Using a Venturi to Measure Flow Rate

Using a Venturi to measure flow rate

P2:

By John J. Turner

9th March 2016

(Submission deadline – 11th March 2016)

Contents:

Glossary of Notation                                                                                3

Abstract                                                                                        3

Introduction                                                                                        3

Background / Theory                                                                                4

Method                                                                                        5

Results                                                                                        6

Experimental errors and error analysis                                                        12

Discussion                                                                                        12

Conclusion                                                                                        13

Glossary of Notation:

Abstract:

The purpose of this report was to test the use of a Venturi to measure flow rate from the change in pressure when a fluid is accelerated.

Observations of the changes in the pressure head between the inlet and the throat of the venturi are taken and combined with an understanding that changes in the velocity head must be balanced by a change in the pressure head to keep the total head constant. Relating the velocities and flow areas at the different points allows the determination of the mass flow rate.

The observations of the pressure profile of water flowing through a venture are measured and compared with those predicted by the Bernoulli equation. Additionally, the venturi is calibrated for use as a flow measurement device and the coefficient of discharge, Cd, determined.

From the generated graphs it can be seen that (hp1 - hp4)0.5 rises steadily with respect to the flow rate (ṁ). Additionally, it can be seen that the flow rate rises steadily with respect to the differential head.

The observed results and calculations / comparisons made clearly show that a rise in differential head causes the flow rate of the liquid to increase, proving the Venturi effect.

Introduction:

The Venturi tube is a device used for measuring the rate of flow along a pipe, as visualised in figure 1.

[pic 3]

Figure 1: Labelled venturi image

A fluid moving through the venturi from the cylindrical inlet accelerates in the direction of the tapering contraction with an increase in the velocity from the convergent entrance to the throat. This is accompanied by a fall in pressure, the magnitude of which depends on the rate of flow. The flow rate may therefore be inferred from the difference in pressure in as measured by piezometers placed upstream of the throat, in the divergent outlet. The effect that the meter has on the pressure change is termed as the Venturi effect. A venturi can also be used to mix a liquid with a gas. If a pump forces the liquid through a tube connected to a system consisting of a venturi to increase the liquid speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), the gas will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of liquid and gas will appear.