test2.txt

This case has a wall of "cardboard" with a hot, burning particle next to it. The case runs in a minute. Look at the wall temperature T1. When solid grid cells are removed, the surface temperature changes significantly. I understand that there is a redistribution of heat, but this seems like a large change.

This is a simple example derived from the NIST_NRC_Transient_Combustibles cases called box_2x2x2. It was running last January, but now fails with instabilities. I bisected and found that when we changed the defaults for RHOMIN and RHOMAX these cases started to fail. But I think the underlying reason is the erratic changes in temperature when solid cells or entire obstructions are removed. This is not unique, and we've seen examples before. I just posted this issue because the case is relatively simple and fast.