Systematic approach based on holographic interferometry measurements to characterize the flame structure of partially premixed flames
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Partially premixed flames (PPF's) represent a class of hybrid flames that contain multiple reaction zones. A detailed understanding of the temperature distribution in PPF's is important from both practical and scientific considerations. Path-integrated or line-of-sight measurement techniques, such as holographic interferometry (HI), that are based on the change in the optical phase of a light beam can be used to reconstruct the refractive index n in flames and thereafter to infer the temperature distribution. Therefore to describe the flame structure in the context of these measurements requires that a systematic approach be developed that relates the density, the temperature, and the composition to the refractive index. We demonstrate that a conserved scalar xi that transforms the flame structure from a spatial to a generic distribution can be inferred from the refractive-index distribution. Thereafter measurements of the density, the temperature, and the composition in two-dimensional PPF's become feasible. We report the first application, to our knowledge, of this method to HI. Specifically, we used HI to measure the refractive-index distributions in methane-air PPF's. One PPF is a double flame that has two reaction zones, and the other is a triple flame that contains three reaction zones. We have applied the procedure to infer the distribution of the modified mixture fraction and thereafter the local temperature and the local mass fractions. We find the local temperature differences, DT(x, y) = |T[n(x, y)] - T?[xi(x, y)]|, to be relatively small. We conclude that it is possible to use HI to infer the mixture-fraction distribution and thereafter the flame structures by the application of state relations in the case of PPF's.
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