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September 18, 2008, 05:37 
energy equation in terms of entropy

#1 
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Hi is it possible to have energy equation in in terms of entropy. thanx.


September 18, 2008, 09:09 
Re: energy equation in terms of entropy

#2 
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just as a quick thought: I know you can get it in terms of enthalpy and enthalpy has Q, which is in the entropy integral. I have never seen it or derived, so I am not sure if there would be any snares when trying to derive it.


September 19, 2008, 03:56 
Re: energy equation in terms of entropy

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Of course, it is. It should be (for a Newtonian  Fourier fluid):
rho * T * DS/Dt = div(Kh * grad(T)) + k * div(u)^2 + 2 * mu * (Sij  1/3 * div(u) * dij) * (Sij  1/3 * div(u) * dij) + rho * E where: rho = density mu = dynamic viscosity u = velocity vector S = entropy E = source term DS/DT = dS/dt + div(uS) = material (or substantial) derivative Kh * grad(T) = thermal flux (following fourier) k = lambda + 2/3 * mu = bulk viscosity (usually set to zero following Stoke's Hypotesis) dij = i,j component of the Kronecker's delta tensor Sij = 1/2 * (dui/dxj + duj/dxi) = i,j component of the symmetric part of grad(u) (Sij  1/3 * div(u) * dij) * (Sij  1/3 * div(u) * dij) = Aij * Aij = trace of the tensor A*A (A squared) It is difficult to distinguish the different operators but should be clear 

September 20, 2008, 04:49 
Re: energy equation in terms of entropy

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The Energy equation, is a statement of the first law of thermodynamics,i.e. the principle of conservation of energy, on the contrary, entropy, is a property of matter that is always increasing as long as there is a change of state. You can write a transport equation for the entropy (Check the book bt Vedat Arpaci Convection Heat Transfer)but not a conservation principle. I hope you get the picture and see the difference. Good luck


September 20, 2008, 05:11 
Re: energy equation in terms of entropy

#5 
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Of course, i was not clear about this. This is a second law of thermodynamics statement, nothing to do with first law.
In fact, the equation writed is not a conservation principle. 

September 22, 2008, 09:52 
Re: energy equation in terms of entropy

#6 
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Hi, Thanks a lot !!!


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