Analysis And Measurement Of Fluid Flow Using The Characteristics Of Vortex Street

The fluid flow in conduits is almost seen in all variety of industrial processes andfacilities. As the main medium for heat, mass, momentum transfer and chemicalreaction, the fluid flow state and its parameters are crucial to the quality of productionprocesses. Knowing about the fluid flow state in time with pertinent parametersmeasured accurately is one of the necessities for effective control and optimization ofproduction operations. However, due to the wide variety of fluids and the complex ofworking conditions, although numbers of methods and instruments for fluid parametermeasurement have been devised, there still exist many urgent practical problems to besolved. For all the time, it is well recognized that acquisition of fluid flow state andmeasurement of its parameters is one of the most difficult problems in the area ofprocess measurement and control.The vortex street is a special flow phenomenon which occurs under uncertainconditions when viscous fluids flow around bluff bodies. More and more researcheshave shown that there is a close coupling relationship between a vortex street and theflow state in which it sheds, and the characteristics of the vortex street can reflect itsfluid flow state to a significant extent. Therefore, through applying proper acquisitionand processing approaches to the vortex street signals, a flow-information-intensivesource, analysis and measurement of the fluid flow in conduits can be partiallyrealized. However, so far studies on this topic are fairly limited despite its importancein both theory and practice. Undoubtedly, investigations on this interdisciplinaryfrontier area will promote the development of process measurement theory andtechnology, and at the same time they may also motivate the rapid-evolving theory ofvortex in fluid mechanics.In this dissertation, investigations on the analysis and measurement of the fluidflow in conduits using the characteristics of a vortex street were carried out. Theproperties of single-phase and gas-liquid two-phase flows were analyzedsystematically based on the duct-wall differential pressure obtained at certainpositions in the vortex wake. The coupling relationship between the vortex street andits flow state was studied thoroughly, and some key fluid parameters including flowrate and flow pattern were measured successfully. The main contributions andinnovations of this dissertation are summarized as follows:(1) Based on the essential characteristics of a vortex street, the duct-walldifferential pressure method (DDPM), a new non-intrusive approach for detectingvortex signals using the static pressure fluctuation on the duct wall, was put forwardfor the first time. Three-dimensional numerical simulations of the pipe vortex flowfield were performed adopting the RNG k-εmodel, on the basis of which thefeasibility of the DDPM was demonstrated primarily. The focus was placed ondepicting the generation and shedding of the Karman vortex street, and the analyses ofthe evolution of the static pressure and velocity with the vortex shedding process wereemphasized. Circumferential and axial DDPMs were devised for various detectionpurposes, and corresponding signal processing systems were also developed.(2) The flowrate of single-phase flows in conduits were accurately measuredthrough the vortex street characteristics extracted from the DDPM. According to therelationship between the vortex frequency and flow velocity in a vortex street, theflowrate was obtained via the frequency extracted from the DDPM signals, and thusthe DDPM vortex flowmeter was taken form with stable performance, stronganti-disturbance ability and low measuring limit. The influence of noise, pressuresampling position and tube on the performance of the DDPM vortex flowmeter wasdiscussed, the uncertainty of the measuring system was evaluated, and measures toreduce the uncertainties were recommended to optimize the measurement. Moreover,the lower measuring limit of the DDPM vortex flowmeter was further extended by theuse of a converging-diverging conduit.(3) The idea and techniques of flow analysis using the statistical characteristicsof vortex street signals was presented, on the basis of which partial fluid flow states inconduits were diagnosed properly. The power spectrum based vortex energy ratio wasput forward through the power spectrum analysis, which could be used as a criterionfor selecting the optimal position of vortex flowmeter sensing element. The maximumvalue of bispectrum from higher-order statistical analysis revealed the relationship ofthe non-Gaussian degree and the flowrates, which contributes to a more in-depthunderstanding of the vortex street phenomenon. Furthermore, the intrinsic modefunctions of vortex street sigals under non-steady states were drawn using the empirical mode decomposition (EMD) technique, and then the EMD vortex energyratio was brought forward to realize the quantificationally judge whether the vortexstreet was shed in a normal single-phase flow.(4) The characteristics of the DDPM signals acquired from gas-liquid two-phasevortex streets and flows around a cylinder were studied, the stability of the gas-liquidtwo-phase vortex street was determined quantificationally, and the influence of flowpattern on the pressure drop in the flow around a cylinder was explored. Based on theDDPM signals of a gas-liquid two-phase vortex street, a new dimensionless stabilityindex using the average velocity and vortex frequency was put forward toquantificationally judge the stability of a gas-liquid two-phase vortex street, and it isfound that the volume void fraction has the dominant effect on the stability. At thesame time, the dynamic pressure drops in the flow around a cylinder were measuredwith the axial DDPM, the relationship between the dynamic pressure drop and theflow pattern was discussed, and results show that the dynamic pressure drop mainlydepends upon the flow pattern.(5) An approach for the flow pattern identification using the frequency domaincharacteristics of gas-liquid two-phase flow around a cylinder was proposed, whichwas applied to vertical upward gas-liquid two-phase flows successfully. After thequantification processing of the DDPM signal frequency domain characteristics ofgas-liquid two-phase flow around a cylinder, the relationship between the frequencydomain characteristics parameters and the flow patterns was investigated. Athree-layer feed-forward neural network was designed and trained through theback-propagation learning algorithm, which adopted frequency domain characteristicsparameters as inputs and numerical-valued flow patterns as output. It is proved thatthis approach predicts the single water, bubble, slug and churn flows in a verticalupward pipe successfully with an accuracy rate up to about 90%.(6) Based on the characteristics of gas-liquid two-phase vortex streets, the vortexflowmeter-Venturi tube combinative method and the single vortex flowmeter methodwere introduced, which realized the measurement of some key parameters of bubbleflows including the flowrate and void fraction. After analyzing the performance anduncertainty of the DDPM vortex flowmeter in gas-liquid bubble flows, the vortexflowmeter-Venturi tube combinative method using the vortex flowmeter and the Venturi tube as measuring cells was put forward, the measurement model based on thevortex frequency and the Venturi differential pressure was established, and it showsthat the relative errors of this method for the mass flow measurement of bubble flowsare within±5%. Furthermore, the flowrate and volume void fraction of bubble flowswere obtained using a single DDPM vortex flowmeter through the relationshipbetween the frequency and amplitude of the circumferential DDPM signal, and resultsdemonstrate that the relative errors of the flowrate and volume void fraction of bubbleflows are within±5% and±10% respectively, which indicates the broad applicabilityof this method to large numbers of engineering fields.This dissertation solves the primary problems of flow measurement and statediagnosis for single-phase flow, flow pattern identification and parameter estimationfor gas-liquid two-phase flow in a unified framework based on the idea of vortexstreet characteristics. The above research results have obtained broad recognitions andpositive appraisals. Nearly thirty related papers on these results have been publishedor accepted for publication by authoritative journals including Measurement Science& Technology, Sensors & Actuators: A. Physical, Measurement, Flow Measurement &Instrumentation, Chemical Engineering Communications etc. Seven national patentshave been applied or authorized, and parts of the researches have received attentionsfrom some renowned national instrument manufacturers who are negotiating with theauthor for cooperation and technique transfer at present. These researches willcontribute to the development of process measurement theory and technology, canadvance the application of vortex street characteristics to practical engineering, andmay also motivate the progress of their related interdisciplines such as the vortextheory in fluid mechanics.