FIN DESIGN FOR FIN-AND-TUBE HEAT EXCHANGER WITH MICROGROOVE SMALL DIAMETER TUBES FOR AIR CONDITIONER

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1 FIN DESIGN FOR FIN-AND-TUBE HEAT EXCHANGER WITH MICROGROOVE SMALL DIAMETER TUBES FOR AIR CONDITIONER Yfeng Gao (a), J Song (a), Jngdan Gao (b), Guolang Dng (b)* (a) Internatonal Copper Assocaton Shangha Offce Shangha , Chna, frank.gao@copperallance.asa (b) Insttute of Refrgeraton and Cryogencs, Shangha Jao Tong Unversty Shangha , Chna, gldng@sjtu.edu.cn ABSTRACT Optmal louver fns sutable for 5 mm dameter tubes are desgned by Computatonal Flud Dynamc-based method n ths study. Based on the desgn result, a set of fn-and-tube heat exchangers wth 5 mm dameter tubes are tested to develop correlatons to predct the performance of new fn-and-tube heat exchanger. Accordng to the expermental results, t s found that water brdge occurs at the bottom of fn wth hydrophlc coatng, whch dd not occur n fn-and-tube heat exchangers wth 7 mm or mm dameter tubes n prevous studes. Based on the data, correlaton of j s developed to predct the heat transfer rate of fn-and-tube heat exchanger wth 5 mm dameter tubes. The mean devatons of the proposed j correlaton are 6.5%. 1. INTRODUCTION Fn-and-tube heat exchangers are wdely used n ar condtoners. For lower materal cost, fn-and-tube heat exchangers wth mcrogroove small tubes (dameter s smaller than or equal to 5 mm) gradually replace those wth 7 mm or larger dameter tubes. When tube dameter decreases from 7 mm to 5 mm, the tube cross-sectonal area can be reduced by 49%, and the refrgerant charge can be decreased accordngly. Due to the lower refrgerant charge, greenhouse effect caused by applcaton of hgh GWP refrgerants may be reduced, and the exploson rsk of ar condtoners usng flammable refrgerants (e.g. R290) can be also obvously decreased. Moreover, wth the decrease of tube dameter, the heat exchanger can be more compact f a constant heat exchanger capacty s requred; or the heat exchange capacty can be enhanced f a constant heat exchanger sze s requred. However, the present fn confguraton wth large dameter tubes cannot be drectly used for smaller tubes due to dfferent confguratons of heat exchangers. In fn-and-tube heat exchanger, the permtted fn ptch depends on the tube dameter, and the fn sze s related to the balance of fn sde heat transfer resstance and tube sde heat transfer resstance, so fn confguraton wth smaller dameter tube s dfferent from that wth larger dameter tubes. The msmatchng of fn confguraton wth tubes may decrease heat transfer rate and ncrease ar-sde pressure drop. Thus, t s necessary to desgn a set of sutable fn confguratons wth smaller dameter tubes. For fn confguraton desgn, emprcal equatons developed on the expermental data of heat exchangers are employed due to short tme consumpton and less resource requrement. However, large devatons wll occur between predcted values and expermental data, when the emprcal equatons (Wang et al., 2002; Ma et al., 2007; Ma et al., 2009) for 7mm or larger dameter tubes are used for predctng performance of heat exchanger wth mcrogroove small tubes. Untl now, there are no emprcal equatons for heat exchanger wth mcrogroove small tubes n lterature. Therefore, emprcal equatons of heat exchanger wth mcrogroove tubes should be developed. In ths study, a set of fns wth hgh heat exchange rate and low ar-sde pressure drop for 5 mm dameter tubes was desgned by computatonal flud based method. Based on the desgn results, heat transfer rate of fn-and-tube heat exchanger wth 5 mm dameter tubes were tested n a closed loop. The effects of fn confguraton, ncludng fn sze, fn ptch, on the heat transfer rate and ar-sde pressure drop were analyzed. Moreover, new correlatons of heat transfer for fn-and-tube heat exchanger wth 5 mm dameter tubes were developed, and correlatons agree wth the experment data well. 1

