Effects of Fluid Properties and System Pressure on Convective Boiling in Microtubes

Transcription

1 The 1st Internatonal Symposum on Mcro & Nano Technology, March, 24, Honolulu, Hawa, USA, VII-1-3 Effects of Flud Propertes and System Pressure on Convectve Bolng n Mcrotubes Tzu-Hsang Yen, Nobuhde Kasag and Yuj Suzuk Department of Mechancal Engneerng, The Unversty of Tokyo, Bunkyo-ku, Hongo 7-3-1, Tokyo, ABSTRACT An expermental nvestgaton of convectve bolng n mcrotubes s carred out. Wth ethanol, and FC72 beng employed as a workng flud, the local heat transfer coeffcent and the pressure drop are measured n.19,.3 and.51 mm ID tubes. It s found that the heat transfer coeffcent s nsenstve to the heat and mass fluxes, whle strongly dependent on the nner dameter, surface roughness, system pressure and the latent heat of the workng flud. The pressure loss characterstcs are smlar to those n conventonal-sze tubes. These observatons can be explaned by the nucleate bubble growth n confned space. INTRODUCTION Hghly effcent heat exchangers have become even more mportant because of the rapd ncrease of the heat dsspaton rate n hgh-end electronc devces. Snce Tuckerman and Pease [1] obtaned large overall heat transfer coeffcent n mcrochannels fabrcated on a S wafer, heat transfer research n mcro conduts for both sngle and mult-phase flows have attracted much attenton. In the last decade, pecular heat transfer characterstcs of convectve bolng have been reported n mntubes (.6 ~ 3 mm) and mcrotubes ( <.6mm, e.g., Kandlkar [2] ; Mehendale et al., [3]). In mcrotubes, hgh lqud superheat phenomena were also dscovered. Peng et al. [4] carred out convectve subcooled bolng experments n 2 ~ 6 mm ID mcrochannels and reported that nucleaton bubbles were hardly observed n the bolng regme. Recently, such hgh lqud superheat was attrbuted to the lack of actve nucleate stes. However, the domnant parameters for the onset and the magntude of the superheat are stll unknown. Snce accurate measurement of temperature, pressure, heat flux and mass flow rate n a sngle mcrotube s extremely dffcult, the heat transfer characterstcs n mcrotubes of less than 1 mm ID stll remans unclear. Ravgururajan [5] measured the average heat transfer coeffcent n parallel mcrochannels of 425mm ID. Yen et al. [6] made a seres of experments n a sngle mcrotube of mm ID and examned the lqud superheat and the local heat transfer characterstcs n detal. The objectve of the present study s to extend the expermental study of Yen et al. [6], and obtan local heat transfer coeffcents and pressure losses of convectve bolng n mcrotubes for dfferent refrgerants under dfferent system pressures and surface roughness. We carred out a seres of experments on saturated convectve bolng n mcrotubes of.19,.3 and.51 mm ID at low heat (1~13 kw/m 2 ) and mass (5~35 kg/m 2 s) fluxes. Expermental apparatus Fgure 1 shows the expermental loop of the mcrotube system [6]. A twn plunge pump (Moleh, MT-2221) was employed n most experments n order to provde extremely low mass flux from 5 to 35 kg/m 2 s (about 1-5 ~1-6 kg/s). The uncertanty nterval of the flow rate s wthn + 1%. The system pressure of the test secton s controlled by a helum tank, whch s connected to the pressurzed workng-flud reservor. The system pressure s measured by a daphragm pressure transducer (Druck, PMP-14). Two types of the mcrotubes made from SUS34 stanless steel and ttanum are used. Fgure 2 shows SEM mages of the nner surface of two dfferent mcrotubes. The SUS34 surface tube has grooves of 2~3 µm wdth and cavtes of 4~6 µm dameter. On the other hand, the ttanum tube surface s much smoother wth only grooves of 1~2 mm wdth sparsely dstrbuted. The nner