Negative mass Fraction on cold boundary
I am trying to simulate the chemically reacting flow> And I found that the formula of reactive NS always gives the negative mass fraction (for product) or over unity mass fraction (for reactant), if the mass diffusion is taken into account. For instance, in 1d steady case.
Be grateful to any body could give me some hint. 
Hi, this is usually due to the numerical dispersion associated to your numerical scheme. This kind of schemes lead to undershoots and overshoots. If you are mixing, say, pure fuel with air then the code will encounter a pretty stiff gradient at the interface, which will result in some wiggles as the fuel diffuses into the oxidiser (Yfuel>1 and Yox<0).
Are you using a centered scheme? Try then using a higher order scheme with lower numerical dispersion. I hope it helps. Cheers, Michujo. 
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Thank you for your answer. I believe what you said is quite often based on your experience, and I appreciate that. But what I am facing is the kind of like the cold boundary difficulty (e.g. described in 'Combustion Theory' [F. A. William]). Let me try to clarify this problem. We have mass fraction Y and let's just talk about reactant. Then from species eqn. (1d, steady state), we have,: dY/dx(D/V)d_sup{2}Y/dx_sup{2}=w, where D, V and w are diffusion coefficient, velocity and reaction rate respectively. For the reactant, RHS w must be <0 and then based on the eqn above, the calculated Y must be larger than original value. This is my confusion. In MAdding.... 
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Any big guy could tackle this problem further?

up up. hope there are people could help

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