Hi Hasibul,

if it is possible to neglect the effect of the fraction of gas mixed  with the liquid (I mean that that gas does not affect the thermodynamic prop of the liquid) you can treat this problem as a two-phase flow heat transfer problem. You can correct the thermodynamic properties at the gas side considering the mixture of the flow of inert gas and that from the evaporation (known from an energy balance since the heat flux at the wall is imposed)

Hence, you have to calculate the void fraction (cross section occupied by the gas over the total), the pressure drop and the heat transfer coefficient through correlations for 2-phase flows with the adjusted thermodynamic properties for the gas side and those for the saturated liquid

The presence of inert gas would increase pressure drop and heat transfer, increasing the temperature lift and reducing the difference of temperature between the wall and the fluid

I hope this could help

In open literature you can find references for the heat transfer and pressure drops calculations for 2-phase flows. For thermodynamic calculations refprop allows to define proper mixtures

If something is not clear, please ask

best regards

Alfonso William Mauro

Hello Alfonso,

Thank you for your answer. I have evaporated water in a closed container with the application of heat flux and I also have flowed inert gas in a container which contains water vapor. My problem is to combine these two in single simulation. I am having difficulty combining these two separate conditions. I have some problems like if the water layer top is to be considered as interface or mass flow outlet.....

Hi Hasibul,

I am not sure I get you right... Do you have to run a CFD code? or your own model? Could you please add a schematic and some extensive, intensive data? otherwise it is difficult to give you suggestions

I am using my own model, it has a container which is half filled with water and air on the other half. Its a closed container. The container is being heated from the bottom. I am having difficulty setting the boundary condition of water-air surface. Should I define it as a interface? 

If it is a closed container which is small or the air is mixed quite well, you may assume thermodynamic equilibrium. Then it should be quite simple for you to calculate the fraction of water in the air.

If it is more complicated ANSYS has an evaporation model within one of their CFD codes (CFX). It is available on request.

Hi Roland,

I am using fluent's condensation-evaporation model for doing this. My main problem is the air-water surface boundary condition. Do I have to declare the surface as interface? or pressure outlet? and another thing is, the water will be evaporating with time and the air-water surface will be moving down as the amount of water will be reducing and volume of vapor will be increasing. So how can I define that motion of air-water surface or boundary?

Do not know how it is done in fluent. I am sorry. The appropriate model depends on the objectives of your simulation.

Hi Roland,

I also dont have experience on this. I am trying to learn. Till now I have successfully boiled off water but when I am trying more complex situation its not giving me solutions. Can you share me your codes in CFX? so that I can learn from that and imply it in Fluent maybe.

Hi Hasibul,

about your question I am not able to give you a good advise, since I have not direct experience with Fluent models for interfaces. About the problem you are solving, if the system is closed or vented, it is quite simple to treat it with an algebraic system of equations to get temperature (mean) of the water, pressure/temperature/composition of the moist-air. A CFD approach will help you to get a local description of what happens.

If it is not strictly required or if it is not an excercise, I suggest you to change the method to solve this problem. It will be much easier to solve it by an algebraic way, with a zero-dimensional approach for each fluid/mixture.

Best regards,

William

Hi Alfonso,

I have a container half filled with water and other half has nitrogen gas initially. I will be applying heat from the bottom of water surface and I want to calculate the vapor fraction in the upper portion (Nitrogen gas) of the container after certain period of time. How can I do it? I have added a simple model herewith.