Equipment for Engineering Education

Transcription

1 Equipment for Engineering Education Instruction Manual Venturi Flume G.U.N.T. Gerätebau GmbH Fahrenberg 4 D-885 Barsbüttel Germany Phone: ++49 (40) Fax: ++49 (40) sales@gunt.de Web:

2 Instruction Manual Please read and follow the safety regulations before the first installation! Publication-no.: A 6 3 (A) DTP_3

3 Table of Contents Introduction Unit description Components Assembly Safety Theory and experiments Subcritical flow and supercritical flow Functional method of the Venturi measuring unit Calculating the flow rate Flow rate measurement experiment Other experiments with the Venturi channel meter Appendix Technical Data Venturi profile

4 Introduction The Venturi flume HM6.5 is the fully-functional model of a Venturi channel measuring unit for flow rate measurement of a river course. The unit is designed for installation in the Modular Flow Channel HM6. The Venturi effect is achieved by way of a horizontal constriction of the channel cross section. The measuring unit consists of two plexiglass side elements, giving a very good insight into the constriction cross section. The flow processes subcritical and supercritical flow can be observed very clearly. In addition to the actual flow rate measurement, when combined with the accessory unit Pitot Static Tube HM6.50 on the Instrument Carriage HM6.59, it is possible to clearly carry out an energy comparison using the Bernoulli method. The Venturi measuring unit covers the following subject areas: Flow rate measurement of currents in open flumes Flow processes of water Application of the Bernoulli equation Introduction

5 Unit description The unit HM6.5 Venturi flume consists of two plexiglass elements for constricting the channel cross section at the sides. The unit is designed for installation in the Modular Flow Channel HM6.. Components Fig.:.. Assembly The unit. Venturi flume consists of the following individual parts: Base plate () Venturi side elements made of plexiglass () Fixing plate (3) Plastic hose for sealing (4) Hexagon socket-head screws M0 (5) Crossbar for securing the pressure plate (not shown in this picture. First, secure the base plate () to the channel bed of the Modular Flow Channel HM6 with 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 doing so, pay attention to the flow direction Fig.:. Crossbar Secure the side elements () by clamping the fixing plate (3) between the side elements with the groove facing down (Fig..3) 3 Because of the huge size of the venturi flume, the fixing plate (3) has to be secured by the crossbar as shown in Fig..3. Fig..3 Unit description

6 3 Safety In all circumstances, it is essential to prevent screws or other small parts from being rinsed into the outlet opening of the Modular Flow Channel HM6 by water. This would destroy the centrifugal pump! Therefore, always follow the safety instructions below: Assembly and disassembly of the Venturi measuring unit should only be carried out with the water drained off After assembly, do not leave any tools in the flow channel! Always securely fasten channel fittings in order to prevent damage to the fittings by coming loose! The side elements of the measuring unit are made of fragile and non-scratch-resistant plexiglass. Therefore, do not use any abrasive cleaning agents for cleaning! Never drop the side elements! 3 Safety 3

7 4 Theory and experiments 4. Subcritical flow and supercritical flow v Water Fig.: 4. To understand the functioning method of the Venturi measuring unit, it is necessary to be familiar with the two fundamental flow states of a water course: Subcritical flow Supercritical The subcritical flow is characterised by the fact that the wave velocity which, as we know, is dependent on the water depth, is greater than the flow velocity. In contrast to this, with supercritical flow reverse conditions apply: the wave velocity is less than the flow velocity of the water course. This means that interference under water caused by a wave cannot influence the upstream water in the case of supercritical flow or, expressed in another way: waves cannot expand upstream in the case of supercritical flow. 4. Functional method of the Venturi measuring unit Fig.: 4. v Wave Channel seen from the top supercritical Narrowing of the cross-section Depth of water subcritical flow Width of channel Like the Venturi meter in pipes which is generally used in pipeline construction or flow mechanics, the Venturi measuring unit in open water courses is used to measure the flow rate. It consists of a constriction of the outflow cross section, as shown in Fig. 4.. This profile brings the water to supercritical flow within the constriction. The result of this is, as outlined in section 4., that the downstream water level can have no influence on the outflow process above, as is the case with an underwater weir (e.g. HM 6.33). 4 Theory and experiments 4

