In Simscape, is there an equivalent to the Two-Phase Fluid System components that support Phase-Change-Materials (solid-liq​uid-transi​tion)?

Hi everyone,

I am currently modelling a heat pump cycle in Simscape (Fluids) with the goal of simulating the behaviour of a phase-change material emulsion.

I started out by designing a heat pump cycle based on an ethylene glycol and water mixture, so I used a Thermal Liquid fluid system. I have access to experimental data on which I based the design, with the goal of thus validating my simulation. My plan was to eventually change up the system parameters in a way to 'insert' PCM, hoping to easily change the coolant at will. The PCM parameterization (cp, viscosity, density...) is also based on experimental data.

Unfortunately, I have run into issues in changing the system to accomodate PCM, as the Thermal Liquid system does not seem to offer an option to directly simulate phase changes.

Additionally, as far as I can tell, Two-Phase Liquid Systems do not accomodate solid-liquid-transitions as they are meant for liquid-vapor transitions.

My first question to you is: am I correct in the assessment that there is no pre-defined liquid system to simulate a solid-liquid phase change?

Second: do you have any suggestions as to how I could accomplish such a simulation? I have tried several approaches using the Thermal Liquid system as a base with an additional subsystem to approximate the effects of the phase change on the system parameters, but the model struggles with numeric instabilities, which I assume are caused by sudden changes in coolant parameters around the melting point of the PCM.

Hopefully you understand what my issue is and that you can help me out with your insight, thanks!

Philipp

[Yifeng Tang]

Does this PCM actually flow after it changes phase to liquid? Or it stays in place (like in pouches) after melting?

[Philipp]

The model is supposed to simulate a mobile heat pump intended for use in vehicles, i.e. city buses. The PCM is being used with a carrier fluid (30% glycol), as a coolant is necessary for usage in vehicles.

Thus I assume the PCM to actually be flowing after it changes to liquid, albeit with an increased viscosity.

Is it possible to simulate this behavious realistically inside a Thermal Liquid System?

[Yifeng Tang]

I think it's possible to manipulate the property table of the thermal liquid, such that it has a sharp jump of specific internal energy to represent the melting heat. Other properties, like viscosity and thermal conductivity can be treated in a similar fashion. I'll find a time to give it a try in the next few days.

[Philipp]

I tried setting up the PCM emulsion via the Thermal Liquid Settings block. I have experimental data of the emulsion (everything except thermal conductivity, which I approximated based on online research). My internal energy data (enthalpy differences are also available) is comprised of both sensible and latent heat.

Unfortunately, my simulation fails to run properly with those settings, whereas it works with the exakt same setup, but with 50/50 water glycol mixture as thermal liquid. My assumption was that the sharp spikes in the parameters around the melting point causes the simulation to run into numeric difficulties.

Answer 1

[Yifeng Tang]

Hi @Philipp,

I attached a simple proof-of-concept.

The mlx script add an offset to the u(T) property after T is greater than the melting temperature, to represent the latent heat.

The manipulated properties are brought into a simple test model using the Thermal Liquid Settings (TL) block. I chose to use the 1D vector based properties option here as it's easier to manipulate a 1D vector than a 2D table :p

The test model is just some flow through a pipe, and a heat source will remove an increasing amount of heat.

As the heat removal happens, the outlet temperature will drop. Near the phase change temperature, the temperature change becomes much more gradual, reflecting the the jump in the u(T) vector.

You may increase the resolution of the T vector near the melting temperature to make it more like a flat line. But for engineering purposes, I suspect +/- 0.5 deg is OK?

Do you think this will be a reasonable workaround?

Thanks,

Yifeng