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

CATEGORY: FLAMELESS OXIDATION
Experimental Thermal and Fluid Science, Volume 44, January 2013, Pages 75–81
Importance of the inlet air velocity on the establishment of flameless combustion in a laboratory combustor
A.S. Veríssimo, A.M.A. Rocha, M. Costa,
Mechanical Engineering Department, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal
Abstract
Authors explain the importance of the inlet air velocity (V_air) on the establishment of flameless combustion in a 10 kW laboratory scale combustor. Researchers obtained variations in V_air by changing the air nozzle diameter while maintaining constant all remaining input parameters. Initial evaluation of the combustor flow aerodynamics under non-reacting conditions was performed using laser-Doppler anemometry.
Subsequent flue-gas composition data and hydroxyl radical chemiluminescence (OH^*) imaging were obtained as a function of V_air. For two of these combustor operating conditions, spatial distributions of temperature, recorded with fine wire thermocouples, and of O₂, CO₂, unburned hydrocarbons, CO and NO_x concentrations, measured with the aid of a sampling probe, were obtained, as well. The OH^* images revealed that as V_air 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. However, the OH^* intensities decrease, suggesting 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 would indicate 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.
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