Unsteady oscillation distribution model of liquid jet in supersonic crossflows

Unsteady distribution of spray is experimentally studied when a round liquid jet is injected into a supersonic crossflow vertically. An oscillation distribution model for the liquid column and spray is established. Tyndall scattering caused by the sol medium is put forward to eliminate the interference effect of monochromatic laser passing through the supersonic gas flow field. The scattering causes the disordering of laser propagation direction and phase, thus makes the planar light source uniform and eliminate the interference effect of laser at the same time. Then a uniform light source is formed and can be set as the uniform background with a pulse width of 7 ns. The camera, with dimension of CCD pixel space of 4000 x 2672 pixel, is located directly in front of planar light source, and the shooting area is between both. The frozen liquid jet/spray images with high spatiotemporal resolution are captured using the pulsed laser background imaging (PLBI) method in supersonic crossflows. And the drag phenomenon caused by the too-long exposure time in the ordinary and traditional high-speed imaging process is avoided. Based on the maximizing inter-class variance method (Otsu) and Canny method, the out boundary of liquid jet/spray are extracted from an instantaneous image. A dimensionless parameter named intermittency factor (the logogram is r) is defined and used to quantitatively analyze the oscillation distribution characteristics of jet/spray. The intermittency factor of the whole spray field could be calculated by sample probability statistic method. An empirical jet/spray oscillation distribution model, in supersonic crossflows, is summarized based on parameter studies. Various conditions are studied, including stagnation pressure range of gas (642 kPa to 1010 kPa), practical pressure range (0.36 MPa to 4.61 MPa), nozzle diameters (0.48 mm/1.0 mm/1.25 mm/1.52 mm), distances down from nozzle (10 mm to 125 mm), and jet-gas momentum flux ratio range (0.11 to 7.49). The empirical model is used to predict the oscillation distribution of water jet penetrated in a Ma2.1 supersonic crossflow. It is indicated that the predictive result matches well with the experimental result. It could be concluded that the PLBI method presented in this paper reasonably utilizes the high energy and short pulse characteristics of the laser to successfully complete the "frozen" image of liquid jet/spray under the condition of supersonic crossflow. The dimensionless parameter 'r' defined in the study can be used to quantitatively analyze the oscillation distribution characteristics of jet/spray well. This study has important significance for understanding the diffusion characteristics of liquid jet in supersonic crossflows.