If you want to move the interface like that you need to read up on moving mesh. You'll also need to provide heat transfer effects to get heat from the gas to liquid and account for latent heat. I'd then be very wary of taking any data from the model as it'll do exactly what you code in: ie you may as well just write the report to suit the rate you want it to move at.
Learning how the multiphase models work, and with that understanding doing it properly will give you more information.
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