[Kronecker delta]

Greetings to you all.

I am currently running a fairly simple 2D axisymmetric simulation of heat transfer w/ melting inside a latent heat storage. I am using water as HTF (inner tube) and paraffin as PCM (outer tube). However, no matter which parameters I change, I can never achieve isothermal melting as it was in the experimental research I am validating my model against. It is a vertically oriented storage, so I am currently modelling the problem without natural convection, as was done in the research, but have also used the Boussinesq approximation to include the effects of natural convection, to similar results.

I have defined a point in FLUENT by coordinates of thermocouples provided in the research, and am then getting the temperature value from Facet Average for the defined point.

I have varied solidus/liquidus temperature (from S32-L35 °C to SL35 °C), thermophysical properties (constant and piecewise polynomial), discretization schemes, geometries (2D/3D) and mesh density, and got fairly similar results and temperature variation over time.

Not only the temperature variation over time looks odd (fairly linear throughout; no distinctive changes in temperature increase while heating solid, melting, and heating liquid), but the PCM melts in a matter of minutes, all the while its temperature increasing at the same rate as when solid PCM was heated, i.e. only sensible heat was exchanged (visible from the images I am providing), yet the experiment clearly shows that melting occurs at nearly constant temperature and in approximately two hours. I am attaching the comparison of my numerical analysis and experimental research.

For the temperature distribution provided, I have used the following properties and schemes:
- density 770 kg/m3 (constant)
- thermal conductivity 0.1 W/mK (constant)
- heat capacity 1800S/2400L (piecewise-polynomial)
- dynamic viscosity 0.023 Pas (constant)
- solidus-liquidus 35 °C
- latent heat 206 kJ/kg



- solution algorithm: SIMPLE
- pressure: Second order
- momentum and energy: Second order upwind
- transient formulation: First order implicit



I would appreciate if you would point me in the right direction or advise me what to do.

[ANTONY A R V]

Pls Choose the solidus as 35 C and liquidus as 36 C with same latent heat in the melting and solidification model.

[Kronecker delta]

@ANTONY: I have done that, and I have gotten almost identical results. It's as if anything I do makes absolutely no difference to the process.
Again there is no change in slope of temperature over time curve when PCM is completely melted. By all reason the temperature should rise more quickly since sensible heat is being exchanged. However, as evident from the picture, there is no discernible difference in temperature profile when sensible or latent heat is exchanged.

Perhaps what I haven't specified yet is that I am running FLUENT on parallel mode and am modifying the setup in the same case (I don't import mesh and define properties and case setup for each individual case), which shouldn't cause problems, but I don't know what to try anymore.


I have several issues that need resolving:

1. There is a sudden change in slope at about 29-30 °C when my melting range is 35-36 °C. It does not seem to be influenced by melting at all. It happens regardless of thermal properties.

2. PCM temperature still constantly grows during melting, when it should be constant or fairly constant. It seems like FLUENT is calculating pure conduction, and then just adds latent heat during and after melting to increase the enthalpy.

3. I still have yet to figure out how to achieve an increase in temperature once PCM is completely melted, as was observed in the experiment. I have tried varying specific heats, but to little effect.


I am not sure what to do next, I feel like I've tried everything, but I would be very pleased if you would prove me otherwise.
Thanks in advance.

[sa har]

Originally Posted by Kronecker delta

@ANTONY: I have done that, and I have gotten almost identical results. It's as if anything I do makes absolutely no difference to the process.
Again there is no change in slope of temperature over time curve when PCM is completely melted. By all reason the temperature should rise more quickly since sensible heat is being exchanged. However, as evident from the picture, there is no discernible difference in temperature profile when sensible or latent heat is exchanged.

Perhaps what I haven't specified yet is that I am running FLUENT on parallel mode and am modifying the setup in the same case (I don't import mesh and define properties and case setup for each individual case), which shouldn't cause problems, but I don't know what to try anymore.


I have several issues that need resolving:

1. There is a sudden change in slope at about 29-30 °C when my melting range is 35-36 °C. It does not seem to be influenced by melting at all. It happens regardless of thermal properties.

2. PCM temperature still constantly grows during melting, when it should be constant or fairly constant. It seems like FLUENT is calculating pure conduction, and then just adds latent heat during and after melting to increase the enthalpy.

3. I still have yet to figure out how to achieve an increase in temperature once PCM is completely melted, as was observed in the experiment. I have tried varying specific heats, but to little effect.


I am not sure what to do next, I feel like I've tried everything, but I would be very pleased if you would prove me otherwise.
Thanks in advance.

Hello friends. I have a same problem exactly. How do you solve it?
Thanks