Liquid surface breakup of nonturbulent and turbulent liquids.

Abstract

An experimental study of the primary breakup in the near-injector region of large diameter (3.6-9.5 mm) liquid jets in still gases is described. The experiments involve the injection of different test liquids at a variety of velocities into various gas environments, with slug flow and fully-developed turbulent pipe flow at the jet exit. Measurements included flash photography, pulsed shadowgraphy, and single- and double-pulse holography. Drop sizes after primary breakup were found to satisfy Simmons' universal root-normal distribution with the ratio of the mass median diameter (MMD) and Sauter mean diameter (SMD), MMD/SMD = 1.2; therefore, the drop size distribution can be characterized solely by the SMD. For nonturbulent primary breakup, the SMD increased with distance from the jet exit and then remained nearly constant within a fully-developed primary breakup regime. SMD measurements in the fully-developed regime did not agree with existing unstable wave growth theories, however, the SMD could be correlated with an expression involving the thickness of an injector passage wall boundary layer at the jet exit. Drops formed by this primary breakup mechanism are intrinsically unstable to aerodynamic secondary breakup. When the liquid/gas density ratio is larger than 500, turbulent primary breakup is controlled by liquid properties alone and mass weighted streamwise and crosstream drop velocities were comparable to mean streamwise and crosstream rms fluctuating velocities in the liquid, respectively. The drop sizes at the onset of breakup could be correlated by comparing the surface tension work required to form a drop with the kinetic energy of radial velocity fluctuations. The SMD variation of turbulent primary breakup with distance from the jet exit could be correlated with a Rayleigh breakup mechanism which provides the time required to complete the drop detachment process from a liquid surface. When the liquid/gas density ratio was less than 500, aerodynamic effects affected turbulent primary breakup and final drop sizes were reduced by merging of turbulent primary breakup and aerodynamic secondary breakup; a good correlation of the final drop sizes due to this behavior was obtained by combining correlations for turbulent primary breakup and aerodynamic secondary breakup.