Real Time Imaging of Conical Liquid Film Breakup From a Research Simplex Atomizer as Viewed From the Spray Centerline

Film disintegration from a standard research simplex atomizer (RSA) is characterized using a novel imaging approach. Various builds of the same RSA have previously been studied by several research groups. Due to the inherent challenges in simulating and interpreting primary liquid breakup from these atomizers, there is a need to accurately capture the dynamics and three-dimensional nature of the discharging conical film. To address this, a new investigative method has been developed. A rigid borescope is used for nonintrusive high-speed video imaging from a novel viewpoint, located on the centerline of the atomizer at some distance downstream of the nozzle, thereby providing a frontal wide-angle unobstructed view of the entire disintegrating conical film. Subsequent analysis of the 360 deg images enables interpretation of the observed dynamics. Canny edge detection is used to characterize the film edge and its dynamics, showing that sheet breakup does not occur circumferentially symmetric. Dynamic mode decomposition (DMD) is used to analyze the emergence of spatiotemporal coherent structures. Extracted dominant DMD modes together with optical image analysis provide insight into film instabilities and the overall film disintegration process. Data analysis for a selected RSA operating point and comparison with linear stability theory suggests film rupture to result from spontaneous local hole formation near the atomizer exit; whereby hole formation is triggered by disturbances originating from the interaction between liquid phase and air core at locations on the film, where the film has thinned sufficiently due to the superposition of streamwise and spanwise developing waves. Using the new imaging method to assess symmetry of the spray formation process is demonstrated to provide an efficient way for an initial assessment of nozzle performance.