INVESTIGATIONS ON THE CONDENSATION OF REFRIGERANTS INSIDE A HELICAL COILED TUBE

Abstract

An experimental investigation has been performed to find the heat transfer coefficients and frictional pressure drops during the condensation of R-134a and R-600a inside helically coiled tubes as well as smooth straight tube placed inside a shell. The cooling water is flowing on the shell side. The test-section comprise of one smooth helical coiled tube, one dimpled helical coiled tube, one micro-fin helical coiled tube and one smooth straight tube. All of them are copper tubes. The helically coiled tubes were made by wrapping a straight copper tube in the grooves of the wooden pattern. Moreover, a mild steel frame was constructed to hold the test-section at particular angle by means of two rod. One end of the rods was welded on middle of the test-section, while, another end was threaded which was used to fix the test section at the particular inclination position by mean of a metal nut. The straight tube was a hard drawn copper tube having 8.92 mm inside diameter and 1000 mm in length. It was fixed concentrically inside the copper pipe with 43 mm inner diameter. The refrigerant was condensed inside the inner tube and the coolant water was passed through the annular space of the test-condenser in counter flow direction. The flow rate of refrigerant was monitored by operating the rpm of the magnetic gear pump through frequency inverter. The coriolis mass flow meter followed by magnetic gear pump was accommodated to measure the refrigerant flow rate and the water flow rate in annulus tube was measured by a turbine flow meter. The outer wall temperature of inner tube of test-section was measured with T-type thermocouples at four axial stations on the smooth straight tube and at six stations on coiled tube. At each position, four thermocouple were soldered 90o apart over the top, bottom and two side positions. The pressure and temperature of refrigerant were measured at the entry and exit of test-section, and at the inlet and outlet of pre-heater as well. In order to control the vapor quality at the inlet of the test condenser, an appropriate pre-heater was designed from a 6 m long U-bend stainless steel tube of 19 mm. By supplying high current and low voltage through step down auto transformer, liquid refrigerant was evaporated by heating the tube. Also Pressure drop between the test-section was measured by a differential pressure transducer. All thermocouple and pressure transducer signal was transferred to the multichannel data acquisition system with PXI controller. The steady state condition was assumed when reading of temperatures, pressure and mass flow rate remain constant for at least 20 min. The experimental measurements were carried out at mass flux of 75, 115, 156 and 191 kgm-2s-1 and saturation temperature between 35o-45oC. In present studies, the effect of the mass ii flux, vapor quality, saturation temperature and inclination helix angle of helical coiled tubes on the average heat transfer coefficient and frictional pressure drop of R-134a and R-600a have been examined and analyzed. In order to check integrity of experimental set-up, the experimental heat transfer coefficients for the smooth straight tube were compared with the data predicted by different correlations viz. Dobson & Chato (1998) and Shah (2009) for the R-134a refrigerant and Cavallini et al. (2006) and Shah (2009) for R-600a refrigerant, respectively. The established correlations are predicted the heat transfer coefficient data for the