Partial premixing can be induced by design in combustors, occurs inadvertently during turbulent nonpremixed combustion, or arises through inadequate fuel-air mixing. Therefore, it is of interest to investigate the effect of partial premixing in a burner that mimics conditions that might occur under practice. In this investigation, we report on similitude of partially premixed flames encountered in practical complex and multi-dimensional burners with simpler, less complex flames, such as counterflow flamelets. A burner is designed to simulate the more complex multi-dimensional flows that might be encountered in practice, and includes the effects of staging, swirl, and possible quenching by introduction of secondary air. The measurements indicate that the structure of partially premixed flames in complex, practical devices can be analyzed in a manner similar to that of flamelets, even if substantial heat transfer occurs. In particular, the flame structure can be characterized in terms of a modified mixture fraction that differentiates the lean and rich zones, and identifies the spatial location of the flame.

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