Study On Flow-Induced Vibration Of Elastic Tube Bundle With Shell-Side Distributed Pulsating Flow In Heat Exchanger

Heat exchangers are equipment used to achieve heat exchange. By using flow-induced vibration to achieve compound heat transfer enhancement, elastic tube bundle heat exchanger proposes an innovative research direction in passive heat transfer enhancement technology. At present, elastic tube bundles in heat exchanger have the phenomenon of vibration uneven when using in engineering application, so that, some elastic tube bundles prone to fatigue failure and some elastic tube bundles have poor heat transfer capacity. They affect the service life and heat transfer efficiency of heat exchangers. Starting from the uniform shell-side flow-induced vibration response of single-row or multi-row elastic tube bundles, the actual shell-side flow-induced vibration responses of elastic tube bundles are investigated numerically in this paper. In addition, a distributed pulsating flow-generating device is designed. The flow-induced vibration responses of the elastic tube bundle subjected to shell-side flow and distributed pulsating flow are numerical and experimental investigated. Research works in this paper have great significance to achieve effective excitation and control on the elastic tube bundle vibration in heat exchanger.The main research works of this paper are summarized as follows.(1) The geometric modle of single-row and multi-row elastic tube bundles as well as the uniform shell-side fluid domain have been established. Based on the sequential solution method of bi-directional fluid-structure coupling, effects of uniform shell-side flow velocity, tube structural parameters, tube-side fluid and number of tube rows on the flow-induced vibration responses of elastic tube bundle have been achieved. Numerical results show that the vibration of the elastic tube bundle is mainly induced by the shell-side fluid, and the tube-side fluid has less effect on the vibration responses. The in-plane vibration of the two stainless steel blocks is displayed when the elastic tube bundle subjected to uniform shell-side fluid, and there are significantly harmonic frequencies when the flow velocity is lower. The structural parameters of the elastic tube bundle have significant effects on the flow-induced vibration responses of the monitoring points. The external diameter of the tube bundle both affects the vibration frequency and amplitude. However, the wall thickness only affects the vibration amplitude. In addition, each row of tubes interacts with each other when multi-row elastic tube bundle subjected to uniform shell-side fluid. Bottom tube bundles have bigger vibration intensity and higher vibration frequency, while top tube bundles have smaller vibration intensity and lower vibration frequency.(2) The geometric model of the actual shell-side fluid domain in the heat exchanger has been established. Effects of actual shell-side fluid velocity, tube row spacing and number of tube rows on the flow-induced vibration responses of each elastic tube bundle are achieved. The convective heat transfer coefficient of each elastic tube bundle is also obtained. The meshing strategy dramatically reduces the number of grids, and improves the grid quality. It can easily adjust the structure and grid of each divided domain. The step calculation method not only dramatically reduces the computing time, but also improves the computational efficiency. Numerical results demonstrate that the vibration of each tube bundle induced by the actual shell-side fluid is mainly out-plane vibration. The vibration amplitudes of the two stainless steel blocks are uniformly when the shell-side fluid inlet velocity is lower. The vibration amplitude of the bigger stainless steel block is more intense when the shell-side fluid inlet velocity is higher. Under the condition of different tube bundle number, the vibration amplitudes of the two stainless steel blocks increase first and then decrease with the increasing of the tube bundle number.The maximum relative errors of the same monitoring points on different elastic tube bundles are higher than 10% under different inlet fluid velocity. This indicates that the elastic tube bundles have poor vibration uniformity. The tube row spacing has greater effect on the vibration amplitudes of the bigger stainless steel blocks both and the vibration frequencies of the smaller stainless steel blocks. Furthermore, the convective heat transfer coefficient of each elastic tube bundle has increased significantly at low flow rates (or low Reynolds number) when the actual shell-side flow induces the elastic tube bundle vibration.(3) The shell-side distributed pulsating flow-generating device has been designed which is used for flow-induced vibration of the elastic tube bundle in heat exchangers. The triangular cylinder pulsating element has been selected by studying the flow around various pulsating elements in different flow channel, and the flow channel structure of the branch pipe has been designed. Compared with square cylinder and cylindrical cylinder, the wake flow of triangular cylinder can generate better pulsating flow. The triangular cylinder size can be adjusted easily, which is favourable to the processing and manufacturing. The direction angle of the inlet fluid has been extended by adding a diversion wall in the flow channel, and it is more easily to generate the pulsating flow. Numerical results show that the flow parameters and channel structural parameters have significant effects on the frequency and intensity of the pulsating flow. The pulsating flow frequency has a certain degree of reduction when there is an external flow field with a certain velocity. The bend branch of the distributed pulsating flow-generating device has such features:on the one hand, it makes the fluid of inside the standpipe flows into the branch easily; on the other hand, it weakens the influence of shell-side fluid on the pulsating flow. Under different structural parameters and different inlet fluid velocities, the maximum relative error of the outlet flow rates of each branch pipes is 7.3%, and the maximum relative errors of the frequency and intensity of normal pulsating flow are 8.33% and 5.64% respectively. This indicates that the pulsating flow of each branch pipes have good uniformity.(4) The geometric model of the fluid domain has been established, which includes the distributed pulsating flow-generating device. The flow-induced vibration responses and convective heat transfer coefficients of the elastic tube bundle subjected to shell-side flow and distributed pulsating flow have been achieved numerically. The test-bed of two-field flow-induced vibration of the elastic tube bundle has been set up, and the flow-induced vibration responses of elastic tube bundle subjected to shell-side flow and distributed pulsating flow have been tested. Numerical results show that the rows of tubes have substantially the same vibration intensity. Vibration of two connecting copper tubes induced by the shell-side flow and distributed pulsating flow are mainly out-plane vibration. The vibration displacement curve of the monitoring points has obvious "Twin Peaks" phenomenon. The vibration intensity of the monitoring points increases with increasing fluid inlet velocity. At the same monitoring point, the amplitude of the constant frequency is higher than that of the variable frequency. At the same flow rate, the vibration intensity of the long connecting pipe is higher than that of the short connecting pipe. The convective heat transfer coefficient of the each elastic tube bundle in heat exchanger improved greatly when elastic tube bundles subjected to shell-side flow and distributed pulsating flow. Experimental studies show that high vibration intensity of the elastic tube bundle at higher fluid inlet velocity. The vibration of each row of the elastic tube bundle is not homogeneous when the elastic tube bundle suffers shell-side flow. Part of the elastic tube bundle vibrates severely and another part of the elastic tube bundle vibrates weakly. There two main vibration frequencies when the elastic tube bundle subjected to shell-side flow and distributed pulsating flow. One of two main vibration frequencies is constant frequency, and the inlet fluid velocity cannot affect it. The other frequency is variable frequency, which increases with the increasing of fluid inlet velocity. The distributed pulsating flow-generating device can generally achieve vibration excitation and control effectively. In addition, the experimental data are very agreement with the simulation results. The vibration intensity of the elastic tube bundle is significantly improved, and the rows of tubes have the same vibration intensity substantially.