Research On Cavitation Erosion Of High-Pressure Common Rail Fuel Nozzle And Corresponding Jet Atomization Characteristics

Cavitation occurs inside the diesel nozzle as fuel flow through the hole of the diesel engine with the high common rail system.The occurrence of cavitation not only affects the internal multiphase flow of the nozzle but also damages the surface of the hole and has an influence on the corresponding jet atomization.Therefore,it is necessary to conduct in-depth research on cavitation flow and its impact on the injection system,including cavitation erosion in the hole,the atomization characteristics of the cavitation jet,and the inner structure of the spray with vortex string cavitation.Firstly,one cavitation erosion risk model was created based on the R-P bubble dynamics equation and the mass transfer rate between liquid and vapor during cavitation.According to the R-P bubble dynamics equation,a high-pressure circular area will appear around the collapsing bubble.The collapse pressure is higher than the surrounding liquid pressure.In addition,the collapse pressure jumps sharply and the bubble takes less time to disappear with the increase of the liquid pressure,which indicates that more damage will be caused on the nearby wall.This is the theoretical basis of the cavitation erosion risk model,which has been proved by cavitation erosion experiments in a transparent nozzle.More numerical studies about the cavitation erosion risk have been conducted on nozzles with the double-layer hole.The influence of hole conicity and needle lift on cavitation and corresponding erosion are discussed.Simulations show that the entrance of the low-level hole has a higher risk of erosion,but the danger can be reduced by increasing the conicity of the hole for the cavitation was depressed effectively.Moreover,when the needle lift is quite low and cavitation is severe near the close position,the surface of the needle and the seat are more likely damaged at the start of injection.But under the high needle lift,the risk position is transferred to the entrance and spine surface of the hole.Then,the author discusses the effect of cavitation on jet atomization near the nozzle with multiple holes,where the cavitation was always developed along the upper surface of the hole.As a result,the jet shows a non-axisymmetric structure.Large eddy simulations show that the initial turbulence on the stable jet surface occurs due to the collection between the jet body and the droplets escaped from the jet tip.Then pressure waves are formed on the jet and grow under the aerodynamic drive.What’s more,some bubbles collapse at the hole exit and disrupted the surface balance,thereby triggering the growth of surface waves.However,some vapor bubbles didn’t collapse inside the nozzle but were ejected out of the nozzle.Some of them moved to the jet bottom,and eventually collapse and enhanced the surface breakup there.The movement of the residual bubbles scratched the liquid zone and divides the continuous liquid core into small groups,causing the breakup of the liquid core.The breakup of the jet from the cylindrical hole,the vertical elliptical hole,and the horizon elliptical hole were very different due to their different distribution of cavitation inside the hole.The cylindrical and vertical elliptical hole have low discharge coefficients because of the well-developed cavitation inside nozzles.The breakup rate of the jet from these two kinds of the nozzle was also very poor compared to the jet from the horizontal elliptical nozzle in which many vortices were found.For all jets,the upper semi-spray angles are always greater than the lower ones.As the ellipticity rises,both semi-spray angle increases,but the difference between them narrows.Finally,experiment and numerical studies were conducted to research the formation of vortex string cavitation and corresponding spray.The VOF method is coupled with LES to simulate the vortex string cavitation inside the cylindrical and tapered holes under different needle lifts.The results show that the vortex string cavitation can enhance the primary atomization of the jet.Increasing the needle lift will suppress the formation of string cavitation and reduce the jet breakup rate.The experiment of the vortex string cavitation in one transparent diesel nozzle showed that the vortex string cavitation occurred at the initial stage of injection where the needle lift was very low.When the vortex string cavitation occurred.the spray angle increased sharply.However,no more details can be obtained from the experiment expect for some pictures,which were far from understanding the characteristics of the vortex string cavitation and the corresponding spray structure.Therefore,the coupling simulation between the vortex string cavitation and the spray was conducted.The numerical result repeated the structure of the vortex string cavitation successfully.The pressure distribution in the diesel nozzle showed that the vortex string cavitation occurred in the low-pressure area generated by the vortex flow.The spray simulation indicated that,when vortex string cavitation occurred,some droplets move around as they were ejected out of the nozzle,and the spray angle increased rapidly,resulting a hollow spray with few droplets inside.