How to conduct a both saturated and unsaturated analysis in a computational domain?
Hi!
Recently, I have been focusing on simulating the evaporation of moisture at a cavity surface to see the desaturated process near the cavity surface using TOUTGH2(EOS3). The figure is from Solving the estimation-identification problem in two-phase flow modeling written by Dr. Stefan Finsterle.
My computational domain is large. The cavity is smaller compared with the computational domain. In my understanding, a desaturation process only occurs near the cavity surface. Other area keeps saturated.
If an unsaturated state (e.g., P=2.2E5, X = 0.1, T = 15.0) is assigned to the whole computational domain, a desaturation process can be seen near the cavity surface. However, liquid pressures (P_l=P_g + P_c) are negative everywhere in the computational domain. In my understanding, I think the result is not right.
Following Figure 1, I want to assign an unsaturated state (e.g., P=2.2E5, X = 0.1, T = 15.0) to the unsaturated region and a saturated state (e.g., P=2.2E5, X = 0.001, T = 15.0) to the saturated region. In this way, I think there is negative liquid pressure near the cavity and positive liquid pressure in other regions.
I have two questions.
1. How to divide the computational domain into unsaturated and saturated regions? Because before analysis, it's hard to differ unsaturated and saturated states in the computational domain.
2. Following https://tough.forumbee.com/t/80xhvp/atmospheric-boundary-conditions and the paper Solving the estimation-identification problem in two-phase flow modeling, should a constant air pressure (e.g., 1.013e5) boundary condition to the cavity element(s)? I just assigned an equivalent capillary pressure to it to simulate evaporation.
I hope you can understand my questions and give me some hints.
Thanks!
Best regards,
Jin
1 reply
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Jin,
I'm not sure I understand the questions, but here are a few remarks:
(1) Consider the negative P_l to reflect a water potential (i.e., related to the amount of energy with which water molecules are held by capillary forces) rather than an absolute pressure (which indeed could not be negative). Note that thermophysical properties are always calculated based on the gas pressure.
(2) I obviously don't know what system you are looking at, but in mine, the cavity was at atmospheric conditions, i.e., P_g=1.013E5 Pa. You must have your reasons to set it at 2.2E5 Pa.
(3) Yes, we don't know the extent of the unsaturated zone a priori. Therefore, I calculated (rather than pre-assigned) the advancement of an unsaturated front from a system that is initially fully saturated throughout, but has an evaporation potential in the cavity.
(4) Your set of primary variables for the unsaturated zone is probably thermodynamically not valid. Check the manual for the meaning of each primary variable.
Good luck!
Stefan