CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   too high efficiency for steam turbine (http://www.cfd-online.com/Forums/cfx/23241-too-high-efficiency-steam-turbine.html)

Qiang October 31, 2006 00:11

too high efficiency for steam turbine
 
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?

Ermanno October 31, 2006 02:34

Re: too high efficiency for steam turbine
 
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?

Qiang October 31, 2006 18:05

Re: too high efficiency for steam turbine
 
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.


Ermanno November 2, 2006 18:02

Re: too high efficiency for steam turbine
 
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 ?


Qiang November 2, 2006 19:04

Re: too high efficiency for steam turbine
 
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


HekLeR November 7, 2006 22:48

Re: too high efficiency for steam turbine
 
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

Qiang November 11, 2006 08:18

Re: too high efficiency for steam turbine
 
HekLeR

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

Regards


All times are GMT -4. The time now is 14:42.