HEAT OF COMBUSTION

[Enzo Dugarry]

Hello,

during the design of a Fire Scenario I usually left prescribed the EMPUO, but in some regulations such as  “C/VM2 Verification method: Framework for fire safety design”, New Zealand Building Code I have seen that a Heat of Combustion (HoC) is prescribed (20 MJ/kg). I have never gave much importance to the  HoC  , but now I am trying to better understand  this parameter. If I prescribe the HRRPUA, it is this parameter that impose the HRR curve, and consequently the Energy which is the integrale of the HRR curve. So, in this case of HRR imposed, the Heat of Combustion should have influence only on the Mass Loss Rate of the Fuel, isn't it? I tried apply different HoC to the same burner and I noticed that for very low HoC the HRR is 0. 

Now my question is:  if I do two simulation with two different HoC (18 MJ/kg and 20 MJ/kg) and for both the HRR curve is similar (with some fluctuations due to turbulence) to that imposed in the input, since a lower HoC gives an higher MLR, can, in this case, a lower HoC  be considered more in favor of security in fire design? 

Thanks

[fde]

Lower HoC means more fuel per unit of energy burns. More fuel  => more smoke (also depends on stoichiometry). More smoke with same energy => lower plume temperature. Therefore I believe the consequences should be evaluated as per the objectives and the fire scenario. 

[dr_jfloyd]

Ideally you want the HoC and the EPUMO2 to both be reasonable values for what you are burning. So if you have an HoC of 20 MJ/kg, then you should not use a hydrocarbon for your fuel otherwise the EPUMO2 would wind up very low. You would want to add a little bit of oxygen to the fuel molecule to bring the EPUMO2 back up near 13.1 MJ/kg O2. 

The HoC and the soot yield are also intertwined. A 1000 MW fire with an HoC of 50 MJ/kg and a soot yield of 5 % will produce half the soot mass as the same fire with an HoC of 25 MJ/kg.