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January 19, 2021, 01:41 |
Negative entropy
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#1 |
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Join Date: Aug 2012
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I run a transient simulation with air ideal gas as material but forgot to set reference temperature and pressure. Now when I examine entropy at any location most entropy contours have negative values.
Since default ideal gas material assumes reference specific entropy as zero, does this happen because of wrong reference temperature and pressure? As transient simulations are time consuming, I decided to define a new variable based on two new reference temperature and pressure as: S - S(ref)=c(p)*log (T/T(ref)) – R*log (P / P(ref)) Expression: Specific Heat Capacity at Constant Pressure*log(Temperature/289 [K])-R*log(Pressure/101325 [Pa]) Then, I defined the reference temperature as the mean value between inlet and outlet of a compressor and the inlet pressure as the reference pressure. Now, when I use the variable to draw entropy, I get error vertex values for variable do not exist on geometry /turbo surface. Does anyone know how to fix it? |
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January 19, 2021, 08:13 |
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#2 |
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Unless you want to compare with previous results using different reference values, the absolute value of entropy is irrelevant in nearly all cases.
The provided value is relative to S_ref, so it can be anything and still be correct. Say S_ref = -10^15.. It may be a nuisance, but it is still correct. On the message, I would create the initialize the turbo features after I had created the new variable to see if the turbo post-processor missed those variables. Can you create a contour on an inlet, wall, or another non-turbo locator?
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January 19, 2021, 11:17 |
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#3 | |
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Quote:
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January 29, 2021, 07:04 |
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#4 |
New Member
Seulchan Chu
Join Date: Jan 2021
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In the expression, R has differenct unit with Cp.
From CFD-POST Constants, the unit of R is [ J K^-1 mol^-1 ] But Cp has [m^2 s^-2 K^-1] I think it is needed to make the units same. R should be divided by molecular weight of the gas. |
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January 29, 2021, 14:14 |
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#5 | |
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Quote:
Over the years I have learned that R in the thermodynamics books means "any consistent R" since the equation is valid per unit mass or per unit mol; therefore, it is only when evaluating the expression that the units must be consistent.
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