Effects of a porous honeycomb structure on critical heat flux in downward-facing saturated pool boiling

In this study, the effect of a porous honeycomb structure on downward-facing pool boiling was investigated. The effects of hole diameter and the hole area ratio of a porous honeycomb plate were investigated to determine which honeycomb structure had an optimal effect on critical heat flux (CHF). Through images acquired by a high-speed camera, it was observed that bubbles generated on the honeycomb surface were large and difficult to coalesce. Compared with a bare surface, a heated surface with a porous honeycomb structure has a higher CHF. Within the experimental range, CHF is dependent on the hole area ratio, not the diameter of the honeycomb hole. There exists a turning point for CHF enhancement with hole area ratio, demonstrated by both experimental data and a model, considering water supply and bubble removal ability. Maximum CHF can be obtained with a hole area ratio of 0.4 and a CHF enhancement ratio of approximately 2.3-fold. Capillary theory for downward-facing can explain the experimental results obtained. The results are essential for determining the optimal performance of the porous honeycomb structure.