Flame surface properties of neutrally stable premixed flames in isotropic turbulence.

Abstract

An experimental and theoretical investigation of free turbulent premixed flames propagating in isotropic turbulence at neutrally-stable preferential diffusion conditions is described. Experiments were limited to the wrinkled thin laminar flamelet regime and involved mixtures of hydrogen, air and nitrogen ignited within a fan-stirred combustion chamber. Test conditions included turbulence intensities relative to the laminar flame speed in the range 0-1.6, and turbulence Reynolds numbers in the range 0-4195. Measurements included two-point laser velocimetry, flash schlieren photography and flame tomography. Turbulence properties in the unburned gas were characterized by laser velocimetry. Flame surface statistics measured by flame tomography included mean and r.m.s. fluctuations of the flame radius, the fractal dimensions and turbulent/laminar flame perimeters as a function of time from ignition. Flame surface properties were numerically simulated using a two-dimensional flame propagation and advection algorithm combined with a statistical time series simulation of unburned gas velocities along the flame surface. The simulation provided for mean velocities generated by flame propagation, as well as the probability density functions and temporal and spatial correlations of velocity fluctuations within the unburned gas. For neutral preferential diffusion conditions, measurements showed progressively increasing flame radius fluctuations, flame surface fractal dimensions and turbulent/laminar flame perimeters with increasing mean flame radius. Additionally, the rate of increase of these properties all increased with increased turbulence intensities relative to the laminar flame speed. This implies that models and computations of turbulent premixed turbulent flame properties for free premixed turbulent flames must account for both relative turbulence intensities and the distance or time of propagation from the point of ignition. Simulated flame properties for neutral preferential diffusion conditions duplicated measured trends with respect to relative turbulent intensities and distance or time from the point of ignition. However, the effects of turbulence were underestimated due to the limitations of a two-dimensional simulation.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105394/1/9124038.pdfDescription of 9124038.pdf : Restricted to UM users only