Evaporation-Condensation in a porous zone
I'm in the midst of my bachelor's thesis and I am supposed to model a moistened insulation.
The insulation material is supposed to be foam glas gravel, which I modeled as a porous zone inside a cube with 1x1x1m as dimensions. The cube is modeled as a cube with walls on all sides. There is no in- and outflux.
My goal is to simulate the heat transfer from the heated bottom (fixed temperature) to the cooled top (fixed temperature) of the cube, in order to get the heat flux through the insulation. The heat flux is monitored through the heat flux trough heated bottom wall of the cube. With this information it should be possible to calculate the effective heat transfer coefficient.
The effect of evaporation-condensation of water inside the insulation on the heat transfer is tried to be found. I am using the Mixture-Model with three phases: air, water-liquid and water-vapour. I activated the Evaporation-Condensation Model. I patched the volume with air and water-vapour with a volume fraction of saturated air at the patched temperature of 308.15K.
I do get phase changes inside the cube with the evaporation-condensation model. The standard state enthalpy of water-liquid and water-vapour are changed, so that they fit the actual latent heat of vaporization (4.40e+07 J/kmol). In my tries I also tried different standard state enthapy values for both water-liquid and water-vapour, but the heat transfer didn't change.
My main problems are:
- As soon as I try to simulate in a steady simulation I get temperature values above 5000K and below 1K, and Fluent returns a floating point exception.
- If I simulate the model in a transient simulation and let it run until the transferred heat through the bottom of the cube is constant, the transferred heat is way to high. (According to measurements it should be around 7 watts, but I'm getting something like 30-40 watts).
Has anyone ever tried to model a closed volume with the evaporation model and got it to work? Has anyone got any idea why the heat flux is so high?
Any help would be highly appreciated, as I am on this simulation for 3 months already and starting to get pretty frustrated.