A STUDY OF HEAT TRANSFER DURING POOL BOILING OVER A REENTRANT CAVITY HORIZONTAL TUBE

Abstract

Boiling is a form of convection heat transfer since it involves fluid motion. But boiling heat transfer coefficients and rates are generally much larger than those of convection heat transfer without phase change, because of combined latent heat and buoyancy-driven flow effects. Among different regions of pool boiling curve, nucleate pool boiling region is one of the most efficient mode of heat transfer. Heat transfer coefficient and rate increases very rapidly in this region. Heat transfer in nucleate boiling is highly complex phenomenon as it depends on several parameters like thermophysicals properties of liquid, input heat flux and heating surface cond it ions. 'l'hcrc has been a quest for the researcher to develop new surfaces to increase the heat transfer. Many type of enhanced surfaces have been developed and commercialized. Reentrant cavity surfaces have been found effective in increasing the heat transfer and this motivates the present work. This study presents experimental investigation of nucleate pool boiling of R-134a on enhanced and smooth tube. The enhanced tube was developed with a Reentrant Cavity Surface in the workshop. Pool boiling data were taken for smooth and enhanced tube in a single tube test section. Data were taken at saturation temperature of 35 °C for plain tube and Enhanced Tube I whereas it was 20°C for Enhanced Tube 2 and 3. Each test tube has an outer diameter of 25 mm and an effective boiling length of 152 mm. The test section was electrically heated with a cartridge heater. Heat flux range was 10-80 kW/m2 for both type oftubes in the decreasing order in increments of 10 kW/m 2 . ftc resulting heat transfer coefficient range was 3.82- 12.25 kW/m2 K for plain tube. Heat transfer coefficient was in range of 4.58-16.05 kW/m2K , 10.07-24.54 kW/m2 K and 5.06-14.34 kW/m2 K for Enhanced tube No 1, 2 and 3 respectively. Enhancement factors have been calculated for the enhanced tube as compare to the plain tube. It has been observed that there is significant enhancement in heat transfer coefficient with the enhanced tube. The results were compared with the publish data in the open literature for commercially available tubes.