A Study On The Characteristic Of The Flow-Induced Vibration And Heat Transfer Of Elastic Tube Bundle

The heat exchanger as a heat transfer unit in the high energy consumption industries is an important equipment, which can be used to rationally use and conserve the existing energy sources. The development of efficient and low resistance heat exchanger has important practical significance to save energy and reduce energy consumption. While, using of advanced heat transfer enhancement technology is undoubtedly an effective way to improve energy efficiency. In the70's last century much attention was put on heat transfer enhancement via tube vibration. While, this was suspended by the difficulty in implement of controllable vibration in heat exchangers. With the recent emergence theory of flow-induced vibration of elastic heat transfer element in heat transfer enhancement, this subject is re-valued. This paper conducts detailed research on the characteristic of flow-induced vibration and heat transfer of tube bundle.Cylinder section vibration perturbs the flow field around the cylinder section, which makes the flow field in a turbulent state. The field synergy principle of turbulence fluid flow around the vibrating cylinder section was derived theoretically, and the relationship between Nu and the field parameters were obtained. In numerical calculation, the discrete-average calculation method was proposed by the domain of the instantaneous field parameters out of vibrating cylinder section being defined, and the filed parameters were got by dynamic grid and Udf programming.Heat transfer characteristic of a vibrating cylinder section in two-dimensional fluid flow was numerically investigated by the dynamic meshing and Udf programming technology via the CFD software, and the variation of surface heat transfer coefficient of the vibrating cylinder section with the speed of the flow fluid was achieved. The numerical results show that, comparing with the still cylinder section, when the flow velocity is much smaller than the maximum cylinder section vibrating speed, the cylinder section vibration has a significant influence on heat transfer enhancement, and the influence was reduced with the increase of the flow velocity. In addition, the heat transfer characteristic of a vibrating cylinder section along different directions was numerically investigated, the the variation of the vibration of cylindrical surface heat transfer coefficient with different directions for each fluid velocity was discussed, and the heat transfer performance was analyzed with field synergy principle. It shows that the vibrating direction has little influence on heat transfer enhancement. The characteristic of flow-induced vibration and heat transfer of a tube with different kinds of elastic supporting fluid flow was numerically investigated by Workbench and CFX software, and the variations of the amplitudes and frequencies of cylinder with different elastic supporting vibrating respectively along the transverse direction and longitudinal direction were investigated. The numerical results show that, in the same conditions, the greater elastic support stiffness of the tube is, the larger the range of fluid speed that the vortex-induced vibration of a circular generates. With the same elastic support stiffness, the frequency and amplitude of flow-induced tube vibrating along transverse direction are much greater than that of longitudinal direction. When the max vibration velocity of cylinder is much less than the flow speed, the flow-induced cylinder vibration can not reach the effective range of heat transfer enhancement.The characteristics of flow-induced vibration of tubes in tandem arrangement with elastic supporting in two-dimension fluid flow were numerically investigated by Workbench and CFX software, and the vibration corresponding displacement and flow field of flow-induced double tubes in tandem arrangement were achieved. The characteristics of flow-induced vibration and mechanism of the downstream tubes of double cylinders in tandem arrangement were analyzed. It shows that, within the scope of the study, when flow induces tubes in tandem arrangement vibrate, the space between the double cylinders exists a critical value. When the space is less than the critical value, the amplitude and frequency of the downstream tube decreases with decreasing space; while, when the space is greater than the critical value, the vibration frequency of the downstream tube no longer changes with the increasing space. The characteristics of flow-induced vibration of tube in staggered arrangement with elastic supporting in two-dimension fluid flow were numerically investigated, and it shows that within the same condition and space, the vibration amplitude and frequency of the downstream tube of the flow-induced tubes in tandem arrangement is greater than that of the downstream tube of the flow-induced tubes in staggered arrangement. The characteristics of heat transfer of tubes in tandem arrangement and tubes in staggered arrangement were numerically investigated, and it shows that, when flow induces cylinders in tandem arrangement vibration, the downstream cylinder in the end of the upstream cylinder wake vortex, the vibration amplitude of downstream is small, but the effect of