2 2. FIN CONFIGURATION DESIGN Fn confguraton sutable for 5 mm dameter fn-and-tube heat exchanger s desgned by the method shown n Fg. 1. The fn sze and fn pattern whch manly affect heat transfer and ar pressure drop are desgned by a Computatonal Flud Dynamc (CFD) based method. Fgure 1. The scheme of desgn method of heat exchanger wth mcrogroove smaller tubes 2.1 Determne the optmal rato of P t to P l In ths study, the optmal rato of P t to P l refers to that of fn wth the hghest fn effcency among fns whch have the same area. The fn effcency s defned as the rato of the actual fn heat transfer capacty (Q actual,fn ) to the maxmum possble heat transfer capacty (Q deal,fn ) f the entre fn were at the base temperature, as Eq. (1) shown: Q a ctual,fn Q deal,fn, (1) Both of Q actual,fn and Q deal,fn are calculated by CFD method. In CFD calculaton, the values of P t /P l are selected n the range of commonly used P t /P l of heat exchangers n ar condtoners. The geometrcal models are selected as the fn-and-tube wth 2-rows tubes. The boundary condtons are: 1)Ar nlet temperature s set as 300 K;2)The tube wall temperature s set as 280 K.;3)The fn s coupled wth tube wall n the model of actual fn, and the fn temperature s set as the same wth that of tube wall n the model of deal fn; 4) The upper and under ar surface are defned as perodc surface wthout pressure drop. From the CFD result n Fg. 2, the best P t /P l can be easly determned as 1.23 where the fn effcency reaches the hghest value. Fgure 2. Varatons of fn effcency as functon of wnd velocty 2

3 2.2 Optmze fn sze For evaporator, the louver or slt wll be blocked by condensate flm whch forms n refrgeraton condtons, so the louver or slt fn s lke a plate fn. Thus, ths method employs the correlatons for plate fn to determne fn sze. In fn sze optmzaton, one objectve functon s used to analyze the rato of performance to materal cost as Eq. (2) shown. And two constrant functons, n Eq. (3) and (4), are that the UA of fn for smaller tubes should be equal to or larger than requrement, and the ar pressure drop should be equal to or lower than requrement. UA max w, (2) C UA UA, (3) requre ΔP < ΔP, (4) requre For 5 mm dameter fn-and-tube heat exchanger, The UA should be hgher than that of plate fn for 7 mm dameter tubes, and the ΔP should be smaller than that of plate fn for 7 mm dameter tubes. Based on the above desgn prncple, the varatons of heat transfer coeffcent, ar pressure drop and the rato of performance to materal cost as functon of the P t are calculated and shown n Fg. 3 (a) to (c). From the results, the best P t can be determned as 18 mm whch has hgh w and also satsfes the constrants of UA and ΔP. From the optmal P t /P l, the optmal fn sze s determned as mm. (a) (b) 3

4 (c) Fgure 3. Varatons of w, UA and ΔP as functon of P t : (a. Rato of performance to materal cost-w; b. Heat transfer performance-ua; c. Ar pressure drop-δp) 2.3 Optmze fn pattern In fn pattern optmzaton, because of no emprcal correlaton for predctng the performance of enhanced fn-and-tube wth smaller dameter tubes s publshed, the CFD method s used to smulate the heat transfer capacty and ar pressure drop of heat exchangers n ths study. Louver fn can break ar boundary more easly than slt fn. In ths study, louver fn s chosen to be desgned. For louver fn, the louver angle and louver number are ndependent varables, whle louver heght and louver ptch are determned by two ndependent varables. Wth the lmtaton of manufacture, the louver s usually 25. Thus, the only ndependent varable s louver number. Based on the optmal fn sze, the performances of fns wth 3 louvers and 4 louvers are calculated by CFD method. Fg. 4 shows the temperature dstrbutons on fn surface. Table 1 shows the results of heat transfer capacty and ar pressure drop of heat exchanger. From results, the fn wth 4 louvers has hgher heat transfer capacty, and smaller ar pressure drop than those of fn wth 3 louvers whch are caused by the more louver number but lower louver heght. Thus, the optmal fn pattern s the louver fn wth 4 louvers. (a) Louver fn wth 3 louvers (b) Louver fn wth 4 louvers Fgure 4. CFD results of temperature dstrbuton on louver fns wth dfferent patterns Table 1: Heat transfer capacty and ar pressure drop of louver fns wth dfferent pattern Louver angle Louver number Heat transfer capacty (W) Ar pressure drop (Pa) 25 o

