| In the operation of shell-and-tube heat exchangers,turbulence buffeting is an ubiquitous flow induced vibration(FIV)mechanism in the shell-side deep flow,and its load has strong randomness in time and space.In addition,in the heat exchangers precisely machined according to the relevant heat exchanger structure design standards,there are certain gaps between the tubes and the support plates.In the process of tube bundle vibration,the random rub and impact between the tubes and the support plates will lead to the constraint failure,thus causing random vibration characteristics.Therefore,the two necessary parameters for the direct problem of turbulence buffeting analysis of heat exchanging tubes,the turbulence load and the vibration characteristics of the structure are both random,which makes it impossible to directly predict the buffeting response of heat exchanging tubes.The present researches on the FIV mechanisms of heat exchanging tube bundles are mainly aimed at the fluid-structure-interaction of periodic excitation,while this dissertation fully studies the turbulence buffeting of the tubes from the perspectives of the random natural vibration characteristics caused by the rub and impact between the tubes and the support plates,random turbulence load characteristics and random vibration response of the tubes,which has certain innovation and reference significance for the structural vibration reduction design of heat exchangers.In terms of natural characteristics,the heat exchanging tube is simplified as a continuous Bernoulli-Euler beam model with fixed supports at both ends and multiple simple supports distributed along the beam.Based on the transverse vibration differential equation of pipefluid-coupling and the transfer matrix method,the influences of the inflow velocity on the natural characteristics of the transverse vibration of single-span and multi-span-tube are deduced and calculated.For the tube with low-velocity inflow,the above influence is relatively weak.The gaps between the tubes and the support plates will cause constraint failure.The more support plates there are,the more combination forms of failure will occur.In this dissertation,the random nonlinear problem is decomposed into a number of determined linear problem samples,and every sample in the random event is extracted,which corresponds to a determined support form,natural frequency and vibration mode.When calculating the modes and natural frequencies of each sample,the calculation methods of equal-span form and non-equal-span form in odd or even number of spans cases are completely different.In this dissertation,an improved Assumed Mode Method is proposed for the non-equal-pan forms which have no analytical solutions,the method can greatly improve the accuracy of the calculation results.In this way,the randomness of the inherent characteristics of the tube can be decomposed into a variety of combination forms to determine the constraint state,in which each support form can be regarded as a linear structure,and the turbulence buffeting response can be simplified into a combination form of multiple linear problems with a single random parameter,the random turbulence excitation.In terms of turbulence external excitation of the tubes,this dissertation establishes the shell-side flow domain of a transposed-regular-triangle-arranged tube bundle,and simulates the physics field of the Computational Fluid Dynamics(CFD)model with fixed velocity using the Large Eddy Simulation(LES)turbulence model,and extracts the external turbulence excitation on the tubes downstream the shell-side flow.When dealing with a large number of distributed fluid force data samples generated by multiple times of transient simulations,this dissertation proposes a method to edit and extract the lift and drag forces on densely packed tubes to form a transient physics field matrix.The spectrum analysis of distributed force is carried out at typical locations,which demonstrates that turbulence excitation has strong randomness in both time and space.In the process,the feasibility and accuracy of rapid construction of turbulence force field are discussed in detail by using the reduced order model.Based on the regularization parameter optimization method,the dynamic mode decomposition method applied to the distributed flow force on the heat exchanging tube is optimized,which ensured the accuracy of reconstruction of turbulence excitation force field and further reduced the order of the system,so as to improve the calculation efficiency and save the computer memory.In terms of the calculation of random vibration response,this dissertation firstly statistically analyzes the simulated turbulence load,which includes not only the time domain features of the random lift force on the tube,including the probabilities,correlation coefficients and random process properties,but also the frequency domain feature,or the power spectral density(PSD)of the excitation in both direct method and indirect method.Secondly,the Pseudo Excitation Method(PEM)is introduced into the direct problem analysis of random vibration of heat exchanging tubes,which is used to calculate the response PSD of the tube caused by turbulence random excitation under various constraint samples,so as to two traditional methods,CQC and SRSS.As a result,the PEM has more advantages in accuracy and efficiency than the traditional methods,so it is highly recommended to be included in the general design of turbulence buffeting calculation of tube bundles of shelland-tube heat exchangers.The response spectra under the most relaxed and strict form of all constraint samples are the upper and lower limits of the response under a random process load.In order to measure the randomness of natural characteristics of transverse vibration of the water-filled tube and verify the random vibration response PSD generated by the simulated load acting on the tube,this dissertation designs a transverse vibration simulation test-bed of heat exchanging tube bundle,which is the same as the CFD simulation model,based on which,two parts of experiments are carried out: 1.The natural frequencies and vibration modes of the zero speed water-filled tubes without transverse external load and the tubes with different inflow velocities are measured through the modal hammer test,and the natural frequencies and vibration modes of the tubes.The existence of the combination of support failure caused by random plate rubbing demonstrates the jumping of non-sequential natural frequencies under different constraint conditions,and it is confirmed that the inflow velocity of accumulated tube has little influence on the vibration characteristics of a multispan tube;2.The test-bed is provided with different flow velocity conditions both inside and outside the tubes.Through the air supply vibration test,different transverse distributed excitation forces on the are simulated,and the transverse response PSD of the tube downstream the shell-side flow is measured.The results were compared with the upper and lower limits of the response PSD predicted by the PEM,and the results are in good agreement. |
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