Effect of spill orifice geometry on spray and control characteristics of spill-return pressure-swirl atomizers

[Display omitted] •Several spill-return atomizers were investigated using PDA and high-speed imaging.•The position of the spill-orifice affects the turn-down ratio and spray stability.•Long and short wave breakup modes were observed for several different atomizers.•The spray cone widens with spill-to-feed ratio and inlet pressure increases.•The inlet pressure plays a dominant role on the Sauter mean droplet diameter. Many spray process technologies require variable liquid flow rates or droplet sizes. Frequently used Simplex atomizers, favoured for their simple construction, reliability and fine spray, have a limited regulation range due to their flow rate dependency on the square root of the inlet overpressure, pl. To overcome this drawback, spill-return versions of the atomizer were developed in the past but so far rarely investigated in depth. In this paper, small spill-return atomizers (SRAs) were designed and investigated experimentally using Phase Doppler Anemometry (PDA) and high-speed imaging with the aim to determine the effect of the spill orifice design, e.g. the positioning of the axial and off-axis spill orifices, their number and inclination on the control characteristics, nozzle efficiency and spray characteristics. Such detailed data were not to be found in the open literature. The off-axial spill orifice version produced a stable spray under all flow regimes investigated while the axially positioned spill orifice provided an unstable spray for low spill-to-feed ratios (SFR). However, the axially placed spill orifice was found to be more energy efficient as it required a lower spill flow rate to achieve the same injection flow rate. The radial position of the spill orifices affected the turndown ratio and liquid breakup nature. The atomizers with spill orifices placed close to the swirl chamber centreline generated a liquid sheet which disintegrated in short-wave breakup mode while the other atomizers demonstrated a long-wave breakup mode. This mode produced longer liquid breakup length and formed droplets with smaller Sauter mean diameters. Atomization efficiency was found to decrease linearly with SFR and almost inversely proportional to pl. These findings have produced practical guidelines and recommendations for atomizer designs to suit specific goals and are addressed to both atomizer designers and application engineers. The experimental data form a significant base to validate advanced numerical simulations of the SRA sprays.