HEAT TRANSFER RATES DURING FORCED CONVECTION BOILING OF R-134a IN HELICAL EVAPORATOR

Abstract

In present work the heat transfer rates during evaporation of R-134a in helical coils have been studied. The experimental set-up consists of a well instrumented vapour compression system driven by an open type compressor. The system consisted of a compressor, condenser, liquid -refrigerant receiver, drier-filter, solenoid valve, by-pass system, hand operated expansion valve, test evaporator and an after-evaporator. The test section coils are made-up of hard drawn copper tube of 9.525 mm O.D. and 6.64 mm. I.D. and the length to be used was estimated with the help of Seban and McLaughlin [25] correlation. This came out to be 7 in. The source of heat supply to refrigerant is through water which is filled in a copper tank of 300 mm dia. x 600 mm. height. The measurement of the wall temperature were done at 9 equidistanced locations. At each location two thermocouples were mounted at top and bottom positions. Experiments were conducted with the following range of parameters. Working fluid = R-134a Tube geometry = 9.525 nun OD, 6.64 mm ID & 7 m length Refrigerant mass velocity, G = 80,21, 120.31, 160.42, 200.52 and 240.62 kg/s in2. Time interval = 60, 90 and 120 min. Helix angles = 5°, 100 and 150 Refrigerant temperatures = -5 °C to 6 °C Vapour quality = 0.13 to 3 °C super heat The average heat transfer coefficient, heat flux, Coefficient of Performance, Reynolds number and average vapour quality were calculated for each run and for all flow conditions. The variation of heat transfer coefficient, heat flux and Coefficient of Performance with main parameters such as helix angle, Reynolds number and time have been studied. The heat transfer coefficient and heat flux increase with increase in Reynolds number but Coefficient of Performance decreases. Heat flux, heat transfer coefficient and Coefficient of Performance increase with increasing helix angles and a decreasing behaviour is observed in these parameters with increasing time interval. A comparison was made between the experimental data collected and predicted data obtained with the help of the correlation given by various researchers. It was found that best agreement occurs with the correlation predicted by Jensen and Bergles [12] and it has been modified to fit for R-134a as: B° = 2.87 x 10-6 (Re) 0,446 x-0.47 (i) This correlation is in -10.6 % to 11.33 % agreement with experimental results. The experimental results do not correlate well with results given by other researchers because their tests were performed under different operating conditions and working fluid used was either water or air. iv