Abstract:

To improve the double-pipe heat exchanger with helical fins on outer wall of inner tube，the flow characteristic of helical duct with rectangular cross section was investigated numerically and experimentally.The computed velocity profiles with aspect ratio（the ratio of height to width of the cross section）of 3.5 were in good agreement with the measured profiles.The axial velocity，secondary flow，stream function and vorticity of the flow field were analyzed at different aspect ratio based on the orthogonal helical coordinate system.The maximum of the axial flow shifts toward the outer wall.The secondary flow is in two-vortex structure，in which the direction of the secondary flow in the upper section is clockwise while that is adverse in the lower section.There are two vorticity in opposite direction in the center besides the stronger vorticity at the boundary.As the aspect ratio increases，the maximum of the axial flow shifts toward the upper section，the two vortices of the secondary flow shift toward upper and lower boundaries respectively，and the vorticity in the center decreases.Therefore，in order to improve heat transfer further，for the double-pipe heat exchanger with helical fins on outer wall of inner tube，in which the heat transfer is mainly enhanced by the secondary flow，the secondary flow in the center of the section should be improved when the aspect ratio is large.

Key words:

螺旋通道, 矩形截面, 流动特性, 二次流动

摘要：

为合理强化内管外壁带螺旋翅片的套管式换热器，对矩形截面螺旋通道内流体的流动特性进行了实验和数值研究。高宽比为3.5的矩形截面螺旋通道速度场的实验测量值与模拟值吻合较好。采用正交螺旋坐标系统分析了不同高宽比矩形截面螺旋通道内的轴向速度、二次流速度、流函数以及涡量的分布规律。结果表明，矩形截面螺旋通道内轴向速度最大值偏向外壁;截面上出现两个二次涡;上半截面流体粒子的二次流动方向为顺时针，下半截面流体粒子的二次流动方向为逆时针;除了壁面上涡量较大以外，截面中心出现了两个相反的涡量。随高宽比增大，轴向速度最大值逐渐向上半截面移动;两个二次涡分别向两端移动;截面中心处的涡量逐渐变弱。因此，对于利用二次流动来强化传热的螺旋翅片管换热器，当矩形截面高宽比较大时，应该采取措施改善其中心处的二次流动以进一步提高换热效果。

关键词:

螺旋通道, 矩形截面, 流动特性, 二次流动