Experimental Study of the Combustion Regimes Occurring in a Laboratory Combustor

This article examines the combustion regimes occurring in a small-scale laboratory cylindrical combustor, in which the burner and the exhaust port are mounted at the top end of the combustion chamber. Flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging are presented as a function of the air inlet preheat temperature and excess air coefficient, which in the present configuration implies also changes in the air inlet velocity. For three of these combustor operating conditions, detailed in-combustor measurements of temperature and of O2, CO2, CO, unburned hydrocarbons, and NOx concentrations are also reported. The flue-gas data reveal that, for a given air inlet preheat temperature, the air inlet velocity has an important impact on the NOx emissions, which decrease as the air inlet velocity increases. Furthermore, as the air inlet temperature increases, the burner is able to operate with higher excess air coefficients. For a given air inlet temperature, the combustion regime in the present combustor develops from conventional lean combustion at lower air inlet velocities (or lower excess air coefficients) to flameless combustion at higher air inlet velocities (or higher excess air coefficients). The OH* images and both the temperature and the chemistry fields indicate that the reaction zone for the conventional lean combustion regime is well defined and established near the burner with relatively high gradients along the combustion chamber length. In contrast, the flameless combustion regime, which occurs for higher reactants jet momenta, is characterized by a more distributed reaction zone, located far from the burner, as revealed by the chemiluminescence images that show relatively small gradients disperse over a large volume of the combustion chamber. In line with this, the spatial temperature and gas species concentration gradients are much lower in this case than in the case of the conventional lean combustion regime.