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Dual Cell Model NTU value

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Old   November 23, 2022, 01:34
Default Dual Cell Model NTU value
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yusuf
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Hi,

I am trying to analyze a heat exchanger(as in first image) that have too much fins. One of the sides is air and the other is water. Because of the complexity of geometry, i decided to analyze with dual cell model. I took a tiny slice from heat exchanger geometry and analyzed parametrically (the parameter was mass flow rate). Then to use dual cell model, i simplified complex geometry taking an example from second image, i removed fins from air side and removed tubes from water side on the geometry. I defined porous zone both air and water sides, I took mass flow-dependent heat transfer results and transfer to heat transfer data tab(raw-data tab). Then i analyzed the simplified geometry. But i encountered a error that says:


NTU value at max. (50) iteration = 4.97639, percent error = 3.9016e-12


Program gives that error every iteration. Although there is an error iterations continue. There is something wrong but i don't know what it is.

I wrote that issue at Ansys Learning Forum (https://forum.ansys.com/forums/topic...del-ntu-value/). I got an answer:

The error message indicates that the maximum number of iterations for solving NTU is reached without convergence to the specified tolerance. It can be safely ignored if simualtion is throwing error in the inital stages. If not, you will have to check your input conditions and other setup.

After that answer, i guess i understand the reason of error. I simulated a slice of exchanger geometry and because of that i defined very small mass flow rate at the inlet compared to full exchanger geometry's mass flow rate. I know that Fluent scales the NTU value depending on mass flow rate. When i transfer flow-dependent heat transfer data from case of slice geometry to case of full geometry, mass flow rate data might be too small to interpolate. I guess Mass Flow Rate point of full heat exchanger exceeds area of can be interpolated (as in third image, representative graph).

Is my detection wrong?

Is the method that i following to perform the dual cell model right?

If it's not, how should be the method?

Thanks
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Last edited by ysfuzn; November 25, 2022 at 07:55. Reason: images, accidentally, didn't uploaded.
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cfd analysis, cht modelling, compressible air, heat exchanger, heat exchanger model

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