dameter of the test secton was chosen as.51 mm,.3mm and.19 mm for SUS34, and.5 mm for ttanum. Ther outer dameter was respectvely.81 mm,.51 and.41 mm for the SUS34 tubes and.7 mm for the ttanum tube. The length of all the test sectons s about 28cm. The pressure loss of the test secton was measured by a daphragm pressure transducer (Sokken, PZ-77-D) through nlet and outlet pressure ports. The test secton was heated by a drect current, and the heat loss to the envronment was compensated [6]. Twelve K-type thermocouples of 25 µm OD were glued onto the tube outer wall wth thermally conductve slcon (k=.9w/(mk), Shn-Etsu Slcones, KE3493). One K-type thermocouple was nserted nto the nlet manfold for the measurement of the nlet flud temperature. Cold junctons of the thermocouples were submerged nto a standard temperature bath (Komatsu Electroncs, Model ZC-114), n whch the temperature was mantaned at ~.2 K. Calbraton of the thermocouples was made between 2 and 9 C. Standard estmaton errors of the thermocouples were wthn +.1 K. Data reducton procedure The heat loss to the envronment s assumed to be as a functon of the wall temperature T wout. The nner wall temperature T wn s calculated by solvng the onedmensonal heat conducton equaton wth the boundary condtons at the outer surface of the tube. After onset of the saturated bolng, the workng flud temperature T ref s assumed to be at the saturated temperature. The saturated temperature s determned usng local pressure, whch s estmated by a lnear nterpolaton of the pressure dfference over the saturated regon. The heat transfer coeffcent s then 1

2 The 1st Internatonal Symposum on Mcro & Nano Technology, March, 24, Honolulu, Hawa, USA, VII-1-3 Flow drecton Flter Pressure loss transducer Absolute pressure transducer Condenser pump Test secton Pump Crculator (a) Pressurzed Helum Tank Tank Tank Fg. 1 Expermental loop. Inlet pressure port 28 cm V Outlet pressure port (b) P n I Voltage suppler D =.19 and.51mm, SUS34 and ttanum P out Flow drecton x K-type thermocouples Fg. 3 Test secton. Fg. 2 SEM mages of the nner surfaces of SUS34 (a) and ttanum (b) test sectons. calculated as q h sat= T wn -T ref. (1) Snce the flud s heated from subcooled lqud to superheated vapor states n the test secton, the total pressure loss P total s composed of Ptotal = Psub + Psat + P sup, (2) where P sub, P sat, and P sup are the pressure losses n the subcooled lqud, saturated bolng, and superheated vapor regons, respectvely. The length of the subcooled regon l and the pressure loss DP sub are calculated as follows. We frst assume an arbtrary value for l, and calculate P as sub Psub = f ρu, (3) 2 D where f, r and U are the frcton factors of the lamnar Poseulle flow, the lqud densty and the bulk mean velocty, respectvely. The saturaton pressure at x = l s gven by ( ) P l = P P, (4) sat n sub where P n s the nlet pressure. Then, the saturaton temperature T sat s calculated from the saturaton table of the refrgerant (REFPROP). Fnally, the new value of l s obtaned from the energy balance,.e., l ( ) π = p( sat( ) n) qx Ddx MC T l-t. (5) The teratve calculaton usng Eqs. (3-5) s repeated untl the value of l converges. The local vapor qualty n the saturated regon s calculated as ( x) = x l ( ) π qx Ddx Mh lv. (6) The length of the saturated regon s s determned from the above ntegraton n such a way that = 1 at x=l+s. The pressure loss n the superheat regon P sup (l + s< x < L) s determned by the lamnar flow soluton for vapor usng physcal propertes at the local pressure and temperat ure. Fnally, the pressure loss n the saturated bolng regon, P sup, s determned by Eq. (2). The local pressure P sat (x) n the saturated bolng regon s assumed