Insights into primary liquid atomization

Previous work employing Volume-of-Fluid simulations of high-speed liquid injection has shown that the most unstable modes obtained from instability theory are not the ones responsible for fragmenting the liquid core. Their associated wavelength is much smaller than the liquid jet diameter; hence, their action is limited to stripping the jet surface leaving the liquid core intact. A much larger sinuous mode develops many diameters downstream of the nozzle orifice, and it is ultimately responsible for fragmenting the liquid jet. The genesis of this sinuous mode is studied in the present work by focusing our attention first on its 2D manifestation. By employing a spatial instability analysis based on a two-phase Orr-Sommerfeld system, it is discovered that as the gas boundary layer grows, the peak growth rate obtained from the dispersion curve shifts to a new maximum located at a much larger wavelength. And this much larger wavelength coincides directly with the onset of atomizing sinuous mode. It is shown that the smaller varicose or sinuous modes do not contribute in a significant way. A subsequent analysis is presented where it is shown that the pressure fluctuations in the gas are the key agents promoting the growth of the sinuous mode.