Experimental and Numerical Investigation of the Influence of the Air Preheating Temperature on the Performance of a Small-Scale Mild Combustor

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 NOx concentrations are also reported. The combustor yields low CO (< 10 ppm @ 15% O2) and NOx 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.