A planar laser-induced fluorescence study on the effects of unsteadiness and fuel Lewis number in hydrogen laminar diffusion flames

Studies on the effect of transport properties coupled with the interaction of fluid dynamics and combustion in acoustically forced laminar hydrogen jet diffusion flames have been performed using the planar laser-induced fluorescence (PLIF) technique. These unsteady diffusion flames are of particular interest because they are reproducible turbulent-like events that can be investigated to gain insight into turbulent combustion. Results reported herein add to the ongoing effort of understanding the complex transport processes taking place in the flames encountered in most modern heat-producing and power-producing devices.; Fuel transport properties (i.e. fuel Lewis number, Le_F) were varied by fuel dilution with various levels of helium (He) or argon (Ar). The fuel stream of Burke-Schumann type hydrogen flames was acoustically excited by using a loudspeaker and the two-dimensional OH and temperature fields were measured. PLIF measurements were performed using an intricate two-laser, two-camera system; digital image analysis was implemented to reduce the large image data set obtained.; The temperature of the unsteady flames departed significantly from the steady-state temperature as predicted by previous researchers. It was found that, regardless of Le_F, unsteady He-diluted flames had maximum flame temperatures at some point during the speaker oscillation that were always higher than the maximum temperatures of the H₂-Ar flames. This was contrary to the trends seen in steady flames. An increased H₂ mass flux to the flame zone in the unsteady H₂-He flames was the reason for this observation since mass diffusion becomes important in the driven flames due to increased mass gradients and the difference in diffusivity of hydrogen in the diluents used.; Low turbulence intensities (i.e. low frequency) allowed the flames to respond steadily to the changing flowfield. The structure of the reaction zone of unsteady flames at this low frequency was altered (i.e. stretched or compressed) slightly and, in general, these flames resembled the steady flames structurally. At high frequency, however, the flames responded to the imposed flow oscillation by considerable reaction zone stretch/compression. Results obtained from the present experiments suggest that, depending on the Lewis number, the flame temperature responds differently to the stretch imparted on the flame by the unsteady flowfield. These Lewis number effects were evidenced by both the low and high frequency flames; however, they were most obvious in the high frequency cases. The temperature of flames with Le _F ≥ 1 increased/decreased when the reaction zone thickened/thinned. On the other hand, flames with Le_F < 1 increased/decreased in temperature when the reaction zone thinned/thickened. These trends competed with the thermal and mass transport processes present in the highcurvature regions of the flames.