to be lnearly dstrbuted along the tube, ( ) ( ) P x = P l P sat sat sat x l s. (7) Expermental results Fgure 4 shows the dstrbuton of the absolute wall temperature n the.19mm ID SUS34 tube when q = 5.5 kw/m 2, where the wall temperature measured between x =.4 and.2 m s n accordance wth the saturated temperature. It decreases wth ncreasng x due to the pressure drop n the test secton. Thus, conventonal saturated bolng should occur under ths heat flux condton. On the other hand, the wall temperature for a smaller heat flux of q = 2.4 kw/m 2, monotoncally ncreases wth the axal dstance and reaches 11 C at x =.27 m. The pressure drop s also n good agreement wth the estmate based on the 2

3 The 1st Internatonal Symposum on Mcro & Nano Technology, March, 24, Honolulu, Hawa, USA, VII-1-3 T wn ( O C) q = 2.4 kw/m 2 q = 5.5 kw/m 2 6x Bo 3 2 FC72 x (m) Fg. 4 Wall temperature dstrbuton n.19mm ID SUS34 tube at 2..1 m =145kg/m 2 s. saturated temp m (kg/m 2 s) Fg. 6 Onset of lqud superheat for dfferent refrgerants n.19 mm ID SUS34 tube. 6x D=.19 mm D=.51 mm D=.3 mm 6x D =.51 mm, Ethanol 1.1 atm 1.9 atm Bo 3 Bo m (kg/m s) Fg. 5 Onset of lqud superheat n dfferent nner dameter SUS34 tubes for. Open symbols represent saturated bolng, whle close symbols superheat lqud. sngle-phase lamnar flow [6]. Snce n the sngle-phase flow, the temperature dfference between the nner wall and the refrgerant s only 1~3 O C, the lqud nsde the mcrotube under the above expermental condton should be n the superheat lqud state. Ths phenomenon s also observed for a stagnant flud n mcro capllares by Brereton et al. [7] Fgure 5 shows the condton map for the superheat phenomenon as a functon of the bolng number Bo and the mass flux. When s fxed, the lqud superheat occurs at smaller Bo. On the other hand, when the mass flux ncreases, the crtcal Bo for the onset of bolng decreases and seems to be ndependent of the tube nner dameter. Fgure 6 shows the map for FC72 and n the.19 mm ID tube. The superheat regme s almost the same for the two dfferent refrgerants, whch have dfferent latent heats, but smlar surface tenson. Fgure 7 shows the map for ethanol n dfferent system pressures, the superheat regon s also smlar under dfferent system pressures. Fgure 8 shows the heat transfer coeffcents of at the same mass flux n the mcrotubes of 1 2 m (kg/m 2 s) 3 Fg. 7 Onset of lqud superheat under dfferent system pressures n.51 mm ID SUS34 tube. dfferent nner dameters. The saturated bolng heat transfer coeffcent decreases as the nner dameter decreases. It also decreases as the vapor qualty s ncreased to =.3, then remans almost constant toward = 1. Such varaton of the heat transfer coeffcent s completely dfferent from those n small or tradtonal-sze tubes, where the convectve bolng effect s domnant and the heat transfer coeffcent ncreases wth ncreasng at <.9. The decrease of the heat transfer coeffcent wth the ncrease of the vapor qualty can be explaned by the flow patterns n the mcrotube [8]. In the mcrotube, the man flow patterns are the slug flow and the annular flow. Partal dryout usually occurs n the local regon covered by slugs. When the vapor qualty ncreases, the length of the slug ncreases and the dryout regon becomes larger. Fgure 9 shows the heat transfer coeffcent of and FC72 n the.19 mm tube at smlar mass fluxes, heat fluxes and system pressures. The heat transfer coeffcent of s larger than that of FC72. The physcal propertes of and FC72 are smlar, but has about two tmes 3

