Research On Cavitation In Blocking Flow Based On Orifice Plate

As one of the three major components of the automatic process control system,the control valve(control valve)is widely used in power plant,petroleum refining,chemical industry and other continuous process industries.In the working process of the regulating valve,when the pressure difference between the front and back of the valve increases to a certain extent,the flow coefficient shows a nonlinear change,and then reaches the limit flow and enters the blocked flow state.At the same time,the cavitation,noise and vibration of the pipeline are significantly intensified,which will cause serious impact and erosion on the valve trim.In this thesis,the orifice plate is used to replace the fixed opening regulating valve.Through the technical routes such as bench experiment,theoretical derivation and CFD fluid simulation,the process of the occurrence,development and change of the blocked flow when the fluid passes through the throttle is studied.The conditions under which the erosion changes provide basic research support for the most critical technical problems in the regulating valve industry such as the detection and evaluation of regulating valve cavitation and the design of the anti-cavitation structure of the regulating valve.The main work includes the following three aspects:1).Design and construct cavitation experiment plan.Aiming at the difficulty of achieving the high temperature and high pressure conditions required for cavitation in the laboratory environment,an experimental bench was designed and constructed.The test bench can supply high adjustable pressure before the valve,adjustable medium temperature,and adjustable pressure after the valve to establish a condition environment in which clog flow occurs.At the same time,four kinds of throttle plates with different specifications were designed to replace or simulate the fixed opening regulating valve,and the design of stainless steel test pipe section and transparent quartz glass pipe section was adopted to achieve the purpose of experimental sensor sampling and visualization research.Finally,a cavitation occlusion flow test and data acquisition and processing system integrating various sensor detection and signal processing are designed,which have both software and hardware such as control,data acquisition,and data processing functions.2).According to the experimental study of the orifice plate,it was found that the flow curve has the characteristics of "step" and "limitation" when the blocked flow occurs.At the same time,it is found that different orifice plates have different blocking flow characteristic curves.Thick orifice plates exhibit two stages: the linear section before the blocking flow occurs and the "platform period" after the blocking flow occurs,and the thin orifice plate generates blocking flow.The critical point has an obvious non-linear transition part,which is embodied in a three-stage type.And through the analysis of theoretical flow characteristics,a reasonable coordinate system is established.Based on the concepts of pressure recovery coefficient,flow coefficient,flow characteristics,etc.,the limitations and stepped characteristics obtained in the experiment are demonstrated,and the " sloping ladder triangle flow curve diagram " is proposed.3).Finally,based on the CFD simulation technology,the pressure and gas volume data of the fluid passing through the throttle under the corresponding experimental conditions are given,and the change of the blocked flow state is studied.The transparent quartz glass test tube section was used for visual research,and the images obtained by high-speed macro photography were processed and analyzed.The relationship between cavitation flash vaporization and the three stages in the development process of occlusion flow is clarified.