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Measurement quantities and techniques

Measurement setup of PDA

The size and velocity of droplets in the spray were determined using a two-component fibre-based PDA measurement system (Dantec Dynamics A/S Skovlunde, Denmark). The arrangement of the measurement setup is shown in Figure 9. A multiline $Ar-Ion^{+}$ laser produced a horizontally polarised light beam with 0.35 W output power. The green (514.5 nm) and blue (488 nm) wavelength components were extracted and used to provide two-component velocity measurements (in the Y and Z-axis) in the coincidence mode simultaneously with droplet sizing. Both the beams were split into a pair of parallel beams with a separation of 38 mm which were consequently expanded by a 1.98× beam expander and symmetrically intersected using transmitting optics. The frequency of one beam from each pair was shifted by 40 MHz. The intersected beams formed an elongated ellipsoidal measurement volume with the axes length of 0.123 × 0.123 × 1.63 mm. The measurement volume length was truncated by a 0.1-mm wide spatial filter. The positioning of the receiving optics at 48° from the forward direction was used to collect the light scattered from droplets dominated by the first order of refraction and to minimise reflections from windows. Both transmitting and receiving optics used lenses with 500 mm focal lengths. The principle of phase Doppler anemometry, data processing, measurement precision and uncertainties and other features are explained in [59]. Three planes in the spray were probed at axial positions Z = 10, 15 and 20 mm from the nozzle exit. For each plane, seven lines perpendicular to the flow direction and one line parallel with the flow direction in the atomiser axis were measured as seen in Figure 10. At each point, either 50 000 droplet samples were acquired or a 10-second acquisition duration was achieved. The Dantec BSA software 5.2 was used to control the measurement. The PDA configuration is described in Table 3. Each sample contains information on diameter and velocity components in the Y and Z-axis as detailed in the section Measurement data/results.