Abrupt transition from slow to fast melting of ice

How fast ice melts in turbulent flows is key to many natural and industrial processes, most notably the melting of ice in the polar regions. To get a better quantitative understanding of the physical mechanics at play, as a model system we pick vertical convection, consisting of ice and fresh water, and examine the lateral melting behavior through numerical simulations and theory. We find that the melting rate of ice as a function of an increasing heating temperature undergoes an abrupt transition from a slow- to a fast-melting state, contrary to the intuition of a gradual transition. The abrupt transition of the ice melting rate is due to the emergence of a reversed buoyant flow, due to the density anomaly of water near the melting point. A theoretical model based on energy conservation gives rise to a universal expression to relate the global heat fluxes and the ice melting rate which is consistent with our data. Besides their fundamental significance, our findings improve our understanding of how phase transitions couple to adjacent turbulent flow.