圆管流速骤增对局部阻力的影响

Research on the Effect of a Sudden Current Velocity Increase on Circular Tube Local Resistance

为探究流速骤增引起的阻力特性以及相关受制因素的受控特性,采用速度骤增比分别为25:1和4:1的透明有机玻璃圆管和不同来流紊流程度的试验装置进行了局部阻力测试试验。试验结果表明,速度骤增管的阻力系数先随雷诺数增加而急剧减小,而后随雷诺数的增加逐渐减少直至不变。随着上游来流的紊流强度增加,在试验所测雷诺数范围内,局部阻力系数因分离流再附点提前而减小;速度骤增管的局部阻力系数不仅与速度骤增比和突缩结构有关,同时在设计管内流速下与雷诺数成负相关。这一试验结果与经典教科书对突缩阻力系数与雷诺数无关的传统经验公式并不相同,对管网设计过程中必须进行的总阻力准确评估,具有一定的参考意义。

In view of an investigation of the resistance characteristics brought about by the rapid increase of velocity and the related factors,the local resistance test has been carried out by using transparent plexiglass circular tubes with sudden velocity increase ratios of 25:1 and 4:1 respectively,with experimental devices of different incoming turbulence degrees adopted as well.The experimental results show that the drag coefficient of the tube decreases sharply with the increase of Reynolds number,subsequently decreasing gradually with the increase of Reynolds number until it remains constantly unchanged.With the increase of the turbulence intensity of upstream flow,the local resistance coefficient decreases due to the advance of reattachment point of separated flow within the range of Reynolds number measured in the test.The local resistance coefficient of the tube with sudden velocity increase is not only related to the sudden velocity increase ratio and sudden contraction structure,but also negatively related to the Reynolds number under the design velocity in the tube.The experimental results subvert the traditional empirical formula that the coefficient of resistance of sudden shrinkage is independent of Reynolds number in the classical textbook,but rather it has a certain reference significance for the accurate evaluation of the total resistance in the process of pipe network design.