ID#: 83
Abstract Title: Oscillations in the Flame Speed of Globally Homogeneous Two Phase Mixtures
Session Title: Multi-phase Combustion I
Session Date: 7/31/01
Session Start Time: 3:40 PM
Contributing Author: Lawes, M.
Organization: University of Leeds
Country: U.K.
Authors: F. Atzler, F.X. Demoulin, M. Lawes, Y. Lee
Short Abstract: The combustion of clouds of fuel droplets is of practical importance in gas turbines, diesel and spark ignition engines, furnaces and hazardous environments. Under certain conditions, spherically expanding spray flames, following spark ignition, exhibit strong periodic fluctuations in flame speed which, are not acoustic in origin. These pulsations can significantly affect the burning rate and are not yet understood. In the present work, the combustion of initially quiescent premixed sprays was experimentally and theoretically investigated. Sprays were generated, thermodynamically, in an optically accessed combustion vessel by the Wilson cloud chamber technique. Centrally ignited explosions were observed by high speed Schlieren photography to yield flame speeds and oscillation characteristics. The affects of overall and gaseous phase equivalence ratio were studied for fuel droplet diameters up to 20 mm. It is shown that overall lean or stoichiometric mixtures with large liquid fractions were most likely to have pulsations in flame speed. A mechanism to explain the oscillations is proposed in terms of variations in equivalence ratio due to phase lags between flame and droplet velocities. These are due to droplet inertia within an accelerating flame. A model of this process is presented and results are compared with experiments. It is shown that, because droplets are initially stationary, the effective equivalence ratio is higher than that of the overall mixture. However, as the droplets accelerate, the effective equivalence ratio changes, which, in a feedback loop, affects droplet velocities such that it is possible for the flame to exist in an unstable, oscillating regime.