Abstract
The method of combining experiment and numerical simulation was used to study the cavitation and gas backflow phenomena during nozzle off-flow stage and the influence of residual bubbles on the initial jet in the near field. An equal-size optical nozzle based on acrylic material is designed, and the injection process of the fuel nozzle is photographed using high-speed photography technology. Establish a cavitation mathematical model to analyze the details of internal flow and initial jet. The results show that after the needle valve starts to close, cavitation occurs in the orifice and the sac in sequence, and the amount of cavitation in the sac is large. The collapse of cloud of cavitation bubbles will cause the outside air to flow back into the nozzle. The volume of the backflow air is slightly larger than the total volume of cloud of cavitation bubbles. The study found that the initial position of the residual bubbles has a significant effect on the initial atomization shape. When the residual bubble was in the front of the orifice, the initial tip was formed at the front of jet, and then, it stretched into a thin ligament due to vortex ring motion around the jet.
Highlights
The better atomization quality of the diesel engine nozzle makes the combustion in the cylinder more full, which can improve the engine’s emission performance
Lopez et al [4] found that testing cavitating injection nozzles shows a strong reduction in mass flow rate when cavitation appears, while the momentum flux is reduced to a lesser extent, resulting in an increase in effective injection velocity
The results show that the cylindrical nozzle orifice has a strong tendency to induce cavitation, and the convergent nozzle orifice has a strong tendency to suppress cavitation
Summary
The better atomization quality of the diesel engine nozzle makes the combustion in the cylinder more full, which can improve the engine’s emission performance. Lopez et al [4] found that testing cavitating injection nozzles shows a strong reduction in mass flow rate when cavitation appears (the flow is choked), while the momentum flux is reduced to a lesser extent, resulting in an increase in effective injection velocity This is related to the vapor appearance inside the injection hole produces a decrease in the viscosity of the fluid near the wall. In order to study the relationship between the cavitation phenomenon and the backflow of external air in the diesel engine nozzle at the end of injection, the transparent nozzle is made of acrylic transparent material and installed on the head of the fuel injector to visualize the fuel flow inside the nozzle. The formation mechanism of the “needle”-shaped liquid filament on the head of the jet mushroom is explained
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