Temperature measurements in steady two-dimensional partially premixed flames using laser interferometric holography

The local flow temperature is a key consequence of combustion and must, therefore, be accurately measured. Holographic interferometry can be employed to accurately determine the refractive index in flames and, thereafter, to infer the temperature distribution. This investigation focuses on the utility of laser interferometric holography as a tool to measure the temperature of two-dimensional partially premixed flames (PPFs). Methane–air PPFs are established on a rectangular Wolfhard-Parker slot burner. These flames contain two reaction zones, one in an outer nonpremixed region and the other in an inner rich-premixed region. We examine flame structure effects (that produce a varying composition in the flame) on the local refractive index and show that a relation that contains relatively minor errors, which depend upon the rich-side equivalence ratio, can model the refractive index in PPFs. This is the first investigation to discuss the effects of a realistically varying composition due to combustion on the refractive index distribution in flames. The maximum error in the temperature is 6–34% when the rich-side equivalence ratio lies in the range 1.5–∞, while the corresponding average error is 2.4–12.3%. Relatively large discrepancies arise in the case of nonpremixed flames. We discuss the experimental configurations required to reconstruct clear interferometric fringe patterns. Image plane holography is employed because it offers two advantages: (1) the holograms can be read with white light, and (2) the fringe count may be considered to occur in a straight line as though no refraction had occurred. We confirm the inefficacy of using thermocouples as a measurement tool in two-dimensional PPFs. Questions related to the beam path length are resolved. The portion of the flame–beam interference length that lies between the high and low temperatures is accounted for by assuming a uniform composition and by introducing a density weighting function. Thereafter, we examine the holographic fringe patterns, discuss the discrepancies that arise in the interpreted temperatures as the PPF equivalence ratio is varied, and present the inferred temperature distributions. The temperature distribution is found to correlate with the heat release in the inner rich premixed and outer nonpremixed reaction zones. Large temperature gradients exist in the inner premixed reaction zone, whereas the outer nonpremixed region contains smaller gradients, since it is transport limited. The spatial temperature resolution required to resolve the higher gradients is ≈400 K mm−1.