The influence of subcooling on nucleate boiling heat transfer in an immersed confined space

In this paper, we report an experimental investigation of saturated and subcooled pool nucleate boiling of water at the atmospheric pressure in submersed vertical channels. The rectangular channel consisted of a heated surface (10 mm in width and 50 mm in height), a transparent parallel glass plate, and two spacers in lateral sides. Both the top and bottom sides are open to the pool water. Four different gap sizes (delta=5.0, 3.0, 1.0 and 0.5 mm) were tested under different subcooled conditions (Delta T-sub=0, 10, 25, and 40 degrees C). A high-speed video graphic motion analyzer was used to visualize the boiling phenomena. The experimental results show under saturated conditions, the heat transfer was enhanced with the decrease of the gap size from 5.0 to 1.0 mm. However, a further reduction in the gap size (0.5 mm) caused a degradation of heat transfer. A comparison of the boiling curves under saturated conditions with those under subcooled conditions show that the gap size has relatively smaller influence on subcooled nucleate boiling than on saturated nucleate boiling. The bubble behaviors corresponding to the boiling curves are presented and discussed.