Experimental studies of hot water depressurization show that the pressure in a vessel, just after it is opened, drops much lower than the pressure of saturation but the liquid does not boil uniformly throughout the vessel. Essentially, liquid boiling begins only on the arrival of a ‘slow wave’ of rarefaction which moves at a low speed of approximately 10 m s−1 from the open end deep into the vessel.

To explain this phenomenon we suggest a model that takes into account the difference in phase velocities. Although this difference in bubble flows is only about 1 m s−1 it proves to be sufficient to cause bubble breakup. Using this model we obtained the ‘slow wave of boiling’ in a numerical experiment that was in good agreement with the physical experiment.