Characterization of Liquid Sheet Breakup Using Numerical Experiments

Abstract

For the plain orifice nozzle configuration, breakup mode analysis of the issuing liquid jet has been extensively, over the years, undertaken. The works of Rayleigh, Haenlein, Ohnesorge, Reitz and others have used an Ohnesorge-Reynolds chart to clearly characterize breakup into four distinct modes. These include: (1) Rayleigh, (2) first wind induced, (3) second wind induced, and (4) prompt atomization. Planar liquid sheet flows have not undergone such intensive characterization analysis. In this work a non-expanding (nor thinning) liquid sheet is injected into a quiescent volume of gas from a planar nozzle of constant opening height. The flow has no co-flowing gas stream nor air-assistance to drive the disintegration. The nozzle configuration and subsequent liquid primary breakup is somewhat similar to an outward opening fuel injector having an annular outlet with a large radius to needle lift height ratio. The numerical experiments in this work use a high fidelity Computational Fluid Dynamics (CFD) modeling approach. This includes the Volume-Of-Fluid (VOF) two-phase method to represent the liquid and gas fluids both considered to be incompressible, coupled with the Large Eddy Simulation (LES) treatment of turbulence modeling. The initial primary breakup of the liquid sheet into large droplets, ligaments and other structures is the main focus of the modeling. As such, secondary breakup and possible evaporation of the liquid is not considered. Due to the configuration of the planar nozzle, upstream cavitation of the liquid within the nozzle is also not considered. Breakup studies were conducted within the Reynolds number range of 3,000 to 23,400, and Ohnesorge number range of 0.004 to 0.1. Results are extensively validated with the works of Heukelbach and Scholz. In-nozzle velocity profiles are characterized with Reynolds number showing laminar, semi- and fully-turbulent states in the flow boundary layer and core. Velocity profile relaxation is studied as the liquid sheet transitions from a wall-bounded flow within the nozzle to a free shear flow surrounded by gas. Particularly, the axial velocity component is seen to weaken, whilst the sheet normal velocity component strengthens and aids in disintegration of the liquid sheet.