Surface Breakup of A Liquid Jet Injected Into A Gaseous Crossflow

• Main Author: Behzad Jazi, Mohsen
• Other Authors: Ashgriz, Nasser
• Language: en_ca
• Published: 2014
• Subjects: shear layer instability; two-phase flows; liquid jet; atomization; large eddy simulations; 0543; 0548; 0759
• Online Access: http://hdl.handle.net/1807/65640

The normal injection of a liquid jet into a gaseous crossflow has many engineering applications. In this thesis, detailed numerical simulations based on the level set method are employed to understand the physical mechanism underlying the jet ``surface breakup''. The numerical observations reveal the existence of hydrodynamic instabilities on the jet periphery. The temporal growth of such azimuthal instabilities leads to the formation of interface corrugations, which are eventually sheared off of the jet surface as sheet-like structures. The sheets finally undergo disintegration into ligaments and drops during the surface breakup process. Temporal linear stability analyses are employed to understand the nature of these instabilities. To facilitate the analysis, analytical solutions for the flow fields of the jet and the crossflow are derived. We identify the ``shear instability'' as the primary destabilization mechanism in the flow. This inherently inviscid mechanism opposes the previously suggested mechanism of surface breakup (known as ``boundary layer stripping''), which is based on a viscous interpretation. The influence of the jet-to-crossflow density ratio on the flow stability are also studied. The findings show that a higher density jet leads to higher wavenumber instabilities on the jet surface and thereby subsequent smaller drops and ligaments. The stability characteristics of the most amplified modes (i.e., the wavenumber and corresponding growth rate) obtained from stability analyses and numerical simulations are in good agreement. The stability results of the jet also show that the density may have a non-monotonic stabilizing/destabilizing effect on the flow stability. To investigate such effect, the concept of wave resonance are employed to physically interpret the inviscid instability mechanism in two-phase flows with sharp interfaces and linear velocity profiles. We demonstrate that neutrally stable waves are formed due to the density jump in the flow, in addition to the well-known vorticity (Rayleigh) waves. Under certain conditions, such neutral waves are capable of resonating and generating unstable modes. The resonance of different pairs of neutral waves, therefore, results in either stabilizing or destabilizing effect of density variation. We predict similar reasoning behind the density behavior in the jet in crossflow configuration with smoothly varying velocity and density profiles.