Comparative Study of Fuel-Air Mixing in a SPRF and a Conventional Cylindrical Combustor Using a New Unmixedness Index

In diffusion combustion systems, fuel and oxidizer (usually air) are admitted into the combustion chamber in separate streams. In these systems, turbulent mixing of the oxidant with fuel is typically the rate-limiting step of the combustion process. Making any changes to the burner and combustor design to enhance the mixing process can help to tailor the performance of the flame. The stagnation-point reverse-flow (SPRF) combustor is one of the recently-developed combustors which have proven to be efficient. In the present work, a SPRF cylindrical combustor is numerically studied and compared with a conventional one, under isothermal conditions. The importance of this study is the definition of a new simple non-dimensional criterion for assessing the mixing efficiency of burners and combustors, namely, the unmixedness index. This index is used to judge the fuel-air mixture quality in the two combustors. Numerical results are validated against the available experimental data and reasonable agreement is observed. According to the results, the SPRF combustor can improve the fuel-air mixing process in parts of the chamber and the newly-defined unmixedness index can be readily employed to evaluate the fuel-air mixture quality.