Fluids

Raoult’s law and Multiphase (mixture) species transport

• Z.PAN-2
Subscriber

Hi, Fluent Specialists!

I am using Raoult’s law to model the vaporization of liquid water below the boiling temperature by using (1) multiphase (mixture) species transport, (2) energy model, and (3) k-e model with enhanced wall function (thermal effects).

I have some problems with the model: (1) the model won’t converge by using either the steady solver or the transient solver (even when the time step size reduces to 1e-7), and (2) the temperature of the vapor mixture reaches 400+ Kelvin or even 1000+ Kelvin, while the temperature of the liquid drops to something like 1 Kelvin. Initially, the whole system is 353 Kelvin.

The system is a 2-D triangle, with an outlet on top and no inlet. There are two phases: liquid water (primary phase) and mixture (air + water vapor). The saturation pressure is defined by piecewise linear method. The backflow is 300 Kelvin pure air. The gravity is activated. I am using PISO, Presto! schemes and second-order upwind for spatial discretization.

What settings should I change? I appreciate any feedback!

• Danica
Ansys Employee

Hi,

Is this a close up on part of the model, where is the triangle? Also, where is the heat coming from and has it got the right units? Have you changed any of the values for enthalpy?

Could also be beneficial to check mesh refinement.

Danica

• Z.PAN-2
Subscriber

Hi, Danica. Thank you for the feedback.

(1) It's a typo. There is no triangle, it's a 2d rectangle.
(2) There's no heat addition in the system. We expect the vaporization process and backflow will cool down the system. (At the beginning, the bottom half of the rectangle contains the liquid and the upper half of the rectangle contains the mixture (0.02 mass fraction of vapor))
(3) I didn't change any enthalpy. Should I change it?
(4) I think the mesh is fine. Minimum Orthogonal Quality = 1.0 Maximum Aspect Ratio = 1.00245e+01.

By the way, I found out that the residuals of continuity and energy always diverge first. Is there any way to fix it?

Thanks