Net Heat of Combustion or Chemical Heat of Combustion

[Sven]

Hi there,

I would like to clear a basic value in FDS. In literature you find values for net heat of combustion as well as chemical heat of combustion. I understand that chemical heat of combustion is lower than net heat of combustion because complete combustion is not achieved. Because of incomplete combustion products such as CO and smoke are produced.

I know that heat of combustion has impact on how much fuel is released in my model to archieve my defined HRR.

In FDS, I also define values for CO_Yield and SOOT_Yield (products of incomplete combustion). That's why I would always choose "net heat of combustion" in FDS. Is that correct?

I have several FDS-files from different Users where net heat of combustion as well as chemical heat of combustion is used...

Thanks in advance for you time and awnsers.

Sven

[dr_jfloyd]

If you are using a predefined fuel species you don't need to do anything. FDS will compute the heat of combustion from the heats of formation of the fuel and products .

Otherwise you can use either heat of combustion, see the discussion for IDEAL on the REAC line in the FDS User's Guide. If you specify the net, you should also set IDEAL=.TRUE. so that FDS adjusts the heat of combustion for minor products.

[Randy McDermott]

You have to be careful too to make sure you distinguish between the Lower and Higher Heating Values.

https://en.wikipedia.org/wiki/Heat_of_combustion

In handbooks you see these listed as Net and Gross Heats of Combustion, respectively.  And this has nothing to do with incomplete combustion but whether the latent heat of vaporization of the water is included in the HOC.  Since the H2O is a gas within the fire plume, you want the LHV or "Net" heat of combustion.  This is the value reported in the .out file from FDS.

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[Sven]

Thanks both of you for the quick response.

"If you are using a predefined fuel species you don't need to do anything. FDS will compute the heat of combustion from the heats of formation of the fuel and products ."

In that case FDS is using EPUMO2 to calcuate my heat of combustion (I guess an EPUMO2 value of 13100 kJ/kg ?)

However, I would like to define heat of combustion by myself and have more questions about the discussion for IDEAL on the REAC line in the FDS User's Guide:

"Setting IDEAL=.TRUE. will reduce the (ideal) HEAT_OF_COMBUSTION based upon the inputs for SOOT_YIELD and CO_YIELD"

"The second approach to determining the heat of combustion is to burn a known mass of the material in a calorimeter and divide the heat release rate by the mass loss rate (known as the effective heat of combustion). In this approach, represented by IDEAL=.FALSE., the measured value of the heat release rate includes the effects of any CO or soot that is produced and no adjustment is needed."

In SFPE Handbook of fire protection engineering I can find 2 terms:

- net heat of combution (water produced being in the vapor state)

- chemical heat of combustion (ratio of chemical heat release reate and mass loss rate)

In this case:

Do I use net heat of combustion and set IDEAL=.TRUE to adjust the heat of combustion for minor products?

Is it also correct to use chemical heat of combustion and set IDEAL=.FALSE. (default)?

Would the outcome be approximately the same?

For my simulations, smoke generation is important (HoC and Soot_Yield). That's why I would like to get more unterstanding for this topic.

Thanks again for your help. I really appreciate your time.

Sven

[dr_jfloyd]

For a predefined fuel FDS does not use EPUMO2.  As I stated in my post, FDS will compute the heat of combustion from the heats of formation of the fuel and products.

Based on how the SFPE Handbook defines the terms net and chemical heat of combustion, the net heat of combustion would represent an IDEAL=.TRUE. value and the chemical does not.