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 NO_x 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 NO_x 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.

This article presents experimental and computational results of a small-scale combustor operating in the mild combustion regime. On the experimental side, flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging are reported as a function of the preheat temperature of the combustion air. For two of these combustor operating conditions, spatial distributions of temperature and of O₂, CO₂, unburned hydrocarbons, CO, and NO_x concentrations are also reported. The combustor yields low CO (< 10 ppm @ 15% O₂) and NO_x emissions (<14 ppm @ 15% O₂) regardless of the inlet air temperature; the OH* images reveal that, as the inlet air temperature increases, the main reaction zone moves progressively closer to the burner; the chemiluminescence images also show that the OH* gradients increase as the level of air preheating increases; and the detailed measurements made inside the combustor confirmed the expected uniformity of the measured fields over the entire combustion chamber, as expected when operating under mild conditions. On the modeling side, the calculations were carried out using the commercial code Ansys-Fluent. Turbulence was modeled using the realizable k-ε model. The eddy dissipation concept was employed along with a skeletal chemical mechanism comprising 13 transported species and 73 chemical reactions. Additional calculations carried out using a global single-step reaction are also reported. The more detailed reaction mechanism is able to accurately predict the temperature and the concentrations of O₂ and CO₂ over most of the combustor, but the temperature field is overestimated in the vicinity of the burner. Discrepancies are found in the prediction of the CO concentrations.