HEAT TRANSFER AUGMENTATION BY CORRUGATED STRIPS DURING FORCED CONVECTION BOILING OF R-134a IN A HORIZONTAL TUBE

Abstract

In the present work heat transfer augmentation has been studied experimentally during evaporation of R-134a liquid inside horizontal tube with corrugated tape inserts. The plain tube data were also obtained for the sake of comparison. The experimental set-up is mainly a well instrumented 1.5 ton refrigeration capacity (5.28 kw of refrigeration) vapour compression system driven by an open type compressor. The system consisted of a test-evaporator, after-evaporator (water-tank), condenser, drier, solenoid valve, expansion valve, needle valve and a receiver which permitted a wide range of operating parameters. The test evaporator consisted of a hard drawn copper tube of 15.875 mm OD, 12.875 mm ID and 4.04 m in length. The heat flux required for boiling the refrigerant is provided by three flexible electrical heating tapes wrapped around test-section tube. The portion of test-section where electric heating was applied, was 3.66 in long. The outside wall temperature 'measurements were done in the middle 2.14 m portion of test-section, at eight locations at an interval of 0.3048 m (1 foot) starting 76 cm from the first heating tape. At each of these eight locations, three copper constantan thermocouples were mounted at the top, the side and the bottom positions. Experiments were conducted in the followin ranges1of parameters. Working fluid = R-134a / Tube geometry = 12.875 mm ID, 15.875 mm OD, 4.04 in leng (iii) 1 Refrigerant mass velocity,G = 76.80, 153.62, 230.43 and 291.28 kg/s-m2. Heat Flux, q = 5069.52, 10139.05, 15208.56, 21629.94 and 25347.60 W/m2 Width of tape = 1 cin Twist Ratio, y = P/2D = 3.10, 5.83, 11.65 and 17.48 Average evaporating temperature= 4.8 - 7.5 OC Vapour quality range = 0.02 - 0.98 The average heat transfer coefficient and average vapour quality were calculated for each run and for all flow conditions. The variation of heat transfer coefficients with the main parameters, such as mass velocity, heat flux and vapour quality,has been studied. The heat transfer coefficient increases with the increase of mass velocoty, heat flux and vapour quality. A comparison was made between the plain tube experimental data and predicted heat transfer coefficients calculated with the help of 'correlations suggested by (i) Dembi et al. [6] (ii) Lavin and Young [2], (iii) Chaddock and Brunemann [3] (iv) Gungor and Winterton [18]. It was found that the best agreement occurs with the correlation of Dembi et al., A generalized best fitting correlation for flow of - Refrigerant given by Dembi et al. [6] has been modified for the flow of R-134a as given below : 2 0.2338 Nu = 7.36394 x 10-5 [Prf]0'7 1XI 0.1i G ht, gaff Experimental data during swirl flow with corrugated tape inserts were also obtained for four twist ratios, i.e. 3.10, 5.83, 11.65 and 17.48. The insertion (iv)