Fire load, brand last in MJ/m²

[Katharina Michaelis]

Today I was told about a fire load or brand last in the units MJ/m² which should show how much material burns per m². How can I show or see that with CFast or information from CFast?

[Kevin McGrattan]

Combustible load is not an input for CFAST. The combustible load does not indicate the HRR of the fire. 

[dr_jfloyd]

Tabulations of combustible load for different occupancies are sometimes used in engineering assessments as a means to estimate how much total energy (the integral of the HRR) a compartment fire could release.  If you had a 10 m2 compartment and assumed that you had a combustible load of 1,000 MJ/m2, then a fire in that compartment that results in complete burnout would release 10,000 MJ. You could then design a HRR curve accounting for a likely fire growth rate and ventilation that would result in 10,000 MJ when integrated over time. 

[Katharina Michaelis]

Thank you. So the 10000 MJ I would see as the integral of the heat release rate in cFast?

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[dr_jfloyd]

The HRR curve is something you are responsible for defining as the user. CFAST does not decide on the HRR. If you want to model a fuel load that represents 10,000 MJ of energy, then you need to define an HRR curve whose integral is 10,000 MJ.

[Katharina Michaelis]

Thank you - so I understood that burned during a time is what I see below the HRR curve? So the result of the integral has the unit MJm², which is kWsm²?

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[Katharina Michaelis]

And I guess with the integral and MJ I can only calculate an area m², but no mass which burns during that time?

[dr_jfloyd]

How did you get MJ m2 from integratig the HRR? The units of HRR in CFAST are kW.  kW is kJ/s.  If you have a function that has units of "something/s" and you integrate it in time, you get units of "something"; therefore the integral over time of kW is kJ.

If you want to go from kJ of energy release to kg of fuel, what is the difference in units between these two? You would mulitply kJ of energy release by kg of fuel/kJ of energy release to get kg of fuel.  What basic fire related property has units that look like kg of fuel/kJ of energy release? The heat of combustion has units of kJ/kg. 

[Katharina Michaelis]

Thank you. So now I know that the HRR is kW, which is kJ/s. And the integral is kJ. Only the last thing I did not get correctly so far... I have to multiply with kg? But kg is what I want to get?

[Katharina Michaelis]

Or which equations would I concretely have to solve to get the mass that burns? 

[Katharina Michaelis]

Also the Qm = MJ/m^2 is a mobile fire exposure. I haven't mentioned or recognized that yet.

[Katharina Michaelis]

MJ/m^2 describes the fire compartment area. Is this what I can also find with the integral of the HRR?

[dr_jfloyd]

Understanding how to go from an HRR curve and an assumed type of fuel to the mass that is consumed by the fire or how to take a fuel load and occupancy type and create an HRR curve is the type of knowledge you should have before using CFAST. If you lack this background, we cannot teach you this. You may wish to study a basic text on fire dynamics such as Drysdale's Introduction to Fire Dynanmics or Karlsson and Quintiere's Enclosure Fire Dynamics. 

[Katharina Michaelis]

Thank you for the recommendation, I am always glad to learn new useful things. It's a pity that those are books which I would have to pay for...

[Katharina Michaelis]

Is the integral of the Heat Release Rate the mobile fire exposure in kJ/m² multiplied with the area in m²? Or is thar integral the mobile fire exposure in MJ/m² multiplied with the area in m²? Or only the mobile fire exposure in MJ/m² or kJ/m²? I thought I need kJ as the HRR also is kW/s?

I am sorry to have to ask so much. I don't have the money or time to buy books...