5 3. EXPERIMENT Based on the desgn results, a set of fn for fn-and-tube heat exchanger wth 5 mm dameter tubes s tested to develop correlatons to predct the performance of fn-and-tube heat exchanger wth 5 mm dameter tubes. The test samples are 11 fn-and-tube heat exchangers whch consst of alumnum fns and copper tubes wth dfferent fn szes, fn ptches and rows, but wth the same louver number because of manufacture lmtaton. The detaled fn confguratons are shown n Table 2. Table 2: Geometrc dmenson of the tested fn-and-tube heat exchangers P t P l (mm mm) δ (mm) D c (mm) F p (mm) Row number Fn pattern and detals / louvers /1.2/ louvers /1.3/ louvers /1.3/ louvers 3.1 Expermental apparatus The expermental apparatus ncludng an ar flow loop, a water flow loop, a data acquston system and the test heat exchangers are shown n Fg. 5. Fgure 5. Schematc of expermental system The ar flow loop s a close type wnd tunnel. A varable speed centrfugal fan (0.75 kw) s used to crculate the ar passng through the nozzle chamber, the ar condtoner box, the mxng devce, the straghtener, and the test heat exchanger orderly. A pressure transmtter (GE Druck, model PTX 1400) wth ±1.0 kpa precson and a dry bulb and wet bulb temperature transducer (CHINO, model R220-30) wth ±0.3K precson are used to measure the nlet ar condtons of nozzles. Ar pressure dfference across nozzle s measured by a dfferental pressure transmtter (GE Druck, model LPM 9000) wth ±5.0 Pa precson. Multple nozzles based on the ASHRAE 41.2 standard (1987) are used to measure the ar flow rate. The ar condtoner box s used to control the temperature and humdty of ar at test secton nlet, whch are allowed ±0.2 K and ±3% fluctuaton range. A 20 mm thck thermal nsulaton materal s used to nsulate the test secton to avod heat transference between the heat exchanger and ar. The dry bulb temperature and relatve humdty of ar at nlet and outlet of test secton are measured by two temperature and humdty transducers (VAISALA, model HMP 233) wth ±0.1 K and ±1.4% precson. Sx K-type thermocouples wth ±0.1 K precson welded on the tube surface are used to measure the fn base temperature. A dfferental pressure transmtter (GE Druck, model LPM 9481) wth ±0.2 Pa precson s used to measure the pressure dfference across the heat exchangers. The water flow loop conssts of a thermostat (ADVANTEC, model TBH 127AA), a centrfugal pump and a magnetc flow meter (TOKYO KEISO, model MGM 1010K) wth ±0.15 L/mn precson. Cold water s used 5

6 as heat transfer flud on the tube sde. Water loop s amed to provde the cool capacty of the test heat exchangers. Water s pumped out of the thermostat, delvered to the heat exchanger and then returned to the thermostat, when t reaches the requred temperature. The water temperature dfferences between nlet and outlet of heat exchangers are measured by two K-type thermocouples wth a calbrated accuracy of ±0.1 K. All sgnals are regstered by a data acquston system and fnally averaged over the elapsed tme. Total 14 test condtons are lsted n Table 3. Table 3. The test condtons RH n (%) T a,n (K) T w,n (K) V (m/s) /1.0/ /0.8/1.0/1.2/ /1.0/ /1.0/ Data reducton The reducton process s based on the Threlkeld (1970) method whch s an enthalpy-based reducton method. Some mportant reducton procedures are descrbed as follows. More detals can be found n prevous study (Wu et al., 2012). In the experments, only those data that satsfy the ASHRAE (2000) requrements (the energy balance condtons, Q w Q a /Q ave 0.05) are consdered n the fnal analyss. The total heat transfer coeffcent of heat exchanger can also be calculated as Eq. (8), where the fn effcency s defned under partally wet condton: Heat transfer coeffcent n tube can be calculated as: D b A ln ' c ' p o ' 1 b A D b A r o w, p o U h A 2πk L h A η o, w p, p p o, w p, o f, wet A f, (8) where, j factor s presented as: k h D 1 f Re D1000 Pr f / 2 2/ f / 2Pr lnRe D, (9), (10) hs j G c c p, a Pr 2 / 3, (11) 3.3 Results and dscusson Fg. 6 depcts the effect of fn ptch on ar-sde heat transfer performance of heat exchanger wth 5 mm dameter tubes. The Colburn j factors decrease wth the ncreases of fn ptch. Moreover, water brdge occurs at the bottom of fn wth hydrophlc coatng as Fg. 7 shows, whch dd not occur n fn-and-tube heat exchanger wth 7 mm or mm dameter tubes n prevous studes (Ma et al., 2007 and 2009). The occurrence of water brdge may be due to the smaller fn sze and fn ptch. When fn sze decreases, the maxmum coverage area of flm decreases accordngly whch leads to the heght of condensate flm ncreases. Then, the smaller fn ptch allows hgher condensate flms at adjacent fn surface to combne to a water brdge. However, because of the small coverage area and the bottom locaton of water brdge, the effects of relatve humdty and fn ptch on heat transfer performance are smlar to prevous research (Ma et al., 2007 and 2009) n whch no water brdge occurred on the fn surface. 6

