Flowfield impact on distributed combustion in a swirl assisted burner

Colorless distributed combustion (CDC) is a novel method to enhance flame stability and thermal field uniformity, increase combustion efficiency and reduce pollutants emission, including noise. CDC is achieved through the use of a carefully prepared oxidizer mixture of reduced oxygen concentration through added high temperature 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 N-2, and 17% using CO2 dilution. Emission of NO and CO were determined under conditions approaching CDC. NO levels of only 2 or 1 ppm were achieved using N-2 or CO2 dilution, respectively under CDC condition. In order to determine how the flow velocity structure and eddy size effect the stability and emissions a high speed (3 kHz) particle image velocimetry (PIV) system was used. 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 NOx emission. Investigation of Reynolds stress showed that dilution with CO2 provided stronger impact on stress than N-2 due to the increased density and reduced viscosity of CO2.