4 The 1st Internatonal Symposum on Mcro & Nano Technology, March, 24, Honolulu, Hawa, USA, VII-1-3 (W/m 2 K) D [mm], q [kw/m 2 ] D =.51, q =12.56 D =.19, q =5.5 1x1 3 8 (W/m 2 K) 6 4 D =.51mm, Ethanol, m =15kg/m 2 s, q =63 kw/m 2 P sys =134.7 kpa P sys =182.4 kpa Fg. 8 Heat transfer coeffcent versus vapor qualty under dfferent ID of the tube at m =145kg/m 2 s. Fg. 1 Heat transfer coeffcent under dfferent system pressures n.51 mm ID SUS34 tube. (W/m 2 s) D =.19 mm, P sys =19~2 kpa, m = 2 kg/m 2 s, q = 7.4 (kw/m 2 ) FC72, q = 8.9 (kw/m 2 ) 3x dp (Pa) 15 1 Ethanol, D =.51mm m =15kg/m 2 s, P sys =13~14 kpa P sys =18~19 kpa Fg. 9 Heat transfer coeffcent of dfferent workng fluds n.19mm ID tube. larger latent heat than FC72. Therefore, t s conjectured that, n mcrotubes, the heat transfer coeffcent s sgnfcantly affected by the latent heat of the workng flud. Ths s because the evaporaton n the mcro layer regon beneath the slug bubble s consdered to be the man mechansm of the convectve bolng n mcrotubes [8]. Thus hgher latent heat of a workng flud results n hgher heat transfer coeffcent [9]. Fgure 1 shows the heat transfer coeffcent of ethanol n the.51mm ID tube under dfferent system pressures. The heat transfer coeffcent of ethanol s much larger than that of and FC72 shown n Fg. 9. Agan, ths s probably because the latent heat of ethanol s about 5 tmes larger than that of. When the system pressure s elevated, the heat transfer coeffcent also becomes larger. Ths phenomena can be consdered as the nteracton between the bubble growth and the confned space n the mcrotube, whch wll be descrbed later. Fgure 11 shows the pressure loss of ethanol convectve bolng versus the ext vapor qualty n.51mm ID tube under dfferent system pressures. The Fg. 11 Pressure loss versus vapor qualty n.51mm ID SUS34 tube under dfferent system pressures. effect of system pressure to the pressure loss seems to be mnor. Fgure 12 shows the heat transfer coeffcents of FC72 wth dfferent tubes. As shown n Fg. 2, the ttanum tube has a smoother surface than SUS34. It s found that the surface roughness has a strong effect on the heat transfer coeffcent. The heat transfer coeffcent n the SUS34 tube (rougher surface) s about two tmes larger than that n the ttanum tube (smoother surface). The present fndngs n mcrotubes are n accordance wth prevous results of convectve bolng n conventonal tubes [1], the number densty of the cavtes becomes smaller on a smoother surface. Comparson of the heat transfer and pressure loss data to the vsualzaton experment Nasu et al. [11] made a mcrochannel, of whch hydraulc dameter s 2 mm, wth the ad of the MEMS technologes. They made flow vsualzaton of forced convectve bolng under the constant heat flux condton. Fgure 13 shows ther flow vsualzaton mage and the conceptual schematc of the flow pattern. The flow regon can be dvded nto three regons; the 1. 4

5 The 1st Internatonal Symposum on Mcro & Nano Technology, March, 24, Honolulu, Hawa, USA, VII-1-3 (W/m 2 s) m = 313kg/m 2 s, q =15kW/m 2 s, P sys =11~13kPa Ttanum, D =.5 mm SUS34, D =.51 mm Intal bubble Bubble growth regon regon Slug and annular flow regon Fg. 13 An nterpretaton of the heat transfer mechansm of convectve bolng n mcrotubes wth reference to the flow vsulzaton by Nasu et al. [9] Fg. 12 Heat transfer coeffcent versus vapor qualty n tubes of dfferent materals. ntal bubble regon, the bubble growth regon and the slug and annular flow regon. In the ntal bubble regon, the bubble s stll small compared wth the nner dameter of the tube. It s conjectured that ths s why the onset of superheat phenomena s ndependent of the system pressure and tube ID. On the other hand, n the bubble growth regon, the bubble dameter becomes comparable to the tube ID, and the confned space start to restrct the bubble to slug flow. Thus, the tube ID and the system pressure affects the heat transfer coeffcents as descrbed n