Simultaneous 2-D Imaging Measurements of Reaction Progress Variable and OH Radical Concentration in Turbulent Premixed Flames: Instantaneous Flame-Front Structure

Improved experimental techniques for the joint two-dimensional measurement of reaction progress variable and OH mole fraction, detailed in a companion paper (Chen and Bilger, 2001), are applied here to the investigation of the instantaneous flame-front structure in turbulent premixed flames. The flames are of natural-gas/air mixtures and are pilot-stabilized on the Bunsen-type burner used by Frank et al. (1999). Multiple joint image pairs show distortion and folding of the preheat zone in the reaction progress variable images at the higher turbulence levels in the range investigated. A modified interaction length scale, ϱm, is defined to describe this phenomenon which occurs when ϱm > δth, where δth is the thermal flame front thickness of an unstretched laminar flame determined from the maximum temperature gradient. Significant departures from flamelet structure are found for spatial derivatives of the progress variable and for OH concentrations. It appears that these quantitative effects persist for ϱm < δth and that a new regime of “flame fronts with complex strain” exists where δth > η and ϱm < δth with η being the Kolmogoroff scale. In this regime, the effects of the turbulence are not sufficient to distort the lamellar-like appearance of the flame front. At scales just larger than the Kolmogoroff scale, strong spatial and temporal variations in the strain-rate tensor occur. These are evidently sufficient to cause significant departure of the quantitative structure of the front from that found for laminar flames subjected to the simple straining used in laminar flamelet modelling of turbulent premixed combustion.