High-Fidelity Numercial Simulations of Impinging Jet Atomization

Abstract

High fidelity numerical simulations are performed to study the atomization process of impinging jets over a broad range of operating conditions. An improved volume-of-fluid (VOF) method augmented with adaptive mesh refinement (AMR) is used to simulate the formation and breakup of the liquid sheet formed by two impinging jets. In addition, a thickness-based refinement method is developed and implemented to automatically and efficiently resolve the drastic changing of requested grid resolution. The behaviors in various Reynolds and Weber number regimes are studied systemically. The predicted liquid sheet topology, atomization, and droplet size distribution agree well with experimental measurements. Several different patterns of sheet and rim configurations are obtained, including liquid chain, closed rim, fish-bone, disintegrating sheet, disintegrating rim and impact wave. The instability mechanisms of sheets and rims are studied based on the concepts of absolute and convective instabilities. New knowledge is acquired about the onset of sheet and rim instabilities. This locking-on feature of Strouhal number of impact wave is found based on the previous detailed study. Finally, schematic diagrams of all kinds of instabilities happens in impinging jet atomization are obtained. I. INTRODUCTION Collision between two cylindrical jets is one of the generic configurations for the generation of liquid sheets, the dynamics and stability of which have attracted a great deal of attention due to their relevance to the spray atomization and combustion in liquid propellant engines.[1-4] The impingement of liquid jets is a very efficient method for atomization and mixing whereby the dynamic head of the propellant is used to destabilize an opposing liquid propellant stream. This in turn results in fragmenting the liquid into ligaments and droplets.[5] The oblique collision of two cylindrical laminar jets at low jet velocities leads to the liquid flowing outward from the impingement point, producing a leaf-shaped expanding sheet which lies in a plane perpendicular to the plane containing the two liquid jets,. A rich variety of flow structures, from single oscillating jet obtained at low flow rates, to the violent disintegration of flapping sheets obtained at higher flow rates, have been observed depending on the Weber and Reynolds numbers of the jets. In the specific usage of liquid propellant rocket engine, the liquid sheet shows full-developed violent breakup with quickly growing waves. The liquid sheet destabilizes, breaks and eventually disintegrates into ligaments or droplets under the influence of surface tension, viscous, inertial, and aerodynamic forces. The impinging jets can provide rapid mixing and atomization. This unstable hydrodynamic wave is usually called impact wave[6]. Impact wave dominates the breakup and atomization process in most rocket combustors using impinging jet injectors.[7] Different from another capillary wave, impact wave shows a nonlinear behavior that cannot be described linear stability analysis.