Importance of the inlet air velocity on the establishment of flameless combustion in a laboratory combustor

Abstract

This article examines the importance of the inlet air velocity (Vair) on the establishment of flameless combustion in a 10kW laboratory scale combustor. Variations in Vair were accomplished by changing the air nozzle diameter while maintaining constant all remaining input parameters. Initially, laser-Doppler anemometry was employed to evaluate the combustor flow aerodynamics under non-reacting conditions. Subsequently, flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging were obtained as a function of Vair. For two of these combustor operating conditions, spatial distributions of temperature, recorded with fine wire thermocouples, and of O2, CO2, unburned hydrocarbons, CO and NOx concentrations, measured with the aid of a sampling probe, were also obtained. The OH* images showed that as Vair increases at a constant excess air coefficient (λ) of 1.3, the main reaction zone, typical of flameless combustion condition, remains approximately in the same region of the combustor, because of the flow aerodynamics similarity, but the OH* intensities decrease, which indicates higher entrainment ratios of the fuel and burned gases by the central air jet. For λ greater than 1.7, however, flameless oxidation could not be established regardless of the air jet momentum. This suggests that the establishment of the flameless combustion condition in future gas turbines through the dilution of the reactants with a substantial amount of flue gases in configurations where the combustion air is provided by a central high-momentum air jet that is surrounded by a number of low-momentum fuel jets may be problematic.