Heat Transfer Enhancement Mechanism Of Wall With A Cylinder In Transitional Flow

Efficient conversion and utilization of energy is an important link to achieve the strategic goal of “carbon peak and carbon neutrality”.Driven by the demand of energy saving and emission reduction,much attention has been paid on the research and development of miniaturized and highefficiency heat exchange equipment.At present,the commonly used heat transfer enhancement technology includes the surface structure-changing of the channel,longitudinal vortex generatoradding,etc.,in which the basic principle is to generate the vortex by changing structure of the flow field near the wall,to thinning or destroy the wall boundary layer to increase the disturbance of the fluid,and to enhance the mixing cold and hot fluid,at last to enhance heat transfer.The characteristic length of small heat transfer device is relatively small,and it usually works in a laminar flow state with low Reynolds number.However,this flow state is not helpful for the improvement of heat transfer efficiency.It is necessary to further study how the periodic and quasiperiodic vortexes affect the near wall flow and heat transfer in the transition flow state.The heat transfer performance of the small heat exchanger can be effectively improved by means of the proper enhancement measures to form periodic vortices,destroy the boundary layer on the wall and make the laminar flow enter the transition state.As the experiment and the numerical model of flow around a cylinder near wall can intuitively explain how the periodic vortices near the wall lead to the heat transfer enhancement.The near-wall cylinder flow model was selected to systematically study the effects of periodic or quasi-periodic vortices on the wall flow and heat transfer in the transition flow state,in order to reveal the effective heat transfer enhancement mechanism of small-sized heat transfer equipment.In this study,a numerical simulation model based on finite volume method was established with self-programmed program in FORTRAN.A low-water-velocity circulating flume flow experimental rig was built.Then the comparative study between numerical calculation and visual flow experiment can be made.In this paper,it is emphatically discussed on the characteristics of flow and heat transfer in transition flow state.And the mechanism of periodic vortex and wall surface heat transfer enhancement is explained.Finally,it is found of the key factors and effective means to reduce the frictional resistance and improve the heat transfer efficiency.The main research work of this paper is as follows:Firstly,the influence of Re(50?Re?1200)on the heat transfer and flow around a near-wall cylinder is investigated.When Re is small,the flow is the laminar flow.Except for the wall directly below the cylinder,the heat transfer is enhanced due to the fluid acceleration.The heat transfer on the downstream wall of the cylinder is not enhanced but worsened due to the flow around the cylinder.When Reynolds number increases to a critical value,which size depends on the geometric parameters of the cylinder,cylindrical wake begins to fluctuate,indicating transition flow state.The interaction between the periodic or quasi periodic wake vortex shedding and the wall boundary layer leads to the heat transfer enhancement of downstream wall of the cylinder,and the increase degree is significant.The results show that the cylinder can move the flow field near the wall into the transition flow state in advance.After the flow from laminar flow state to transition flow state,the heat transfer of the cylinder downstream is obviously enhanced and the frictional resistance is reduced,but there is non-similarity in the time-averaged flow and heat transfer.Secondly,the effect of geometrical parameters(such as clearance ratio and blocking ratio)of the flow around a near-wall cylinder on the flow and heat transfer are studied in the transition flow state.First,the flow heat transfer characteristic under different clearance ratio C/D is investigated.There are three modes.When the clearance ratio is small,the vortex shedding in the wake is inhibited by the wall,its frequency is very small,and there is no heat transfer enhancement in the downstream of the cylinder.When the clearance ratio is moderate,periodic and quasi-periodic vortexes shedding occur in the wake.The vortex strength and shedding frequency increase with the increasing of C/D,and remain unchanged after reaching the maximum at the optimal clearance ratio,and the wall heat transfer of the cylinder downstream is enhanced and the frictional resistance is decreased.When the clearance ratio continues to increase,the cylinder is further away from the lower wall,and the wall surface effect gradually decreases.The wake flow gradually presents a cylinder-less flow mode,the vortex shedding frequency does not change after a small decrease.And the wall heat transfer enhancement gradually weakens.The optimal clearance ratio exists when the Reynolds number and the blocking ratio are fixed,in which the heat transfer enhancement on the wall reach to the best level.Second,the flow heat transfer characteristic under different blocking ratio is investigated.It is found that wall heat transfer enhancement has regional characteristic.The larger the blocking ratio is,the larger the heat transfer enhancement area is;The larger the blocking ratio is,the greater the forced acceleration of the fluid is,and the larger the periodic vortex in the wake is,the greater the near-wall heat transfer enhancement is.At the same flow velocity,the larger the blocking ratio is,the smaller the optimal clearance ratio is.Thirdly,the periodic flow and heat transfer characteristics of the flow around a near-wall cylinder is studied.A typical case within transition flow is selected to do analysis on the instantaneous values at eight equal divisions in a velocity fluctuation period.It is found that the interaction between cylindrical wake and wall boundary layer leads to the result that the local heat transfer on the downstream wall of cylinder is enhanced and frictional resistance is reduced.For heat transfer enhancement,the cross motion of positive and negative vortexes is generated in the wake of a near-wall cylinder due to the wall effect.The negative vortex in clockwise rotation in the cylindrical wake falls off and contacts with the wall boundary layer near-wall vortex and protrusions in clockwise rotation too.With their interaction,cold fluid is brought in,and hot fluid is taken away.It is the main mechanism of heat transfer enhancement on the local wall downstream of a near-wall cylinder.For the frictional resistance reduction,due to the impact of the cylindrical wake,the near-wall vortex rotating clockwise and the protrusion moving clockwise are generated in the wall boundary layer,which can push the fluid to the near wall area from the downstream,reducing the local flow velocity.Moreover,the frictional resistance reduction caused by the clockwise rotation of the near-wall vortex is more significant.It is the main reason of resistance reduction on the local wall downstream of a near-wall cylinder.Finally,a comprehensive performance evaluation index ? was adopted to evaluate the comprehensive performance of heat transfer enhancement under various conditions around a nearwall cylinder in the range of transition flow.The conclusions on the optimal clearance ratio in the previous sections are supported with index ?.There is an optimal clearance ratio,which means the wall heat transfer enhancement reaches the best.when the Reynolds number and blocking ratio are fixed.In the transition flow range,the larger the Reynolds number is,the smaller the optimal clearance ratio is.At the same flow velocity,the larger the blocking ratio is,the smaller the optimal clearance ratio is.