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specific heat=f(temperature)-how to do???

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Old   June 12, 2003, 09:06
Default specific heat=f(temperature)-how to do???
  #1
Micha
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Hello,

I would like to model the specific heat as a function of temperature. Therefore I calculated the specific heat of my gas-mixture at different points with an other programm, calculated the quotient Cp/R according to the manual and also fitted a polynom to these values (4th degree). So I got 5 polynomial coefficients.

I tried to define the polynom with command popcp (without the n+1 coefficient), but Unfortunately PROSTAR calculates the wrong values for the specific heat...!

Probably the reason is, that I have to define another coefficient for enthalpy (-->methology equation 1-41 or star-commands-help: popcp). Why do I need this one? Where can I get it from?

What do I have to do to define the polynom for cp=f(t) correctly?

Thanks for every advice!

Micha
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Old   June 12, 2003, 10:21
Default Re: specific heat=f(temperature)-how to do???
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m-hack
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Try a Fortan code in the CONVTE.f subroutine. You define both enthalpy and cp there.
Greetings, Marc
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Old   June 13, 2003, 06:29
Default Re: specific heat=f(temperature)-how to do???
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4xF
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Hmmm.. Quite tricky this one... The polynomial definition is set accordingly to the definition of H/RT used by the Chemkin database. The relation to be used corresponds to the following formula:

H/RT = Cp/R = C1 + C2 T /2 + C3 T^2 /3 + C4 T^3 /4 + C5 T^4 /5 + C6/T

So, you have to calculate your coefficients so that they match the definition above.
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Old   June 13, 2003, 07:48
Default Re: specific heat=f(temperature)-how to do???
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Micha
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Hi 4xF,

I got a polynom for cp/R fitted with MS-Excel to something like Cp/R=a1+a2*T+a3*T^2+a4*T^3+a5*T^4 with cp...specific heat capacity and R...specific gas constant.

So according to you, I can't use this simple polynomial fitting, but I have to do another one which includes also the term: a6/T .

Then I have to calculate my values for PROSTAR like this: C1=a1 C2=a2*2 C3=a3*3 C4=a4*4 C5=a5*5 C6=a6

Is that correct?

I just want to make sure I got it...!

Thanks. Micha
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Old   June 13, 2003, 10:07
Default Re: specific heat=f(temperature)-how to do???
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4xF
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This is correct, when you do it like that. You can set a6 to 0. if you want to. a6 should be used to calculate the reference enthalpy at T =298.15. For a pure specie, this value should give you a value of H/RT that is nearly 0.
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