受二次流涡流影响的螺旋盘管内传热特性研究

Investigations on heat transfer characteristics in helical coils under the effects of secondary flow vortices

为改进传统螺旋盘管的换热效率,通过实验和数值模拟研究了湍流水流经均匀加热下螺旋盘管的流动和传热特性。研究重点为垂直于盘管轴向横截面内二次流的特性及其对工作流体水和加热盘管壁间传热的影响。结果表明,仅在实验结果中发现了盘管局部努塞尔特数振荡,但模拟结果的努塞尔特数的模拟结果绝对值与实验结果匹配良好。其原因可能是尽管二次流存在并沿盘管轴向不规则地演变,但与主流速度相比过小,对局部传热的影响可忽略不计。为进一步确认盘管中二次流特性,在每180°盘管转弯处,加接一个360°外部反向回路环。研究发现:额外的反向回路环加强了重新流入主盘管的二次流速度,使该横截面积内二次流涡流的数量增加。

In order to improve heat transfer efficiency of the traditional helical coil,the turbulent flow and heat transfer characteristics of a helical coil under uniform heating of water flowing are experimentally and numerically investigated.The main study focus is on the characteristics of local secondary flows in the axial cross-section perpendicular to the coil and their heat transfer effects between the working fluid water and the heated coil wall.It is found that the absolute value of the Nusselt number of the simulation results match well with the experimental results although local Nusselt number oscillation of the coil is discovered only in the experiments rather than in the simulation results.The reason may be that,though the secondary flows indeed exist and evolve irregularly along the coil axis,their effects on local heat transfer are almost negligible since the secondary flow velocity is relatively too small compared with the main flow velocity.To further confirm the characteristics of the secondary flows in the coil,a 360°external reverse loop is added to each 180°coil turning.It is found that the additional reverse loops increase the velocity of the secondary flows reflowing into the main coil and increase the number of the secondary flow vortices in a cross-sectional area.