Assessment of MILD combustion in co/counter-swirl configuration using syngas as a fuel.

Moderate or intense low oxygen dilution (MILD) combustion is investigated in a co/counter-swirl using OH∗-chemiluminescence, species measurements, OH planar laser-induced fluorescence (OH-PLIF), two-dimensional particle image velocimetry (2D-PIV), and microphone measurements. Experiments are performed in a two-stage combustor, where the first stage of combustor is catalytic stage and the second stage is the swirl stage (co/counter-swirl configuration). A fuel-rich syngas (20 % H 2 , 20 % CO, 12 % CO 2 , 2 % CH 4, and 46 % N 2) reacts with air in the catalytic stage. Then, the mixture of unconsumed syngas and products of catalytic combustion is supplied to the swirl stage where the mixture is burnt with oxidizer with varied oxygen concentration. The flame in the swirl stage is established by two concentric swirling streams where the inner stream is the hot gases from the catalytic stage, and the outer stream is the oxidizer. Co/counter-swirl flames are generated by changing the swirl direction of the inner stream. After achieving a stable flame, the macrostructure, flame steadiness, CO and NO x emissions, reaction zone distribution, and sound pressure level (SPL) are investigated at several oxygen concentrations (O 2 = 13.13 %–21 %) with an aim to assess the MILD combustion mode in co/counter-swirl configuration. As the oxygen percentage decreases, the flame luminosity decreases for both co/counter-configuration. However, the reduction in luminosity is profound for co-swirl configuration. Clear distinctions between co and counter-swirl configurations are observed regarding flame height and stand-off height. Two-dimensional particle image velocimetry (2D-PIV) is utilized to understand these trends. The steadiness of the flame is investigated using standard deviations (SD) of OH∗-chemiluminescence images and global luminosity (I(t)). The flame steadiness is found to be improved as the oxygen concentration decreases. The OH-PLIF indicates the distributed nature of combustion. The NO x emission is found to be extremely low in all studied cases; however, the CO emission shows an increasing trend when O 2 reduces. Finally, the sound pressure level and the dynamics stability are investigated using microphone measurements. The SPL decreases by ∼3 dB and ∼7 dB for the counter-swirl and co-swirl configuration, respectively. Furthermore, the frequency domain analysis suggests that the fundamental axial mode of the combustor is excited at high oxygen concentration. However, the unsteady combustion and chamber acoustics become decoupled at lower O 2 concentrations. Thus, the present paper, for the first time, confirms that MILD combustion can be achieved in co/counter-swirl configuration, provided the oxygen concentration is low (∼13 %). The present study also establishes that the co-swirl configuration is more suitable than the counter-swirl for achieving the MILD combustion mode. • Moderate or intense low-oxygen dilution (MILD) combustion is achieved in co/counter-swirl configuration. • OH∗ chemiluminescence intensity decreases with O 2 concentration. • High-speed chemiluminescence indicates improved stability at low O 2 concentrations. • Sound pressure level decreases with O 2 concentration, and NO x is below 2 ppm. • PIV and PLIF highlight the difference between co and counter-swirl flames. [ABSTRACT FROM AUTHOR]