| With the improvement of productivity and industrialization level,industrial applications such as radiators and heat exchange equipment have put forward higher requirements for high efficiency heat transfer.Forced convection in channels is the focus of innovation of heat transfer methods.For the plate heat transfer channel whose working medium is air,the heat transfer capacity is mainly limited by the fluid flow state between the plates and the mixing effect of hot and cold fluids.In order to further improve the heat transfer performance of plate heat exchanger,it is an effective method to enhance fluid mixing and improve heat transfer efficiency by installing elastic fins inside the channel and realizing continuous vibration with fluid-induced elastic fins.In this paper,the fixed fin gap of plate heat exchanger channel or tube fin heat exchanger is taken as the research object.Based on two-way fluid-structure coupling method,three kinds of structures such as new three-dimensional flexible fin,flat polyethylene film with wire thickness,vortex generator and elastic fin series element are numerically calculated,and the process of fluid induced structural vibration and the flow field after development are obtained.Using Spaceclaim software to draw models and divide grids,the boundary conditions of fluid computing domain and solid computing domain were exchanged in real time in Workbench module to realize two-way coupling of flow field and solid domain.The vibration characteristics of the structure under the multi-field synergistic bidirectional fluid-structure coupling were revealed.The reasons for the periodic vibration of the fluid-induced element were clarified from the perspectives of driven pressure and restoring force.The influences of the vibration modes of the three structures on the flow and heat transfer characteristics were analyzed respectively.The law of convection-induced vibration and enhanced heat transfer effect,such as inlet Reynolds number,structure,size,material parameters and arrangement of internal components,was studied by control variables.The study of the new three-dimensional flexible plate installed in the channel shows that:(1)The new three-dimensional flexible plate generates periodic vibration under the action of fluid dynamic pressure and its own restoring force.Compared with the traditional flexible plate and rigid plate,the new flexible plate has the advantages of generating continuous vibration to promote the mixing of hot and cold fluids and weaken the hot mass in the shielding area,which proves the feasibility of fluid-induced vibration to enhance heat transfer.When the height of the new flexible plate accounts for 13%-15% of the height of the channel,the heat transfer enhancement effect is the best.When Re=1500,the maximum PEC of the channel can be 1.25.(2)When the hollowed part height of the new flexible plate accounts for 0.2 of the total height of the flexible plate,the heat transfer enhancement effect is the best,and the PEC is increased by more than 15%.(3)The new three-dimensional flexible plate will show continuous and stable periodic vibration after being impacted by air.The flutter mode is divided into three types: vibration mode,unilateral deflection mode and static mode.Under the same conditions,three modes enhance the heat transfer: vibration mode > unilateral deflection mode > static mode.The study on the placement of wire-grade polyethylene film in the channel shows that:(1)the fluid-induced vibration of the flat polyethylene film enhances the heat and mass transfer in the channel.The maximum PEC is 1.2,which proves that the placement of flat polyethylene film with the thickness of wire level in the channel can improve the heat transfer capacity.When Re is in the range of 2300 to 7150,PEC first increases and then decreases with the increase of Reynolds number,and reaches the maximum value when Re=4400,when PEC is1.27.(2)At the same Reynolds number,the longer the film is,the larger the fluid-induced amplitude will be,and the Nu and pressure drop will also increase.When Re=5500,the channel heat transfer capacity is the best when the ratio of film length to channel height is 1.(3)The thickness of the film affects the flapping mode of the fin.Under the same conditions,the thicker the film,the smaller the amplitude and frequency of the fin.The strengthening ability of flutter mode on heat transfer effect of the film: vibration mode > unilateral flutter mode > static mode.The study of vortex generators and elastic finned series elements installed in the passage shows that:(1)The series elements will generate periodic vibration under fluid induction,providing power for heat and mass transfer in the mainstream area and boundary layer,enhancing the heat transfer effect in the passage.The pressure field and temperature field within the vibration period of the fin are obtained,and the mechanism of fluid-induced vibration of the fin is revealed from the perspective of hydrodynamic and restoring force.Vortices will be generated when the fluid flows through the series element.Vortices develop on both sides of the fin and eventually fall off at the tail of the fin,which enhances the mixing of hot and cold fluids in the channel.(2)The fins have three flutter modes in the channel,namely,vibration mode,irregular mode and stationary mode.The enhanced heat transfer effect of vibration mode and irregular mode is greater than that of stationary mode.Both Reynolds number and Young’s modulus will affect the vibration mode of the fin.With increasing Reynolds number or decreasing Young’s modulus,the fin will change from static mode to vibration mode,and then develop into irregular mode.(3)Under the same conditions,the enhanced heat transfer capacity of the staggered arrangement of adjacent fins is higher than that of the parallel arrangement.In the same case,when the distance between the location of the two elements and the adjacent heat exchange wall takes up 40%-45% of the channel height,the vibration of the fin has a stronger promoting effect on heat exchange. |
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