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Old   August 29, 2004, 23:05
Default about efficiency of compressor
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when i compute efficiency of compressor with data which comes from my own cfd code, it always higher than 1.i have processed mass average. i use total pressure ratio and total temperature ratio to compute efficiency.How can i correct it. i think the total pressure ratio is right.
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Old   August 30, 2004, 00:04
Default Re: about efficiency of compressor
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what is the formula for mass you are using. Mass at exit should be A*Vnormal. Is it same or not. This can be the only reason for it. Another thing is pressure should be area averaged quantity not mass averaged in my view.

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Old   August 30, 2004, 06:07
Default Re: about efficiency of compressor
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i use




to compute efficiency. P*ratio is total pressure ratio. t*ratio is total tempreure ratio. total pressure ratio is averaged by energy average and total tempreure ratio is averaged by mass average. thank you for help!!
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Old   September 1, 2004, 06:06
Default Re: about efficiency of compressor
Jonas Bredberg
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According to Cumpsty there are two ways to compute efficiency for compressors, either isentropic or polytropic efficiency. I've used eff=ln(PR)/k/ln(TR). k is the ratio of kappa as kappa/(kappa-1)=(air)=3.5. The difference between the two methods are around 1%. When the efficiency is over 1, your solution may not be converged, or you are using less suitable planes. For instance you are comparing the inlet of a stator to the outlet of a rotor (rotating). Use planes some nodes from both inlet and outlet, to reduce numerical errors. If you are uncertain of the averaging technique, use all: massflow/flux-averaging, mass-averaging and aera-averaging.

Regards Jonas
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Old   September 1, 2004, 07:34
Default Re: about efficiency of compressor
Jörn Beilke
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what about

\frac{\dot m Y}{P_{impeller}}\,=\,
\frac{\dot m \left[\displaystyle{ \frac{\kappa}{\kappa - 1}}
RT_inlet \left\{
\left(\displaystyle{\frac{p_outlet}{p_inlet}}\righ t)
^{\textstyle{\frac{\kappa - 1}{\kappa}}}-1\right\}\right]}
{{\mbox M_{impeller}}*2 \pi n}

with M_impeller = F x r [Nm]
n = ... rpm
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