ID 390

Focusing Schlieren Imaging in Close-Coupled Atomization: Comparison of Experimental Results with Numerical Simulations

 

Abstract:

Gas-assisted atomizers are often used for the atomization of very viscous liquids, for example liquid metals. One kind of such atomizer, a close-coupled atomizer, consists of two coaxial annular openings - the inner for the injection of a liquid and the surrounding for the generation of a conical high-speed gas stream. The liquid jet is disintegrated by the gas flow in the vicinity of the atomizer orifice. The gas nozzle of a close-coupled atomizer can be designed as a Laval nozzle, resulting in supersonic flow conditions with shock and expansion waves. In order to investigate their significance for the atomization process, a visualization of these effects is imperative. By means of Schlieren photography density variations become visible. This technique helps to better understand the underlying physics of the compressible gas flow and especially its interaction with liquid jets. A conventional Schlieren system, in which the refraction of light is integrated over the entire optical path can lead to misinterpretations of concentric flow systems. In this study a focusing Schlieren setup (also known as a lens and grid Schlieren) was designed to characterize the geometry of the gas flow, to determine the positions of the shock and expansion waves and to identify the main flow regions involved in the atomization performance. In this experimental study the operating pressure has been varied. A very good agreement has been achieved between Schlieren images and corresponding numerical simulations of a pilot plant atomizer.