Experimental and numerical investigations of mechanisms in fluidic spray control

A fluidic control method in an axisymmetric spray orifice is investigated experimentally and numerically. In this method, a nominally steady secondary flow is introduced through an annular slot placed near the vena contracta along the orifice wall to control the cavitation, and thus the spray, at pressures up to 550 kPa driving pressure difference. Images of cavitation, measurements of droplet sizes and discharge coefficients, and CFD modeling are combined to explore the flow physics leading to the production of small droplets. Experimental results suggest that the secondary flow is incapable of confining cavitation to the region upstream of the slot, and generally a larger secondary flow rate results in a lower discharge coefficient, and a larger fraction of small droplets. The homogeneous model-based CFD code of Chen and Heister was employed to model the internal flows, which indicated that a high pressure region upstream of the slot, large pressure fluctuations in the orifice, and long cavitation lengths are the favorable conditions for atomization. The CFD simulations, together with experimental measurements, correlate the orifice geometry and flow structures to droplet sizes. Understanding the relationship between flow structures and droplet sizes helps to design orifices in favor of production of small droplets.