矩形自支撑缩放管换热器强化传热的结构优化

在新型换热器--矩形自支撑缩放管换热器的基础上,通过FLUENT软件利用三维数值模拟的方法分别研究在缩放段长度比例保持不变的情况下,缩放节距及缩放肋高对换热器管程、壳程及整体综合传热性能的影响,并得出缩放管的优化尺寸。研究表明:对于换热器管程和壳程,缩放节距l越小,换热效果越好,阻力也越大,壳程在l=16.5 mm 时综合传热性能达到最佳,而管程则在l=9 mm时综合传热性能最好;缩放肋高h越大,二者的换热效果越好,阻力也由于管子的粗糙程度增加而变大,此时综合传热性能管程在h=1.25 mm时最好,壳程则在h=0.5 mm时最好。引起这些变化的原因主要是由于随缩放节距与缩放肋高的增加,管程和壳程通道内的回流区不断增加,在回流区的增加造成阻力增加的同时,也改善了速度场与温度场的协同性,从而使二者的传热性能增强。最后将管程和壳程作为一个串联的整体进行综合考虑,得到整个换热器的综合传热性能在l=15 mm,h=0.75 mm时达到最佳,综合因子η=1.136~1.155(壳侧Re=27900~41900)。

Structural optimization of converging-diverging tube based on heat transfer enhancement for self-support rectangle heat exchanger

For the new heat exchanger,self-support rectangle converging-diverging tube bundle heat exchanger,3-D numerical simulations were performed with FLUENT software for turbulent heat transfer and fluid flow characteristics in the heat exchanger.To obtain the optimal structure of converging-diverging tube,the rib length (l)and height (h)were examined while retaining converging length ratio.The effects on heat transfer performance of tube side,shell side and whole heat exchanger were studied.With constant rib length,the effect of rib height was examined.The numerical results indicate that smaller rib length and larger rib height lead to better performance of heat transfer and larger flow resistance on both shell side and tube side.Performance evaluation criteria (η)is adopted to evaluate the overall heat transfer performance.When l=16.5 mm,η is the largest on the shell side,while when l=9 mm it is the largest on the tube side.For rib length of 15 mm,the overall heat transfer performance is the best when h=0.5 mm on the shell side,while it is the best when h=1.25 mm on the tube side.The reason is that the recirculation region increases with the increase of rib length and height on both shell side and tube side. With the increase of recirculation region,the friction in the channel increases.However,the recirculation region reduces the average intersection angle between the velocity vector and the temperature gradient,which is one of the essential factors influencing heat transfer performance.For rib length of 15 mm and rib height of 0.75 mm,the performance evaluation criteria η of the whole heat exchanger is 1.136—1.155 against the Reynolds number of shell side in the range of 27900—41900 as compared with the smooth tube bundle heat exchanger.