[Tobia]

Hello everybody!

I am doing a 3D steady simulation of the interior of a 16 MWth biomass grate-fired boiler.

The biomass burning bed and the grate have been excluded from the geometry and in their place a mass-flow-inlet boundary has been set. This boundary has to be divided in 6 different zones, one for each section of the underlying grate. Each sections have different values of massflux. To set these conditions I have used an UDF specifying a step-wise massflux profile (6 steps).

(So far I have only done simulations without reactions, as if no combustion occurs. The fluid is pure air for now.)

My problem is that when I hook this UDF to my case residuals stabilize with little fluctuations around 10^-2 - 10^-3 after more than 5000 iterations. This can't obviously be accepted as a converged solution! On the other hand when I use a uniform value of massflux on the boundary, without using any UDF, then residuals drop even under 10^-6.

The reference settings of the simulation are:
type: 3D, double precision, steady
solver: pressure-based
turbolence model: k-epsilon standard
energy: activated
species: not activated
fluid: air as ideal-gas
pressure discretization scheme: PRESTO!
all other discretization scheme: First Order
p-v coupling: SIMPLE

I have already done several tests trying to change different things such as type of mesh, turbulence models and type of solver.
Mesh tested: full hexa, full tetra, hybrid
Turbulence models tested: k-epsilon realizable, k-omega standard, k-omega SST
p-v couplings: SIMPLE, PISO, Coupled

I have also tried to bypass the UDF usage by directly dividing the single boundary in 6 different boundaries, but the problem is always the same: uniform massflux value residuals drop, stepwise profile high residuals values.

Any suggestions?

Thank you for the attention.