The effects of buoyancy on turbulent nonpremixed jet flames in crossflow

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Boxx, Isaac G.

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An experimental research study was conducted to investigate what effect buoyancy had on the mean and instantaneous flow-field characteristics of turbulent jet-flames in crossflow (JFICF). The study used an experimental technique wherein a series of normal-gravity, hydrogen-diluted propane JFICF were compared with otherwise identical ones in low-gravity. Experiments were conducted at the University of Texas Drop Tower Facility, a new microgravity science laboratory built for this study at the University of Texas at Austin. Two different diagnostic techniques were employed, high frame-rate digital cinematographic imaging and planar laser Mie scattering (PLMS). The flame-luminosity imaging revealed significant elongation and distortion of the large-scale luminous structure of the JFICF. This was seen to affect the flame-tip oscillation and burnout characteristics. Mean and root-mean-square (RMS) images of flame-luminosity were computed from the flame-luminosity image sequences. These were used to compare visible flame-shapes, flame chord-lengths and jet centerline-trajectories of the normal- and low-gravity flames. In all cases the jet-centerline penetration and mean luminous flame-width were seen to increase with decreasing buoyancy. The jet-centerline trajectories for the normal-gravity flames were seen to behave differently to those of the low-gravity flames. This difference led to the conclusion that the jet transitions from a momentum-dominated forced convection limit to a buoyancy-influenced regime when it reaches ξC ≈ 3, where ξC is the Becker and Yamazaki (1978) buoyancy parameter based on local flame chord-length. The mean luminous flame-lengths showed little sensitivity to buoyancy or momentum flux ratio. Consistent with the flame-luminosity imaging experiments, comparison of the instantaneous PLMS flow-visualization images revealed substantial buoyancy-induced elongation and distortion of the large-scale shear-layer vortices in the flow. This effect became apparent in the JFICF at around ξy = 3.1 and grew in influence to become a dominant flow-field characteristic approximately ξy = 4.3. The PLMS images also yielded physical-insight into the nature of the fore-aft asymmetry of JFICF characteristics noted by previous researchers. Ensemble-averages of PLMS images were used to investigate centerline mixture fraction decay. Consistent with previous studies of non-reacting JICF studies, the mixture-fraction of the JFICF showed a power-law decay profile which scaled with (rd)-0.66. Over the region these measurements were made (ξy = 0 – 1.9), the mixture fraction decay scaling showed little sensitivity to buoyancy. Taken as a whole, these measurements show that buoyancy has the potential to significantly modify both the mean and instantaneous flow-field of a turbulent JFICF, even at relatively modest length-scales.



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