Development of a two-point high-speed laser-induced fluorescence technique and its applications to turbulent jet flames for hydroxyl concentration

Jiayao Zhang, Purdue University

Abstract

In turbulent combustion, scalar fluctuations are characterized by a wide spectrum of time and length scales. To fully characterize fluctuating scalars and to elucidate nonlinear turbulence-scalar interactions, we have developed a novel two-point, high repetition-rate, laser-induced fluorescence technique, specifically for simultaneous time-series measurements of minor-species concentrations at two spatial locations. Its applications for OH concentration in turbulent flames facilitate the study of hydroxyl integral time and length scales, and the understanding of turbulence-combustion interactions. Optical aberrations of the detection system prove to be important for spatial resolution and for the measurement of two-point statistics, and can be minimized through an optimal design. The dominant Poisson noise is effectively corrected for using two independent procedures. For the flames under investigation, fluctuations in the fluorescence lifetime have a negligible effect on the measured statistics; the measured integral length scale only weakly depends on the sampling rate. Radial two-point OH measurements are applied to a series of H2/N 2 nonpremixed jet flames. The OH integral time scale exhibits a strong dependence on jet Reynolds number and fuel dilution. The OH radial integral length scale is determined by local turbulence and depends only weakly on Reynolds number. Different trends are manifested at radial [OH] peaks and along the jet centerline. Partially premixed H2/CH4 jet flames show different radial two-point OH statistics as compared to nonpremixed jet flames. For flames with a double-flame structure, the presence of a thick high-temperature region between the two flame fronts prolongs regions with coherent vortices and local laminarization. The premixed flame front is characterized by much more rapid fluctuations in [OH] and larger radial integral length scales compared to the nonpremixed front at the same axial distance. Axial two-point OH measurements in a standard H2/N2 nonpremixed jet flame show axial convection of OH structures approximately at the flow velocity. The axial length scale is generally larger than the radial length scale, indicating elongated structures. Along the axial direction, two-point OH statistics are primarily dominated by motion of the flame front, with appreciable deviation in thick OH regions.

Degree

Ph.D.

Advisors

King, Purdue University.

Subject Area

Mechanical engineering

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