7 Fgure 6: Varatons of Colburn j factor as functon of ar Reynolds number Fgure 7: The locaton of water brdge on fn for 5 mm dameter tubes 4. CORRELATIONS The multple lnear regresson technque n a practcal range of expermental data (350 < Re Dc < 4500) s carred out, and the sutable correlaton of j s gven as follows: j Re P F t p Dc N, (12) Pl Range for applcablty for Eq. (12) s gven as follows: D c =5.2 mm, P t =18-19 mm, P l = mm, F p = mm, N=1-2, Re Dc = The proposed heat transfer j factor correlaton, Eq. (12), can descrbe 85.7% of the test data wthn the devaton of ±15%. The proposed correlaton of j has a mean devaton of 6.5%. D c 5. CONCLUSIONS (1) Optmal louver fn confguraton for 5 mm fn-and-tube heat exchanger s desgned by Computatonal 7

8 Flud Dynamc method. (2) Water brdge occurs at the bottom of fn wth hydrophlc coatng, whch dd not occur n fn-and-tube heat exchangers wth 7 mm or mm dameter tubes n prevous studes. (3) Correlaton of j s developed to predct the heat transfer rate of fn-and-tube heat exchanger wth 5 mm dameter tubes. The mean devaton of the proposed j correlaton s 6.5%. NOMENCLATURE A heat transfer area (m 2 ) C materal cost (kg) D dameter (m) F fcton factor fb F p fn ptch (m) G c mass flus (kgm -2 s -1 ) h convecton heat transfer coeffcent (Wm -2 K -1 ) enthalpy (kjkg -1 ) I 0 Bessel functon soluton of the second knd, order 0 I 1 Bessel functon soluton of the second knd, order 1 j Colburn heat transfer factor K 0 Bessel functon soluton of the frst knd, order 0 K 1 Bessel functon soluton of the frst knd, order 1 N Row number of tubes P t transverse tube ptch (m) P l longtudnal tube ptch (m) ΔP pressure drop (Pa) Q heat transfer capacty (W) Re Dc Reynolds number based on the outer tube dameter δ fn thckness (m) Subscrpts a ar f fn fb fn base ft fn tp nner n nlet o outer r refrgerant s saturate w water REFERENCES Threlkeld T., 1970, Thermal Envronment Engneerng, Prentce-Hall, New York, p Ma, X.K, Dng, G.L., Zhang, Y.M., et al., 2007, Arsde heat transfer and frcton characterstcs for enhanced fn-and-tube heat exchanger wth hydrophlc coatng under wet condtons, Int. J. Refrg., vol. 30: p Ma, X.K, Dng, G.L., Zhang, Y.M., et al., 2009, Arsde characterstcs of heat, mass transfer and pressure drop for heat exchangers of tube-n hydrophlc coatng wavy fn under dehumdfyng condtons, Int. J. Heat and Mass Transfer, vol. 52, no : p Wang, C.C., Lee, W.S., Sheu, W.J., et al, 2002, A comparson of the arsde performance of the fn-and-tube heat exchangers n wet condtons wth and wthout hydrophlc coatng, Appl. Thermal Eng, vol. 22: p Wu, W., Dng, G.L., Gao, Y.F., et al, 2012, Expermental nvestgaton of fn-and-tube heat exchanger wth smaller dameter tubes. The 6th Asan Conference on Refrgeraton and Ar Condtonng, August 26-28, 2012, X an, Chna. 8