Heat of combustion

[DRAKE JONES]

Hi,

      I am doing a compartment fire simulation using HRRPUA model. The materials in the compartment starts to burn once and when they reach their ignition temperature following a ramp. I did perform two cases:

CASE 1: I did mention the heat of combustion of all the materials which are burning other than the fuel on their respective material IDs. The fuel's heat of combustion is mentioned on the REAC line. Provided ramps for the burning materials.

CASE 2: I did mention the heat of combustion of the fuel only on the REAC line. Provided ramps for the burning materials

        

      Though both the cases are running, which case is right? TIA

[Randy McDermott]

From page 108 of user guide:

For consistency, the HEAT_OF_COMBUSTION(i,j) can also be specified for each species, i, in each reaction, j. These values are used only if the corresponding heats of combustion for the gaseous species are greater than zero. Note that HEAT_OF_COMBUSTION(i,j) discussed here is not necessarily the same as the HEAT_OF_COMBUSTION of the surrogate FUEL used for gas phase combustion (e.g., ’PROPANE’). When the heats of combustion differ, the mass flux of the surrogate FUEL in the gas phase is adjusted so that the correct heat release rate is attained. This is further discussed in the next section.

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[DRAKE JONES]

Thank you Randy. I did read that page. As per my understanding, the heat of combustion of different products that are produced when a Fuel is burnt can be specified.  Is this true for all the materials in the model other than the REAC fuel?

[Randy McDermott]

Look in your CHID.out file.  There the gas phase reactions will be listed.  The FUELs for those reactions (maybe just one for simple chemistry) have an HOC designated by the REAC line.  The total mass loss rate of that surrogate fuel times that HOC gives the gas phase heat release rate.  If you specify any combination of MATL with HOC, the mass loss rates will be adjusted so that the MLR for that MATL times the MATL HOC equals the total surrogate gas MLR  times the REAC HOC.

To view this discussion on the web visit https://groups.google.com/d/msgid/fds-smv/6faf9fef-ef52-4204-b611-432cfd949138n%40googlegroups.com.

[dr_jfloyd]

In a typical simulation, most users are just using a single REAC line. This means if you have more than one MATL, then typically the REAC line corresponds to either the most common MATL or the average of the MATLs. This means the actual MATL heat of combustion may not match the REAC. We want fds to preserve both the solid phase fuel mass (so objects burn away at the right time) and the total gas phase energy release.  We do this by letting you specify a heat of combustion both on REAC and on MATL.  The REAC value is used in the gas phase combustion. The MATL value is used to scale the computed MATL mass loss. For example if REAC is 40 MJ/kg and MATL is 20 MJ/kg, fds will predict the MATL mass loss using the pyrolysis model inputs and remove that amount of mass from the solid phase. But in the gas, FDS will inject half the mass loss (20/40) so when it burns the correct amount of energy is released.

[Hannah 11]

May I know why its half the mass loss injected in the gas phase and how it's gonna release correct amount of energy?. And also, could you please through more light on this- the mass loss rates will be adjusted so that the MLR for that MATL times the MATL HOC equals the total surrogate gas MLR  times the REAC HOC.

[dr_jfloyd]

If you were you to have defined REAC HEAT_OF_COMBUSTION of 50 MJ/kg of fuel and also defined a 10 kg block of material with a MATL HEAT_OF_COMBUSTION of 10 MJ/kg. When that 10 kg pyrolyzes, how much mass needs to be added to the gas phase to get 10 kg x 10 MJ/kg = 100 MJ?