8 h Fig.: Calculating the flow rate v, b, h side neck bed threshold supercritical flow The Venturi measuring unit is designated below as the Venturi channel meter, in order to clearly distinguish it from the Venturi pipe meter. In contrast to the measuring weir (HM 6.30), whic is also used to measure the flow rate, the Venturi channel meter has two advantages: only slight gradient loss no danger of sediment deposits In reality, a Venturi channel meter can, in contrast to a measuring weir, also be used in the case of water courses carrying high bed loads. The narrowing of the cross section can be formed with a side neck, through the installation of a bed threshold or even through both measures, as is the case here with the. The flow rate Q calculation by a Venturi channel meter is based on the continuity law between cross sections and of the channel: Q = Q, or (4.) v b h v b h to (4.) Fig.: 4.4 v, b, h v i bi- water velocity at the cross section i of the channel channel width of the cross section i hi - water level height at the cross section i If these findings are combined with the Bernoulli law between cross sections and, we finally obtain the outflow formula for the Venturi channel meter: 3 Q b g C h. (4.3) 4 Theory and experiments 5

9 Whereby: Channel seen from the top b b direction of flow Q Q - - b - g - calculated flow rate of the Venturi channel meter Outflow coefficient, in the case of the Veturi flume is =0.985 side constriction of the Venturi channel meter acceleration due to gravity (g=9.8 m/s) C - Coefficient for constriction h Fig.: 4.5 C 0,75 a h - Channel height before the Venturi inlet The coefficient C depends on the constriction ratio in the horizontal m b b, with (4.4) b - Channel width before the Venturi inlet, and the constriction ratio in the perpendicular h a t h and can be seen in Fig. (4.6). (4.5) 0,70 0,65 0,60 0,55 0,0 0,30 0,40 0,50 0,60 0,70 0,80 0,90 m t Fig.: 4.6 Coefficient C for venturi-channel meter 4 Theory and experiments 6

10 4.3. Flow rate measurement experiment - After you have placed the Venturi flume in the channel, set the flow rate to an average value - The water is flowing supercritically in the vicinity of the constriction. This can be clearly seen - Now measure the water level height in the upstream water sufficiently far from the intake in the Venutri channel meter. To do this, we recommend that you use a Level Gauge HM6.5 on the Instrument Carriage HM6.59. Example calculation: Measured water level height, upstream water: h = 00 mm Set flow rate: Q = 60 m3/h The following values can be found in the appendix: width of the base plate: a = 5 mm (The base plate has a constant thickness. For the calculation, it is approximately dealt with as though the cross section was a Venturi profile.) b = 304mm b = 48 mm First determine the constriction ratio i in the perpendicular (Gl. 4.5) t 00mm 5mm 00mm Theory and experiments 7

11 and in the horizontal (Gl. 4.4): m 48mm 304 mm 0487, With the aid of m t the coefficientc can be read off in Fig. 4 6: C = 0,554 can be deter- The theoretical flow rate Q theo mined: Q theo = 0,985 0,9 3,3 0,56 0, 3/ m 3 /s Q theo = 6,6 m 3 /h There is an actual deviation of Q theo Q 00% =,35% Q to the set flow rate. This is a very good value. It must be said, that this example calculation is just made to show, how the calculation can be made. It may be, that real measurements lead to higher deviations. 4 Theory and experiments 8

12 4.4 Other experiments with the Venturi channel meter If you have the accessory parts Pitot Static Tube HM6.50 and Instrument Carriage HM6.59, we recommend that you perform two further experiments: You could check the continuity equation by measuring the flow velocities in the cross sections and (Fig.4.4) and checking them against equation 4.. You could place a Bernoulli stream thread between cross sections and and directly investigate the corrsponding variables. According to Bernoulli, the following applies: h v v h (4.6) g g 4 Theory and experiments 9

13 5 Appendix 5. Technical Data Base plate: Material: PVC Dimensions (L x W x H) 900 x 304 x 5 mm 5. Venturi profile 0 R Fig.: 5. Venturi side elements ( ea.): Material: Plexiglass Dimensions (L x W x H) 850 x 76 x 430 mm see diagram, basic body (Fig. 5.) t = Appendix 0