Turbulent piloted dilute spray flames: Flow fields and droplet dynamics

This paper presents a comprehensive mapping of the flow and droplet fields in turbulent non-reacting as well as reacting dilute spray jets of acetone and ethanol fuels. The burner is well-designed such that the boundary conditions are characterized with sufficient details and the stability limits of the flames are known. The flow is intentionally simple so that attention is shifted to the study of flow-droplet interactions with and without heat release. Velocity, turbulence and droplet size data are measured using a conventional LDV/PDA system and measurements are reported at a number of axial locations in the flow. Sequences of conditions are investigated to resolve the effects of increasing the droplet loading (at a fixed carrier flow rate, where the carrier is air) and the effects of increasing the carrier velocity (at a fixed liquid flow rate).It is shown that the decay in the mean axial excess velocity on the centerline of acetone and ethanol flames is significantly slower than in non-reacting spray jets. The centerline rms velocity fluctuations in non-reacting jets peak around x/. D= 10 while in the acetone and ethanol flames the peak occurs further downstream at around 80% of the length of the flame. In both reacting and non-reacting jets, large droplets exit the nozzle with negative slip velocities and a changeover occurs further downstream where the slip velocity of large droplets becomes positive. Radial dispersion of droplets decreases with increasing Stokes number while the axial rms fluctuations may remain high due to " memory" effects as well as to droplet shedding from the inner wall of the fuel pipe hence leading to high anisotropy. The extensive data base, noting similarities and differences between the non-reacting and reacting jets, will facilitate model development and validation for dilute sprays.