hm 162.51 venturi flume

G.U.N.T. Gerätebau GmbHFahrenberg 14

D-22885 Barsbüttel

Germany

Phone: ++49 (40) 670854.0

Fax: ++49 (40) 670854.42

E-mail: [email protected]

Web: http://www.gunt.de

Equipment for Engineering Education

Instruction Manual

HM 162.51 Venturi Flume

09/2003

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Instruction Manual

Please read and follow the safety regulations before the first installation!

Publication-no.: 917.000 51 A 162 13 (A) DTP_3

HM 162.51 VENTURI FLUME

Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Unit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 Theory and experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4.1 Subcritical flow and supercritical flow . . . . . . . . . . . . . . . . . . . . . . . . 4

4.2 Functional method of the Venturi measuring unit . . . . . . . . . . . . . . . 4

4.3 Calculating the flow rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4.3.1 Flow rate measurement experiment . . . . . . . . . . . . . . . . . . . 7

4.4 Other experiments with the Venturi channel meter . . . . . . . . . . . . . . 9

5 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.1 Technical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5.2 Venturi profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10


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1 Introduction

The Venturi flume HM162.51 is the fully-func-tional model of a Venturi channel measuring unitfor flow rate measurement of a river course. Theunit is designed for installation in the ModularFlow Channel HM162. The Venturi effect isachieved by way of a horizontal constriction of thechannel cross section.

The measuring unit consists of two plexiglass sideelements, giving a very good insight into the con-striction cross section. The flow processessubcritical and supercritical flow can be observedvery clearly.

In addition to the actual flow rate measurement,when combined with the accessory unit PitotStatic Tube HM162.50 on the Instrument Car-riage HM162.59, it is possible to clearly carry outan energy comparison using the Bernoulli method.

The Venturi measuring unit covers the followingsubject areas:

Flow rate measurement of currents in openflumes

Flow processes of water

Application of the Bernoulli equation

1 Introduction 1


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2 Unit description

The unit HM162.51 Venturi flume consists of twoplexiglass elements for constricting the channelcross section at the sides. The unit is designed forinstallation in the Modular Flow Channel HM162.

2.1 Components

The unit.HM 162.51 Venturi flume consists of thefollowing individual parts:

Base plate (1)

2 Venturi side elements made of plexiglass (2)

Fixing plate (3)

Plastic hose for sealing (4)

Hexagon socket-head screws M10 (5)

Crossbar for securing the pressure plate(not shown in this picture2.2

2.2 Assembly

First, secure the base plate (1) to the channelbed of the Modular Flow Channel HM162with screw M8 (5)

Insert appropriately shortened plastic hoses(4) into the grooves of the side element(we recommend to use vaseline)

Insert the two side elements and, when doingso, pay attention to the flow direction

Secure the side elements (2) by clamping thefixing plate (3) between the side elementswith the groove facing down (Fig. 2.3)

Because of the huge size of the venturiflume, the fixing plate (3) has to be securedby the crossbar as shown in Fig. 2.3.

2 Unit description 2


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Fig.: 2.1

24

35

1

Fig.: 2.2

4

2

5

1

3

Fig. 2.3

3

Crossbar

3 Safety

In all circumstances, it is essential to preventscrews or other small parts from being rinsed intothe outlet opening of the Modular Flow ChannelHM162 by water. This would destroy the centrif-ugal pump! Therefore, always follow the safety in-structions below:

Assembly and disassembly of the Venturimeasuring unit should only be carried out withthe water drained off

After assembly, do not leave any tools in theflow channel!

Always securely fasten channel fittings in or-der to prevent damage to the fittings by comingloose!

The side elements of the measuring unit aremade of fragile and non-scratch-resistantplexiglass. Therefore, do not use any abra-sive cleaning agents for cleaning!

Never drop the side elements!

3 Safety 3


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4 Theory and experiments

4.1 Subcritical flow and supercritical flow

To understand the functioning method of the Venturimeasuring unit, it is necessary to be familiar with thetwo fundamental flow states of a water course:

Subcritical flow

Supercritical

The subcritical flow is characterised by the factthat the wave velocity which, as we know, is de-pendent on the water depth, is greater than theflow velocity.

In contrast to this, with supercritical flow reverseconditions apply: the wave velocity is less than theflow velocity of the water course. This means thatinterference under water caused by a wave cannotinfluence the upstream water in the case of super-critical flow or, expressed in another way: wavescannot expand upstream in the case of super-critical flow.

4.2 Functional method of the Venturi measuring unit

Like the Venturi meter in pipes which is generallyused in pipeline construction or flow mechanics,the Venturi measuring unit in open water coursesis used to measure the flow rate. It consists of aconstriction of the outflow cross section, as shownin Fig. 4.2. This profile brings the water to „super-critical“ flow within the constriction. The result ofthis is, as outlined in section 4.1, that the down-stream water level can have no influence on theoutflow process above, as is the case with an underwa-ter weir (e.g. HM 162.33) .

