The effects of steam dilution on flame structure and stability for a H₂/air micromix burner

The present work is dedicated to understand the effects of steam dilution on the structure and stability of a micromix H₂/air swirl flame. The flame structure and stability were obtained through numerical simulations using Large Eddy Simulation (LES) with the Eddy-Dissipation-Concept (EDC) combustion model. In addition, OH-PLIF images and temperature distribution were used to verity the computed results. The experimental and simulated OH distributions show that great part of OH distributes on midstream and downstream of the flame. From the numerical results, it can be seen that the thermal-dilution effect of steam has the biggest impact on flame temperature. Detailed analysis show that the thermal-dilution effect of steam lowers the OH mass fraction, which is the same as third-body effect, however, the OH mass fraction is increased by the direct chemical effect due to decrease the net reaction rate of R4 (H₂ + OH = H₂O + H). The stability of the micromix hydrogen flame could be reduced due to the thermal-dilution effect, but the direct chemical effect could slightly improve the stability of the micromix hydrogen flame which is the same as third-body effect.