Temperature measurements in steady axisymmetric partially premixed flames by use of rainbow schlieren deflectometry

We focus on the utility of rainbow schlieren as a tool for measuring the temperature of axisymmetric partially premixed flames (PPFs). Methane-air PPFs are established on a coannular burner. The flames involve two spatially distinct reaction zones, one in an inner premixed region that has a curved tip and a spatially planar wing portion and another that involves an outer nonpremixed zone in which intermediate species burn in air. Schlieren images are found to visualize clearly these PPF characteristics through light deflection by steep refractive-index gradients in the two reaction zone fronts. The temperature distributions of two flames established at fuel-rich mixture equivalence ratios of phi(r) = 1.5 and 2.0, with bulk-averaged velocities, Vreac = 60 cm s(-1) and Vair = 50 cm s(-1), are inferred from color schlieren images, and a measurement error analysis is performed. Errors arise from two sources. One lies in the process of inferring the temperature from the refractive-index measurement by making assumptions regarding the local composition of the flame. We have shown through simulations that the average temperature deviations due to these assumptions are 1.7% for the phi(r) = 1.5 flame and 2.3% for the phi(r) = 2.0 flame. Another source involves the local uncertainty in the measurement of the transverse ray displacement at the filter plane that is used to determine the refractive index and thereafter the flame temperature. We have ascertained that a maximum error of 4.3% in the temperature determination can be attributed to this local measurement uncertainty. This investigation demonstrates the capability of the schlieren technique for providing not only qualitative displays of the PPFs but also full-field-of-view temperature measurements that are accurate, spatially resolved, and nonintrusive.