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-   -   specific heat=f(temperature)-how to do??? (https://www.cfd-online.com/Forums/cd-adapco/53350-specific-heat-f-temperature-how-do.html)

 Micha June 12, 2003 09:06

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

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

 m-hack June 12, 2003 10:21

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

Try a Fortan code in the CONVTE.f subroutine. You define both enthalpy and cp there.
Greetings, Marc

 4xF June 13, 2003 06:29

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

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.

 Micha June 13, 2003 07:48

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

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

 4xF June 13, 2003 10:07

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

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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