Introduction介绍
目标下面是文丘里流量计的工作原理。
利用伯努利定理证明能量原理。
·将管道收缩校准为文丘里流量计，以便使用无量纲图表预测给定水头下降的流速。
使用管道收缩作为了解管道中能量变化的概念的工具。
使用管收缩作为一种工具，用于了解管道能量变化的概念。
The Venturi meter is a device which has been used for many years for measuring the discharge in a pipeline. The fluid flowing in the pipeline is led through a short contraction to a throat section, which has a smaller cross sectional area than the pipeline, thereby increasing the fluid velocity and causing the pressure to decrease. This decrease in pressure can be related to the rate of flow, and so by measuring this pressure drop between the upstream pipeline and the throat, the flow rate can be calculated. Beyond the throat the fluid is decelerated in a pipe of slowly diverging section, known as a diffuser, in order to convert as much as possible of the kinetic head at the throat back to pressure head in the pipeline.
Objectives
• Undertand how a Venturi meter works.
• Demonstrate the energy principle using Bernoulli's theorem.
• Calibrate the pipe contraction as a Venturi meter such that the flow rate for a given head drop can be predicted using a non-dimensional graph.
• Use the pipe contraction as a vehicle for understanding the concepts of energy changes in pipes.
Theory
Consider the flow of an incompressible fluid through a convergent/divergent pipe section as shown in Fig. 1. Piezometer tubes at section 1 with area A1 (upstream) and section 5 with area A5 (throat) show the drop in pressure in terms of the piezometric heads h1 and h5 respectively as shown.
For an ‘ideal’ fluid, the Bernoulli’s equation is,
(1)
The Conservation of mass holds that,
(2)
From eq(1) and eq(2), we can obtain the discharge is related to the head drop by
(3)
Because there will always be some energy loss between points 1 and 2 in a real fluid we introduce a correction factor called a coefficient of discharge, CD, thus
(4)
where the discharge coefficient varies slightly from one meter to another and for a given meter varies slightly with discharge. It usually lies in the range of 0.96 to 0.99, indicating that the simple one-dimensional theory based on Bernoulli's theorem presented here is accurate to within 1 to 4%.
Experimental apparatus
Turn on the pump with the control valve closed, so that there is no flow. Adjust the air pressure in the manifold until all the piezometer readings are reading the same at around 250mm for this no flow condition. Open the control valve carefully until the pressure head difference between sections 1 (a) and 5 (d) is the maximum possible, i.e. of the order of 250mm. Record the piezometer readings at all points. Measure the flow rate for this particular flow. The method for doing this will be demonstrated to you. Now decrease the flow rate from this maximum value by 6 to 8 stages, recording the levels of piezometer tubes at the upstream, the throat and the fully expanded pipe section 6. Record all the readings using the example tables at the end of this handout.
Figure 1 The venturi meter
Experimental steps
1. Examine the apparatus and determine how the flow is controlled and measured using the stopwatch.
2. Record the data about diameters and positions of manometer tappings from the plates on the apparatus. Ceck how the letters and numbers map onto each other. [This is because it is possible to change the direction of the pipe section].
3. Seek assistance to set up the manometer readings at zero flow with the pump on but the control valve closed. At this point all the water levels should be the same.
4. Open the control valve slowly and set the largest flow for which the readings fit on the scales.
5. Record all of the readings for this flow using table 1.
6. &n
