Hydroxyl spatio-temporal statistics for turbulent partially premixed opposed -jet flames

Krishna Kumar Venkatesan, Purdue University

Abstract

Turbulent flows display fluctuations spanning a wide spectrum of both length and time scales. The most energetic fluctuations are typically contained in large-scale eddies characterized by integral length and time scales. In turbulent opposed-flows, the predominance of young turbulence and the influence of the stagnation plane complicate the interpretation of spatial and temporal structures. Therefore, it is essential to understand both the temporal and spatial behavior of large-scale structures in such opposed flows. In this context, two-point, picosecond time-resolved laser-induced fluorescence (PITLIF) has been used to resolve both integral length and time scales, thus providing unique spatial statistics in addition to temporal statistics. The interpretation of spatial and temporal structures is further complicated by their dependence on both local turbulent Reynolds number and equivalence ratio, as affected by changes in internal structure and flame thickness. Hence, simultaneous PLIF/PIV measurements were employed to investigate spatial OH structures and flame-velocity interactions. Hydroxyl single-point time-series measurements have been obtained in opposed-jet nonpremixed flames to study the independent effects of jet velocity (U) on Reynolds number (Re) and strain rate (SR) using time-scale correlations. The combined effect of Re and SR on the integral time scale behaves approximately as U-1.35 in these opposed-jet nonpremixed flames. Two-point, time-series experiments with associated PLIF/PIV measurements were employed to investigate flamelet spatio-temporal scales, extinction and reignition mechanisms, and flame-velocity interactions in turbulent opposed jet, partially premixed and double flames. Filtered OH length scales corresponding to radial OH fluctuations, were obtained from the spatial autocorrelation function. The axial change in length scale is more pronounced for flames with lower bulk Re and greater partial premixing. A stochastic time-series simulation, using a combined mixture-fraction and progress-variable approach and based on measured OH time scales, has been performed to extract scalar time scales in opposed jet double flames. In contrast to the partially-premixed and nonpremixed flames, where the ratio of OH to underlying scalar time scales is ∼0.3, the time-scale ratio for the double flames is ∼0.5. As compared to nonpremixed flames, OH in double flames is distributed across a broader spatial and mixture-fraction space, thereby inducing relatively slower OH fluctuations.

Degree

Ph.D.

Advisors

Laurendeau, Purdue University.

Subject Area

Statistics|Mechanical engineering|Optics

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