TDLAS measurements of temperature and water vapor concentration in a flameless MILD combustor

We performed non-intrusive tunable diode laser absorption spectroscopy measurements of the temperature (T) and water vapor concentration (X H2O) on a moderate or intense low-oxygen dilution combustion chamber. The combustor was operating in flameless mode by axially injecting fuel and air at high speeds through separate nozzles, thereby creating a recirculating flow of combusted gas. A pair of water vapor absorption lines near 7185.6 and 7444.36 cm-1 was used to acquire axial absorbance profiles along the height of the combustor. The axial profiles of T and X H2O were first derived using two-line thermometry measurements. Then the T profiles were estimated from each of the single absorption line measurements by assuming spatial uniformity of X H2O. Compared to a thermocouple temperature measured at the combustor outlet, the temperatures given by the two-line thermometry were under-estimated whereas the thermocouple temperature was in between the T range given by the single-line thermometry. Importantly, the measurement successfully revealed that the axial profiles of T and X H2O were mostly flat certainly due to the strong recirculation of the hot combustion gas; however, the different temperature values at a given height implied the existence of a radial temperature gradient in the combustor.