Research On Key Technologies In Fluidic Flowmeter

The fluidic flowmeter with feedback passages induces fluid vibration by Coanda effect. The frequency of the the vibration is proportional with the flow rate to a large extent. The fluidic flowmeter has a favourable future because of its extremely low lower limit in theory, however the present fluidic flowmeter has some shortcomings in engineering applications: lower limit much higher than theoretical; low signal to noise ratio; lack of criteria for structural parameters design and performance optimization, which limited the popularization of this flowmeter.In this dissertation, basic theory and design method of hydrodynamic vibration induced by jet alternating are researched to counter the defects of high lower limit and low signal to noise ratio in fluidic flowmeter. Fluidic flowmeter with vertical inlet passage and double outlets structure is schemed out, prototype is developed and performance test is carried out. Criteria for structural parameters design and performance optimization of the fluidic flowmeter are established.Design method of improving the anti-interference ability of fluidic flowmeter by vertical inlet passage structure is schemed out, which makes the fluid vibraion plane depart from the pipe axis to reduce the interference of the upstream flow pulse to the flow field in fluidic oscillator, therefore induces stable jet attachment and alternate vibration even at low flow rate, improves the anti-interference ability ,decreases the lower limit of the fluidic flowmeter; the double outlets design method is schemed out, which improves the performance of attachment and switching in fluidic flowmeter, strengthen the pressure in the feedback passage, therefore decreases the lower limit and improves the signal to noise ratio.The first chapter of this dissertation generally discusses the profile of fluidic theory, the control characteristics, strongpoints and flaws of fluidic technique, the development, classification and present situation of flowmeter, on the basis of study on lots of domestic and foreign literature and reference, and then the background, characteristics and applications of fluidic flowmeter are generally introduced, the advantages and shortcomings in engineering applications of fluidic flowmeter are emphasized. Research on key technologies in fluidic flowmeter is carried out to solve the flaws. At the same time the background, characteristics and applications in theoretical study and engineering design of fluid numerical simulation are introduced. At the end of this chapter, the project background, significance of the research and main research content of the subject is illustrated.In the second chapter, fluidic theory is studied systematically. Aimed at free submerged two-dimensional turbulent jet, the k-εturbulent model is utilized to calculate and simulate the flow field numerically; distribution characteristics in fluidic area are researched. The LES model is used to study the coherent structure in the jet flow, the large eddy structure and the phenomenon of vortex pairing are simulated, the influence of the generation, merge and elimination of these vortexes on the development of the jet flow along the axis are discussed, which reveal the mechanism of jet flow.In the third chapter, Coanda effect of jet is studied systematically. Mathematic model of jet attachment is established, the relationship between hit angle or attachment point and geometry or physical parameter is discussed, the influence of the width of control nozzle and the influence of the flow rate and pressure of control nozzle on jet attachment are studied. Then the flow field distribution during jet attachment and switching is researched through fluid numerical simulation, the influence of stream splitter on flow recovery, pressure recovery, attachment stability and switching sensitivity are also discussed.In the fourth chapter, the structure design of fluidic flowmeter is discussed, including the design criterion of fluidic oscillator and the design scheme of matching internal passage. Vertical inlet passage and double outlets structure design of fluidic flowmeter is schemed out to solve the flaws existing in the present fluidic flowmeter. According to the design criterion, the fluidic flowmeter structure model used in numerical and experimental research is established, the influence factors of Strouhal number and pressure loss in fluidic flowmeter are discussed.In the fifth chapter, the control equation and simulation model for fluid numerical simulation of fluidic oscillator are established, the internal flow field are calculated numerically, and the phenomena of jet attachment and switching are simulated, the trend of the flow fild distribution is discussed, the mechanism responsible for the fulid vibration induced by jet alternating is revealed. And then the influence of the structural parameter of fluidic oscillator on its performance and Strouhal number are studied, as well as the influence on the performance of fluidic flowmeter.In the sixth chapter, the experimental research on fluid flowmeter is carried out. The experimental bench for fluid flowmeter is established, and the prototype of fluidic flowmeter based on the design criterion is produced. The feasibility experiment is carried out to verify the feasibility of the fluid vibration induced by jet alternating, and the linear relation between the flow rate and the frequency of fluid vibration, the relationship between the pressure loss and flow rate is also discussed. Optimization design experiments on the prototype are carried out, the influence of the structural parameter on the performance of fluidic flowmeter are studied. At last, the fluidic flowmeter proposed in this dissertation is compared with the general present fluidic flowmeter in performance.