I'm studying heat transfer from an electric heater to a cooking pan (Stainless steel) filled with water; specifically, my problem is related to water vaporisation, as I've already found good models to predict radiative and convection losses. The point is that vaporisation losses are easy to calculate after the water reaches the boiling point, as mass of water vaporised times enthalpy of vaporisation of water at atmospheric pressure (which is 2257 kJ/kg). But what happens before boiling? My experiments show that a considerable amount of water already evaporates starting from a temperature of 70°C. In fact, it is well known that water evaporates even at ambient temperature due to mass diffusion, and as the temperature increases the rate of evaporation is also increased. But since the water is not saturated, I guess I cannot calculate this loss by means of any enthalpy of vaporisation. How should I calculate the heat removed from the pan by the water molecules escaping from the upper surface at temperatures lower than boiling?
Thank you very much