Hi
I am simulating a kind of electrolyzer in Fluent. Consider a 2D-box, the bottom boundary is opening, top boundary is degassing, gas is generated from a part of left side boundary and right boundary is wall. I did the mesh dependency test and got dy=1mm is an optimum size. Now, if I set the vertical size of cells close to the degassing boundary equal to 1mm the total amount of gas in the domain will be A%. But, if I change the cell size close to the degassing boundary to a smaller value (e.g. dy= 0.5mm) then the total amount of gas in the domain in the steady-state condition will change. Note that I only change the cell size close to the degassing boundary to see the effect of cell size of the performance of the degassing boundary.
I know that Fluent specifies a mass sink for the gas phase in the cells adjacent to the degassing outlet which is related to the cell size (i.e. mass sink=integral (e_g * rho_g* u_n*dA) ), but I do not know how I can get similar results with degassing boundary when changing the cell size. any idea?
HI Azad,
If I'm not wrong, the sink is defined on the face boundary, not to the entire volume and accounting for in the balance as outlet mass flux. The mesh resolution will affect directly the normal velocity at this boundary condition, as a Finite Volume Method, the face value to the normal velocity is interpolated using elements velocity values (Fluent has a Cell center formulation). Hence, this sink that is the degassing condition is function of the solved velocity field, because dA is the same considering you have only divided the normal distance for some factor 1/2 1/4 1/8 and so on. If the a big element has a velocity value represents an integral average value to the velocity of a big area. If the element is smaller, and the centroid becomes closer to the face boundary and the velocity value on the face will be closer the element value and theoretically, u_n -> u in P when dy ->0. We will notice that U_n -> to U in P assintotically (where P is the centroid of element) as the velocity gradient is better solved over the wall. The same behavior is valid to the volume fractions, because the integral has the volume volume fraction of gas rg. A bad evaluation of volume fractions leads to a bad evaluation of the value at the boundary face.
I Hope help you!
Hi Ricardo
thanks for your reply... that was very useful.... So according to your comment, I should consider "dy" very small for the raw of cells close to the degassing boundary, to get mesh-independent result? right?
The values of the mesh must be ascertained through three methods, that can be identified by the fluent guide
Ahad Zarghami , small enough.... You need to have a good mesh resolution, aspect ratio, element ratio, and all important metrics. If you are using a turbulence model, the mesh resolution must capture the adequate y+, you need to have a minimum number of elements inside boundary layer to capture the correct profile., etc... there are many factor to be accounted for. These mesh spacing must be finer enough to capture smoothly the field, but without to generate a huge number of elements and take a life to run.
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