Liquid-fuel Film Break-up in Unsteady Flow Conditions within an Aero-engine Lean Premixed Burner

The research here presented deals with a detailed experimental investigation of a Lean Premixed Pre-Vaporized (LPP) burner, designed by Avio S.p.A. for aeronautical applications. Main object of the paper is that of achieving an in-depth interpretation of the interactions taking place between the unsteadiness of a complex air flow field and the dynamical behaviour of a liquid-fuel film break-up. The experimental campaigns were focused upon the detailed characterisation of the unsteady air flow field as well as of the fuel spray dynamics in terms of velocities, diameters and spatial distribution of the fuel droplets. In order to perform the required measurements in a most accurate way, a 4.73-to-1 enlarged-scale model of an LPP burner has been realised. Measurements have been carried out by means of multiple laser-based instrumentations, namely: Laser Doppler Anemometry (LDA), Phase Doppler Anemometry (PDA) and Particle Image Velocimetry (PIV). Taking advantage of these measurements, it has been possible to capture and clearly observe the unsteadiness of the fuel release from the premixing-duct exit lip. In particular, the fluctuations, in time and space, have been mapped of the droplets discharged from the liquid film break-up, together with their diameter distributions, both representative of the local fraction of the still un-evaporated liquid fuel mass-flow rate. Correlation and direct dependence of the fuel discharge fluctuations upon the pulsations within the air flow field have been clearly established by all measurements.