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-   -   about alpha in OPenfoam's sutherland part (http://www.cfd-online.com/Forums/openfoam/93489-about-alpha-openfoams-sutherland-part.html)

yiwang25 October 17, 2011 12:00

about alpha in OPenfoam's sutherland part
 
Hello, all
I want someone to help me on openfoam's sutherland part. It is as follows:

inline Foam::scalar Foam::sutherlandTransport<Thermo>::alpha
{
.......................
scalar CpBar =
(deltaT*(this->H(T) - this->H(specie::Tstd)) + Cp_)/(sqr(deltaT) + 1);
return mu(T)*Cv_*(1.32 + 1.77*this->R()/Cv_)/CpBar;
}
I am confused about Cp and alpha. In my opinion, alpha=mu(T)*Cp_.
Why use above fomula to calculate alpha?

yiwang25 October 17, 2011 13:59

Just I type the wrong fomula for alpha. it should be
alpha=kappa/Cp

jason_wy October 18, 2011 10:10

Yes, but you need to compute Kappa first:

Kappa = mu(T)*Cv_*(1.32 + 1.77*this->R()/Cv_)

yiwang25 October 19, 2011 04:37

In my opinion, that is
Kappa=mu(T)*Cp_
=mu(T)*(Cv_+this->R())
=mu(T)*Cv_*(1. + this->R()/Cv_)
which is different from Kappa = mu(T)*Cv_*(1.32 + 1.77*this->R()/Cv_) .
Why?
How can I get 1.32 and 1.77?
if some would like, please give the details. thanks

yiwang25 October 19, 2011 04:40

Additionally, Why is CpBar used in this formula, not Cp? How to derive CpBar?

jason_wy October 19, 2011 10:03

Quote:

Originally Posted by yiwang25 (Post 328531)
In my opinion, that is
Kappa=mu(T)*Cp_

You are making an assumption that Prandtl Number is 1, which is not general.

kappa is computed by modified Euchen Formula

yiwang25 October 19, 2011 12:21

Quote:

Originally Posted by jason_wy (Post 328580)
You are making an assumption that Prandtl Number is 1, which is not general.

kappa is computed by modified Euchen Formula


jason_wy, thank you very much!

Would you like to show me the details of modified Euchen Formula? or how can I get its reference?

jason_wy October 19, 2011 12:26

This is the reference I have. I don't have it in hand. You proabably can find the formula elsewhere too. Good luck.

modified Euchen formula (Thermophysical Properties of Fluids: An Introduction to their prediction, Marc J. Assael, J. P. M. Trusler, Thomas F. Tsolakis, Imperial College Press, 1996)

yiwang25 October 19, 2011 13:22

Quote:

Originally Posted by jason_wy (Post 328600)
This is the reference I have. I don't have it in hand. You proabably can find the formula elsewhere too. Good luck.

modified Euchen formula (Thermophysical Properties of Fluids: An Introduction to their prediction, Marc J. Assael, J. P. M. Trusler, Thomas F. Tsolakis, Imperial College Press, 1996)


Thank you very much!

yiwang25 October 20, 2011 06:16

Additionally, I want to know where CpBar comes from? would you like to show me the reference?

CpBar =
(deltaT*(this->H(T) - this->H(specie::Tstd)) + Cp_)/(sqr(deltaT) + 1);

Yuri Almeida December 28, 2012 20:58

I don't understand the meaning of Cpbar too.

First of all, in this expression, the dimensions doesn't fit:

Cpbar = (deltaT*deltaH + Cp) / (sqr(deltaT) + 1)

Apparently it is a empirical correlation, because as deltaT -> 0 (or T -> Tstd), Cpbar -> Cp.

As described in Poling, B.E., Prausnitz, J.M., and O’Connell, J.P., “The Properties of Gases and Liquids”, McGraw-Hill, New York, 2001, the Modified Eucken Model does not fit very well as the temperature rises (the Eucken factor has in some cases a wrong behavior), so maybe Cpbar should "correct" this deviation.

But this expression for Cpbar I can't find it anywhere. Could someone help us?

By the way, the usage of Cpbar for alpha calculation imply that the kappa and alpha returned by sutherlandtransport does not satisfy the alpha = kappa/Cp condition, resulting perhaps in a poor adjustment.

dkxls March 27, 2013 09:41

In my opinion this whole Cpbar calculation is just wrong.

It might be somewhat justified for constantTransport, but I wouldn't even do it there.

In case of sutherlandTransport or polynomialTransport I think it's just a bug. Both of these transport models are usually used with a advanced model for Cp (e.g. janaf or polynomial) and therefore one gets an accurate Cp from there.

I guess I'm not the only one with that opinion, as the whole Cpbar thing was replaced in 2.2.x by a function call to the thermo class for all three transport models.
So correct Cp values are used there for the calculation of alpha.

-Armin


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