the prevous chapter. Fnally, n the annular flow regon, the pressure characterstcs are smlar to those n conventonal-sze tubes because nucleaton becomes less mportant n these flow regons. Therefore, the pressure loss characterstcs s smlar to that n conventonal tubes [6] and remans unchanged under dfferent system pressures. Conclusons The heat transfer coeffcent and the pressure loss of the convectve bolng n mcrotubes were nvestgated usng dfferent IDs, surface roughness, and workng fluds. The followng conclusons can be derved: 1. The system pressure has large effect on the heat transfer coeffcent. When the system pressure becomes larger, the heat transfer coeffcent becomes larger. 2. Smooth surface nhbts the bubble nucleaton, and the heat transfer coeffcent becomes smaller. 3. Accordng to the vsualzaton experment by Kandlkar [8] and Nasu et al. [11], the slug flow pattern and annular flow pattern are the man flow patterns n the convectve bolng of mcrotubes. Thus, the pecular heat transfer characterstcs can be consstently explaned by the characterstcs of the slug bubble behavor n the mcrotube. Nomenclature Bo: bolng number, Bo=q/h lv G C p : specfc heat at constant pressure [J/(kg K)] D o : outer dameter [m] D : nner dameter [m] f : frcton factor M : mass flow rate [kg/s] m : mass flux [kg/m 2 s] h lv : latent heat [J/kg] : saturated bolng heat transfer coeffcent [W/m K] l : subcooled regon length [m] P sys : test secton pressure [kpa] P sub : pressure loss over the subcooled regon [kpa] P sup : pressure loss over the superheated regon [kpa] P total : total pressure loss of the test secton [kpa] q: heat flux [W/m 2 ] T ref : refrgerant temperature [ O C] T wout : outer wall temperature [ O C] T wn : nner wall temperature [ O C] U: bulk mean velocty [m/s] x: axal coordnate [m] : vapor qualty r :lqud densty [kg/m 3 ] References [1] Tuckerman, D. B., Pease, R. F. W., Hghperformance heat snkng for VLSI. IEEE Elec. Dev. Letters EDL-2, [2] Kandlkar, S. G., 22. Fundamental ssues related to flow bolng n mnchannels and mcrochannels, Exp. Therm. Flud Sc. 26, [3] Mehendale, S. S., Jacob, A. M., Shah, R. K., 2. Flud flow and heat transfer at mcro- and meso-scales wth applcaton to heat exchanger desgn. App. Mech. Rev. 53, [4] Peng, X. F., Wang, B. -X., Forced convecton and flow bolng heat transfer for lqud flowng through mcrochannels. Int. J. Heat Mass Transfer 26, [5] Ravgururajan, T. S., Impact of channel geometry on two-phase flow heat transfer characterstcs of refrgerants n mcrochannel heat exchangers, Trans ASME: J. Heat Transfer. 12,

6 The 1st Internatonal Symposum on Mcro & Nano Technology, March, 24, Honolulu, Hawa, USA, VII-1-3 [6] Yen, T.-H., Kasag, N. and Suzuk, Y., 23. Forced Convectve Bolng Heat Transfer n Mcrotubes at Low Mass and Heat Fluxes, Int. J. Multphase Flow 29 Issue 12, [7] Brereton, G. J., Crlly, R. J., Spears, J. R., Nucleaton n small capllary tubes. Chem. Phys. 23, [8] Kandlkar, S.G., 23, Flow Bolng In Mcrochannels: Non-Dmensonal Groups And Heat Transfer Mechansms." Thermque et Mcrotechnologes Proceedngs of congres de la socete Francas de la Thermque 23, Edtors P. Marty, A. Bontemps, S. LePerson And F. Ayela, June 3-6, 23, pp 3-21 [9] Wayner, P. C., Kao, Y. K., and LaCrox, L. V., The Interlne Heat-Transfer Coeffcent of an Evaporatng Wettng Flm. Int. J. Heat Mass Transfer 19, [1] Yu, J., Momok, S. and Koyama, S., Expermental study of surface effect on flow bolng heat transfer n horzontal smooth tubes. Int. J. Heat Mass Transfer 42, [11] Nasu, N., Suzuk, Y. and Kasag, N., 23. Fabrcaton of Prototype Testbench for Forced Convectve Bolng n Mcrotube. Proc. 4th Natonal Heat Transfer Symp. of Japan, Hroshma, Japan, (n Japanese). 6