HEAT TRANSFER ENHANCEMENT BY INSERTS DURING CONDENSATION OF R-245fa INSIDE A HORIZONTAL TUBE

Abstract

An experimental investigation has been made to study the enhancement in heat transfer coefficient and pressure drop during condensation of the pure vapor of R-245fa inside a horizontal tube. For the purpose of enhancement in heat transfer coefficient, different tube inserts have been tested. The test set-up had two circulation loops, one for the circulation of refrigerant, and another for the circulation of cooling water. The test-condenser contained-two--- double pipe -heat _ exchangers connected in series. Each exchanger had a hard drawn copper tube having 9.4 mm inner diameter, 12.76 mm outer diameter and 1.0 meter length. This tube was concentrically fixed inside a copper pipe of 43 mm inner diameter, thus, forming a counter flow heat exchanger type test-section. The cooling water was circulated in the annular space between the tubes of test-condenser and the refrigerant was condensed inside the inner tube. The turbine flow meters were used to measure the cooling water flow rate in each of the test-sections and a Coriolis effect mass flow meter was used to measure the refrigerant mass flow rate. The cooling water temperature rise was measured in each test-section with the help of thermopiles. The outer wall temperature of the inner tube of test-section was measured using T-type thermocouples at four axial locations at equal distance from each other. On every location the thermocouples were brazed at the top, bottom, left and right sides positions. The temperature and the pressure of refrigerant were measured at the inlet of each test-section, and at the inlet and outlet of steel tube evaporator as well. In order to monitor the pressure in the refrigerant loop the pressure transducers were fixed at the inlet header and the outlet header of the bank of three gear pumps. The refrigerant mass flow rate was regulated by monitoring speed of each pump through a frequency controller. The heat input to evaporator was monitored through auto-transformer to get the desired vapor quality at the inlet of test-condenser. The data were acquired for nine different mass fluxes in the range of 100 kg/m2-s to 500 kg/m2-s. In order to cover the entire range of vapor quality (0.1 to 0.9) nearly 10 test-runs were made for each mass flux. A total of six twisted tape inserts were investigated having the twist ratio in the range 4-9 and six coiled wire springs were used with coil pitch in a range of 3 mm to 17 mm. For all the investigations the coil wire diameter was 1.0 mm and the thickness of the twisted tape was 0.5 mm. A perforated tape of twist ration of 7.1 was also used with the pitch of perforation as 12.5 mm, 25.0 mm and 37.5 mm. iii Abstract First, the integrity of the experimental set-up was established by acquire the experimental data for the condensation of R-134a inside a plain tube. These data were compared with the predictions from different correlations. Subsequently, the data were acquired for the condensation of R-245fa inside a plain tube and a comparison was made between the plain tube experimental data and the heat transfer coefficients predicted by the correlations of other investigators The data were in the best agreement with the Traviss correlation (1973). The pressure drop data were also compared with the predictions of different correlations and the best agreement occured with the correlation of Jung-Radermacher (1979) correlation For the coiled wire inserts the highest heat transfer coefficient has been attained for the coil pitch of 8.4 mm for refrigerant mass flux range 100 to 400 kg/m2-s. However, The coiled wire pitch and the insert having pitch of 11.0 mm gave the highest heat transfer coefficient for mass flux range from 450 to 500 kg/m2-s. For all the tubes the heat transfer enhancement is more in high vapor quality region as compared to low vapor quality region. In fact, the heat transfer enhancement is a function of coil pitch and refrigerant mass flux and with the rise in mass flux the heat transfer coefficient increases, however at the same time, the pressure drop penalty is also high. The experimental heat transfer data for coiled wire inserts are in agreement with those predicted by Akhavan-Behabadi et al. (2008a) in an error band of +20 percent to -40 percent. The experimental pressure drop data for the flow with coiled wire insert are also compared with the prediction by Akhavan-Behabadi et al. (2008b). The model predicts the data in the range of +20 percent to -10 percent for the condensation of R-245fa. In the case of twisted tape inserts the highest heat transfer coefficient has been attained for the twist ratio of 4.3 for all the mass flow rates. The twisted tape is more effective at the low mass flux. Further, the heat transfer coefficient is higher in low vapor quality region as compared to high vapor quality region. Regarding the pressure drop an increase has been noted with rise in vapor quality and decrease in twist ratio. At higher mass flux the turbulence level is high therefore, the twist ratio has little bearing on the enhancement of heat transfer coefficient and pressure drop during condensation of R-245fa.. A number of correlations have been used to compare the experimental data and the best agreement of data has been found with the correlation of Akhavan-Behabadi et al. (2008b). The pressure drop data are also compared with the Akhavan-Behabadi et al. (2008b) correlation. The perforation in the twisted tapes has improved the performance, and the perforated twisted tape of 12.5 mm pitch of perforation has improved the heat transfer coefficient in a range of 10 to 20 percent over that predicted by the insert of same twist ratio of 7.1 but without iv Abstract any perforation. Though heat transfer coefficient enhancement is marginal beyond the mass flux of 350 kg/m2-s. In fact, the perforated twisted tape is effective only in low vapor quality region at high mass flux. It is observed that the coiled wire insert provides the highest enhancement ratio at each mass flux. The twisted tape of lowest twist ratio of 4.3, gives highest enhancement in heat transfer for entire range of mass flux. The enhancement factor decreases as mass flux increases - for all inserts. The Perforated tape_ gives. better -enhancement in heat transfer as compared to twisted tape of same twist ratio. The perforated twisted tape with pitch of perforation of 12.5 r mm gave the highest enhancement in heat transfer coefficient The pressure drop rates are less for twisted tape inserts in comparison to that for coiled wire inserts. However, the pressure drop for perforated twisted tape inserts is the lowest amongst all inserts...