Experimental Study Of String Cavitation In Real-size Optical High-Pressure Common Rail Injector Nozzles And Its Effects On Spray

The diesel engine is widely utilized in passenger cars,heavy trucks,engineering machineries,ships and power generation industry owing to its good fuel consumption and power performance.However,it also brings the problem of high NO_x and PM emission.All kinds of new clean combustion modes put forward higher requirements for the performance of fuel injection system.The electronic high-pressure common rail fuel injection system is one of the three breakthroughs of the internal combustion engine industry in 20th century.Certainly,it is also one of the gordian technologies that need to be urgently solved in China.The internal nozzle flow plays a vital role in the spray atomization and the formation of air-fuel mixture in diesel engine.Therefore,it is indispensable for a precisely control of spray and high reliability injector nozzle design to well understanding the internal flow characteristics in the common rail multi-hole injector nozzle.The geometry induced cavitation,caused by the fierce transition of nozzle flow channel,has been widely investigated by many researchers.In contrast,the string cavitation induced by vortex flow shows a more significant influence on diesel spray.Whereas,it is rarely to see the investigations on string cavitation in diesel injector nozzle.In this paper,the common rail injector equipped with the real-size optical nozzle was used to investigate the effects of string cavitation characteristics in the nozzle and its influence on spray characteristics.The main research contents and innovative points are as follows:(1)The visualization testbed of the real-size optical nozzle and spray based on the high-pressure common rail fuel injection system was designed and constructed.Then,the nozzle material and the production process of real-size optical nozzle were illustrated.Furthermore,the post-processing of cavitation,needle lift and spray images were clearly introduced in MATLAB.The transient intensity of string cavitation,the of needle valve motion,the spray cone angle and the assemble average images of internal flow and spray were solved by code,respectively.(2)By means of shadowgraph method,the paper designed several groups optical nozzles with different geometry for exploring the effects of nozzle sac type,sac size,nozzle hole position,nozzle hole number and nozzle hole geometry on the string cavitation and spray.The results indicated that above factors played vital roles in string cavitation.Additionally,the transition process of string cavitation from vortex flow were analyzed.Compared with the geometry induced cavitation,the string cavitation has a much stronger influence on spray atomization.(3)The string cavitation could promote the spray cone angle markedly under low needle lift.Therefore,the different internal spray structures caused by different internal nozzle flow was studied by a plane with a slit.The results indicated that the string cavitation with strong intensity,usually stretches to the nozzle outlet,could induce the special hollow cone spray with a large spray cone angle.Benefit from larger specific surface area,this spray could improve the formation of air-fuel mixture efficiently under the condition of small injection mass flow.(4)The needle lift adjustment shims were successfully assembled in common rail injector for the sake of quantitatively adjusting the injector needle lift.Subsequently,the string cavitation experiments under different constant needle lift were carried out in three nozzles with different nozzle geometry.The results present that the internal nozzle flow is closely related to the needle valve eccentric under the condition of low needle lift and the internal nozzle flow and spray exist obvious difference between right hole and left hole.In addition,the string cavitation characteristics is tightly associated with the sac size,the nozzle hole position and the needle position.The stable and strong string cavitation was obtained in the Min-sac nozzle with middle hole position under the needle lift of 0.35mm.At this time,the spray shows a hollow cone structure with large spray cone angle and could stimulate the mixture and reduce the HC and PM emissions.Therefore,when the needle valve tip lies on a suitable position at the stage of the maximum needle lift,the Min-sac nozzle with a middle hole position could induce a stable and strong string cavitation.In this paper,the visualization testbed of the real-size optical nozzle and spray based on the high-pressure common rail fuel injection system was utilized to investigate the relationships between nozzle geometries and the string cavitation based on real-size optical diesel nozzle.The occurrence mechanism of string cavitation was also analyzed.Moreover,two nozzle geometries were given for inducing string cavitation or eliminating string cavitation.In addition,paper analyzed the potential application prospect of the hollow cone spray.Finally,the abundant experiments could provide a reference for cavitation simulation model.Certainly,the investigations also provide basic theory for high consistency,high stability and high precision injector nozzle design.