Divergence happens when only geometry is changed

Hi all,

I am modelling methane catalytic combustion with fluent. Both gas and surface mechanism are included. Also effective diffusion model is incorporated with UDF. 

When the channel diameter is 2.6 and 2.1 mm, reasonable results can be obtained. However, when channel diameter decreased to 1.8 mm, divergence happed after several hundreds of iterations.

I tried to decrease URF and increase mesh quality. However nothing works. I can guarantee that all other settings are the same for different channel diameters. I am wondering why divergence will happen when just geometry is changed for example decreasing channel diameter.

Best regards



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