运用 Fluent 软件对宽高比 ε 为 0． 1 ～1． 0 的矩形微通道层流流动入口段的阻力 特性进行了三维数值模拟． 结果表明:宽高比 ε 和雷诺数 Re 对表观摩擦因子与雷诺数乘 积 f_app Re 有一定的影响． f_app Re 值随着宽高比 ε 的增大而减小，在不同的 Re 数范围内呈现 了不同的影响． 当 Re 数小于 300 时，f_app Re 值随 Re 数的增大而减小;Re 数大于 300 后， f_app Re 值随 Re 数变化很小． 当流动达到充分发展后 f_app Re 值保持不变，且与 Re 数无关． 此 外还研究了 ε 和 Re 数对水力进口长度的影响． 发现当 Re 数小于和等于 20 时，无量纲进 口长度随着 ε 的增大逐渐减小;当 Re 数大于 20，无量纲进口长度随着 ε 的增大呈先增后 减的趋势，且在宽高比为 0． 3 ～0． 4 时达到峰值． 基于得到的结果，提出了新的层流流动进 口长度关系式，该式对微型设备的设计和优化具有一定的指导意义．

The friction factor characteristics of rectangular microchannels with aspect ratio ε from 0. 1 to 1. 0 in hy- drodynamic entrance region were investigated by using the Fluent software． The results reveal that the aspect ratio ε and Reynolds number Re have effects on the product (f_app Re) of the apparent friction factor and Reynolds number． The f_app Re of rectangular microchannels decreases as the aspect ratio ε increases，and different effects are presented in the different range of Re． The f_app Re decreases with the increase of Re when Re is less than 300; while the f_app Re varies slightly when Re is more than 300． When the flow is fully developed，the f_app Re remains unchanged and be- comes nearly independent of Reynolds number． The effects of ε and Re on the hydrodynamic entrance length is also studied． It is found that when Re is less than 20，the dimensionless entrance length gradually decreases with the in- crease of the aspect ratio ε． When Re is more than 20，the dimensionless entrance length first increases and then decreases with the increase of the aspect ratio ε，and reaches the peak value as the aspect ratio is between 0. 3 to 0. 4． Based on the obtained results，a new hydrodynamic entrance length correlation for laminar flow was proposed， which may provide guidance for the design and optimization of microchannels．