heat transfer is the best. Within a certain distance, the heat transfer effect of flow-induced vibration cylinders in tandem arrangement is better than that of cylinders in staggered arrangement.Based on above results, the characteristics of flow-induced vibration and heat transfer of four cylinders in tandem arrangement with4.5d space were numerically investigated, and the vibration corresponding displacement, frequency and the surface heat transfer coefficient were obtained. It shows that, when flow induces four cylinders in tandem arrangement vibration with4.5d space between every cylinder, the frequency of every cylinder is equal, while the amplitude and the surface heat transfer coefficient of cylinder increase with the upstream and downstream sequence.The natural characteristic of the planar elastic tube bundle with different size (A bundle has small size, and B bundle has large size) was numerically investigated, and the vibration modes were obtained. It shows that, the vibration mode of planar elastic tube is complex, which contains in-plane vibration and out plane vibration, and the two modes appear alternately. The characteristic of the flow-induced vibration of the planar elastic tube bundle was numerically investigated by Workbench and CFX software, and the vibration corresponding displacement and frequency of tube free ends were obtained. It shows that, the lower flow fluid is able to induce the planar elastic tube bundles vibration along the longitudinal direction. When the flow rate increases, the planar elastic tube bundle forms a helical bundle state, while the vibration of the elastic tube bundle in plane impact over the vibration of various points on elastic tube bundle along each direction, and become multi-frequency control vibration. The vibration in the Z direction is particularly significant. The two free ends have different frequencies, but they are all around their first three orders natural frequencies. The characteristic of the flow-induced vibration of the planar elastic tube bundle was investigated with the same numerical calculation method. It shows that, because of the size of B bundle is big than that of A bundle, and the stiffness of B bundle is small, the multi-frequency controlling vibration of the free end is more significant, and the amplitude of the free end of B bundle along each direction are more than that of A bundle.The heat transfer characteristics of the single row plane elastic tube bundle A via the flow-induced vibration in condition of different flow velocity were numerically investigated, and the surface heat transfer coefficient of elastic tube bundle in vibrating and still state were obtained. It shows that, in the low flow speed, the flow-induced A tube bundle vibration frequency is high, but the amplitude of A tube bundle is small, comparing to the fluid flow, the vibration velocity of A tube bundle is relatively small, thus the influence of flow-induced vibration of A tube bundle on heat transfer enhancement is slightly, and the maximum is only5.7%. With the increase of flow velocity, the influence of tube vibration on heat transfer enhancement decreases greatly. The heat transfer characteristics of the single row plane elastic tube bundle B via the flow-induced vibration in condition of different flow velocity were numerically investigated, and the surface heat transfer coefficient of elastic tube bundle in vibrating and still state were obtained. It shows that, due to the large size and small stiffness of B tube bundle, in low flow rate, the amplitude of B tube bundle is larger, and the vibration frequency is between5Hz to8Hz. In this condition, the vibration of tube bundle has a great influence on tube heat transfer, and comparing with the still tube bundle heat transfer in the same condition, the heat transfer enhancement is about11.4%. With the increase of flow velocity, the influence of flow velocity on tube bundle heat transfer increases, while the effect of flow-induced vibration on heat transfer enhancement decrease. Therefore, the heat transfer enhancement of B tube bundle via flow-induced vibration decreases in condition of large fluid flow speed.The characteristics of the single row plane elastic tube bundle at flow-induced vibration were experimental studied with flow-induced vibration test table. The frequencies of the free ends were obtained, the results of which are in moderate agreement with that of numerical simulation.The characteristic of flow-induced vibration of the planar elastic tube bundle in tandem arrangement with the space of4.5d between tube bundles was numerically investigated, and the vibration corresponding displacement and frequency of the free ends were obtained. It shows that, in a low flow velocity, the flow fluid is able to induce the small free end of the three row of planar elastic tube bundles continuous vibration along the transverse direction. While, for the deep rows, under the common action of vortex shedding, the vibration of small free end along longitudinal direction is random and weak, due to the turbulent buffeting and elastic excitation. The vibration factors of the small free end of B tube bundle were analyzed, which were associated with the vibration factors of the small free end of flow-induced single row tube bundle vibration.