Application of Multispectral Imaging to Understand Wall Heat Flux Measurements in a Backward-Facing Step Combustor

As gas turbine combustors become smaller to achieve efficiency goals, understanding the heat flux to the wall becomes imperative for proper combustor design. As part of an on-going effort to measure and analyze convective and radiative heat flux in a backward-facing step combustor, wall temperature, heat flux, and near-wall flame behavior are investigated using a suite of diagnostics, including multi-spectral infrared (IR) imaging, heat flux sensors, and OH planar laser-induced florescence (PLIF). Backward-facing step test sections have many of the flow features of gas turbine combustors (recirculation, flame/wall impingement, and boundary layer recovery), but in a nominally two-dimensional flow. IR imaging shows consistent contributions of carbon dioxide and water vapor to wall heat flux over a range of Reynolds numbers (Reℎ = 4199 − 8323). Gaseous contributions to heat flux increase further downstream; as the flame expands through the combustor, higher concentrations of radiating products are produced, increasing the radiation to the wall. Through-flame IR imaging of the wall temperature showed high temperature gradients along the combustor, with local cooling as a result of the water-cooled heat flux sensors. The increased radiative load is reflected in higher plate temperature measurements. Importantly, the sensor face temperatures were measured and found to be dramatically lower than the wall temperature due to the water cooling of the instruments. Interestingly, increases in near-wall progress variable do not correspond to increases in heat flux, indicating the non-local impact of the flame on heat flux measurements. Additional analysis of heat flux the wall beneath the recirculation zone showed the increase in total heat flux is likely driven by flame wrinkling. Overall, the work presented here compliments previous testing and provides additional insight into combustor heat transfer.