Impact of Flowfield on Pollutants’ Emission from a Swirl-Assisted Distributed Combustor

Colorless distributed combustion (CDC) is a novel method to enhance flame stability and thermal field uniformity, increase combustion efficiency, and reduce pollutants’ emission. CDC is achieved through the use of a carefully prepared fuel–oxidizer mixture along with reactive species. In this study, a partially premixed, swirl-assisted cylindrical combustor utilized a propane–air flame with either nitrogen or carbon dioxide gas in order to reduce the oxygen concentration of the oxidizer. OH* chemiluminescence signatures were used to determine transition to distributed combustion condition. The results showed transition to CDC at approximately 15% using N₂ and 17% using CO₂ dilution. Emission of NO and CO under CDC condition showed NO levels of only 2 or 1 ppm using N₂ or CO₂ dilution, respectively. High-frequency PIV (3 kHz) was used to determine the flow velocity structure and eddy size effects on flame stability and emissions. Increase in dilution enhanced both the radial and axial mean and fluctuating velocities under CDC that foster mixing. Additionally, the Kolmogorov length decreased with increase in dilution resulting in smaller eddy size particularly in the swirl lobe region, which enhanced turbulent dissipation that resulted in lower peak temperatures and reduced thermal NO_x emission. Reduced viscosity using CO₂ dilution provided a stronger effect in reducing NO as compared to N₂ as the diluent.