Construction Of Effective Slip Surfaces In Rectangular Channels And The Influence On Flow And Heat Transfer Characteristics

With the gradual emergence of the energy crisis,the issue of energy efficiency has attracted more and more attention from all over the world.In engineering applications,a large amount of energy is needed to overcome the flow resistance in the process of fluid flow,so drag reduction is one of the effective methods to reduce energy consumption in the process of fluid transport.In recent years,the drag reduction of effective slip surface has been paid more attention by many researchers.The research on the drag reduction mechanism of effective slip surfaces and the exploration of simpler and more efficient effective slip surface structures have significant values to academic and engineering application in the efficient use of energy.Inspired by the idea of "effective slip surface",two physical models which can produce effective slip effect are constructed in this paper: a rectangular channel with miniature rectangular groove on the bottom and a rectangular channel with miniature triangular groove on the bottom,and two physical models are studied by Large Eddy Simulation.The effects of two effective slip surfaces on fluid flow,flow structures,temperature distribution and heat transfer performance in the rectangular channels are analyzed to explore the mechanism of drag reduction and heat transfer enhancement of the miniature grooves.Meanwhile,the influence ranges of the two kinds of miniature grooves on the flow field are also analyzed,and the influence mechanism of the Reynolds number on the drag reduction and heat transfer enhancement of the miniature grooves is preliminarily explored by comparing the thermal performance factor under different Reynolds numbers.Firstly,the numerical method used in this paper is verified.The results show that the numerical simulation results are in good agreement with the experimental data and Direct Numerical Simulation results in literatures,which proves the accuracy and reliability of the numerical method used in this paper.The numerical simulation results in this paper show that effective slip occurs on the bottom surfaces of the two kinds of rectangular channels under fully developed turbulence.At the same time,low-speed rotating spanwise vortices exist in the two kinds of miniature grooves,and its main recirculation zone covers most of the upper region of the two kinds of miniature grooves.The spanwise vortices act as a virtual interface.When the fluid in the channel flows across the groove,the fluid does not penetrate the spanwise vortices but skims over it.The fluid rarely enters the groove,and the fluid inside the groove is also trapped.Therefore,when the fluid flows across the miniature grooves,it is in direct contact with the spanwise vortices rotating in the grooves,rather than the wall with non-slip boundary condition,resulting in the effect of effective slip,thus reducing the shear stress on the fluid flow.It is also found that the rotation direction of the spanwise vortices in the miniature grooves are the same as the movement direction of the fluid in the upper channel.The roles of the spanwise vortices are similar to rolling bearings,which apply an additional impetus forward to the fluid flowing above it to push the fluid forward and achieve drag reduction effect.In addition,the results show that the effective slip length generated by the miniature triangular groove is larger than that of the miniature rectangular groove,so the drag reduction effect produced by the miniature triangular grooves is better.In addition,it is indicated that the heat transfer performance of the channel is closely related to the intensity of the spanwise vortices at the corresponding position.The intensity of the spanwise vortices at the leading edge and trailing edge of the miniature grooves increases slightly,which means that the fluid mixing is enhanced and the heat transfer performance is also improved correspondingly.The effective slip effect produced by the miniature grooves has an impact in their upstream and downstream affected ranges respectively.In the affected range,the friction resistance is significantly reduced and the heat transfer performance is improved.The effect of miniature triangular groove is better than that of miniature rectangular groove in heat transfer enhancement and drag reduction.The results also revealed that the thermal performance factor in both kinds of miniature grooves decreases slowly with the increase of Reynolds number,which indicates that the increase of Reynolds number will inhibit the effect of miniature grooves.