Study On Flow-induced Vibration Of Tube Bundle In Hairpin Heat Exchanger

Hairpin heat exchanger,as a new type of efficient shell and tube heat exchanger,is widely used in the field of nuclear power.Similar to shell and tube heat exchangers,hairpin heat exchangers also have structural failure and damage cases caused by flow induced vibration.Therefore,the flow field characteristics of shell side of hairpin heat exchanger and the flow-induced vibration response of tube bundle are studied by numerical simulation.According to the frequency difference between the in-plane and out-plane of the tube bundle in the bend section,the experimental study of water tunnel was carried out with unequal stiffness bundle,and the fluid-elastic instability was obtained,in order to provide reference for the design and manufacture of hairpin heat exchanger.This study is intended to provide reference for the design and manufacture of hairpin heat exchangers.In this paper,based on the porous media model,ANSYS CFX software was used to simulate the shell side flow field of hairpin heat exchanger,and the flow law of shell side fluid is analyzed.The site of vibration hazard was obtained.The results show that the fluid flows in the straight section through the baffle repeatedly,the turbulence intensity is high,and the flow characteristics are periodic,while the flow field in the bend section is relatively uniform.The pressure loss of shell side flow field is mainly generated in the straight pipe segment.The flow rate near the outermost tube bundle is high,and the tube bundle is subject to the strong transverse scour action of the fluid,which is prone to the danger of vibration.The modal analysis and numerical simulation of the turbulent excitation response of the tube in hairpin heat exchanger were carried out by using the beam element model.The results show that the natural frequency of the bend section is much lower than that of the straight section,and the in-plane part is lower than the out-plane part.The vibration intensity of the bend section is much higher than that of the straight section,and the amplitude of the maximum span is the largest.The critical velocity of the bundle was calculated by fluid-solid coupling method.Under the calculation conditions in this paper,the critical velocity of the straight and bend sections are 6.84m/s and 1.9m/s,respectively.In order to provide data support for the check of flow-induced vibration of hairpin heat exchanger tube,the relevant parameters were calculated.Critical velocity proportionality coefficient K of the straight section and the bend section is respectively 4.5 and 3.8,the Strouhal number is about 0.54～0.58,the lift coefficient is about0.14,and the resistance coefficient is about 0.038～0.079.In this paper,the fluid elastic instability water tunnel experiment was carried out by using the elastic tube with unequal stiffness as the tube bundle in the bend section.The effect of the arrangement of elastic tube and the frequency ratio in two directions was studied.The results show that the critical velocity of the single elastic tube array is higher than that of other arrays,and the fluid-elastic instability only occurs in the cross flow direction.The vibration characteristics of the minimum element elastic tube array and the extended element elastic tube array are basically the same.Vortex shedding and fluid-elastic instability are controlled by the direction downstream flow and the cross flow direction,respectively.It is recommended to focus on the natural frequencies in both directions to avoid fluid-induced vibration.