## 关于静态和动态水头损失原因的探讨：Static Head Loss&Dynamic Head Loss

- 发布日期：2013-01-17
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Discussion讨论

Static Head Loss静态水头损失

For static head loss, it can be see clearly from figure 1 that the free jet when tilted plus the height of tap is almost exactly the same as free jet when the pipe is horizontal. This proves that th建筑工程期刊杂志e static head loss at the same distance from entrance of pipe is caused by elevation of the pipe. 对于静态的水头损失，它可以从图1中看到的显然是由于自由射流的高度，致使水龙头歪了，几乎是完全一样的自由射流当管道是水平的。这意味在相同的距离出现静态水头损失是由于从入口管道的管道引起的高程导致的。

On the other hand, the head loss between tap 1 and tap 2 and the head lose between tap发表建筑工程论文 2 and tap 3 is different in both horizontal and tilted pipes. This head loss should be caused by friction and also influenced by the free end of the pipe. This head loss should be considered as dynamic hea建筑工程师职称论文发表d loss. There are two different types of streamlines for the free end and the free jet. The distance between free end and free jet and flow rate determine the friction which, in turn, affects the pressure distribution. It will strike a balance and the head loss alone the pipe is not linear. However, the exact relationship need further experiment to be found out.

There could be large error, approximately 0.01m, when measuring the height of the tap when titled. Since the two distances between the three taps are same, the differences between the heights of the three taps when titled should also be same. In experiment, however, the height differences are 0.17m and 0.05m which suggest that the pipe is not perfect straight. This may causes errors when measuring the actual height free jet in static head loss test.测试实际高度自由射流在静态水头的损失试验中的影响因素时,可能会引起测量误差，

Dynamic Head Loss动态水头损失

The pipe is assumed to be smooth pipe. Figure 2 use log axis for flow rate which is linear to Reynolds Number for the same pipe, so the shapes of the curves are similar to the shape of the smooth pipe curve in Moody Chart. 被假定为光滑管的管。图2使用日志轴的线性流动速率，这是同一管道的雷诺数，所以该曲线的形状是与光滑管图表曲线的形状相似。

Figure 2 clearly shows that the experimental results of Laminar flow, Low Flow Rate and High Flow Rate are pretty close (in error bar) to the theoretical friction values. This proves that when the flow rate is not high (Reynolds Number is less than approximate 30000), the dynamic head loss is caused by the friction between water and pipe. The High Flow Rate tests (Reynold’s Number is greater than approximate 30000), however, gave much smaller friction values than theoretical ones. It is a big sudden drop which should be caused by other factors, such as significant turbulence in flow. It can be seem from figure 3 that the assumption of smooth pipe may not be valid, because the curve of relative roughness of 0.0002 could be chosen to give more accurate predicts.

Figure 2 clearly shows that the experimental results of Laminar flow, Low Flow Rate and High Flow Rate are pretty close (in error bar) to the theoretical friction values. This proves that when the flow rate is not high (Reynolds Number is less than approximate 30000), the dynamic head loss is caused by the friction between water and pipe. The High Flow Rate tests (Reynold’s Number is greater than approximate 30000), however, gave much smaller friction values than theoretical ones. It is a big sudden drop which should be caused by other factors, such as significant turbulence in flow. It can be seem from figure 3 that the assumption of smooth pipe may not be valid, because the curve of relative roughness of 0.0002 could be chosen to give more accurate predicts.

Absolute roughness depends on the type and age of the material of pipe, while relative roughness also depends on the diameter of the pipe. In assumption, the pipe is smooth pipe which has zero absolute and relative roughness. In theory, a new drawn copper pipe should have absolute roughness, e, of 1.5 x 10-6m. In experiment, the absolute roughness of pipe is 3.2 x 10-6m, which indicates that the pipe is an old pipe.从理论上讲，应该有一个新的铜管画绝对粗糙度，E，1.5×10-6M。在实验中，管道的绝对粗糙度为3.2×10-6M，这表明LA管道是老管道。

For a particular pipe, Re is linear to the flow rate. Laminar flow occurs when a fluid flows in parallel layers, with no disruption between the layers (Batchelor, 2000). Laminar flow usually has low flow rate which gives low Re. Turbulent flow usually occurs when flow rate is high and has many unsteady vortices which appear on many scales and interact with each other. For pipe flow, a Reynolds number above about 4000 will most likely correspond to turbulent flow, while a Reynold's number below 2100 indicates laminar flow. The region in between (2100 < Re < 4000) is called the transition region (Fox, Robert W. 2009). In laminar flow, f=64/Re is used to predict the friction factor, while in turbulent flow, the curves in the Moody chart are used to predict the friction factor.

In this experiment, there is no test fall in the transition zone where more and more parts of the flow begin to change from laminar flow to turbulent as the flow rate increases. In transition zone, the linear interpolation could be used to predict the Re, although there could be large error.在这个实验中，有越来越多的水分开始流动，从层流到紊流的流速提高进而产生变化，测试的过渡区中没有下降。在过渡区中，可能是用来预测的线性插值，虽然有可能是范围的错误。

When measuring the head at three tap, there is sometimes few air bubbles in the plastic pipe. This may causes error when reading the height. In the two high flow rate tests, 40kg water should be collected, instead of 20kg, to give more accurate flow rate.当测头有三个抽头，有时是有FEW气泡的塑料管中。当这个错误可能会导致读取高度的误差。在两个高流量测试，40KG水应有收集20公斤，以提供更准确的流量。

Conclusion结论

Tow experiments were done to find what factors cause the static head loss and dynamic head loss. 拖车做过实验，发现导致静态和动态的水头损失的原因。Static head loss is caused by elevation of the pipe. Dynamic head loss is caused by friction and should be considered as potential energy loss of flow. The assumption that the pipe is smooth is not valid. The copper pipe has relative roughness of 0.0002 and absolute roughness of 3.2 x 10-6m. The pipe is old while a new drawn copper pipe should have absolute roughness, e, of 1.5 x 10-6m. When Reynold’s number is less than 2100, the flow is laminar and experimental results agree with the theoretical result. When Reynold’s number is greater than 4000, the flow becomes turbulent, but the curve of relative roughness 0.0002 in Moody Chart still provides accurate predicts of friction factor until Reynold’s number exceeds 30000. When the flow rate is very high, Reynold’s number greater than 30000, the experiment results gives much smaller friction factor than that in theory. The reason could be the significant turbulence in the flow which makes the assumption of parabolic curve of speed distribution not valid.

Reference参考资料

Batchelor, G.K. 1973. An Introduction to Fluid Dynamics. Cambridge University Press http://www.51fabiaowang.com/jzgcqk/

Fox, Robert W. 2009. Introduction to fluid mechanics. 7th Edition. John Wiley. Hoboken, N.J.

McKEON, B. J. A. S. SHARMA and J. Fluid Mech(Editor). 2010. A critical-layer framework for turbulent pipe flow. vol. 658, pp. 336–382. Cambridge University Press

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