The role mechanism of vapor-liquid behavior on boiling crisis triggering

•The role mechanism of vapor-liquid behavior on CHF triggering is revealed.•Rapid evaporation of liquid film is essential for the eventual CHF triggering.•An expression for CHF temperature is developed with good accuracy. Revealing the physical mechanism of critical heat flux (CHF) triggering is an essential basis for building accurate prediction models. Photographic observations assist in comprehending the physical mechanism of CHF during subcooled flow boiling. The vapor-liquid behavior obtained from experimental observations shows that, when heat fluxes extremely approaching CHF, a large amount of vapor condenses into a wavy vapor layer; simultaneously, a wavy liquid film layer forms beneath the vapor layer that propagates along the heating wall. The wavy vapor layer directly impedes liquid supplement from the liquid core to the liquid film adjacent to the heating wall. As the heat flux increases, the bubble departure diameter increases and the residual vapor phase in the detachment process directly leads to the formation of local dry spot, which prevents the rewetting of the adjacent liquid due to the vapor recoil effects. At approaching CHF conditions, dry patches are formed after the remaining wetted areas rapidly evaporate. Then, boiling crisis is postulated to be triggered when the dry spots initially form, grow and finally merge into dry patches. This observation sheds light on the triggering mechanism of the flow boiling crisis. The role mechanism for the CHF triggering is proposed through studying the boiling behavior of subcooled flow boiling near a heating wall, including just before CHF (CHF-), during CHF onset (CHF transient), and after CHF appearance (CHF+). Besides, a new correlation is presented for predicting critical heat flux temperature of water under a wide range of operating conditions with a mean absolute error of 3.81%. These results will provide guidance for developing mechanistic theoretical model and enhancing CHF.