"Is it possible to find an equivalent heat flux for a given constant surface temp or vice versa?" yes it is possible, but not practical or useful. You can find an average temperature for a given heat flux during the melting process, if you have total heat transferred, mean temperature of PCM, and averaged heat transfer coefficient.

Selection between the constant temperature or constant heat flux boundary conditions, is related to real application of your simulations.

For PCM heat transfer, they produce different results. Also the results are different for laminar fluid flows, but for turbulence applications the results are similar. Turbulence flow is less sensitive to these boundary conditions.

These two type of boundary conditions produce different results as the upper and the lower values for realistic heat transfer coefficients.

For your case, I suggest constant temperature boundary condition as it seems more realistic.

Dear, Zahid A.

No different boundary conditions.

Prescribing either the temperature or the heat flux is different both in the physics and in the mathematics. Fixed temperature (Dirichlet BC) let the heat flux to accomodate while the prescribed heat flux (Neumann BC) let temperature to accomodate.

A mixed condition is possible, the so called Robin BC where you prescribe a combination between temperature and heat flux.

I full agree with your answer Mr. Filippo M.

Before trying to define your mathematical formulation, you should define what the actual and prevailing conditions are of your experimental/physical set up. How are you going to control the bottom plane heating conditions? Are you providing heat to melt the PCM from both top and bottom? Just the top? The real world exp conditions will dictate your analysis.

Constant heat flux and constant temperature surfaces do not give the same

results. also If one of them be fixed, the other one can not be fixed. an example illustrates the issue clearly. I hope the following link to be useful :

yon.ir/bNJio