Hi Prashanth. Thanks for your help.
What you said makes sense but my problem is that liquid fraction of 1 is showing where there’s no water phase but only air phase. Again, I have both ‘Multiphase (VOF)’ and ‘Melting and Solidification’ both enabled.
I investigated a bit more and found that this issue only happen if I set the mixture temperature for the air to high temperature. It seems like to some degree, the liquid/solid interface tracks the temperature for ice melting (273.15K) regardless the phases. In other words, it only cares where the 273.15K contour is but not the liquid fraction.
Below is a new example I did. Instead of making the air temperature high to melt the ice, I shined a laser beam from the top boudnary to the ice to melt it (plot 1). As can be seen, at certain time, the liquid/solid interface again went above the water/air interface (the head of the 'red mushroom' in plot 2) and showed that there is liquid water in the region where there’s no water phase at all (from the volume fraction of air, plot 3).
Any idea why this is happening?