Comparison of high-speed videography and phase Doppler anemometry applied to flat-fan sprays

This aim of this study is to investigate local droplet size and velocity distributions in a near two-dimensional quasi-steady flat-fan water spray injected under quiescent conditions. The objective is to provide accurate experimental data to validate the generalised Fully Lagrangian Approach (gFLA) model for polydisperse droplet flows. To achieve this, droplet size and velocity distributions were characterised using classical phase Doppler anemometry (PDA) and a recently-developed imaging framework based on high-speed shadowgraphy in conjunction with particle image velocimetry (PIV). The latter technique can also be referred to as high speed videography (HSV). Average horizontal velocity components obtained from both techniques are in good agreement within the main body of the spray. However, average vertical velocity components measured using HSV were consistently lower than those from PDA by up to 20%. This discrepancy is primarily attributed to the broader dynamic range of PDA, allowing it to capture a wider spectrum of velocities, whereas HSV is constrained by the temporal and spatial resolution of the optical setup. Both techniques showed that droplet number median diameter decreases downstream, due to secondary and tertiary atomisation, though PDA consistently reported larger values compared to HSV by up to 15%. These differences stem from the limited size range of each method, as HSV measures droplet diameters from 10–300 µm, while PDA is restricted to 2–120 µm. These findings highlight the importance of selecting an appropriate combination of measurement techniques for spray characterisation. PDA is well-suited for velocity measurements due to its high temporal resolution, while a hybrid approach incorporating both PDA and HSV for droplet size distributions offers a more comprehensive representation of the spray field.