The Role of Fuel Droplets in Laminar Flame Propagation
In all likelihood, combustion will be needed in the foreseeable future to supply energy to a growing population. Yet the burning of fossil fuels to power modern life is both harmful and unsustainable. Faced with this dilemma, the design of clean and efficient combustion systems is a global imperative. Advances in computational modelling have provided valuable predictions informing this optimisation problem. However, combustion models have outpaced their experimental validation. This thesis explores the role of fuel droplets in laminar flame propagation. In particular, the effect of fuel droplets on the burning velocity of strained laminar premixed flames was investigated via experimentation and simulation. The twin premixed counterflow configuration was used to measure reference flame speed as a function of strain rate and nominal equivalence ratio (φ_0) for acetone/air mixtures. Two opposing stagnation flames were studied: a gaseous flame with prevaporised fuel, and a spray flame with fuel in liquid and vapour form. Gas phase velocity was measured using particle image velocimetry, from which the reference flame speed was approximated. Liquid phase properties, including droplet diameter, velocity, and concentration, were measured using phase Doppler anemometry. The droplet Sauter mean diameter ranged between 65-75 μm, and the estimated fuel liquid fraction ranged between 6-22%, which increased with the nominal equivalence ratio. The results suggest that droplets affect the flame speed of fuel-lean (φ_0 < 1) to fuel-rich (φ_0 > 1) mixtures by contributing to an offset in the effective gas phase equivalence ratio. Gas phase velocity profiles, droplet measurements, and simulations along the axial centreline suggest that some droplets do not vaporise ahead of the flame, causing the spray flame to burn leaner. The droplets that evade combustion in the spray flame may cross the stagnation plane, and may contribute to the gaseous flame burning richer and cooler. The effect is pronounced for nominally rich mixtures owing to high liquid fractions. The findings suggest that the reference flame speed is influenced by the effective equivalence ratio resulting from incomplete droplet vaporisation. This thesis contributes the first suite of measurements characterising the effect of fuel droplets on laminar flame propagation in the twin premixed counterflow configuration.
Spray combustion, Flame propagation, Two-phase flow, Droplets
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