Experimental Study on Heat Transfer and Resistance Performance of Elliptical Finned Tube Heat Exchanger with Different Air Entrance Angles

Transcription

1 Experimental Study n Heat Transer and Resistance Perrmance Elliptical Finned Tube Heat Exchanger with Dierent Air Entrance Angles Xueping Du 1, Min Zeng 1, Qiuwang Wang 1*, Zhayi Dng 2 1 Key Labratry Thermal Fluid Science and Engineering MOE, Xi an Jiatng University, Xi an , Shaanxi, China wangqw@mail.xjtu.edu.cn 2 Beijing Lngyuan Cling Technlgy C., Ltd, N.11 Tngji Suth Rad, Beijing , China T save space, smetimes the heat exchanger is necessary t arranged bliquely. In view the act that the air entrance lw directin is nt rthgnal t the heat exchanger surace in many cases r the direct and in-direct air cling system, the present study describes the experimental system abut the elliptical inned tube heat exchangers inclined twards the air ncming lw. The inclined angles between the minr axis the val tube and the ncming air lw are 90 and 60 separately. The heat transer and pressure drp characteristics are investigated. The results shw that the verall heat transer ceicient 60 is abut 9.96 % smaller than that 90 when the ther cnditins are the same, while the pressure drp has a signiicant change when the air velcity is big t sme extent. At last the crrelatins the cnvectin heat transer ceicient and resistant ceicient the air side are acquired. And sme cnclusins are drawn upn the eects the air entrance angles n the heat transer and resistance perrmance the elliptical inned tube heat exchanger. 1. Intrductin Smetimes, a heat exchanger is necessary t arranged bliquely t save space. The air ncming lw directin has a majr impact n the heat and resistance characteristics the air-cled radiatr. It is a widespread prblem that the air inlet directin is nt rthgnal t the radiatr. Circular tubes are used in mst the inned tube heat exchanger. It is prven that using elliptical inned tube can eectively reduce the lw resistance luid in the in side, and lwer the energy cnsumptin. Therere, the research and develpment elliptical tube-in heat exchanger is paid mre attentin. The in-and-tube heat exchangers with circular tubes r val tubes were studied experimentally and numerically (Jang and Yang, 1998). The results shwed that the mean heat transer ceicient an elliptic inned tube was % the crrespnding circular inned tube with the same tube perimeter, while the pressure drp the val inned tube was nly % the rund inned tube cniguratin. Erek et al. (2005) had researched the inluence the gemetry parameters n the heat transer straight in-and-tube heat exchangers, using the CFD technlgy. They indicted that the heat transer wuld be enhanced while the pressure drp culd be reduced thrugh increasing the ellipticity the elliptical ined tube.

2 In recent years, there are many studies n circular inned tube heat exchangers (Wang and Chi, 2000; Tang et al., 2009; Xie et al., 2009), while ew studies n elliptical inned heat exchangers, especially r the blique arrange. S the present paper studied the dierence the heat transer and resistance perrmance when the air entrance angles are 60 and 90 elliptical inned tube heat exchangers. 2. Experimental Apparatus and Prcedures The experiment is cnducted in an pen wind tunnel. The main eatures the experimental apparatus in the system are schematically shwn in Figure 1, including tw lps: ht water lp and air lp. The ht water lp prvides ht luid at the temperature 60 C t the val tubes the test cre. The air lp is supplied t blw air acrss the inned tube heat exchanger the test cre, which is psitined in the test sectin. In the experiment prcess, the air lw rm the labratry rm is induced by the blwer, t pass successully thrugh the pre-duct, test sectin, ater-duct and pitttube meter F Water tank 1-Entrance; 2-Transitin sectin; 3-Cntractin sectin; 4-Straightening sectin; 5-Test heat exchanger; 6- Straightening sectin; 7-Cntractin sectin; 8-Flw metering duct; 9-Expansin sectin; 10- Blwer; 11-Electric heating rd; 12- valve; 13-Water pump; 14-Turbine lwmeter; 15-Data acquisitin system; 16-Thermcuples grid; 17-U tube manmeter Figure 1: Schematic experimental apparatus and system Air is induced t the wind tunnel by a requency mdulatin blwer. The air temperature inlet and utlet acrss the test cre are measured by tw sets multipint T-type cpper-cnstantan thermcuple grids. Each set cntains twelve calibrated thermcuples within the accuracy 0.2 C. Meanwhile, the air velcity is measured by a Pitt-tube meter, which is lcated in the lw metering duct ar dwnstream the test cre, and the wall static pressure the air bere and ater the test cres are surveyed by a U-tube water clumn manmeter. The Pitt-tube meter is cnnected t the micrbarmeter (at small lw rates) r U-tube water clumn manmeter (at high lw rates). Ht water is brught t the heat exchanger by a pump rm a ht water tank, which can heat the water t 60 C with electric heating rds. The E-type armured thermcuples are used t measure the water temperature, and there are ur thermcuples placed in 16 11