condensation of R-134a and R-600a with the agreement of experimental data. The experimental results showed that for the all helical coiled tube and smooth straight tube, the average heat transfer coefficient and frictional pressure drop of the R-134a and R-600a increased with the rise of the vapor quality and mass flux. Moreover, an increase in saturation temperature decreased the both average heat transfer coefficient and frictional pressure drop of the R-134a and R-600a for all the mass fluxes. The average heat transfer coefficient and frictional pressure drop data of helically coiled tubes (smooth, dimpled and micro-fin helical coiled tubes) during condensation of R-134a and R-600a were compared at saturation temperature of 35oC. The experimental results of all helically coiled tube are also compared with that of smooth straight tube at same mass flux and saturation temperature. Also, the effects of inclination angle of the helical coiled tubes (smooth, dimpled and micro-fin helical coiled tube) on the heat transfer coefficient during the condensation of R-134a and R-600a at the saturation temperature of 35oC were examined. The highest and lowest values of the heat transfer coefficient take place at inclination angle of +30 and -90 degree, respectively, while, the inclination angle of helical coiled tubes has a small effect on frictional pressure drop of R-134a and R-600a. In addition, the experimental data of horizontal helically coiled tubes have been plotted on the El Hajal et al. (2003) flow pattern map (Mass flux, G vs vapor qaulity, x). In present study, the photographs of the flow patterns during the condensation of refrigerants (R-134a and R-600a) inside the horizontal helical coiled tubes are captured by using high speed camera having speed of 1000 frame per second. The transitions between different flow regimes inside helically coiled tubes have also been discussed. In current investigation, the three flow regimes are observed: the stratified-wavy, intermittent and annular flow patterns, as well as their transition. Finally, the new empirical correlations have been proposed for prediction of the two-phase flow Nusselt number and pressure drop during condensation of R-600a and R-134a iii inside horizontal positioned helically coiled tubes. The new developed correlation predicts the experimental results within ±20%. The following correlation has been developed to predict the experimental results for different cases: Cases Correlations Smooth helical coiled tube Heat transfer coefficient For condensation of R-134a and R600a: (𝑁𝑢𝑡𝑝)𝑆𝐻𝐿𝑇=0.023 (𝐷𝑒𝐸𝑞)1.056(𝑃𝑟𝑙)0.47(𝜒𝑡𝑡)0.121(𝑝𝑟)−0.069 Frictional pressure drop For condensation of R-134a: (Δ𝑃𝑡𝑝,𝑓Δ𝑃𝑙,𝑓)𝑆𝐻𝐿𝑇= (𝜙𝑙2)𝑆𝐻𝐿𝑇=1.90(1+1.733𝜒𝑡𝑡1.184+1𝜒𝑡𝑡2)𝑝𝑟0.194 For condensation of R-600a: (Δ𝑃𝑡𝑝,𝑓Δ𝑃𝑙,𝑓)𝑆𝐻𝐿𝑇= (𝜙𝑙2)𝑆𝐻𝐿𝑇 =3.17 (1+1.97𝜒𝑡𝑡1.49+1𝜒𝑡𝑡2)𝑝𝑟0.118 Dimpled helical coiled tube Heat transfer coefficient For condensation of R-134a and R600a: (𝑁𝑢𝑡𝑝)𝐷𝐻𝐿𝑇=0.055 (𝐷𝑒𝐸𝑞)1.023(𝑃𝑟𝑙)0.24(𝜒𝑡𝑡)0.105(𝑝𝑟)−0.0785 Frictional pressure drop For condensation of R-134a: (Δ𝑃𝑡𝑝,𝑓Δ𝑃𝑙,𝑓)𝐷𝐻𝐿𝑇=(𝜙𝑙2)𝐷𝐻𝐿𝑇=4.50 (1+4.86𝜒𝑡𝑡1.49+1𝜒𝑡𝑡2)𝑝𝑟0.321 For condensation of R-600a: (Δ𝑃𝑡𝑝,𝑓Δ𝑃𝑙,𝑓)𝐷𝐻𝐿𝑇=(𝜙𝑙2)𝐷𝐻𝐿𝑇=3.18 (1+4.324𝜒𝑡𝑡1.65+1𝜒𝑡𝑡2)𝑝𝑟0.0147 Micro-fin helical coiled tube Heat transfer coefficient For condensation of R-134a and R600a: (𝑁𝑢𝑡𝑝)𝑀𝐹𝐻𝐿𝑇=0.157 (𝐷𝑒𝐸𝑞)0.847(𝑃𝑟𝑙)0.604(𝜒𝑡𝑡)0.054(𝑝𝑟)−0.194 Frictional pressure drop For condensation of R-134a: (Δ𝑃𝑡𝑝,𝑓Δ𝑃𝑙,𝑓)𝑀𝐹𝐻𝐿𝑇= (𝜙𝑙2)𝑀𝐹𝐻𝐿𝑇=4.19 (1+2.32𝜒𝑡𝑡1.34+1𝜒𝑡𝑡2)𝑝𝑟0.246 For condensation of R-600a: (Δ𝑃𝑡𝑝,𝑓Δ𝑃𝑙,𝑓)𝑀𝐹𝐻𝐿𝑇=(𝜙𝑙2)𝑀𝐹𝐻𝐿𝑇=3.55 (1+2.57𝜒𝑡𝑡1.50+1𝜒𝑡𝑡2)𝑝𝑟0.152