title: Heat release parameter
text: # Heat release parameter
  
  In combustion, heat release parameter (or gas expansion parameter) is a dimensionless parameter which measures the amount of heat released by the combustion process. It is defined as
  
      $\alpha ={\frac {T_{b}-T_{u}}{T_{b}}}$
  
  where
  
  * $T_{b}$ is the burnt gas temperature
  * $T_{u}$ is the unburnt mixture temperature.
  
  In typical combustion process, $\alpha \approx 0.7-0.9$. For isobaric combustion, using ideal gas law, the parameter can be expressed in terms of density, i.e.,
  
      $\alpha ={\frac {T_{b}-T_{u}}{T_{b}}}={\frac {\rho _{u}-\rho _{b}}{\rho _{u}}}.$
  
  The ratio of burnt gas to unburnt gas temperature is
  
      ${\frac {T_{b}}{T_{u}}}={\frac {\rho _{u}}{\rho _{b}}}={\frac {1}{1-\alpha }}.$
  