Abstract
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 Φb, and the NH3/air mixture can not be ignited when Uin is higher than 5 m/s. After adding 50% CH4 fuel, the laminar flame speed SL and the extinction strain rate Kext 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 Φb = 0.78 with inlet bulk velocity Uin = 4 m/s can be extended wider by 16.6% to Φb = 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.
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