A laser-based diagnostic, referred to as the Paired Astigmatic Interferometric Particle Imaging (PAIPI), is developed and used in the present study to measure the three-dimensional position, diameter, and velocity of non-reacting and dilute spray droplets. Separate two-dimensional Particle Tracking Velocimetry (PTV) is performed to assess the accuracy of the PAIPI technique for measuring the planar components of droplet velocity. Two operating conditions are tested: either water is sprayed into quiescent air or into a swirling co-flow of air with a mean bulk flow velocity of \(18.2\,\hbox {m s}^{-1}\) . The mean diameter of the spray droplets is in good agreement with previous work. It is shown that, while the mean diameter of the droplets increases in the axial direction for the no co-flow test condition, the interaction of the swirling co-flow with the spray results in a more uniform spatial distribution of the droplet diameter. The probability density functions (PDFs) of the droplet planar velocity obtained using the PAIPI and PTV techniques are in good agreement. The results show that the interaction of the turbulent swirling flow with the droplets influences the PDFs of axial and tangential droplet velocity, while the PDF of the droplet radial velocity remains almost unchanged. The joint PDF analysis of the droplet diameter and velocity shows that, for both test conditions, larger droplets feature a narrow range of velocity variations. It is shown that the presence of the swirling co-flow leads to the formation of slightly smaller droplets across a wide range of velocities. The simultaneous measurement of droplet position, velocity, and diameter, that is performed in the present study, facilitates improved characterization of dilute sprays.
