Transient boiling caused by vapor film collapse at minimum heat flux in film boiling

Abstract

This paper deals with the transient boiling heat transfer caused by vapor film collapse at minimum heat flux in film boiling for a horizontal rod heater in a pool of water under pressures from 100 to 400 kPa. Platinum rods of 2, 3, and 4 mm in diameter are used as the heater. In the transient boiling after the vapor film collapse, the heat flux increases at first from the minimum film boiling heat flux with rapid decrease in heater surface superheat. It arrives at a maximum heat flux value, and then decreases to nearly the same value as the minimum film boiling heat flux, where the heat transfer on the heater becomes again stationary in nucleate boiling or natural convection. The transient maximum heat flux is about 80% of the steady-state maximum heat flux of nucleate boiling independent of the pressure and heater diameter. The heater surface superheat at transient maximum heat flux is higher than the heater surface superheat at steady-state maximum heat flux. After the maximum heat flux is reached, the transient nucleate boiling heat flux approaches the steady-state nucleate boiling curve on heat-flux-versus-superheat graph and coincides with the curve in the lower heat flux region.