射流式涡发生器强化矩形螺旋通道内流体换热机理

Heat transfer enhancement mechanism of jet longitudinal vortex generator in helical channel with rectangular cross section

提出利用射流式涡流发生器(JVG)强化螺旋通道内流体的换热。采用三维激光多普勒测速仪(LDV)测量了曲率为0.134并安装了JVG的矩形截面螺旋通道内流体的流动特性,实验结果与数值模拟结果吻合较好。获得了安装JVG的螺旋通道内复合二次涡旋的演变规律以及射流在螺旋通道内的衰减过程。结果表明,射流的冲击和卷吸作用改变了单一螺旋通道内背离壁面(common-flow-up, CFU)结构的离心二次涡旋,在射流的起始段形成了一对冲向壁面(common-flow-down, CFD)结构的二次涡旋。随着流动的发展,CFD涡旋经历了快速形成、缓慢分解并逐渐耗散的过程。射流速比ε_j在1.48~4.02范围内时,射流在螺旋通道内沿主流方向的作用距离可达40d_h^74d_h(d_h为螺旋通道当量直径)。射流提高了通道换热壁面附近处速度场与温度场的协同性,实现了换热强化。研究范围内,换热壁面平均Nusselt数的最大值相对于单一螺旋通道提高了28%~248%。

This study proposed to use a jet longitudinal vortex generator(JVG) to enhance heat transfer capability of fluid in the helical channel. Fluid flow characteristics in the helical channel with rectangular cross-section and installed with a JVG were measured by a 3D Laser Doppler Velocimeter(LDV). The curvature of helical channel is δ=0.134. The experimental results were in good agreement with the simulated results. The evolution process of the composite secondary vortices in the helical channel with a JVG and the attenuation process of the jet in the helical channel were obtained. The results show that the impact and entrainment of the jet changes the structure of the common-flow-up(CFU) vortices in the smooth helical channel. And a pair of common-flow-down(CFD) vortices is formed in the initial stage of jet. With the development of the flow in the helical channel, the CFD vortices undergo a process of rapidly producing, slowly decomposing and gradually dissipating. When the speed ratio of the jet stream to the main stream is between 1.48 and 4.02, the effect of jet can reach a distance of 40-74 times d_halong the mainstream direction in the helical channel. Here d_his the equivalent diameter of helical channel. The effect of JVG improves the synergy between the velocity fields and the temperature fields in the helical channel, thus heat transfer enhancement can be achieved. Within the scope of the study,the average Nusselt number of the heat exchange wall is increased by 28% to 248% relative to the single spiral channel.