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-   -   How does StarCD calculate the Temperature? (http://www.cfd-online.com/Forums/cd-adapco/56326-how-does-starcd-calculate-temperature.html)

Pete October 24, 2007 07:44

How does StarCD calculate the Temperature?
 
Hello

Can anyone tell me how (formula!) the temperature rise during an adiabatic compression is calculated in StarCD 3.26 and which parameters are important! If i calculate the temperature rise by the follwing formula: dT=T*(dV/dT)*(1/cp)*dp

the results differ from the Temperature in the simulation!

Appreciate your Help!

Pete

Pauli October 24, 2007 11:28

Re: How does StarCD calculate the Temperature?
 
What is the error magnitude? Here are some ideas for small errors.

1) Are your walls adiabatic? The equation assumes they are.

2) Are you using variable gas properties? If so Cp will be different.


Pete October 25, 2007 03:34

Re: How does StarCD calculate the Temperature?
 
The compressed fluid is an oil with a cp=1860 (20C) The calculated Temperature Rise by the formula and described in some papers are: for a pressure rise of 4000 bar ==> dT=+50C

In the simulation: dp=4000bar ==> dT=+100C

The walls are defined adiabatic and use variable cp-properties as well as constant!

So i need to verify the dT=100C by some calculation!

Appreciate your help!

Thx Pete

Pauli October 25, 2007 20:33

Re: How does StarCD calculate the Temperature?
 
So your working fluid is a liguid & not a gas?

If so I'm not sure what equation you use or if StarCD will handle it correctly.


Pete October 26, 2007 02:43

Re: How does StarCD calculate the Temperature?
 
yes its a liquid...


Richard October 26, 2007 03:14

Re: How does StarCD calculate the Temperature?
 
If your working fluid is a liquid, does that mean you are using the constant density option? If so, STAR-CD is solving the energy equation rather than the enthalpy equation, and the specific heat which you need to input is cv, not cp. The error might then be that you are using a certain value as cp in your hand calculations and cv in STAR-CD.

Pete October 29, 2007 06:24

Re: How does StarCD calculate the Temperature?
 
No im using a user coding that defines the working fluid as a compressible medium by:

Density=Density(0)*(Kappa*dp)

Kappa=compressibility (e.c. for water: 5 E-10)

Can you tell me which formula StarCD uses in this case to calculate the temperature?

Richard October 29, 2007 08:24

Re: How does StarCD calculate the Temperature?
 
With densit, it is solving for enthalpy, so your specific heat is cp.

Pete October 29, 2007 09:46

Re: How does StarCD calculate the Temperature?
 
Yes, I calculated the Temperature rise with cp by:

dT=T*(dV/dT)*(1/cp)*dp

but which other effects are considered by StarCD, that would explain the higher Temperature in the calculation (Dissipation?,...)?


Richard October 30, 2007 06:59

Re: How does StarCD calculate the Temperature?
 
If you switch on the conservation checks in the info file, you can see the value of the terms in the enthalpy equation, so you should be able to work out where the difference comes from.


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