The main advantages of air-assisted spray are its high-quality atomization at low injection pressures and insensitivity to the viscosity of atomized liquid. In this study, the droplet size and velocity of a low-pressure intermittent air-assisted spray were studied by using phase Doppler anemometry, and the effects of liquid fuel injection duration on time-resolved spray microscopic characteristics and spray unsteadiness were analyzed. Droplet size-velocity joint probability density functions were employed to characterize the droplet diameter-velocity distribution as well as the probability range. A comparison of the droplet Weber number with an empirical critical value indicates that atomized droplets hardly undergo secondary shear breakup. Based on the ideal spray theory of Edwards and Marx, an improved algorithm is proposed with the concept of iterative rejection of inter-particle arrival times to quantify the unsteadiness of air-assisted sprays by eliminating the dependence of the calculation results on droplet sampling data. The results show that intermittent air-assisted spray is an inherently unsteady process that can be influenced by fuel injection duration and spatial location, while independent of the droplet size. In addition, the spray unsteadiness exhibits noteworthy variations at different spray stages segmented by droplet velocity vs time. The relation between the potential internal gas–liquid two-phase status determined by fuel injection duration and the spray performance is elaborated.

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