My combustion simulation is fully converged. The mass imbalance that Fluent calculates is 3×10^-9. However, if i calculate the mass imbalance from the average surface integrals, as (density×velocity)_avg xArea is the mass flux from inlet and outlet, the resulting mass imbalance is the order of 10^-7. What causes this deviation as its order is around 10^2. Thank you.
The deviation in mass imbalance between the value calculated by Fluent and the value calculated using surface integrals is likely due to a difference in the way mass is being tracked in the simulation.
In Fluent, the mass balance is calculated by considering all mass sources and sinks in the simulation, such as inlets, outlets, walls, and internal mass sources or sinks. The mass imbalance is the difference between the total mass entering the system and the total mass leaving the system.
On the other hand, when calculating mass imbalance using surface integrals, you are only considering the mass flux through the inlets and outlets. This means that you are not accounting for any mass sources or sinks within the domain, such as combustion or evaporation.
Another reason could be the usage of different density values, if you are using an ideal gas model, the density values for the inlet and outlet would be different than the density values used by fluent.
It's also possible that there is an error in the calculation of surface integrals, such as a mistake in the area of the inlet and outlet or in the calculation of the average density and velocity.
It's recommended to double-check your calculations and also make sure that you are using the same density values for the mass imbalance calculations and for the simulation.
You can also check if there is any mass source or sink in the domain, such as combustion or evaporation, that is not considered in the surface integral calculations.
Actually, there is no mass source or sink in the calculation domain. Yet, there is a deviation of mass fluxes which are calculated automatically in the code and surface integrals. The interesting thing is that both methods are performed by Fluent and I was expecting them to match exactly since there is one inlet and outlet without any mass source or sink.
Have you checked the way the velocity is calculated in the code and surface integral?
No, i haven't. I just use the surface integrals command provided by the code.
What kind of average did you use for the surface integrals? Did you try computing mass weighted average of both density & velocity?
I can use the area-weighted average for "density*velocity" only. I don't think the mass-weighted average of density-velocity will work properly for this work.
Mass weighted averages are more meaningful. For combustion simulations, local density and mass depend on temperature... Area weighted average is based on the mesh...
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