Effect of liquid droplets on premixed laminar acetone flames

Abstract

The effects of fuel droplets on premixed laminar counterflow acetone flames are numerically investigated, and parametric studies are conducted for a range of monodisperse droplet sizes, equivalence ratios, and liquid fractions of fuel. A point-source Lagrangian framework is adopted for droplet calculations coupled with the Eulerian gas-phase solution. The particular geometric arrangement is created to mirror an experimental setup for flames with a global stoichiometric air fuel ratio, and a liquid mass fuel fraction of around 10%. The simulated flame generally appears as a one-dimensional structure under stoichiometric conditions for all the droplet sizes studied. Small droplets are found to vaporize in the vicinity of the flame front, while large droplets cross the flame and oscillate about the stagnation plane while vaporizing in the product zone. Under globally lean conditions, the local droplet vaporization leads to local enrichment and slightly higher temperatures and flame acceleration. Under rich conditions and larger liquid fractions, however, temperature decreases from fuel vaporization are not compensated by local heat release, and local deceleration occurs. The vaporization of droplets in the post-flame zone introduces fuel into products, creating a rich area and leading to further reactions to produce more CO and less CO₂. Increases in droplet diameter lead to survival of droplets across the flame, which are eventually dragged back towards the flame for final vaporization and exit. Comparison with experimental measurements of velocities along the centerline with simulations results using incoming measured polydisperse droplet distributions show that agreement is only fair, and that it may be important to capture the cooling effects on the spray side with high accuracy temperature and droplet number measurements, as pre-cooling may affect the initial boundary conditions.