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-   -   Heat and mass balance problem (https://www.cfd-online.com/Forums/fluent/30393-heat-mass-balance-problem.html)

 Laurent October 14, 2002 11:02

Heat and mass balance problem

I have some trouble with Fluent concerning heat and mass balance. I tested the following simple case: a flow of a gas in a tube heated by the wall. The inlet temperature is 300 K and the heat flux at the wall is constant (5000 Watts/m2).

1/ Fluent give me in Report/Area weighted average an inlet temperature of 307.7 K which is not the true specified boundary value (in velocity inlet). The density is then affected by temperature, resulting in a change of the required mass flow.

2/ The heat fluxes given by Report/heat fux are quite different by the fluxes that I can calculate from Inlet flux = Mass flow*Cp*(T inlet-T reference) and Outlet flux = Mass flow*Cp*(T outlet-T reference)

Did you already have the same kind of problem ? How to manage with that ?

 Erwin October 14, 2002 11:39

Re: Heat and mass balance problem

The fluxes given by the Report/FLuxes option are the actual heat fluxes (as calculated by the solver).

The ones reported in surface integrals are backcalculated (and not used in the calculation itself) from node values, and sometimes the cell centers have to be interpolated to get node values. This introduces the error that many people have noticed already.

 cfd man October 15, 2002 10:52

Re: Heat and mass balance problem

i noticed the same think, the inlet temperature comingout 307 insted of 300 as you defined it in B.C because fluent will caculated the the wall B.C as your inlet sheeled by a wall another think it is better to get the temperature from mass weighted average insted of area weighted average thats one of the recomindation at fluent course. can you calculated your heat flux by Q=mass flow rate*Cp*(Tin-Tout)and compare your results with Q=5000*(area subjected to H.T.