Experimental study of the effect of droplets on partially prevapourised opposed stretched premixed flames
Résumé
The effect of fuel droplets on the burning velocity of strained laminar premixed flames was investigated experimentally using optical diagnostics and numerically with one-dimensional gas phase simulations. The reference laminar flame speeds as a function on strain were obtained in a twin counterflow burner configuration creating premixed acetone vapour flames with the addition of fuel droplets. The mixtures were investigated in lean and rich regimes with nominal equivalence ratios of 0.9-1.2 and a (250-550 s -1 ) strain range. Particle image velocimetry was used to measure 2D velocity fields, from which the reference laminar flame speeds and local strains were obtained that were compared to simulated values of the purely gaseous flames. Doppler Phase Anemometry was employed to measure droplet size, velocity and concentration along the axial direction from the spray flame nozzle in reacting conditions. The droplet Sauter mean diameter ranged between 60-69 µm with estimated liquid fractions varying between 3.5-8.5 % at the spray flame nozzle's outlet. This indicated that the majority of the fuel vapourised before the flame front, lowering the droplet-laden mixture temperature by 25 • C on average compared to the vapourised premixed mixture across all of the experimental conditions. The results show that the reference flame speed of the spray flame decreases by 5 cm/s on average relative to the counter pre-vapourised flame. Droplet population measurements along the stagnation streamline suggest that the evaporation is nearly complete. There is a small number of non-vapourised droplets that succeed in penetrating the spray and vapour flame fronts, depending on their size. However, due to their small numbers they do not actively impact the combustion characteristics. The reference laminar flame speeds calculated by one-dimensional simulations with the mixture temperatures of droplet-laden and fully pre-vapourised mixtures at nominal equivalence ratios are in good agreement with experimental results. Thus, the reduction of reference flame speeds is predominantly thermally driven by the enthalpy of evaporation of the droplets.
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