Learning Forum

[Yogesh Sajikumar Pai]

Hello all,

I am simulating the solidification of a metallic PCM using the solidification/melting model in Ansys Fluent. The PCM is enclosed in a solid container with a pipe flow of air beneath it, which removes heat from the PCM. I'm assuming constant volume and density for the PCM. The specific heat capacity and thermal conductivity are defined to be temperature dependent with large changes at the solidus temperature. The PCM liquidus temperature is 579°C and the solidus temperature is 571°C. The Pure solvent melting heat is set to the melting enthalpy of the PCM (478590 J/kg). There is a mass flow rate inlet (1.48e-4 kg/s) and a pressure outlet (zero gauge pressure) in the pipe. The PCM and the solid are initialized using patch tool to 650°C. There are also contact resistances defined at PCM - Solid interface walls using shell conduction layer (temperature (phase) dependent).

The simulation works but the temperature curve of the PCM (see image - red curve) at the end of solidification shows a gradual change, whereas in reality the temperature changes steeply upon reaching the solidus temperature.

The reason is that there are liquid portions in the PCM volume even upto 20°C below the solidus temperature. Below is a contour of the PCM mass fraction at 555°C (15°C below the solidus temperature). Only when the whole volume is solidified, the temperature starts dropping as expected.

I wish to get the liquid fraction to zero closer to the solidus temperature and see a temperature curve similar to the experiment. I've tried the following so far.

1. Reducing the solidus temperature to check if there is some additional latent heat stored. Makes no difference.
2. Tried the following values for the mushy zone parameter, A_mush : 10^5, 10^4, 10^6,10^8. No effect on temperature curve (but affects flow convergence).
3. Delaying the increase of contact resistances after solidification.
4. Changing the temperature at which the specific heat capacity and thermal conductivity of the PCM change sharply (tried setting them to liquidus temperature and solidus temperature)
5. Mesh is converged. I've run this with a million more cells and same result.
6. Tried setting density of PCM to value at initial temperature and also with average value over temperature range.

Some more details about the method:

Laminar model, double-precision enabled, second order bounded implicit transient formulation, Coupled pressure-velocity solver, Second order accuracy for pressure, momentum and energy.

With time step size 1s, I run it for first 5-10 time steps. I'm getting good convergence of solution (1e-6 for continuity, momentum and 1e-12 for energy). Thereafter, I use a time step size of 10s.

Note:  The contact resistances are still to be adjusted. Therefore, the time of heat discharge differs from the experiment.

Which parameter or property controls the range in which the liquid fraction changes from 1 to 0?

 

Thank you!

 

[SRP]

Hi,
An enthalpy-porosity technique is used in Ansys Fluent for modeling the solidification/melting process. In this technique, the melt interface is not tracked explicitly. Instead, a quantity called the liquid fraction, which indicates the fraction of the cell volume that is in liquid form, is associated with each cell in the domain. The liquid fraction is computed at each iteration, based on an enthalpy balance.Computational cells that are undergoing a phase change are modelled as pseudo porous media with porosity, being a function of H and ranging between 1 (fully liquid) and 0 (fully solid).

For more details on enthalpy porosity technique please refer research paper: Enthalpy-Porosity Technique for modeling convection-diffusion phase change:Application to the melting of a pure metal by A.D.Brent, V.R.Vollar, K.J.Reid

Hope you find this useful

Thank you
Saurabh