CHF MECHANISM IN FLOW BOILING FROM A SHORT HEATED WALL .1. EXAMINATION OF NEAR-WALL CONDITIONS WITH THE AID OF PHOTOMICROGRAPHY AND HIGH-SPEED VIDEO IMAGING

Results of flow visualization experiments are presented to clearly identify the trigger mechanism for critical heat flux (CHF) in flow boiling. It is shown that discrete bubbles which form following the onset of nucleation coalesce into a wavy vapor layer at fluxes well below CHF. Depressions in the layer interface were observed to touch the heater surface periodically, painting a thin liquid sub-film on the surface beneath the vapor layer. This sub-film was consumed by a combination of vigorous boiling and interfacial evaporation. As CHF was approached, boiling became more concentrated toward the leading edge of the heater surface and around regions where the vapor layer interface touched the surface, while the remaining parts of the surface were dry. CHF was exceeded when severe vapor effusion from the sub-film normal to the heater surface lifted the wavy interface away from the surface, precluding any sustained wetting and resulting in a global dry-out.