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Enthalpy equation and Cp dependency upon specis

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Old   July 27, 2000, 03:53
Default Enthalpy equation and Cp dependency upon specis
Mads Bang
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Does any of you out there have any experience with solving the enthalpy equation when the Cp, of the gas considered, are dependent upon different species concentrations.

The problem is that "normally", for standard CFD, the Cp is only a function of temperature. However when I make the Cp dependent upon the various sepcies concentration I get some quite unphysical solutions.

Example: (Slow laminar flow with high diffusion rates) A Cell containing water vapour at 300K (Cp ~ 2.000 J/kgK) is experiencing an inflow of hydrogen at 300K (Cp ~ 14.500 J/kgK). Now in real physical life one wouldent experience any temperature change, but in this case the solution yilds a dramatic temperature drop to some 25K or so.....

The problem is obviously that the temperature is calculated from the enthalpy, velocity and Cp: T = (H - 0.5w^2)/Cp So when the Cp is going up the temperature drops. It is somehow not taking into account the change of enthalpy with gas composition.

So I were wondering if any one knows where to find some information on this, or have a straight forward solution......

I'll appriciate any suggestions, which can leed me any closer to a solution.

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Old   July 30, 2000, 06:43
Default Re: Enthalpy equation and Cp dependency upon speci
Steve Amphlett
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Mads, Your basic problem is that you are not making any use of the enlthalpies of formation of the species you are mixing. Effectively, your data (in the sense of datums!?) are different, so you can't just add the values up.
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