A spray of puree: Wave-augmented transonic airblast non-Newtonian atomization

Abstract

Characterization of viscous, non-Newtonian atomization by means of internal waves is presented for a twin-fluid injector. Atomization of such fluids is challenging, especially at low gas–liquid mass ratios. This paper details mechanisms that enhance their disintegration in a “wave-augmented atomization” process. The working fluid, banana puree, is shear-thinning and described by the Herschel–Bulkley model. Unlike a conventional airblast injector, an annular flow of banana puree is injected into a core steam flow, encouraging regular puree waves to form inside the nozzle. A pulsing flow develops with three distinct stages: stretch, bulge, and burst, leading to an annular puree sheet stretching down from the nozzle exit. Rayleigh–Taylor instabilities and viscosity gradients destabilize the surface. During wave collapse, the puree sheet bulges radially outward and ruptures violently in a radial burst. Near-nozzle dynamics propagate axially as periodic Sauter mean diameter fluctuations in a wave pattern. Numerical simulations reveal three atomization mechanisms that are a direct result of wave formation: (1) wave impact momentum, (2) pressure buildup, and (3) droplet breakaway. The first two are the forces that exploit puree sheet irregularities to drive rupture. The third occurs as rising waves penetrate the central steam flow; steam shear strips droplets off, and more droplets break away as the wave collapses and partially disintegrates. Waves collapse into the puree sheet with a radial momentum flux of 1.7 × 105 kg/m s2, and wave-induced pressure buildup creates a large pressure gradient across the puree sheet prior to bursting.