On the phenomenon of combustion-induced rapid phase transition of water

When a liquid reaches its boiling point in the absence of nucleation sites, the boiling process may be delayed so that the liquid reaches superheated conditions. If a limit temperature is reached, homogeneous spontaneous nucleation may induce the liquid to boil in an explosive manner (Rapid Phase Transition, RPT). More specifically, the rapid production of highpressure vapour exerts sudden pressure on the surrounding fluid, thus leading to the formation of strong shock waves. In a previous paper, we have shown that the RPT phenomenon can be observed when igniting CH4/O2/N2 mixtures with high oxygen contents. Indeed, the measured pressure time histories displayed interesting and unexpected behaviour in that very high pressure peaks (~ 240 bar) were recorded. We have attributed such anomalous behaviour (named Combustion-induced Rapid Phase Transition, CRPT) to the occurrence of cycles of condensation/evaporation of the water produced by combustion at the walls of the explosion vessel, followed by superheating of the liquid film due to radiative heat transfer from the flame, hence culminating in the Rapid Phase Transition. In this work, we further verify our thesis by performing explosion experiments in the presence of nucleation sites, thus promoting the heterogeneous boiling mode and hindering the water explosion due to RPT. To this end, we added talc (fine powder) on the vessel walls. The results show that, in the presence of nucleation sites, the pressure peak is significantly reduced to a value close to the adiabatic value.