Simultaneous volume-velocity measurements in the near field of atomizing sprays

Direct visualization of atomizing sprays using backlight imaging is commonly used to obtain both qualitative and quantitative information on spray morphology. Recently, the standard backlight technique was extended to 'dual-angle' imaging by the authors (Kourmatzis et al 2017 Meas. Sci. Technol. 28 035302), to enable quantification of the volume of arbitrarily shaped fragments to accuracies of the order of 10. In this contribution, dual-angle particle-tracking velocimetry (PTV) has been developed, hence extending our capability to measure volume, velocity, and flow rate of atomizing fragments simultaneously, regardless of their shapes. The experimental layout consists of two time-shifted lasers, where each beam is split in two, two long-distance microscope lenses and two cameras oriented 90 degrees towards each other, operating in particle image velocimetry mode. The accuracy of the joint volume-velocity measurements has been carefully assessed using mono-dispersed droplets and microspheres of known size. The technique has also been examined for different air-assisted sprays covering regimes from Rayleigh to multi-mode breakup. By introducing terminologies such as 'fragment residence time' and 'fragment-specific volume flow rate', the overall volume flow rate of both the mono-dispersed drops as well as low Weber number air-assisted sprays may be recovered. This PTV method is a powerful diagnostic tool to simultaneously track and size arbitrarily shaped liquid fragments. It also provides a viable technique to measure liquid mass flux in the near field of sprays.