4 Theory and experiments 4


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Fig.: 4.1

vWave

vWater

Depth ofwater

Fig.: 4.2

Channel seen fromthe top

Narrowing of thecross-section

Widthof channel

supercritical subcriticalflow

The Venturi measuring unit is designated below as theVenturi channel meter, in order to clearly distinguish itfrom the Venturi pipe meter.

In contrast to the measuring weir (HM 162.30), whic isalso used to measure the flow rate, the Venturi channelmeter has two advantages:

only slight gradient loss

no danger of sediment deposits

In reality, a Venturi channel meter can, in contrast to ameasuring weir, also be used in the case of watercourses carrying high bed loads.

The narrowing of the cross section can be formed with aside neck, through the installation of a bed threshold oreven through both measures, as is the case here withthe HM 162.51.

4.3 Calculating the flow rate

The flow rate Q calculation by a Venturi channelmeter is based on the continuity law between crosssections 1 and 2 of the channel:

Q1 = Q2, or (4.1)

v b h v b h1 1 1 2 2 2� � � � � to (4.2)

vi water velocity at the cross section i of thechannel

bi- channel width of the cross section i

hi - water level height at the cross section i

If these findings are combined with the Bernoullilaw between cross sections 1 and 2, we finally ob-tain the outflow formula for the Venturi channelmeter:

Q b g C h� � � � �� 2 1

32 . (4.3)



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Fig.: 4.3

bed threshold

side neck

Fig.: 4.4

supercritical flow

v2, b2, h2

v1, b1, h1

h1

1 2

Whereby:

Q - calculated flowrateof theVenturi channelmeter

� - Outflow coefficient, in the case of the Veturiflume is �=0.985

b2 - side constriction of the Venturi channel meter

g - acceleration due to gravity (g=9.81 m/s2)

C - Coefficient for constriction

h1 - Channel height before the Venturi inlet

The coefficient C depends on the constrictionratio in the horizontal

mbb

�2

1

, with (4.4)

b1 - Channel width before the Venturi inlet,

and the constriction ratio in the perpendicular

th a

h�

�1

1

(4.5)

and can be seen in Fig. (4.6).



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Fig.: 4.5

Channel seen fromthe top

b1 b2directionof flow

h1

a

Q

Fig.: 4.6

0,75

0,70

0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90m � t

0,65

0,60

0,55

C

Coefficient C for venturi-channel meter

4.3.1 Flow rate measurement experiment

- After you have placed the Venturi flume in thechannel, set the flow rate to an average value

- The water is flowing supercritically in the vicin-ity of the constriction. This can be clearly seen

- Now measure the water level height in the up-stream water sufficiently far from the intake inthe Venutri channel meter. To do this, we rec-ommend that you use a Level GaugeHM162.52 on the Instrument CarriageHM162.59.

Example calculation:

Measured water level height, upstream water:

h1 = 200 mm

Set flow rate:

Q = 160 m3/h

The following values can be found in the appendix:

width of the base plate:

a = 15 mm

(The base plate has a constant thickness. For thecalculation, it is approximately dealt with as thoughthe cross section was a Venturi profile.)

b2 = 304mm

b1 = 148 mm

First determine the constriction ratio i in the per-pendicular (Gl. 4.5)

tmm mm

mm�

��

200 15200

093.



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and in thehorizontal (Gl. 4.4):

mmmmm

� �148304

0487,

With the aid ofm t� � 0453.

the coefficientC can be read off in Fig. 4 6:

C = 0,554

The theoretical flow rate Qtheo can be deter-mined:

Qtheo = 0,985�0,291�3,132�0,561 � 0,23/2m3/s

Qtheo = 162,16 m3/h

There is an actual deviation of

Q QQ

theo ��100% = 1,35%

to the set flow rate. This is a very good value.

It must be said, that this example calculation is justmade to show, how the calculation can be made. Itmay be, that real measurements lead to higher de-viations.



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4.4 Other experiments with the Venturi channel meter

If you have the accessory parts Pitot Static TubeHM162.50 and Instrument Carriage HM162.59,we recommend that you perform two further exper-iments:

� You could check the continuity equation bymeasuring the flow velocities in the crosssections 1 and 2 (Fig.4.4) and checking themagainst equation 4.2.

� You could place a Bernoulli stream threadbetween cross sections 1 and 2 and directlyinvestigate the corrsponding variables. Ac-cording to Bernoulli, the following applies:

hv

gh

vg1

12

222

2 2� � � (4.6)



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5 Appendix

5.1 Technical Data

Base plate:

Material: PVC

Dimensions

(L x W x H) 900 x 304 x 15 mm

Venturi side elements (2 ea.):

Material: Plexiglass

Dimensions

(L x W x H) 850 x 76 x 430 mm

5.2 Venturi profile

see diagram, basic body (Fig. 5.1)

5 Appendix 10


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Fig.: 5.1

t = 15

R188

900317

180

7.7°

304 148

20

hm 162.51 venturi flume

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