Impact of internal entrainment on high intensity distributed combustion

Colorless Distributed Combustion (CDC) has shown ultra-low emissions and enhanced performance of simulated gas turbine combustors. To achieve distributed combustion, the flowfield must be tailored for desirable mixture preparation within the combustor prior to mixture ignition. Though CDC have been extensively studied using a variety of geometries, heat release intensities, and fuels, the role of internally recirculated hot reactive gases needs to be further investigated and quantified to obtain the minimum requirement of internal entrainment for achieving distributed reaction condition. In this paper, the impact of internal entrainment of product gases on flame structure and behavior is investigated with focus on fostering distributed combustion and to provide guidelines for seeking distributed combustion. To simulate the recirculated gases from within the combustor, a mixture of nitrogen and carbon dioxide is introduced to the air stream prior to mixing with fuel and combustion. Increase in the amounts of nitrogen and carbon dioxide (simulating increased recirculation) increased the reaction volume to occupy larger volume with an overall enhanced and uniform distribution as revealed from the OH∗ chemiluminescence intensity. At the same time, the bluish flame is replaced with a more uniform almost invisible bluish flame. The increased recirculation also decreased the NO emission significantly for the same amount of fuel burned. Lowering oxygen concentration from 21% to 15% (due to increased recirculation) resulted in 80-90% reduction in NO with no impact on CO emission with sub PPM NO emission achieved at an equivalence ratio of 0.7. The same trend was demonstrated for a range of recirculated gases temperature. The reaction distribution was significantly enhanced with ultra-low emissions for oxygen concentration lower than 16% setting a minimum recirculation requirement for distributed combustion.