In combustion, heat release parameter (or gas expansion parameter) is a dimensionless parameter which measures the amount of heat released by the combustion process.[1][2] It is defined as
![{\displaystyle \alpha ={\frac {T_{b}-T_{u)){T_{b))))](https://wikimedia.org/api/rest_v1/media/math/render/svg/a2f6db2f00ec9a2a7fa45ab3c056d6073e1f1e4e)
where
is the burnt gas temperature
is the unburnt mixture temperature.
In typical combustion process,
. For isobaric combustion, using ideal gas law, the parameter can be expressed in terms of density,[3] i.e.,
![{\displaystyle \alpha ={\frac {T_{b}-T_{u)){T_{b))}={\frac {\rho _{u}-\rho _{b)){\rho _{u))}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/adf51a26629c632b582dff68cc3525e83bca46b7)
The ratio of burnt gas to unburnt gas temperature is
![{\displaystyle {\frac {T_{b)){T_{u))}={\frac {\rho _{u)){\rho _{b))}={\frac {1}{1-\alpha )).}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4c3a02c08fbaf415c0360a3ec34272dbdd6ff8c3)