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Data Quality and Accuracy of Measurements

The spray, in the case without the airflow, is axially symmetrical with low residual irregularities on the velocity, droplet size and concentration resulting from the imprints of the two inlet ports. Also, the airflow is symmetrical along the horizontal and vertical axes of the test section, as seen in Figure 13.
Several repeated measurements were performed to assess the repeatability of the PDA measurements. Based on that, the measurement errors were estimated (A type of uncertainty estimate). The velocity measurement error was under ±0.3 m/s, the linear average droplet diameter ( $D_{10}$ ) error was ±1.1 µm, and Sauter mean diameter ( $D_{32}$ ) error was ±1.8 µm.
The PDA system provides velocity and spherical validations whose results are documented in the measurement files.
The spatial resolution is 46 μm/pix × 46 μm/pix, which restricts its geometric detection limit. Optical distortions of long-range microscopis lens is negligible compared to other errors. The images were processed to estimate the spray boundary. The standard mean deviation of the spray boundary detection was ±0.55 mm.
The wind tunnel was designed and tuned to provide controlled, repeatable and stable flow with low $T_{u}$ and flat velocity profile. The flow conditions in the test section of the tunnel were inspected before the measurement with Laser Doppler anemometry. The velocity and Tu were measured in 20 equidistantly spaced positions 150 mm downstream from the inlet of the test section in the case without the atomiser spraying, see Figure 13. The horizontal velocity profiles at three cross-flow velocity cases in the test section are documented there. The error bar width in Figure 13 is in all cases smaller than the symbol size. The vertical velocity profiles (not shown here) are symmetrical and similar to the corresponding horizontal profiles. The free-stream $T_{u}$ is lower than 0.8%. It was computed as the ratio of root mean square cross-flow velocity to average cross-flow velocity. The positioning error of the 3D computer-controlled system used for positioning the wind tunnel body relative to the measurement volume of the PDA was less than 0.1 mm.