Abstract
ABSTRACTStructures of turbulent Bunsen flames in the corrugated-flamelet regime were investigated by use of a three-color six-beam laser-Doppler velocimetry system. Four different mixtures with identical laminar burning velocities (0.34 m/s) were selected to facilitate comparisons—lean and rich methane, and lean and rich propane. A bimodal distribution, not previously reported in the literature on turbulent Bunsen flames, was observed in the radial component of gas velocity off-axis in the turbulent flame brush. Our previous measurements enabled the low-velocity mode to be identified as velocity fluctuations of the unburned mixture and the high-velocity mode as fluctuations of the burned-gas radial velocity. Favre-averaged and Reynolds-averaged reaction-progress variables were then calculated from these bimodal distributions, identifying an initially unexpected region near the flame tips where, at a fixed radius, the average progress variable decreased (rather than increasing) with increasing height over a short distance, likely through enhanced flamelet flapping, which has not been predicted by modeling but which appears to occur quite generally for sufficiently tall turbulent Bunsen flames in quiescent ambient environments, for the corrugated-flamelet regime. Conditioned and unconditioned Favre-average velocity components and intensities also were calculated from the data for future tests of modeling. The distributions of the progress variables also clearly showed that the turbulent burning velocity of the rich propane flame was appreciably larger than that of any of the other three, as was its radial flame-brush thickness at any given height, and its high-radial-velocity mode had a higher average velocity magnitude than the others. Similarly, the turbulent burning velocity and flame-brush thickness appear to be smaller for the lean propane flame. These differences can be attributed to influences of preferential oxygen diffusion to turbulence-induced flamelet bulges, not included in existing modeling approaches, for the rich propane flames, and to a corresponding inhibition of fuel diffusion to the bulges in the lean flames. The former phenomenon is related to but different from the well-known cellular-flamelet instability, these effects occurring for flames that are stable to diffusive-thermal disturbances. It was concluded that a greater fraction of the total amount of heat release occurs in the upstream half of the turbulent flame brush in the rich propane flame, producing enhanced flow divergence in the upstream region, while the reduced ability of the slowly diffusing fuel to reach bulges in the lean flame generates the opposite effect. The results point to directions in which turbulent-combustion modeling needs to be improved, and an approach to modeling this type of preferential-diffusion effect is suggested.
Highlights
Structures of turbulent flames differ considerably in different regimes of turbulent combustion (Peters, 2000)
Such flames have been investigated experimentally by Schlieren, shadowgraph, and visible-emission techniques for more than 60 years (Bollinger and Williams, 1949; Hottel et al, 1953; Karlovitz et al, 1953), unexpected aspects of their structures continue to be discovered by experimental methods ranging from the use of electrostatic probes (Furukawa et al, 2002, 2010, 2013a, 2013b) to laser-sheet Mie-scattering tomography (Kobayashi et al, 1996, 1997, 1998), to combined Rayleigh scattering and planar OH laser-induced fluorescence (LIF; Chen and Bilger, 2001, 2002; Frank et al, 1999), to laser-Doppler velocimetry (LDV; Furukawa et al, 2002, 2013a, 2013b) and particle-image velocimetry (PIV; Frank et al, 1999; Pfadler et al, 2009; Steinberg et al, 2009)
As indicated in the introduction, in earlier work (Furukawa et al, 2010) on the basis of measurements of flamelet motions, we identified clear differences between the turbulent flame structures of rich and lean propane flames hypothesizing that the differences arise from turbulence-produced flamelet wrinkling, followed by preferential oxygen diffusion to protruding wrinkles, thereby enhancing the average flamelet propagation velocity
Summary
Structures of turbulent flames differ considerably in different regimes of turbulent combustion (Peters, 2000). Ordinary premixed turbulent Bunsen flames of gaseous hydrocarbon fuels in the laboratory often are in the corrugated-flamelet regime (Peters, 2000), as was the case in our earlier work (Furukawa et al, 2002) Such flames have been investigated experimentally by Schlieren, shadowgraph, and visible-emission techniques for more than 60 years (Bollinger and Williams, 1949; Hottel et al, 1953; Karlovitz et al, 1953), unexpected aspects of their structures continue to be discovered by experimental methods ranging from the use of electrostatic probes (Furukawa et al, 2002, 2010, 2013a, 2013b) to laser-sheet Mie-scattering tomography (Kobayashi et al, 1996, 1997, 1998), to combined Rayleigh scattering and planar OH laser-induced fluorescence (LIF; Chen and Bilger, 2001, 2002; Frank et al, 1999), to laser-Doppler velocimetry (LDV; Furukawa et al, 2002, 2013a, 2013b) and particle-image velocimetry (PIV; Frank et al, 1999; Pfadler et al, 2009; Steinberg et al, 2009). Conditions are near the borderline of the wrinkled-flamelet ( called weak-turbulence) regime, far from the thin-reaction-zone regime, so that Markstein numbers should be appropriate for describing effects of the turbulence on the flamelets
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