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 Phi(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 Phi = 0.8, resulting in a more stable flame. And the NH3/air flame Phi(b) = 0.78 with inlet bulk velocity U-in = 4 m/s can be extended wider by 16.6% to Phi(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.

Automated summary

This study investigated the experimental study of unconfined lean premixed NH3/CH4/air flames and found that adding CH4 fuel can improve flame speed and stability, resulting in a wider flame range. However, excessive stretching is the key factor for flame blow-off.