| The jet pipe servo valve is widely used in the high-precision hydraulic servo system of aerospace,warship,nuclear power and all kinds of military equipment.Most studies have been conducted on the use of micro-jet to achieve the purpose of flow field disturbance,pressure replenishment,and heat transfer enhancement,while the true sub-millimeter level micro-impact jet flow laws and energy conversion characteristics have yet to be revealed.The deterioration of the oil quality in the long-term service of the jet pipe servo valve will lead to contamination jamming,wear degradation and other failure phenomena of the hydraulic components,which in turn will lead to wall micro-defect in the receiver and split tip.At present,there are few reports on the characteristics of mesoscopic impinging jet flow field under the influence of wall micro-defect,so there is an urgent need to further explore the relevant basic theories and research methods to develop high-performance jet pipe valves.Therefore,this paper will combine Micro-Particle image velocimetry(Micro-PIV)technology and Computational Fluid Dynamics(CFD)method to study the flow field characteristics of mesoscopic short-distance(impact distance H/D=1)submerged impinging jet under the influence of wall micro-defect.A Micro-PIV flow measurement system with 1:1 scale reduction is built by using a modified low frequency 2D-PIV system loaded with mesoscopic microscopic lens and coaxial illuminated light path,and the original scale of the flow field in the front stage of the jet pipe valve is reduced.The mesoscopic high speed short distance(impact distance H/D=1)submerged impinging jet under the influence of micro-defect on the wall in strongly confined space is directly observed.The transient velocity field and the characteristics of pulsation and time averaging are analyzed,and the identification and distribution law of vorticity field and Ω vortex core is analyzed.The flow coherent structure of square cavity is analyzed by using Proper orthogonal decomposition(POD)analysis from the point of view of energy.The influence of the change of gap height and geometric characteristics of micro-defect on the vortex shape and evolution law of square cavity is explored.The mechanism behind the splitting and normalization of the vortex core in the cavity under the mesoscopic scale of the submerged impinging jet in a strongly confined space is revealed.The accurate flow modeling is realized for the whole pilot stage basin of the jet pipe servo valve,including the jet pipe and receiver,and the mesoscopic submerged impinging jet flow law under the influence of near-field constraints,wedge micro-defect and surface micro-defect is obtained.it provides a theoretical basis for the design and manufacture of high-performance jet pipe servo valves for machining defects,serious erosion wear,erosion"weight gain",cavitation cavitation control and prediction.Based on the above experimental and simulation studies,the flow mechanism and vortex dynamic behavior of high-pressure high-speed impinging jet at the pilot stage and mesoscopic scale in the jet gap are expounded.The research results are expected to break through the bottlenecks of theory,experimental technology and simulation methods in the development of high-performance jet pipe electro-hydraulic servo valve. |
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