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-   -   Problem with total heat transfer rate (https://www.cfd-online.com/Forums/fluent/175222-problem-total-heat-transfer-rate.html)

aswathy_raghu July 26, 2016 07:41

Problem with total heat transfer rate
 
I am doing a 2D simulation of catalytic combustion of propane-air in a straight channel microreactor. Catalyst is coated on the wall. Rate equation is supplied using a UDF. Flow is laminar.

My problem is that, the net term of the total heat transfer rate (Reports/Fluxes/Total heat transfer) doesn’t approach 0. In certain simulations it is even more than 100 W (Error is very high)! The net term of sensible heat transfer rate, however, approaches 0. The monitors remain constant for several thousand iterations and residuals are very low, while the total heat transfer term continue to remain high.

Please help me understand how heat transfer rate is calculated by fluent. The flow rate of total enthalpy at outlet matches the value of total heat transfer rate at outlet but the flow rate of total enthalpy at inlet is not even close to the total heat transfer rate at inlet.

By the way, when I disable the species inlet diffusion (in Models), the net term of total heat transfer is approaching 0. But then, the residuals have a tendency to oscillate.

It would be a great help if you could help me figure this out.

nvarma July 26, 2016 16:52

Does the udf include heat source? Any other heat source in the domain? All boundaries included in total heat flux calculation?

aswathy_raghu July 27, 2016 00:28

Thank you for your concern.

The UDF contains a rate expression for the reaction. The expressions for the corresponding rate constants and activation energies are also included in the UDF. There is no other heat source other than this reaction (given by the UDF) at the wall.

All boundaries were considered for calculating total heat transfer.

I am unable to figure out how the total enthalpy is calculated by fluent. For the same inlet conditions, I get different values for enthalpy at inlet for the cases with and without species inlet diffusion. Why does it happen?

nvarma July 27, 2016 15:50

>The UDF contains a rate expression for the reaction.

This might be the net Q you are seeing. Total heatflux will not approach zero if you have an energy source term. Instead it should tend to the energy source value.

> I get different values for enthalpy at inlet for the cases with and without species inlet diffusion.

I have no idea about that. But my guess is with different mass fraction of each species entering, the enthalpy WILL be different?

aswathy_raghu July 28, 2016 09:06

Thank you very much for considering my problem.
Total "sensible" heat transfer rate is approaching a value that is equal to the reaction source term. I believe net total heat transfer rate has to approach 0 for energy conservation. Energy in has to be equal to energy out, right?

The inlet conditions are same in both the cases. Same mass fraction is entering. I am not sure how species inlet diffusion option works.

nvarma July 28, 2016 09:26

The heat flux is integrated of "surfaces" in report>flux>total heat flux.

So volumetric heat generation is not included. For heat balance:

Heat out (domain boundary)=Heat in (domain boundary)+ Qsource (domain).


If you define a UDM, and do volume integral of your source terms it should match the total heat flux calculated by fluent.

SUBHANKAR November 6, 2019 07:51

Hi raghu,
Could you solve the issue? I am also getting the same error. I am using non-premixed combustion model. Total heat transfer rate is coming negative for all surfaces. Thus, the net is coming a larger negative value.

regards
Subhankar

aswathy_raghu November 6, 2019 12:08

Hey Subhankar

Is your mesh structured and uniform? Sometimes unstructured mesh gives problems.
My issue was solved when I switched off inlet diffusion.

SUBHANKAR November 6, 2019 12:40

Thanks for the reply. I am using unstructured mesh. I then converted to polyhedra mesh. I had inlet diffusion disabled.

regards

Edit: Inlet diffusion should be enabled. I suppose.


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