Experimentally investigating heat transfer and boiling characteristics for the downward facing wall heating

This paper experimentally investigates the heat transfer and boiling characteristics for the downward facing wall heating since this heating design has been gradually adopted in the industry, especially in the energy and heat transfer fields. The pool boiling and forced boiling cases are considered. These results include temperature/heat flux histories, snapshots of bubble sliding/breakup/coalescence characteristics, relationship of Nusselt (Nu) number vs Rayleigh (Ra) number for the natural convection, and boiling curves in the nucleate boiling regime. Based on the comparisons of heat flux histories and boiling curves, the heat transfer can be enhanced for the forced boiling case due to bubble disturbance by the injection flow. More oscillations in the temperature history and boiling curve are also revealed in the forced boiling case than the pool boiling case. Under the present experimental conditions, appropriate correlations can be derived using the least-square-error regression method. The exponent of fitting curve for the forced boiling is higher than that for the pool boiling, which quantitatively confirms enhancement of heat transfer by the forced boiling under the downward facing heating condition. In addition, the snapshots of boiling characteristics are also provided, clearly revealing the bubble dynamic behaviors. These results are seldom shown in the previous researches.