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August 29, 2004, 23:05 
about efficiency of compressor

#1 
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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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August 30, 2004, 00:04 
Re: about efficiency of compressor

#2 
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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.
Apurva 

August 30, 2004, 06:07 
Re: about efficiency of compressor

#3 
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i use
(p*ratio)**((r1)/r)1  t*ratio1 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!! 

September 1, 2004, 06:06 
Re: about efficiency of compressor

#4 
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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/(kappa1)=(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/fluxaveraging, massaveraging and aeraaveraging.
Regards Jonas 

September 1, 2004, 07:34 
Re: about efficiency of compressor

#5 
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what about
\begin{equation} \eta_{num}\,=\, \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} \end{equation} with M_impeller = F x r [Nm] n = ... rpm 

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