Experimental investigation of very rich laminar spherical flames under microgravity conditions

Very rich premixed outward propagating spherical flames of n-heptane (phi = 3.5) and iso-octane (phi = 3.9) spherical flames were experimentally investigated under microgravity conditions at 420 K and initial pressures of up to 30 bar. Inert gases with significantly different molecular weights were used as diluents leading to mixtures with varying effective Lewis numbers. The experimental setup consisted of a spherical closed pressurized vessel that enabled optical access. The experiments were performed in a drop tower under microgravity conditions to prevent the influence of buoyancy on the expanding flames. A Schlieren measurement technique combined with a high-speed CCD camera-was used to track the expanding flames. During the entire experiment, the flames were laminar and smooth with no wrinkles due to combustion instabilities observed. This allowed for accurate measurements of the propagation speed and its dependence on stretch, which was then compared to theoretical predictions based a hydrodynamic model for relatively thin flames. The experimental and theoretical results show good agreement with each other. Investigation of such rich Mixtures that are close to the upper flammability limit has not been reported before. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.