Fuel Nozzle Geometry Effects on Cavitation and Spray Behavior at Diesel Engine Conditions
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Cavitation dynamics of diesel and gasoline injection nozzles has been a topic of ongoing research due to the effect of cavitation on the characteristics of the fuel spray, including the discharge coefficient, outlet velocity, spray angle, and atomization process. Additionally, repeated collapse of vapor cavities can damage nozzle surfaces, permanently changing the boundary conditions of the fluid flow field. Understanding the evolution and behavior of cavitation can therefore allow more precise control over its presence, as well as improved predictability of the corresponding fuel spray distribution.
Studies have shown that the inception and persistence of fuel vapor inside the spray hole is sensitive to geometric features of the injection nozzle, such as the degree of taper and inlet corner radius of curvature. For example, a hole with a cylindrical profile and sharp inlet corner more readily supports cavitation formation as compared to a monotonically converging hole with a rounded inlet. To better understand the effect of these geometric features on cavitation formation, and by extension, on the associated fuel spray, we compare the nozzle geometry and spray characteristics of two single-hole diesel injectors procured through collaboration with the Engine Combustion Network (ECN). The Spray C injector, specifically designed by the ECN for the express purpose of studying cavitating flows, features a cylindrical hole with a slight divergence near the outlet and a relatively sharp inlet corner. Its non-cavitating analog, the Spray D injector, contains a rounded inlet corner and a gently converging hole profile.
High-resolution x-ray tomography measurements coupled with optical microscopy images of both injectors provide the nozzle geometry with 𝒪(1) μm spatial resolution. Analysis of the measured geometries reveal that the radius of curvature of the hole inlet varies azimuthally for the modestly hydroground Spray C injector. To elucidate the effect that this asymmetric inlet condition has on cavitation formation during operando conditions, the fuel flow inside the nozzle hole was recorded using high-speed x-ray phase contrast imaging. These images reveal the formation of an asymmetric sheath of fuel vapor that persists throughout the injection event. Complementary x-ray and optical diagnostics of the downstream fuel spray further highlight the effect that this cavitation layer has on the spreading angle of the spray in comparison to the non-cavitating Spray D injector.