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Gas turbine efficiency.

Gas turbine efficiency.

Gas turbine efficiency.

(OP)
Hello,

I am an student, and doing a project involving a gas turbine, and for that we need to calculate the different efficiencies of the gas turbine (compressor, thermal etc.)

I am currently stucked at the turbine efficiency.

Whenever i try to calculate the effieciencies I either get a number greater than one, or a number smaller than zero.

I am currently using this formule:

eff=(T[3]-T[4])/T[3]*(1-(1/(P[3]/P[4])^(gamma-1)/gamma

This one is equal -0.626 = -62.6%

I have also tried to use the formula using the polytropic efficiency. Doing so:

T3/T4 = (P3/P4)^p.eff*(gamma-1)/gamma solving for p.eff

eff = (1-(1/(P3/P4))^p.eff*(gamma-1)/gamma)/1-(1/(P3/P4))^(gamma-1)/gamma

which results in a negative number.

The formaly is from 50 years of gas turbines, a book I picked up at our library.
So far the system has been calculated as a brayton cycle. I have temperatures and pressures.
Anyone who can help? :)
Hope this post wont clasify me as a lazy student.

RE: Gas turbine efficiency.

I think you need to add a few parentheses, and make sure you are using absolute temperatures and pressures

"Nobody expects the Spanish Inquisition!"

RE: Gas turbine efficiency.

for isentropic efficiency, I find:
eff,i = (T3-T4)/(T3-T4,ideal)
T4, ideal = T3*(P3/P4)^[(g-1)/g]
where g= Cp/Cv

for polytropic efficiency , I find
T4/T3=(P4/P3)^[(g-1)*eff,p/g]
so eff,p=g/(g-1)*Ln(T4/T3)/Ln(P4/P3)

absolute P and T should be used.
In real turbines that use staged combustion and cooling air inflow there sometimes is controversy on the value of T3 to use.

"Nobody expects the Spanish Inquisition!"

RE: Gas turbine efficiency.

(OP)
Great, thanks!

The results seems much better now.

Mathias Kjeldahl

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