Modeling Fire Spread Between Wood Cribs – High Temperature Issue

[hgggu]

Dear all,

I'm trying to model the spread of fire between wood cribs in a large compartment. Each crib is modeled as a solid obstacle with a surface material. The surfaces are assigned thermal and combustion properties such as ignition temperature, heat release rate per unit area (HRRPUA), and burning duration, which are defined using a time-based ramp function.

Some cribs are configured to ignite first, acting as the initial ignition sources, while neighboring cribs are designed to catch fire later through thermal heating from the burning ones.

The problem I'm facing is that, based on this approach, the temperature of the wood cribs keeps rising, resulting in very high temperatures. These in turn radiate into the environment, leading to extremely high temperatures throughout the entire domain.

I tried introducing a heat of vaporization to reduce the temperature, but this led to negative temperatures at the beginning of the simulation. I'm wondering if anyone has experienced a similar issue, or if you have any suggestions on how to tackle this problem.

Thanks in advance for your help.

Best regards,

[drjfloyd]

If you are using an HRRPUA RAMP then the burning rate isn't coupled to the actual heat feedback. Getting negative temperatures implies your predicted flux is low compared to that needed to sustain the burning rate. One possible solution is to not use a heat of vaporization and instead define a temperature dependent specific heat. You could make that specific heat very large after reaching some notional post-ignition surface temperature.

[hgggu]

Thank you Jason for your reply. So my ambition to simulate wood with its “real” thermal properties is not the right idea. My aim is also to look at the propagation and contribution of the column-beam elements in the compartment, this will be a bit complicated.

[Vojta-Salek]

How did you obtain your HRRPUA? In general, it's questionable to combine even very 'realistic' thermal properties with a prescribed HRR that doesn't account for the conditions within the domain. It can work well in some scenarios, but fire propagation is arguably one of the most complex phenomena to model.

[johodges]

I would set the initial temperature of your initiating cribs to be ~300 C. Make sure you also set the TMP_INNER so that the interior parts of the surfaces are set as well. That should avoid the negative temperature issue with the heat of vaporization when you have HRRPUA from the start of the simulation.