a similar study (Fig 5.b) was performed in 1998 by Yan and in, ited in [1]. From te grapial representation, Fig. 4 an be observed te tendeny to derease of te eat transfer oeffiient TP and te onvetion number Cox depending on te vapour quality, due to te laminar onditions and te dominane of nuleate boiling effets. a. 36

6 TEHNOMUS - New Tenologies and Produts in Maine Manufaturing Tenologies b. Figure 5: Te variation of te eat transfer oeffiient TP wit respet to te title of te x vapours: a Te variation of TP obtained by Tu and Hrnjak, [6]; b Te variation of TP obtained by Yan and in, [7]. By omparing te alulated values for te Freon R134a from Fig. 4, wit tose determined by oter autors, Fig. 5, it an be observed te same way of variation and te same variation grapi. It is sown tat teoretially, tere an be reaed ooling apaities of 700 W/m, in te situation wen all te refrigeration agent vaporizes, [8]. Te aquired results, represented in Fig. 4, sow te same variation tendeny and also te same variation urve of te eat transfer oeffiient as tose obtained by oter autors in Fig. 5, 6 [1, 7, 8]. Tus, te obtained results are validated. ACKNOWEDGMENT Tis paper was supported by te projet Improvement of te dotoral studies quality in engineering siene for development of te knowledge based soiety - QDOC ontrat no. POSDRU/107/1.5/S/78534, projet ofunded by te European Soial Fund troug te Setorial Operational Program Human soures ferenes Figure 6: Te variation of te eat transfer oeffiient on bot pases wit respet to x, for Freon R13, [1]. Tere ave been arried out alulations for determining te eat transfer oeffiient for oter types of freons, like R13. In Fig. 6, [1], an be observed, even in tis ase, a similar tendeny of evolution of te eat transfer oeffiient wit respet to te vapours title. Consequently, on te basis of te omparison between te results obtained by alulation wit tose obtained by oter autors, te alulation model is valid. 5. Conlusions Te ooling systems wit pase ange represent a ig-quality solution for te future. [1] Kandlikar, G. S., Heat transfer and fluid flow in miniannels and miroannels, (Elsevier) OX5 1GB (006). []Freon-Cooling-informatii, 01, ttp://forum.razyp.ro/sowtread.pp/4091. [3] Asetek apoill Miro Cooler - PC Garage, ttp:// asetek/vapo ill-miro/, 01. [4] Arne, J. and oters CoolPak - a olletion of simulation tools for refrigeration, University of Denmark (DTU), 001. [5] Raznjevi, K., Tabele şi diagrame termodinamie, Buureşti, Publising Tenology, [6] Tu X., Hrnjak, P.S. Flow and Heat Transfer in Miroannels 30 to 300 Mirons in Hydrauli Diameter - Illinois: University of Illinois at Urbana-Campaign ACRC, 004. [7] Wu, J. and oters Tree-Dimensional Numerial simulation for Annular Condensation in tangular Miroannels - ondra: Taylor & Franis, 009. [8] Cristea, A., Sientifi ontributions to te study of refrigeration systems for miroeletroni omponents, Summary of PD Tesis,