[abc197]

Dear all,
I am working on melting of a phase change material in an open cup (material glass) using a heat source at bottom. Initially cup is filled with solid PCM to 80 % height.

(1) I am not using any multiphase model, but 'solidification/melting' option is ON. I wish to know what should be the appropriate Boundary condition at top of PCM as it is exposed to atmosphere. (should it be a pressure_inlet, pressure_outlet or an insulated wall). If I use Pressure_inlet, there is some reverse flow at each iteration & timestep.

(2) Does the flow becomes turbulent, once PCM melts. Or it is laminar, since velocities are very low. I have seen in many works, turbulence is used.

(3) In this situation (PCM simulation) How should one estimate appropriate time-step.

Any suggestions are highly appreciated.

[Martin_Sz]

Here are some suggestions for your simulation of melting a phase change material (PCM) in an open cup:
Boundary Condition at Top (1):
* Insulated Wall is the most appropriate boundary condition for the top of the PCM since it represents an open cup exposed to still air. This avoids introducing an artificial pressure or flow at the top.
Turbulence Modeling (2):
* Laminar flow is likely a reasonable assumption for this scenario. PCM melting typically involves low velocities due to the high viscosity of the molten material. Turbulence might be relevant if you have strong forced convection due to external factors, but for natural convection in the cup, it's likely negligible.
Time Step Estimation (3):
* You can estimate the time step based on the Courant-Friedrichs-Lewy (CFL) condition, which is a rule of thumb for stability in transient simulations. Here's a simplified approach:
* Identify the characteristic velocity (v) in your system. For melting, this could be the estimated velocity due to thermal expansion as the PCM heats up. You might need to make an initial assumption based on material properties.
* Find a characteristic length scale (L). This could be the diameter of the cup or the height of the PCM.
* Choose a CFL number (typically between 0.1 and 1). A lower value ensures stricter stability but smaller time steps.
* Calculate the time step (Δt) using the formula: Δt = CFL * L / v
Additional Tips:
* Monitor Results: Pay attention to the temperature gradients and ensure they are reasonable within the PCM.
* Refine Mesh: If you see unexpected behavior, consider refining the mesh near the solid-liquid interface for better accuracy in capturing the phase change process.
* Convergence Criteria: Set appropriate convergence criteria for temperature and other relevant variables to ensure the simulation reaches a steady state.
By using an insulated wall at the top, assuming laminar flow, and estimating the time step using the CFL condition, you should be able to get a good starting point for your simulation. Remember to monitor the results and adjust parameters as needed to achieve accurate modeling of the PCM melting process.

[abc197]

Thank you Martin for a detailed explanation and important tips. This really helps. I hope I can ask a few follow-up questions:

(1) For boundary condition of PCM top face, if I use wall condition then I have to assign wall solid material as well, This material should be some low conductivity material right? OR any material can be given, material properties dont matter at this boundary as its zero heat flux.
(2) Also, is 'Insulated' the best option among all wall conditions. Or convection OR combined external radiation & convection can be considered as PCM will have high temperature w.r.t ambient. Although for convection, I would need heat transfer coefficient.