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Old   August 14, 2003, 09:12
Default CHT problem
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Hi, I am simulating a problem involving heat transfer to the cooling oil by pipe surface subjected to heat flux. The rise in oil temperature should be near to delta T = q/(m*Cp). But is not giving that value. There is around 15% error. What could be the reason? Can anybody guide? help requested. Thanks,
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Old   August 14, 2003, 10:53
Default Re: CHT problem
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Look at your mesh (y+) values at the interface of fluid & solid. What turbulence model are you using?

What differencing scheme are you using?

Do you have your oil properties set correctly?

Are you modeling the correct length of pipe (is the flow fully developed)?
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Old   August 17, 2003, 07:05
Default Re: CHT problem
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This is quite a typical discrepancy that is not due STAR calculating the wrong values, but due a crude approximation of the temperature rise.

The formula you use Delta T = Q /(m*.Cp) is only valid when: 1) Cp is constant 2) the velocity profile is constant over every section of the pipe.

In the case of a boundary layer flow (if you use an inlet with a block profile that wiil change because of the flow developing) and if Cp is not constant, you will end up with a quite big error since Q will not be equal to m*.Cp. Delta T.

You should look at the enthalpy rise instead, since it is the integral value that has to be considered. Up to the convergence criteria, it should be equal the heat flux you input into your computational domain.
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Old   August 18, 2003, 02:36
Default Re: CHT problem
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Thanks cfdweirdo and 4xF. I think turbulence model has very little effect on temperature calculations. I am using inlet block profile and Cp is changing with temperature. But in case of hand calculations i am using average Cp at inlet and outlet T. Looking into the enthalpy rise, Y+ values is good suggestion. Thanks again, SSP
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