Flow Measurement-Venturi meter

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来自 Georgia Institute of Technology 的课程

工程考试复习基础

207 个评分

Georgia Institute of Technology

工程考试复习基础

207 个评分

The purpose of this course is to review the material covered in the Fundamentals of Engineering (FE) exam to enable the student to pass it. It will be presented in modules corresponding to the FE topics, particularly those in Civil and Mechanical Engineering. Each module will review main concepts, illustrate them with examples, and provide extensive practice problems.

从本节课中

Fluid Mechanics

This module reviews the basic principles of fluid mechanics particularly the topics covered in the FE Exam. It first discusses what a fluid is and how it is distinguished from a solid, basic characteristics of liquids and gases, and concepts of normal and shear forces and stresses. The major fluid properties are then discussed. Next fluid statics are addressed: pressure variation in homogeneous and stratified fluids and application to manometers; forces on submerged plane surfaces and buoyancy forces on fully and partially submerged objects.Flowing fluids are then covered. This includes the equations for conservation of mass (the continuity equation) and energy (the Bernoulli equation). These are then applied to velocity and flow measuring devices: the Pitot tube, and Venturi and orifice meters.The final topic is similitude and dimensional analysis. This includes concepts of fundamental dimensions and dimensional homogeneity, the Buckingham Pi theorem of dimensional analysis, and the conditions for complete similitude between a full-scale prototype flow situation and a small scale model.In all cases, basic ideas and equations are presented along with sample problems that illustrate the major ideas and provide practice on expected exam questions.Time: Approximately 6 hours | Difficulty Level: Medium

Fluid Properties- Introduction6:29

Fluid Properties-Density and Pressure10:37

Fluid Properties-Stresses Viscosity15:46

Fluid Properties-Surface tension10:21

Fluid Properties-Units and other properties4:16

Fluid Statics- Introduction and Pressure Variation12:13

Fluid Statics-Application to manometers and barometers14:55

Fluid Statics-Forces on submerged plane surfaces10:19

Fluid Statics-Forces on submerged plane surfaces continued9:28

Fluid Statics-Buoyancy and stability9:12

Continuity and Energy Equations: Continuity and mass conservation7:45

Continuity and Energy Equations: Energy equation9:12

Continuity and Energy Equations: Energy equation examples9:24

Flow Measurement-Pilot tubes10:51

Flow Measurement-Venturi meter4:27

Flow Measurement-Orifice meter9:17

Flow Measurement-Dimensions and units, Pi theorem12:24

Flow Measurement-Similitude9:26

Flow Measurement-Similitude examples13:06

与讲师见面

Dr. Philip Roberts
Professor
School of Civil and Environmental Engineering

I want to continue our discussion of flow measurement by

today looking at venturi meters.

A venturi meter is basically a constriction or

reduction in area and it enables us to measure flow rate in pipes

by observing the pressure difference at the throat.

Here is the relevant segment from the reference manual

which describes the variables here, and

to analyze the Venturi meter we can apply the Bernoulli equation.

Now, for a horizontal pipe where the elevation is different, is constant,

the Bernoulli equation reduces to p over gamma plus v-squared over 2g

equals constant.

From which we can see that where the velocity is high, the pressure is low and

vice-versa.

So if I think about the pressure distribution in a Venturi like this,

then the variation of pressure in the pipe upstream here, assuming there's

net energy losses are negligible where the area is constant the pressure is constant.

Then as the area begins to converge here, the velocity increases,

therefore the pressure decreases, reaches a minimum at the throat where the area

is a minimum, and then, as the pipe gradually expands again,

the pressure recovers back to almost its original value.

And if energy losses are negligible, which they are very small

in a venturi meter like this, we should have a perfect pressure recovery,

the pressure recovers back to its original value.

Now we can relate the pressure difference at the throat here and

the upstream pressure to the flow rate by continuing with the Bernoulli equation.

So if I have station one upstream here, and

station two at the throat, we have the Bernoulli equation as shown.

If the meter is horizontal, then the elevations Z1 and Z2 are the same,

so they can sollout leaving us with this equation.

Next I invoke continuity for an incompressible fluid,

the volume flow rate Q is equal to A1 V1 which is equal to A2 V2 and

for round pipe, the areas are as shown.

Combining these equations, we end up with this equation.

But the volume flow rate Q is equal to the throat area multiplied by the square

root of this expression, where (p1- p2) is the pressure

difference between the upstream section and the throat.

And also we recognize that the areas are proportional to diameter squared.

So we can, in turn, write this equation in this form in terms of the upstream and

the flow diameters.

Now this equation assumes no energy losses.

So let me do an example on that, so in this question we have Kerosene

of specific gravity .85 flowing, we're given the pressure difference

between measured on the pressure gauge is 116 kilo pascals.

The density of water is given,

the flow rate is most nearly which of these alternatives.

So to solve this we invoke the Venturi meter equation that

I gave on the previous slide as shown here, and the area is 5 by 4d squared.

So continuing on I can substitute in the numbers.

The diameter is .06 meters at the throat.

The pressure difference is 116 kilopascals or 116 times 10 cubed pascals.

The density of kerosene is the specific gravity .85

times the density of water so substituting in the answer

is .05 cubic meters per second and the closest answer is C.

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