| With the increasing requirements for energy conservation at home and abroad,the development of efficient heat transfer methods has became a research focus.Addding a vortex generator on the heat transfer surface is a common enhanced heat transfer technology.The vortex generator is used to induce vortex in the channel to destroy the flow boundary layer,reduce the heat transfer resistance,and improve the efficiency of the heat transfer equipment.For these reasons,the vortex generator obtain extensive research and application.This paper mainly use the simulation method to analyze the enhanced heat transfer and flow resistance characteristics of different concave vortex generators added in rectangular channels under steady and pulsating flows.The concave vortex generators are analyzed in different flows and gained the enhanced heat transfer mechanism.This paper first use the trapezoidal vortex generator flow field information in a rectangular pipe to verify the calculation grid and numerical simulation model.And fitting formula is also to verify the boundary conditions and turbulence model.The flow and heat transfer of concave,convex,trapezoidal,and punched holes vortex generators in rectangular channels were numerically simulated.Velocity,temperature distribution,dimensionless vortex strength of the vortex core structure,Nusselt number and flow resistance are analysed.It is found that the concave vortex generator can reduce the flow resistance while increasing the maximum heat exchange capacity in the tube,for that it can enhance heat exchange efficiency,and reduce energy consumption.Secondly,this paper study the effect of changing the structure of the concave vortex generator on the enhanced heat transfer and flow resistance characteristics.This paper calculate the characteristics of the width and height of different concave vortex generators at different inlet Reynolds numbers(5 inlet Reynolds numbers,5 groups of width,13 groups of height,total of 325 datas).PEC compared to the smooth pipe maximum strengthening ratio is 93%.This paper analysed the heat transfer mechanism of concave vortex generator under steady flow conditions and vortex structure in the tube.It is found that the concave vortex generator forms different velocity gradients through different shear layers,and forms a strong shear stress on the wall,which affects the distribution of turbulence dissipation near the wall leading an enhanced heat transfer performance in the tube.The formula of nusselt number(Nu),friction factor(f),comprehensive heat transfer factor(PEC)base on slot width and depth are obtained.Finally,the composite enhanced heat transfer characteristics of the concave vortex generator structure under pulsating flow conditions are analyzed.And in this paper,the enhanced heat transfer characteristics under different inlet Reynolds numbers and different pulsating frequencies were studied in simulation.It was found that there is an optimal frequency for pulsating flow enhanced heat transfer,and the optimal frequency is 1 Hz.The pulsating flow can improve the enhanced heat transfer on the basis of steady-state flow.Compared with steady-state flow,the ratio of composite pulsating flow to enhanced heat transfer is 17.52%.The enhanced heat transfer characteristics of pulsating flow are related to the pulsation frequency and the period of speed change.The heat transfer is enhanced during the half cycle of speed increase,and the heat transfer is weakened during the speed decay half cycle.The research in this paper shows that the concave vortex generator can generate multi-longitudinal vortex structure downstream of the flow.And it can increase the heat transfer intensity under the premise of reducing the resistance loss.The research in this paper introduce a new type of concave vortex generator in steady state and pulsation flow.The optimization of the concave structure provide some theoretical guidance to improve the heat transfer performance and the flow resistance characteristics. |
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