| Heat exchangers are one of the indispensable process equipment in the industry and are widely used in many industrial fields.The shortage of energy makes it more important to improve the efficiency of heat exchangers.The enhanced heat transfer technology which is widely used to improve the performance of heat exchangers is one of the most effective means of improving efficiency and reducing operating cost of heat exchangers.The enhanced heat transfer technology can be divided into active heat transfer technology,passive heat transfer technology and composite heat transfer technology.The passive heat transfer technology is widely used in industry due to its easy implementation and low energy consumption.Longitudinal vortex generator(VG)is one of the most commonly used passive heat transfer technologies and has become the typical representative of the third generation of enhanced heat transfer technology.Since the longitudinal vortex generated by the vortex generator can greatly enhance the heat transfer capacity of the heat exchange surface with a relative small pressure drop,the heat transfer enhancement using longitudinal vortex has been a hot research topic in the world and a large number of research results have been published.Most of these papers mainly introduce the characteristics of fluid flow and heat transfer of longitudinal vortices,as well as the parametric effects of vortex generators on the heat transfer enhancement.But little research focuses on the mechanism of longitudinal vortex induced heat transfer enhancement.At present,the mechanism of the longitudinal vortex induced heat transfer enhancement can be summarized as follows:(a)boundary layer theory,the longitudinal vortex can enhance the exchange of momentum and heat between the vortex and the boundary layer,can increase the instability of the fluid and also can increase mixing of fluid with different temperature;(b)absolute vortex flux theory,the value of the absolute vortex flux determines the strength of the longitudinal vortex.These two interpretations are still controversial.The boundary layer theory cannot explain the different convective heat transfer strengths under different thermal boundary conditions.The absolute vortex flux is a dimensional number and the relationship between the value of the absolute vortex flux and the longitudinal vortex intensity varies with the change of structural parameters.The absolute vortex flux cannot explain the reason of large heat transfer coefficient at the beginning of the boundary layer.In order to enrich the mechanism of longitudinal vortex heat transfer enhancement,present paper expounds the mechanism of longitudinal vortex heat transfer enhancement in another perspective which is based on the heat flux convection transfer equation.The transmission of q is determined by the velocity gradient,the velocity and the combination contribution of both velocity gradient and velocity.This method can be used to reveal the mechanism of convection-enhanced heat transfer.In this paper,the mechanism of heat transfer enhancement of the secondary flow generated by the longitudinal vortex generator is revealed based on the heat flux convection equation.Two heat transfer channels are numerically studied with Re=200 in present paper.For one channel,vortex generator pairs(VGs)are installed on the lower wall of the channel.And there are no vortex generator pairs in another channel.The differences in convection contributions are compared between the channels with and without VGs.At the same time,because the convective heat transfer capacity on the wall also depends on the thermal boundary conditions,the differences of longitudinal vortex heat transfer mechanism under the two conditions of uniform heat flow(UWHF)and uniform wall temperature(UWT)are also studied.The research results show that:(1)For channels without VGs,the local heat flux and velocity gradient contribution,velocity and heat flux gradient contribution,and the combination of the two have zero contributions,and have no contribution to the local transmission of q_y.Thus zero of the local value of q_y indicates that these contributions do not contribute to the local transfer of q_y in the y direction.For channels with VGs,the longitudinal vortex greatly enhances these contributions.The local values of these contributions enhance the local transmission of q_y.(2)The local velocity gradient contribution,velocity contribution and the common contribution of both lead to an increase in q_z/(35)T on the upper and lower walls,thus enhancing the heat transfer enhancement by the longitudinal vortex.(3)The studied physical problem is periodically symmetrical in the y direction,so that there is no heat transfer in the mean sense along the y direction.The zero-average(average along the y-direction)of the velocity gradient contribution,the velocity gradient contribution,and the contribution of both agree with the zero lateral average of q_y.(4)Under the uniform wall temperature and uniform heat flow boundary conditions,in the channel with VGs,the enhancement of the corresponding contribution of q_x and q_z transmission is caused by the local velocity gradient contribution and velocity contribution and the mutual contribution of the two in the y direction.Thus,the enhancement of q_y transmission causes an increase in q_x and q_z transmission.(5)Under uniform wall temperature and uniform heat flow boundary conditions,the height and attack angle of VGs are the main parameters affecting the local transmission velocity contribution and velocity gradient contribution of q_y,and then affect the strength of q_z transmission.The length of the VGs has little effect on the velocity contribution and velocity gradient contribution.(6)The contribution of velocity has a positive contribution to the transmission of heat flux in the mainstream direction under the uniform wall temperature boundary condition,while the velocity has a negative contribution to the transmission of heat flux in the mainstream direction under the UWHF boundary condition.(7)Under the uniform heat flow boundary condition,the longitudinal vortex enhances the transmission of q_z by increasing the lateral average(average along the y direction)of the velocity and velocity gradient contributions,but mainly increases the contribution of the velocity gradient.(8)Under the boundary conditions of uniform wall temperature and uniform heat flow,the significant difference between the mechanisms of longitudinal vortex convection heat transfer enhancement is caused by the contribution of the velocity gradient to the heat flux transmission perpendicular to the channel wall.That is,along the main flow direction,the heat flux exists on the wall under the uniform heat flow boundary condition,so that the mainstream velocity component gradient in the vicinity of the channel wall surface plays a positive role in the heat flux perpendicular to the wall surface. |
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