# How to calculate Entropy in CFD as a state variabl

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 July 7, 2008, 11:01 How to calculate Entropy in CFD as a state variabl #1 Vinayender Guest   Posts: n/a Hi all, Can any one help me how to calculate Entropy in CFD as a state variable instead of calculating change in entropy between two states. Thanks, Vinayender.

 July 7, 2008, 15:22 Re: How to calculate Entropy in CFD as a state var #2 Ananda Himansu Guest   Posts: n/a In classical thermodynamics, absolute entropy enters only in a very restricted way via the Nernst postulate of the pure crystalline state of some substances. I would not play around with this. Otherwise, in classical thermodynamics, entropy is indeed a state variable, but it is defined only to within an arbitrary constant (ignoring the Nernst postulate / third law). Hence, you can only know entropy relative to an arbitrary reference state, which is equivalent to calculating the change in entropy between two states. Just define the entropy of the pure substance you are considering to be zero in some reference state. After that, its value in any other state is well defined by any valid reversible process that will take the substance from the reference state to the final state. You have consistently take into account changes in entropy during phase changes and changes in chemical composition, etc.

 July 8, 2008, 06:42 Re: How to calculate Entropy in CFD as a state var #3 Vinayender Guest   Posts: n/a Hi Ananda, Thank you very much for your comment. Thanks, Vinayender.

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