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

Abstract

Abstract. Coherent anti-Stokes Raman spectroscopy (CARS) is an established technique for acquiring highly accurate, nonintrusive gas temperature measurements. CARS was selected for this study because other temperature measurement approaches would lose their accuracy in the extreme environment of the liquid-fueled rich-burn, quick-quench, lean-burn combustor configuration that was studied. These measurements were motivated by a goal 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 axial 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. Motorized translation stages were used to move the probe volume to different spatial locations in the combustor without misaligning the CARS system. The x = 40 mm results demonstrate that the average temperature in the central recirculation zone (CRZ) 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, which suggests that the temperature distribution is influenced more by the proximity to LBO than the global equivalence ratio. 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. A single test case was captured where LBO happened during the data acquisition process. This confirmed that LBO is preceded by significant entrainment of cool gases. Particulate matter (PM) emissions measurements were also acquired as part of this work. The PM measurements showed a nonmonotonic 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. This flame shape change was evaluated in the authors' previous work, where it was found to result from a helical vortex structure that develops in the flow at certain conditions and disrupts the fuel spray.