Upward single-nozzle sprays: Droplet count, size, and velocity under flash pressures with directional conditional filtering

Abstract

This study investigates the influence of pressure variation on droplet characteristics (count, size) and dynamics (velocity) in upward atomised full-cone sprays. The challenges of understanding the fundamental droplet behaviour in spray-flash desalination systems further imposes a need to explore methodologies such as directional conditional filtering in better resolving droplet behaviour. Directional filtering distinguishes upward-moving droplets (driven by momentum) from downward-moving droplets (entrained by gravity) thereby considering the differing residence times (downward falling droplets have a longer history). Particle Image Velocimetry (PIV) and Shadowgraph were applied to 75° full-cone nozzle at 1.75 L/min. Large Eddy Simulations (LES) were performed in two flash-environments (-0.50 bar gauge and -0.75 bar gauge) and compared to atmospheric pressure (1.01 bar absolute) sprays. Firstly, and in non-conditionally filtered droplets (raw data), droplet counts significantly increase with decreasing spray chamber pressure, with the highest counts observed under −0.75 bar gauge chamber pressure; attributed to enhanced flash-induced atomisation and secondary breakup associated with the larger pressure differentials. Along the spray centreline, unfiltered droplet counts at −0.75 bar gauge increase by 17.26%, 10.61%, 7.43%, and 6.40% at further downstream locations of Y = 0.1, 0.2, 0.3, and 0.4 m, respectively, relative to the −0.5 bar gauge chamber pressure. Counts at -0.75 bar gauge are also substantially much larger compared with atmospheric pressure sprays, with increases of 57.92%, 73.06%, 73.05%, and 78.17% at the same locations. Secondly, and with regard to droplet size relative to unfiltered droplet sizes in an atmospheric pressure environment, droplet size decreased much more appreciably at pressure reductions, reflecting intensified atomisation and flash-assisted breakup. At -0.50 bar gauge, the upward-filtered droplet size decreased by 21.3%, while at -0.75 bar gauge, it decreased by 30.55% compared to unfiltered droplet size at 1.01 bar. In case of downward-filtered droplet size and in comparison to unfiltered droplet size at 1.01 bar, it was reduced by 23.74% at -0.50 bar gauge and reduced by 29.92% at -0.75 bar gauge. Thirdly, and relative to unfiltered droplet velocity at 1.01 bar chamber pressure, upward droplet velocity increased at lower pressures. Initial upward-filtered droplet velocities rose by up to 32% at -0.50 bar gauge and 54% at -0.75 bar gauge compared with unfiltered droplet velocity at 1.01 bar. Upward-filtered droplets retained higher positive velocities near the spray core, whereas downward-filtered droplets exhibited stronger negative velocities under vacuum, indicating intensified gravitational recirculation. The results show that flash pressure and conditional filtering strongly impact the interpretation of results and so affect droplet count, size and velocity.