a.) Will the entropy of steam increase, decrease or remain the same as it flows through a real adiabatic turbine ?
b.) Will the entropy of the working fluid in an ideal Carnot Cycle increase, decrease or remain the same during the isothermal heat addition process ?
c.) Steam is accelerated as it flows through a real, adiabatic nozzle. Will the entropy of the steam at the nozzle exit be greater than, equal to or less than the entropy at the nozzle inlet ?
3 comments:
For part (C) I was a bit confused on how to approach this. It seems there are multiple ways to approach this, some more obvious than others.
A less obvious approach, I was thinking using the nozzle relationship:
dH + dv^2/2gc = 0
(v in this case being velocity)
Then using Gibb's 1st eqn:
dH = TdS + VdP
From these two eqns we get:
TdS + VdP + dv^2/2gc = 0
Solving for dS we get:
dS = -1/T * (VdP + dv^2/2gc)
which is counter intuitive because of the negative. Where am I going wrong?
Sleepless:
This is not quite the correct differential form of the 1st Law:
dH + dv^2/2gc = 0
You could try dH + dEk = 0.
Then: dS = -1/T * (VdP + dEk)
dP < 0 in a nozzle so VdP must be a bigger negative number than dEk.
I think this is all correct, but it doesn't really help us solve the problem because it will be tough tp integrate dEk/T since T is not constant.
Instead consider dS = dQ/T + dSgen
Changes in entropy can be caused by heat transfer or entropy generation (irreversibilities). Apply this eqn to each part of this problem. Think about the sign of each term. It seems odd, but there is no work term in this eqn. Work does not directly effect S.
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