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Old   October 31, 2006, 00:11
Default too high efficiency for steam turbine
  #1
Qiang
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I performed a simulation of one stage in a steam turbine, but I got a unreasonable high total-total efficiency about 0.97(the design is 0.8),steam table I used is iapws if97.

Is there anyone know the reason?
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Old   October 31, 2006, 02:34
Default Re: too high efficiency for steam turbine
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Ermanno
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Hi,

Have you used the steam table to evaluate the efficency (but you used a different fluid model in your simulation, such as Redlich-Kwong...etc), or have you loaded the steam table as fluid model in CFX?

How have you evaluated the efficency?
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Old   October 31, 2006, 18:05
Default Re: too high efficiency for steam turbine
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Qiang
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Dear Ermanno

I used the iapws IF97 steam table in the material labrary in CFX, also for calculating efficiency, but the total-total efficiency I got is 0.95(actual enthalpy drop/isentropic enthalpy drop),the design is 0.80

How about your case? Did you get a turbine efficiency lower than 0.85 before? I have not.

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Old   November 2, 2006, 18:02
Default Re: too high efficiency for steam turbine
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Ermanno
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Hi,

My latest simulation was one HP stage of steam turbine. The efficiency I got is 0.94 about (higher than design efficiency 0.90 about). In my simualtion I have a low Mach number (0.2), therefore I can neglect differences between static and total enthalpy: have you evalueted the velocity (isentropic) correctly in your simulation?

During the use of steam table I had a problem (as I wrote few weeks ago): static pressure was higher than total pressure. Have you checked it ?

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Old   November 2, 2006, 19:04
Default Re: too high efficiency for steam turbine
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Qiang
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The key problem of me is the efficiency from CFX is too much higher than designed condition(0.95/0.8),I think that's due to fewer flow losses in the stage in simulation. I don't know why the efficiency of my simulation is so high.

Which steam table do you use? Virial or ispws if97 ?

Which version of CFX do you use? I learned it from Ansys support that version 10 had bugs on this issue(I also faced this pressure problem before),you can use CFX-5.7 or 5.7.1, that will be OK.

I'm an engineer in turbomachinery. Maybe we can have technical discussion by e-mail or MSN. my e-mail: lu.qiang@doosan.com

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Old   November 7, 2006, 22:48
Default Re: too high efficiency for steam turbine
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HekLeR
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Qiang,

If you are using IAPWS then you are using CFX 11. IAPWS was not released in versions of CFX earlier than that.

CFX 11 can automatically calculate efficiency as a global (overall) device based quantity or local field (at every node) based quantity for you based on mass average inlet entropy & enthalpy. Look in the output control -> export tab and read the doc on it as well in the basic capability modelling section.

HekLeR
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Old   November 11, 2006, 08:18
Default Re: too high efficiency for steam turbine
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Qiang
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HekLeR

Thank you very much for your kind advice, I will check it tomorrow.

Regards
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