3 the water inlet and utlet n each side respectively. Furthermre, the lw rate water is tested by a turbine lw meter. At the same time, the static pressure water side is acquired by Rsemunt pressure transmitters. The experimental samples are cvered by the am insulatin abut 10 mm thick t reduce the heat lss t surrundings. Mre attentin is paid t the heat balance t ensure the steady state heat exchange bere writing dwn the data in the experiments. During the data-acquisitin prcedure, each measured value is read at least ive times, starting reading data n less than rty minutes later when a wrking cnditin is changed. And the arithmetic mean each recrded data is used r checking the heat balance between the energy gain the air and the energy lss the water. Then the data with the smallest thermal equilibrium errr between air side and water side will be saved. In all the remaining data the tests, the errr is less than 7 %. Tw air entrance angles (90, 60 ) are tested in present study. The test sample is an elliptical inned tube heat exchanger shwed in Figure 2, which is nly ne small part the air cling twer used in pwer plant r chemical actry. All tubes and ins are made carbn steel. Due t the parameters the air side having an imprtant eect n the heat transer and resistance perrmance the heat exchanger with dierent air entrance angles, s the present paper nly describe the air side in detail. Figure 2: Duble tube pass elliptical tube inned heat exchanger 3. Data Reductin T acquire the air side heat transer and rictin characteristics, sselt number () and rictin actr ( ) the heat exchangers, and ind ut crrespnding pwer-law crrelatins vs. Re, and vs. Re r each air entrance angle, the experimental data shuld be reduced. The thermal prperties the air are determined by the average value the inlet and utlet temperatures the air side and the thermal prperties the water are evaluated by the inlet and utlet temperatures the water side. The data reductin prcess is illustrated in detail as llws. The heat transer rate Q w water side is given as Qw = ρw qw cpw Δ Tw (1) where qw is the vlume lw rate water.

4 The heat transer rate Q a air side is calculated by Qa = ma cpa Δ Ta (2) where ma is the air mass lw rate. The average heat transer rate the air and the water side is regarded as the ttal heat transer rate, which is cmputed by Qm = ( Qw + Qa)/2 (3) The verall heat transer ceicient based n the heat exchange area the air side is given as Qm k = (4) Δ T A m where Δ Tm is the lgarithmic mean temperature dierence, and A is the heat transer area the air side, including the in area and the base tube area. The water side heat transer ceicient h i, is calculated rm the Gnielinski rmula (Yang and Ta, 1998) 2/3 ( / 8)( Re 1000) Pr d = 1 + 2/3 ct (5) / 8 ( Pr 1) l where the parameters can be und in reerence (Yang and Ta, 1998). T get the cnvectin heat transer ceicient the air side, thermal resistance separatin is adpted. The verall heat transer resistance can be deined as d 1 ln ka = hia + i 2πλl d + i hη A (6) In Equatin (6), h is the air side cnvectin heat transer ceicient, and η is the verall inned surace eiciency, which can be acquired rm the rmulas in reerence (Wang, 2001). Only h and η are unknwn parameters. S they will be gt thrugh sme iteratin prcesses by prgramming with Brland C++ Builder 6.0. The heat transer and rictin characteristics the heat exchanger are presented in the llwing dimensinless rms = h D λ (7) c / Re = v D μ (8) ρ / max c 2Δp D c = (9) 2 ρvmax L where v max is the velcity at the minimum ree lw area the air side. 4. Results and Discussin All the measurement instruments had been adjusted t veriy the test results and methds bere the experiment started t run. Figure 3 shws the crrelatins with Re and with Re when the air entrance angle is 90. And Figure 4 gives the heat transer and resistance perrmance when the air inlet angle is 60. Frm Figure 3 t Figure 4, it can be seen that mst the experiment data are in the range the it errrs which are less than 8 %. Figure 5 shws the cmparisn the verall heat transer ceicient and pressure drp with dierent air entrance angles (90, 60 ) when

