Experimental study on structure and blow-off characteristics of NH₃/CH₄ co-firing flames in a swirl combustor

Experimental study on the unconfined lean premixed NH₃/CH₄/air flames with three CH₄ fractions of 0, 50%, and 100% stabilized on a bluff-body and swirl burner was investigated. The flow fields and instantaneous OH distributions were captured by synchronous PIV/OH-PLIF measurement. The macrostructure and critical mechanism of blow-off for the compact NH₃/air flame at conditions far away and near blow-off, and the effect of CH₄ addition on the topology and flame stabilization was discussed. Results show that the NH₃/air flame possesses a poor lean blow-off limit Φ_b, and the NH₃/air mixture can not be ignited when U_in is higher than 5 m/s. After adding 50% CH₄ fuel, the laminar flame speed S_L and the extinction strain rate K_ext are approximately 3 times and 7 times that of the NH₃/air flame with equivalence ratio Φ = 0.8, resulting in a more stable flame. And the NH₃/air flame Φ_b = 0.78 with inlet bulk velocity U_in = 4 m/s can be extended wider by 16.6% to Φ_b = 0.65. The NH₃ flame front has been locally extinguished due to excessive stretching, which is the key factor of triggering the overall blow-off. Stretching is mainly caused by shear strain. CH₄ addition makes the flame front more wrinkled. Furthermore, the flame is strengthened due to the enhanced resistance to stretching by blending CH₄ with NH₃. But the root of the NH₃/CH₄/air flames with methane fractions of 50% and 100% also have excessive straining near the blow-off conditions, which induces flame blow-off. The increase of burning velocity and temperature by adding CH₄ is also conducive to improving flame stability. The Inner Shear Layer (ISL) vortexes promote the uniform mixing of combustion products in the Inner Recirculation Zone (IRZ) and downstream combustion gas, which is conducive to the stability of CH₄/air flame. For NH₃/air flame, the ISL vortexes accelerate the flame blow-off by entraining the cold reactants into the IRZ.