HRR and Temperature

[ShehabGamrah]

I have a question regarding HRR and temperature. I modeled a car park with an 8 MW fire and a 30 MW fire, but the resulting temperatures in both simulations were quite similar. I expected the temperature to increase significantly for the 30 MW fire. In both cases, the burner surface thermal temperature was set to 300°C. Does this setting control the temperature?

[drjfloyd]

It is always a good idea to think about what kind of back of the napkin calculation can you do to estimate what you might expect from your simulation.

Depending on what you have set for your REAC you will have a heat of combustion somewhere between 20 and 50 MJ/kg. So order of magnitude your 30 MW fire is 1 kg/s of fuel. Gas species have a specific heat around 1 kJ/kg. So every degree C above ambient that you set your burner will add about 1 kW of enthalpy to the domain. 300 C is going to add ~300 kW. Your fire size is 30,000 kW. What impact on temperature would you expect if you added 30,300 kW of heat vs. 30,000 kW of heat?

[ShehabGamrah]

So, the temperature boundary condition has minimal impact on the temperature output, with HRR being the primary factor? However, why were the temperatures similar when comparing the 8 MW fire to the 30 MW fire?

[drjfloyd]

There are many reasons based on fire dyanmics as to why this could be. For example, perhaps 8 MW and 30 MW both result in postflashover conditions. Look in detail at your results. You should be able to figure out why this is happening. You have access via outputs to pretty much every quantity that FDS predicts. Looks at temperatures, where heat is being released, O2 levels, extcintion, wall temeperature, etc.