5 the water velcity v in each val tube is 1.1 m s -1. It is bserved that a marked decrease the heat transer while the pressure drp has a slight increase when the air entrance angle changes rm 90 t 60. Cmparing the verall heat transer ceicient 60 with that 90, the rmer reduces by 9.96 %. Fr the pressure drp, it can be seen that the pressure drp varies slightly at a small air velcity, while the rmer becmes bigger and bigger than the latter when the air velcity is mre than 1.8 m s -1, and it increased by 6.66 % when the air velcity reached at 4.4 m s Linear Fit = 2.52Re % -7% % % Linear Fit = / Re k 6k 10k 14k 18k Re k 5.7k 9.7k 13.7k 17.7k Re Figure 3: The crrelatins with Re and with Re at air entrance angle= Linear Fit = 3.02Re % -7% % +7% Linear Fit = / Re k 5.7k 9.7k 13.7k 17.7k Re k 5.7k 9.7k 13.7k 17.7k Re Figure 4: The crrelatins with Re and with Re at air entrance angle=60 k /W m -2 K ( ) ( ) v = 1.14m / s 90 v = 1.13m / s 60 Δ p/pa v = 1.13m / s( 60 ) v = 1.14m / s( 90 ) v = 1.13m / s( 60 ) v = 1.14m / s( 90 ) v /m r s -1 v /m r s -1 Figure 5: Cmparisn verall heat transer ceicient and pressure drp with dierent air entrance angle at v=1.1 m s-1

6 Because it is nt distinct pressure drp shwed in lgarithmic crdinates, the trend pressure drp with the air velcity is shwn in Figure 5 in linear crdinate system. The decrease the verall heat transer ceicient 60 % may be caused by the nnunirm air lw when the heat exchanger is placed bliquely. 5. Cnclusins Experimental studies the air side thermal hydraulic perrmance elliptical inned tube heat exchanger with the air entrance angle 90 and 60 are cnducted. The verall heat transer ceicient has a remarkable decrease when the air entrance angles change rm 90 t 60, while the pressure drp des nt change bservably. S it is nt wrth placing the heat exchanger bliquely i nly rm the cnsideratin enhancing heat transer. Otherwise, in rder t save space in practical applicatin, the heat exchanger needs t be psitined bliquely. And nly tw air entrance angles are studied in the present wrk. S the heat transer and resistance perrmance ther air entrance angles heat exchangers need t be urther studied t get the prper angles. Acknwledgment This wrk was supprted by the Fundamental Research Funds r the Central Universities China. Reerences Erek A., Özerdem B., Bilir L. and Ilken Z., 2005, Eect Gemetrical Parameters n Heat Transer and Pressure Drp Characteristics Plate Fin and Tube Heat Exchangers, Applied Thermal Engineering, 25(14-15): Jang J.Y. and Yang, J.Y., 1998, Experimental and 3-D merical Analysis the Thermal-Hydraulic Characteristics Elliptic Finned-Tube Heat Exchangers, Heat Transer Engineering, 19(4): Shi M.Z. and Wang Z.Z., 2003.Principle and Design Heat Transer Device. Press Sutheast University, Nanjing, China; pp. 289 (in Chinese). Tang L.H., Zeng M., and Wang, Q.W., 2009, Experimental and merical Investigatin n Air-Side Perrmance Fin-And-Tube Heat Exchangers with Varius Fin Patterns, Experimental Thermal and Fluid Science, 33(5): Wang C.C. and Chi K.Y., 2000, Heat Transer and Frictin Characteristics Plain Finand-Tube Heat Exchangers, Part I: New Experimental Data, Internatinal Jurnal Heat and Mass Transer, 43(15): Wang Q.C., 2001, Heat Exchanger Design. Wunan Bk C. Ltd, Taipei, China; (in Chinese). Xie G.N., Wang, Q.W. and Sunden B., 2009, Parametric Study and Multiple Crrelatins n Air-Side Heat Transer and Frictin Characteristics Fin-And- Tube Heat Exchangers with Large mber Large-Diameter Tube Rws, Applied Thermal Engineering, 29(1): Yang Sh.M. and Ta W.Q., 1998, Heat Transer (4th Editin). Higher Educatin Press, Beijing, China; (in Chinese).