HEAT TRANSFER OF REFRIGERANT DURING POOL BOILING OVER A HORIZONTAL STRUCTURED TUBE

Abstract

Boiling is phase change process which involves the process of convection heat transfer because there is fluid motion takes place at the time of energy transfer. Fluid motion occurs due to densities difference caused by temperature gradient. In case of boiling rate of heat transfer as well as the heat transfer coefficient have a larger value when the convection occurs without phase change because of in boiling flow due to buoyancy and latent heat happen simultaneous so the combined effect of these increase the value of heat transfer coefficient as well as heat transfer rate. The pool boiling curve shows the different modes of pool boiling in these modes, nucleate boiling has a great interest in the researchers to study it. Because of it is a very resourceful mode of heat transfer. This region shows the high rate of heat transfer which followed by high value of heat transfer coefficient and these increase with high rate. In nucleate pool boiling the heat transfer is a very complicated process. It depends on many factors such as input heat flux, heated surfaces conditions and most on thermo physical properties of refrigerants. The most important parameter that affects heat transfer very much is enhancement of heated surface. So the development of heating surface has a significant area of research for scholars. This is the reason that there have been a lot of types of enhanced surface developed and commercialized. The present study is motivated by the effectiveness of re-entrant cavity in enhancement of heat transfer. This present work mainly shows the experimental study of refrigerant R-600a on smooth surface as well as enhanced surface of testing tube. The enhancement of testing tube is done by making the re-entrant cavity on the surface in the instrumentation workshop. The experimental data of pool boiling was noted for both enhanced and smooth tube in single tube test section. The saturation temperature at which the experimental data was taken was 35°C. The dimensions of the tubes were as OD of each tube was 25mm and effective length of each tube is 130mm. For electrical heating of tube a cartridge heater is used. The range of heat flux was 10 to 80kW/rn2. VIII The range of heat transfer coefficient was 2.11 to 6.24kW/m2K for plain tube. While the heat transfer ranges of enhanced tube one and enhanced tube two was 2.52 to 7.75 kW/m2K and 4.6 to 10.45 kW/m2K respectively. The enhancement factors of enhanced tubs have been - calculated with respect of plain tube. It shows that there is a much increment in heat transfer coefficient for enhanced tubes. The results obtained in this study were compared to the published data in the open literature of commercially available testing tubes.