In combustion, heat release parameter (or gas expansion parameter) is a dimensionless parameter which measures the amount of heat released by an adiabatic combustion process.^[1][2] It is defined as

${\displaystyle q={\frac {T_{ad}-T_{u}}{T_{u}}}}$

where

• ${\displaystyle T_{ad}}$ is the adiabatic flame temperature
• ${\displaystyle T_{u}}$ is the unburnt mixture temperature.

In typical combustion process, ${\displaystyle q\approx 2-7}$. For isobaric combustion, using ideal gas law, the parameter can be expressed in terms of density,^[3] i.e.,

${\displaystyle q={\frac {T_{ad}-T_{u}}{T_{u}}}={\frac {\rho _{u}-\rho _{ad}}{\rho _{u}}}.}$

The ratio of burnt gas to unburnt gas temperature is

${\displaystyle {\frac {T_{ad}}{T_{u}}}=1+q.}$

The gas expansion ratio is simply defined by

${\displaystyle r={\frac {\rho _{u}}{\rho _{b}}}}$

which is related to ${\displaystyle \alpha }$ by ${\displaystyle r=1+q.}$

• Zel'dovich number

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