Analysis On Mechanism Of Heat Transfer Enhancement By Vibration And Experimental Research On A New Type Of Vibrational Heat Transfer Component

In face of increasingly serious energy shortage and environmental pollution, to develop new energy sources and to improve energy utilization efficiency have been internationally recognized as important approaches to solve the problem. Theories and methods on this topic have been research focus in the field of energy and power engineering. In the 1950s and 1960s much attention was put to vibration enhanced heat transfer, which was suspended by the difficulty in implement of controllable vibration within heat exchangers. However, with the recent emergence of flow-induced vibration heat transfer components, transformative development has appeared for the implementation of vibration, which draws new attention to this topic. This paper conducts detailed research on the mechanism of vibration enhanced heat transfer from the aspects of theorectical analysis, numerical simulation and experimental study. A flow-induced vibration enhanced heat transfer component—a new type of elastic tube bundle is put forward, and systematic study is carried out on its inherent vibration, heat transfer and fluid-structure interaction characteristics.In this paper, CFD moving mesh model is established on the characteristics of the flow and heat transfer outside the vibrating tube and velocity vector chart outside the tube at different time phase is obtained during a quarter cycle. Compared with vibration-free conditions, it is found that vibration can enhance the relative velocity of fluid in near-wall region and can form effective erosion on the both sides of the wall in the direction parallel to vibration. By comparing the temperature distribution of vibration and vibration-free condition it is found that, with the same heat transfer temperature difference, the thinner thermal boundary layer and the greater temperature gradient occur under vibration, showing that the vibration can enhance heat transfer.The concept of effective velocity is put forward, which is regarded as the evaluation index of heat transfer performance. Then cosine of the field synergy angle and effective velocity at different time phase and surface phase are calculated near the wall of the vibrating tube.The results show that at every certain time-phase in the surface phase intervalβ=0-360°, there exists two variation periods for both cosine of the field synergy angle and the effective velocity, during which two peak values respectively appear at the left and right side of the direction parallel to vibration while two valley values at upside and downside of the direction vertical to vibration. In a quarter period, cosine of the average field synergy angle outside the tube and effective velocity increase with the time-phase goes up.Effective velocity and cosine of the average field synergy angle outside the tube under different vibration frequencies and amplitudes are calculated in this paper. For the same tube type under the same amplitude, cosine of average field synergy angle outside the tube varies little with frequency, while the effective velocity of fluid outside the tube varies approximately linearly. Under the same vibration frequency and amplitude, cosine of average field synergy angle and effective velocity both goes up in the order following:elliptic tube vibrating in the long-axis direction, tube vibrating in diameter direction and elliptic tube vibrating in the short-axis direction, showing that the tube type is also an important factor on heat transfer besides frequency and amplitude. In a half period, the effective velocity reaches the maximum when the time phase is 90°.PIV test bench is set up for visualization research on flow field outside different types of vibrating tube. Flow field vector charts outside tubes under different conditions are obtained, the results of which are in moderate agreement with that of numerical simulation.Single-tube vibration heat transfer experiment bench is set up and reliability analysis is carried out on the instruments and test system. The convective heat transfer coefficient is obtained under different amplitudes, frequencies and the vibration tube type, and the influence rules of the three factors on heat transfer outside the tube are summarized. Under the same vibration frequency and amplitude, the heat transfer performance improves in the order following:elliptic tube vibrating in the long-axis direction, tube vibrating in diameter direction and elliptic tube vibrating in the short-axis direction. Adopting the range method and variance method, it is shown that the tube shape is also an important factor on heat transfer and is consistent with the simulation results. Meanwhile, the fitting formula of vibration parameters for convective heat transfer coefficient is obtained. Analysis is carried out on resonance phenomenon in the experiment, which can increase the convective heat transfer coefficient by times. Within permissible range of experimental instruments credibility, resonance enhances heat transfer.A new elastic tube bundle is presented in this paper. Compared with the original one, the load conditon of the new bundle improves moderately. The heat transfer area per unit volume increases by 24.7%. Modal analysis on the new elastic tube bundle shows that its vibration mode is more complex, which is three-dimensional combined with in-plane and lateral vibration. Compared with the original one, the new bundle has lower natural frequency and the vibration of 2# and 3# tube of the new bundle is stronger than the other two tubes, the reason for which is that the intermediate two tubes possess a relatively free boundary condition. Finite element analysis on the new bundle reveals that simulation results are moderately consistent with experiment data, showing that the methods adopted in experiment and simulation have good reliability.The constant heat flux electric-heated heat transfer experiment bench for the new elastic-bundle heat exchanger is set up and reliability verification of the instruments and test system is carried out. Under the condition of flow-induced vibration outside the tube, the average convective heat transfer coefficient of the new bundle reaches more than three times as high as that of fixed one, which means remarkable improvement of heat transfer performance.Electric motor driven and flow induced pulsation devices are designed, and average convective heat transfer coefficients outside the tube of the new bundle at different frequency are obtained. Analysis on flow resistance and comprehensive heat transfer performance under different pulsation conditions is carried out, the results of which show that low frequency pulsation condition is benificial to heat transfer enhancement. Average convective heat transfer coefficients outside the tube of the elastic tube bundle in different arrangement mode are obtained. By comparing the results it is found that in most conditions, the order of heat transfer performance follows:bilateral-distribution-staggered arrangement>unilateral-distribution-staggered arrangement>unilateral-distribution-in-line arrangement. Meanwhile, fitting formulas for different experiment conditions are obtained, the error of which compared with experiment data is lower than 5%. Local convective heat transfer coefficients are obtained and it is found that those of the two intermediate tubes are apparently higher than those of the other two. The average heat transfer performance order can be briefed as follows:2#>3#>1#>4#. Among which the convective heat transfer coefficient of the small free end of the two intermediate tubes is a bit higher than those of other sections.Water-water and steam-water heat transfer experiment benches are set up for the new elastic-bundle heat exchanger and heat transfer experimental study is conducted for the two conditions. The in-tube and external-tube heat transfer coefficients are separated and the experiment results support the reliability of the separation method. The pulsation device is installed at the flow entrance of new elastic tube bundles and the in-tube and external-tube convective heat transfer coefficients under different flow and pulsation frequency are obtained in water-water condition. The results show that pulsation inside the tube has no effect on heat transfer characteristics of the tube bundle. Compared with the constant heat flux condition, the external-tube convective heat transfer coefficient increases significantly under both water-water and steam-water conditions, between which that of steam-water condition is even higher. This shows that the medium in tube exerts important influence on both vibration and heat transfer characteristics.Simplified model for fluid-solid interaction (FSI) is set up to obtain the change curve for flow maximum deformation with other parameters of fluid and structure. By range analysis, the influence order of fluid and structure parameters is obtained, among which the tube length, fluid velocity, tube thickness and fluid density are the main factors on structural deformation. The fitting formula for maximum structural deformation with fluid and structure parameters is obtained, the average error of which with calculation results is 8.2%. Meanwhile, proper selection of specific values of the parameters can make an effective control of structural deformation, which provides beneficial reference for the operation and design of engineering equipment.