High Schmidt Number

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 October 28, 1999, 17:29 High Schmidt Number #1 Jon Johnson Guest   Posts: n/a I am using CFD with mass transfer to investigate a 2-D internal flow that is laminar but has some recirculation zones. I'm looking at the convection and diffusion of a trace component that has a very low diffusivity in the mixture. The Schmidt number (analogous to the Prandtl number in heat transfer) is about 700 for this component. The surface mass transfer coefficients continue to decrease as I refine the grid, and there appears to be no end in sight. What are the special tricks to modeling high Sc (Pr) flows?

 October 28, 1999, 18:00 Re: High Schmidt Number #2 John C. Chien Guest   Posts: n/a (1). I don't have any idea about what you are trying to do. And I don't have time to figure out your problem. (2). But I think you are talking about three things,: one is the high Schmidt number ( similar to Pr) =700 for that component, the other is the mesh refinement, and the last is the decreasing surface mass transfer coefficients. (3). I think, before you start the computation, you need to define your problem first. Then size your computational domain. Then estimate your final flow field distributions, before creating your mesh. (4). What I am saying is, you need to identify the areas such as the boundary layer first so that you can arrange your mesh to properly capture the solution and the wall properties. You have to know something about your flow field before attempting the mesh generation. Sorry, I can only give you this very general hint.

 October 28, 1999, 19:50 Re: High Schmidt Number #3 Sung-Eun Kim Guest   Posts: n/a Hi, I think; For problems involving very small diffusivity hence negligible diffusion, numerical (false) diffusion can be much larger than physical diffusion. The numerical solutions then are much more sensitive to mesh and discretization scheme you use. Perhaps that's why the solutions keep changing as you refine the mesh.

 October 28, 1999, 23:31 Re: High Schmidt Number #4 Jin Wook LEE Guest   Posts: n/a Dear Jon Johnson I had nearly similar situation more than ten years ago. My problem was so called double diffusive natural convection with Pr=7.0 and Sc=700. Reminding my old experience, there was no trick except concentrating grid in the boundary layer. Only what I could was to increase grid number and to use supercomputer. It was crazying computation for 2D unsteady problem. Following is for your reference for laminar natural convection in the cavity. Ratio of thermal and solutal(mass) boundary layer is, approximately, delta thermal / delta solutal = [(Ras/Rat)*(Sc/Pr)]**(0.25) Rat : thermal Rayleigh number, Ras : solutal(mass) Rayleigh number Considering, in general, Ras is much larger than Rat, mass boundary layer is very very thin. For forced convection, I can not remember exactly but mass boundary layer is more thin, maybe, power of 0.5 rather than 0.25(not clear value). Sincerely, Jinwook

 October 30, 1999, 10:00 Re: High Schmidt Number #6 Sergei Chernyshenko Guest   Posts: n/a Jon and Jin, sorry, I posted an answer in a wrong bracnh, it was meant for Jon. Sergei

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