Experimental investigation of turbulent flames in uniform dispersions of ethanol droplets

Abstract

A turbulent flame in an ethanol droplet-laden uniform mixture is investigated at overall equivalence ratios (ϕov) of 0.62, 0.72 and 0.82, using a piloted Bunsen burner. Imaging of OH* chemiluminescence and simultaneous imaging of OH PLIF and Mie scattering, both at 5 kHz, and imaging of CH2O-fuel PLIF at 5 Hz, were used to obtain instantaneous and time-averaged images, temporal sequences and 2-D estimates of flame surface density and curvature. 1-D PDA and LDA measurements were used to obtain droplet size and velocity statistics. At ϕov = 0.62, the flame takes a cylindrical shape, and changes to a cone shape with increasing fuel loading to obtain higher ϕov. Larger droplets are generally observed to have lower average and RMS axial velocities than smaller droplets. Profiles of droplet size distributions indicate a decreasing droplet number density downstream together with a shift to larger droplet diameters. The flame structure is observed to be relatively smooth at locations near the burner exit, and becomes more contorted with distance downstream. In general, droplets are observed to coincide with low-to-intermediate regions of OH. Occasionally, droplets appear to penetrate the flame front, and are detected in regions of intermediate-to-high OH. This occurs particularly at the downstream locations where the flame closes across the jet, with no significant averaged droplet penetration observed past 2 mm in the direction normal to the flame front. Measurements show a gradual reduction in flame surface density and higher flame front curvature with both distance downstream and increasing fuel loading. Estimates of the average droplet evaporation rate increase with both distance downstream and ϕov, as droplets appear in higher mean progress variable regions. The measurements reported here are useful for model validation of flame propagation in dilute sprays.