Heat Transfer and Critical Heat Flux in Transient Boiling, (I)

Heat transfer and critical heat flux in saturated pool boiling were experimentally studied under transient power condition. The heating elements were flat plates of nickel submerged facing vertically in stagnant water. The heat generation rate in the test section was increased linearly in time, upon which, under certain conditions, the heat flux was found to reach a maximum point located in the nucleate boiling regime. The heat flux of this critical point increased with mounting sharpness of the transient, and the mechanism that occurs such a high critical heat flux may be the rapid formation and evaporation of thin liquid film at the bases of vapor bubbles. Examination of high speed motion pictures reveals that all bubbles on the heating surface are still in the phase of the first generation until the critical condition is reached. Compared to the case of steady boiling, the effect of differences in the heat capacity of the test section upon the critical heat flux was found to be less marked under the present experimental conditions.