7.1.1.6. FAQ

1) What should I do if the methane-oxygen failed to be ignited?

• Check if the diffusion mechanisms and turbulence model are turned on.

tmodel   = 'des',      ; turbulence model (none: no model | des: DES turbulence model)
idiffmom = 1,          ; momentum diffusion (0: off | 1: on; must be 1 if tmodel /= 'none')
idiffme  = 1,          ; mass and energy diffusion (0: off | 1: on)

• Check if the ignitor model and ignition temperature has been set properly.

ignitemp = 3000,                ; ignition temperature
ignitdef(1:5,1) = 27,27,27,1,0, ; ignitor cells
ignitdef(1:5,2) = 28,27,27,1,0,
ignitdef(1:5,3) = 27,28,28,1,0,
ignitdef(1:5,4) = 28,28,28,1,0,
ignitdef(1:5,5) = 27,28,27,1,0,
ignitdef(1:5,6) = 28,28,27,1,0,
ignitdef(1:5,7) = 27,27,28,1,0,
ignitdef(1:5,8) = 28,27,28,1,0,

2) What should I do if the methane-oxygen can be ignited, but the pressure increase rate is too slow?

First, you need to clarify: compare to what the pressure increase is "too slow". Make sure you are using the same ignition condition as the one in the experiment. The treatment of ignition is always tricky.

Reasons could be:

1) the ignition is too weak.

2) the mesh is too coarse. Strong artificial diffusion may rapidly decrease the temperature in the ignitor cells.

Several things you could try:

• If you are using one-step reaction, try to increase the frequency factor, freq_fact. The default value is 6.9e+14.

ireastep  = 1,      ; one-step reaction
ich4reaopt = 1,
freq_fact = 1.0e17, ; user-defined frequency factor

• Impose an initial high temperature in the ignitor cells by using gasdef. In the example below, an initial temperature at 2500 K is used in the ignitor cells. The volume fraction of the gas species is the same as the one close to the ignitor cells.

gasdef(1:18,2) = 26, 28,  26, 28,  26, 28,  1, ; give the initial temperature at 2500 K at the ignitor cell 
                 -25252501, 2500, 2, 0.0, 0.0,   ; time phase should be 0.0, 0.0,
                 'n2', 0.715, 'o2',0.19, 'ch4',0.095,

• Refine the mesh.

 xmrefactor = 2, ; mesh refinement factors
 ymrefactor = 2,
 zmrefactor = 2,

3) Should I turn on the heat and mass transfer models in the methane combustion simulations?

Yes. Any combustion simulation with adiabatic boundary conditions is strongly not recommended because the pressure (increase rate, peak value and decay rate) as well as the temperature can be highly overpredicted. In the predictive combustion simulations, heat and mass transfer mechanisms must be considered in any hydrogen or hydrocarbon combustion simulations, including thermal radiation, convective heat transfer, steam condensation and heat conduction in the solid structures.

4) Should I refine the mesh for the methane combustion simulations?

Yes, at least once. Please make sure your calculation results are mesh independent.

5) How to import the mesh file generated from the STL file?

You need to copy the file "cadarray" to the same folder as the "ingf" file. Then, turn on the option below,

import_cad = 1, ; read from cadarray file generated from CAD

6) How to refine the mesh?

For example, the meshes in x, y and z directions are refined 2 times.

 xmrefactor = 2, ; mesh refinement factors
 ymrefactor = 2,
 zmrefactor = 2,

7) Is there a sample input file?

Yes. You have to change the name of the file to ingf in order to run the GASFLOW code.