Study On The Coupled Vibration Of Square Tube Arrays In Heat Exchangers

Shell-and-tube heat exchangers are widely used in petroleum,chemical industry,nuclear power and other industrial sectors,and with high reliability and wide adaptability,they are dominant in the use of various heat exchangers.Due to the existence of shell-side fluid,the vibration of a tube will excite the vibration of other tubes,and the whole tube arrays will be coupled with each other as a system,showing multiple coupled frequency and coupled mode shapes.Experimental and numerical simulation studies on the coupled vibration of tube arrays have not formed universal experience,and the calculation of tube mass in relevant standards is based on uncoupled vibration which refers to the situation where surrounding tube arrays are still.For this reason,the coupled vibration of tube arrays was studied in this paper by means of experiments and numerical simulation.Reference and bases for the revision of standards were provided.A vibration test system for tube arrays in still water was designed and built in this paper.The appropriate structural type as well as the material,size and natural frequency of tube arrays were determined.For four and nine square tube arrays with a pitch-todiameter ratio of 1.25,tube arrays performed coupled vibration through exciting one tube,and the trajectories of the tubes were got by a video processing software to obtain the coupled vibration modes.Results showed that distinct coupled frequency was corresponding to symmetric mode shapes,while repeated coupled frequency to asymmetric ones.The coupled mode shapes became more complex as the number of tubes increased.Acoustic fluid-structure interaction method in ANSYS was used in dynamics analysis of coupled vibration of tube arrays.It was found that for the coupled vibration model of tube arrays with infinite length,the axial size of tubes made no difference to the calculation results,and that effect of fluid boundary in the radial direction need not be taken into account if the distance between fluid boundary and tube arrays was 5 to 10 times the diameter of tubes.Numerical simulation results showed that for the same number of tubes,the smaller the pitch-to-diameter ratio was,the greater the bandwidth of coupled frequency was,and that for the same pitch-to-diameter ratio,the larger the number of tubes was,the greater the bandwidth of coupled frequency was.Detuning of tube frequency leaded to changes of coupled frequency and coupled mode shapes.Coupled vibration of tube arrays was simulated using rigid wall motion to calculate the velocity and pressure of fluid around tube arrays in ANSYS CFX.Results showed that the velocity and pressure of fluid around changed with the velocity and displacement of the tubes respectively,but fluid velocity at the center and outside of tube arrays and fluid pressure at the center of tube arrays were throughout at low level.Based on the results above,added mass coefficients of the coupled vibration of tube arrays were calculated.The result showed that the relationship between added mass coefficients of coupled vibration and pitch-to-diameter ratios of tubes was an area where the bandwidth decreased with the increase of pitch-to-diameter ratios.Therefore,it was recommended that the added mass coefficient curve be changed into the added mass coefficient band in standards.