| Poppet valve is a very important control component widely used in oil-hydraulics.It even could be said that poppet valve could be found almost in every hydraulic system.Due to its simple but robust structure,poppet valve is extensively used in pressure control application or as pilot-stage for instantaneous feedback of pressure or flowrate in a multi-staged valve.Even though its important role could not be overestimated,related research is still limited,specifically on flow dynamics of internal jet flow,which is a surprising fact.It is reasonable to conject the speculation that performance of poppet valve is highly associated with internal flow structure.Furthermore,the structure of poppet valve involves a small size orifice besides the extremely high pressure drop that it is usually subjected to,thus cavitation is frequently inevitable.Consequently,comprehensive investigation on cavitating jet flow through a poppet valve makes significant sense for improvement of its performance.In order to bring some new insight into the cavitating jet through a poppet valve,a combined numerical and experimental investigation is performed in the present paper.The numerical method is performed on OpenFOAM platform with a personalized solver.In experiment,flow visualization technique is applied for observation of cavitation morphology.High speed cameraing mainly assumes the light-transimission set-up,with an as low as 2ns exposure time and a 200W LED as a backlit light source,in order to capture instantaneous variation of cavity structure.furthermore,the light transimission method could provide extra indication of flow structure such as turbulence,vortices and shearing layer,which is beneficial for comprehensive understanding of the flow structure.the comparison between numerical and experimental method agrees well in terms of flow performance,instantaneous cavitation structure,characteristic structure,and periodic process of cavitation behavior,which provides a valid basis for a combined analysis.The present study is mainly concerned with flow structure and its dynamics behavior in the cavitating jet,including the following aspects:(1)For the numerical method,a new compressible two-phase model with cavitation-prediction capability is developed based on OpenFOAM,with consideration of detailed flow dynamics in a small-sized orifice.The newly developed solver is advantageous,because it is able to model effect of surface tension with incorporation of VOF numerical technique;considers the compressibility of each single phase;involves special treatment for compressibility to improve its ability in dealing sub/super-sonic flow or pressure wave structure.the developed solver is tested with a 2-D square nozzle,and the results are compared with some available experimental measurement,which is supposed to validate preciseness of the model.(2)The governing mechanism for cavitation generation in a poppet valve is yet controversial.Based on characteristic structure of cavitating jet,combined with variation of cavitation morphology with respect to intensification in cavitation extent,and with special consideration of coherent structure,the present study reveals essence of jet flow inside a poppet valve,proposes the formation process of inceptive cavitation,which is proved dependent on essence of j et flow and geometric structure of poppet valve.(3)Both numerical and experimental results confirm that choked flow is accompanied with disintegration of jet potential core in downstream region.in corporation of distribution of cavitation structure as well as the difference of flow structure between non-cavitating and cavitating flow condition,it is proposed that the mechanism for ch oked flow shows some dependence on cavitation-induced fragmentation of potential core.Numerical results are later combined for a quantitive analysis.(4)Comprehensive discussion is carried out with regards to detailed disintegration process of potential core,which reveals the vital role of cavitation.subsequent analysis of periodic behavior of cavitation furthermore reveals the consistence between the periodic cavitation evolution and fragmentation of jet potential core.These two aspects confirm the high correlation between cavitation and potential core.(5)From aspects of flow structure,driving factors for transition in flow pattern is comprehensively analized,which reveals the transition is not due to alteration in coherent structure but because of change in subsequent evolution for coherent structure after its construction.Three different evolution processes determine formation of three flow patterns respectively.(6)In order to evaluate influence of flow pattern on flow performance,cavitating and non-cavitating case are separately discussed.It demonstrates the coincidence between flow pattern and alteration in flow performance.A quantitive estimation is implemented with comparison between the two flow conditions.With introduction of flowrate-pressure drop as a new flow coefficient,the piecewise feature of flowrate variation is remarkably presented.Discussion is extended to its formation as well as its dependence on outlet pressure. |
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