CFD Online Discussion Forums - Need any WHATEVER Help,please. Heat-Flow problem

All,

I am simulating a heat-flow problem that is a real pain. The problem is a 3-D rectangular room (3.4L X 3.3W X 3.05H) in meters with a 2 m high door at the east wall. Fire is supposed to be simulated through a non combustion approach as heat and mass sources of CO2 and H2O. It is desired to calculate the transient distribution of temperature and gases for up to 300 seconds. Volumetric heat and mass sources are presented in the room domain by a separate fluid zone (cube) on the room's floor. My BC's are pressure outlet at the door and a steady heat and mass sources (6 MW /m3) and 0.136 kg/m3 respectively.

I have done so many trails trying to simulate this experiment with no good as follows:

-Without combustion, a steady heat source of 6 MW, issued from a separate fluid zone. -Smoke is alternatively simulated as CO2 and H2O from a "mass flow inlet" with different mass fractions from the bottom face of the "fire zone". -A mixture of O2, N2, H2O and CO2 is set so as all properties are temperature dependant (kinetic theory). -Density is set to be "compressible Ideal gas" or "Incompressible ideal gas", Bousinessque approximation is not used. -Standard k-e model used with standard near wall treatment. -Used "body force weighed", Quick, 1st order discretization schemes for pressure, density and other equations respectively. -Gird is comprised of perfect hexahederals with maximum skewness of 0.2. Spacing is 10 cm all over. Note that gird adaption was used near the fire zone (with hanging nodes) so that grid spacing near fire source was 3 cm. Initial elements count is 35000. -Segregated solver was used. Transient simulation with 0.5 sec, and once with 1e-10 sec for a 600 seconds duration.

1- In the above simulations -while setting a 0.5 sec time step and very low URF's- I was not getting convergence in terms of total mass flow rate inside the domain (mass conservation). Scaled residues were dropping to 1e-3 in the first few iterations with high fluctuations.

2- I noticed that the convergence works okay when I decreased the time step size to 1e-5. For such, even with adaptive time settings, I waited for two whole days and it only made 0.002 sec !!!. This would take weeks to finish my simulation.

3- Have tried to see simulations with reduced heat and mass sources of (10 KW/m3 and 0.013 kg/m3) with the coupled solver "with CFL of 100" and once with the segregated solver. This worked well in terms of convergence and residues though more robust with the coupled solver.

4- Now, till this moment, I failed to simulate the problem with the complete heat and mass sources defined. I am thinking that Fluent is not accepting the sudden introduction of large heat and mass sources even with low URF's. Could UDF's that is increasing till the steady complete value of heat or mass sources be a rescue?

Would so much appreciate any of your advices as what are better practices (or do's and don'ts) that may guide. I am in real need for any help in this transient heat-flow problem. Looking forward to hearing back from you.

Cheers, Amr