Understanding triggering mechanisms for critical heat flux in pool boiling based on direct numerical simulations

•The difference between the triggering mechanisms of critical heat flux (CHF) and film boiling is clarified.•It is demonstrated that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF.•It is shown that a vapor recoil force plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF.