Coherent Anti-Stokes Raman Spectroscopy Temperature Measurements in a Swirl-Stabilized Spray Combustor

Coherent anti-Stokes Raman spectroscopy measurements were acquired in a RQL combustor for the purpose of quantifying changes in combustor primary zone gas temperature as flames approach lean blowout (LBO). Both stable burning and near-LBO data were acquired at spatial locations with (i.e., x=24 mm) and without (i.e., x=40 mm) fuel droplets using two liquid fuels with very different LBO boundaries. The x=40 mm results demonstrate that the average temperature in the central recirculation zone decreases ~300 K between the stable burning and the threshold of LBO operating condition. Furthermore, the measured gas temperature distributions were very similar when both fuels burned near their respective LBO boundaries. Measurements that were acquired further upstream at x=24 mm showed temperature distributions that differed significantly from the x=40 mm data. Specifically, the temperature distribution was much wider at x=24 mm with temperature values ranging from the air inlet temperature to the stoichiometric adiabatic flame temperature. The x=24 mm CARS data showed a notable increase in cool gases and a notable decrease in hot gases near LBO as compared to the stable burning results. Particulate matter (PM) emissions measurements were also acquired as part of this work. The PM measurements showed a non-monotonic dependence on the equivalence ratio where soot production increased to some extent as the equivalence ratio was decreased. This unusual soot production observation is linked in this work to differences in fuel spray patterns that accompany a flame shape change.