Heat transfer, local distributions of Nusselt number, flow structure, and friction characteristics of twisted square ducts are presented. Numerical analysis was carried out to investigate the influence of the twist ratio (TR = p/D = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0) on the thermal-hydraulic performance of twisted square ducts under constant wall heat flux condition for Reynolds numbers based on the hydraulic diameter of the twisted square duct ranging from 3000 to 20 000. The straight square duct was also analyzed for comparison. The numerical results showed that the twisted square ducts were more efficient in heat transfer than the straight square ducts because the swirl flow helped to increase fluid mixing and reduce thermal layer boundary thickness. The decrease of the twist ratio led to the increase in the Nusselt number and friction factor due to the higher frequency of swirl flow. As compared to the straight square duct, the twisted square ducts with TR = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 improved heat transfer by 52, 49.82, 45.85, 42.22, 39.54, 35.41, and 31.77 %, respectively. Among the studied twisted ducts, the ones with twist ratio TR = 3.5 offered the maximum thermal enhancement factor of 1.42 at Re = 3000. In addition, the results also revealed that the twisted square ducts are thermo-hydraulically superior to the straight square ducts.

传热，努塞尔数的局部分布，流动结构和扭曲的方管的摩擦特性。进行了数值分析以研究扭曲比的影响（TR = p / D= 3.0、3.5、4.0、4.5、5.0、5.5和6.0）在恒定壁热通量条件下根据方管的水力直径为3000到20的雷诺数计算的方管的热工水力性能000.还分析了直方管以进行比较。数值结果表明，由于涡旋流有助于增加流体混合并减小热层边界厚度，因此扭曲的方管比直的方管更有效地进行传热。由于旋流的频率较高，因此扭曲比的降低导致Nusselt数和摩擦系数的增加。与直方管相比，带TR的扭曲方管= 3.0、3.5、4.0、4.5、5.0、5.5和6.0分别提高了52、49.82、45.85、42.22、39.54、35.41和31.77％的传热效率。在所研究的扭曲风管中，扭曲比TR = 3.5的那些在Re = 3000时提供了最大的热增强系数1.42。此外，结果还显示，扭曲方风管在热工液压方面优于直方风管。