| Flow and heat transfer models involving multiple blunt bodies are fundamental models for many engineering applications,such as heat exchanger tube bundles,electronic components on printed circuit boards,fuel cells,etc.Especially,heat exchangers are used in a wide range of engineering fields such as petrochemical industry,energy and power.In order to improve the heat transfer efficiency of the heat exchanger,the heat transfer components are gradually using high strength materials,and the wall of the heat exchange tube is designed to be thinner and thinner.Under the action of unsteady fluid force,there will be obvious fluid-induced vibration phenomenon.In addition,the influence of thermal buoyancy driven flow caused by heat transfer temperature difference on the convection pattern cannot be ignored,and the change of flow pattern will affect the fluid-induced vibration response and heat transfer performance of the heat exchanger.Therefore,the interaction between the vibration of the heat exchange tube and the hot fluid becomes a problem that must be considered in the structural design of heat exchanger.In order to study the fluid thermal buoyancy on the tubes vibration and the influence of mixed convection heat transfer characteristics,thermal fluid and interaction mechanism of pipeline vibration,in the light of the three cylinders are triangular arrangement of a typical bundle arrange the basic structure,Fluid-induced vibration and mixed convective heat transfer characteristics of three cylinders under thermal buoyancy were numerically investigated.The effects of the reduced velocity,the Angle between the direction of incoming flow and the direction of thermal buoyancy(δ)and the Richardson number on the flow induced vibration response,wake vortex evolution and heat transfer characteristics of three cylinders are analyzed in detail.In this paper,the vibration response characteristics and the mixed convective heat transfer characteristics of three cylinders arranged in equilateral triangle under the aiding thermal buoyancy(δ=0°)are firstly studied.The effects of Richardson number and reduced velocity on the flow induced vibration of each cylinder and the flow field and temperature field around the column group are discussed.The results show that the vibration of each cylinder is suppressed by the aiding thermal buoyancy(δ=0°),and the vibration suppression effect of cylinder 1 is stronger than that of cylinder 2 and cylinder3.Cylinder 2 and Cylinder 3 appear in various vibration states with the change of reduced velocity.When the vibration of cylinder 2 and cylinder 3 is close to the inverse phase,the wake of three cylinders presents side-by-side vortexes.When the vibration of cylinder 2and 3 is close to the same phase,the wake of the column group presents an alternate single vortex mode.When the vibration of the cylinder is completely suppressed,the wake of the three cylinders is long and narrow.The fluid-induced vibration of cylinder 1 under the action of aiding thermal buoyancy force leads to the deterioration of heat transfer in different degrees,and the average Nusselt number is reduced by 4.57% compared with that of the fixed flow.The heat transfer of cylinder 2 and cylinder 3 is enhanced by vibration in different degrees,and the average Nusselt number increases by 9.74% and9.78%,respectively.Due to the effect of aiding thermal buoyancy,the boundary layer could not be separated smoothly,and the local Nusselt number at the stagnation point appeared the minimum value when the vibration of the three cylinders was completely suppressed.Then,the fluid-induced vibration and mixed convective heat transfer of three cylinders with equilateral triangle arrangement under the action of cross thermal buoyancy(δ=90°)are investigated.The effects of the reduced velocity and Richardson number on the fluid-induced vibration and heat transfer characteristics of the cylinders are discussed.It is found that the maximum amplitude of cylinder 1 increases by 21%under cross thermal buoyancy compared with that without considering thermal buoyancy.When Ri=1,the flow field asymmetry caused by cross thermal buoyancy force causes cylinder 1 to appear a phenomenon similar to galloping.With the rise of Richardson number,the asymmetry of flow field is strengthened.This weakens the amplitude of cylinder 2 and slightly strengthens the amplitude of cylinder 3,resulting in a significant difference in the amplitude of cylinder 2 and 3.When the cross-thermal buoyancy force acts on the cylinder 2 and 3,the opposingd phase region disappears.When cylinders 2 and 3 tend to move in the same phase,the wake flow of the three cylinders presents a "2S" mode.The average flow around cylinder 1 is 10.42% higher than that around cylinder 1.When the reduced velocity U~* is less than or equal to 7,the flow induced vibration causes the heat transfer deterioration of cylinder 2.With the increase of the reduced velocity,the heat transfer is gradually strengthened.The heat transfer of cylinder 3 under the influence of vibration is worse than that of the fixed flow.When Ri=1and U~*=10,the heat transfer deterioration of cylinder 3 is the most serious,and the average Nusselt number decreases by 9.08%.Due to the flow asymmetry caused by cross thermal buoyancy,the local Nusselt number of cylinder 1 on the surface of 0°≤θ≤180° is higher than that on the surface of 180°≤θ≤360°.Finally,the vibration response characteristics and the mixed convective heat transfer characteristics of three cylinders arranged in equilateral triangle under the action of opposing thermal buoyancy(δ=180°)are studied in this paper,and the vibration and heat transfer characteristics of three cylinders arranged in equilateral triangle are compared with those under the action of aiding thermal buoyancy(δ=0°)and cross thermal buoyancy(δ=90°).The effects of reduced velocity,Richardson number and direction of thermal buoyancy on the flow induced vibration and mixed convective heat transfer characteristics of three cylinders are analyzed.The results show that the maximum amplitude of each cylinder is obviously enhanced by the opposing thermal buoyancy.In the case of opposing thermal buoyancy,there exists a critical reduced velocity,which makes cylinder 2 and cylinder 3 vibrate in the same phase,but this critical reduced velocity is smaller than that of cross and aiding thermal buoyancy.When Ri=0.25,the region of antiphase vibration of cylinder 2 and 3 is found.With the increase of Richardson number,the antiphase region of vibration disappears.Although the Richardson number increases with the temperature difference,the Nusselt number of fixed and vibrating cylinders still decreases with the Richardson number increasing.When the cylinder is fixed,the average Nusselt number of cylinder 1,2 and 3 decreases by 3.15%,13.25% and12.94% respectively when Ri=1 compared with Ri=0.25.The deterioration of heat transfer is also observed in the vibrating three cylinders,but this phenomenon is alleviated when the reduced velocity is high(U~*≥10).When the reduced velocity U~*=12,the average Nusselt number of cylinder 1,2 and 3 increases by 5.02%,5.05% and 4.54% respectively when Ri=1 compared with Ri=0.25.When the Richardson number increases under the opposing thermal buoyancy,the heat transfer of the cylinder will be weakened at the facing surface and enhanced at the rear stagnation point. |
