oxygen reduction with water mist.

[Prometeo-ianic]

Hello everyone,
I am running a numerical calculation in which the water mist causes a reduction in HRR (prescribed with HRRPUA) due to oxygen depletion inside the room. However, while analyzing the parameters of this simulation, I noticed that the MLR of my fuel does not decrease in line with the HRR but continues following its input trend, similarly to what was set as the HRR output (see the attached figure). This seems to suggest that smoke continues to be released from the burner, filling the room. Is this consistent with the physics of the event or MLR usually decrease with the HRR?

[mcgratta]

HRRPUA implies that you have a gas burner with a specified flow rate of gas. If there is no oxygen to burn it, the combustion stops as does the production of smoke.

[Vojta-Salek]

I think this may be confusing—my colleagues are often surprised by this as well. With HRRPUA, you are actually setting the mass of released gases, which is calculated by dividing the HRRPUA by the heat of combustion. If sufficient oxygen is available, combustion will consume all the released gases, producing an amount of energy equal to the HRRPUA. If oxygen is insufficient, the combustible gases calculated from HRRPUA will still be released, but they will not fully combust. This is exactly what you observe in your results.

In real fires, suppressing combustion reduces thermal feedback to the decomposing material, which should slow down decomposition. However, this effect is not captured in your model because the decomposition rate is prescribed rather than calculated based on the surrounding environment.