2. venturimeter

experiment - 2Aim : To determine coefficient of discharge(Cd) analytically and graphically by

and plotting a graph between Discharge(Q) v/s (H)1/2

IntroductionA venturimeter is commonly used to measure discharge in closed conduits having pipe flow. It consists of a converging cone, a throat section and a diverging cone. An expression for the discharge is derived by applying the Bernoulli equation to the inlet and the throat using the equation of continuity. The discharge Q = Cd (A1 A2) 2gH

A12 A22

Where is the coefficient of discharge. and are the area of cross-section at the inlet ad throat respectively, H is the difference of the piezometric heads expressed as the height of the liquid.

The converging cone has an angle of about 20. The flow in the cone is accelerating and the loss of head is relatively small. The coefficient takes into account this head loss. The value of is usually between .97 and .99.

In the diverging cone the flow is decelerating. To avoid excessive head loss it is important to keep angle of cone between 6 and 7.

Generally the diameter of the throat is between to times the inlet diameter . The smaller the / ratio, the more is the pressure difference. However, the pressure at the throat should not be allowed to drop to the vapour pressure to avoid cavitation.

For accurate results, the venturimeter should be preceded by a straight and uniform length of about 30. Alternatively, straightening vanes can be used in the pipe.Experimental setupThe setup consists of a pipe connected to a constant head supply tank. A horizontal venturimeter is fitted to the pipe at a distance of at least 30 times diameter. A regulating valve is provided at the exit to vary the discharge.

A measuring tank is provided to determine discharge.

The difference of pressure between the inlet and throat is measured by a U-tube manometer.

TheoryVenturimeter is a device used to measure the discharge and it works on the basis of Bernoullis theorem. It consists of 3 parts:- Convergent cone (inlet), throat and divergent cone for outlet.

Throat has the minimum diameter. Length of outlet is longer to avoid breaking away of liquid stream due to recovery of pressure. A manometer is also attached with inlet as first pressure point and throat as second pressure point. Discharge equation for Venturimeter is:

Q = Cd (A1 A2 ) 2gH

A12 A22

where, Cd = Coefficient of Discharge

A1 = Cross Sectional Area of Inlet

A2 = Cross Sectional Area of throat

H = hm (s2 s1 ) / s1

s2 = Specific gravity of Manometric Liquid

s1 = Specific gravity of flowing fluid

hm = Difference in Manometer reading in mts.PROCEDURE

1. Fill the sump tank with water.2. Connect the main chord to the A.C. power source.3. Open the by-pass valve completely.4. Open the gas cocks at the top of header pipes and also the gas cocks connecting to the selected pipe line.5. Note down pressure drop and time taken, say 15 seconds, for level rise in collecting tank.6. Repeat above procedure for different discharge rate and find coefficient of discharge.OBSERVATIONS AND CALCULATIONS

Area of tank ( A) = cm2 Diameter at inlet, D1 = cm

Diameter at outlet, D2 = cm

Area at inlet, A1 = cm2 Area of outlet, A2 = cm2

Specific gravity of Hg= gm/cm2

Specific gravity of H2O= gm/cm2

Q = Cd (A1 A2 ) 2gHA12 A22

S. No Q (cm3/sec)

hm (cm)H = hm(s2-s1)

s1

(cm) H

(cm)C d

1.

2.

3.

4.

5.

6.

7.

8.

CALCULATIONSExperimental Value =Coefficient of Discharge (Mean) = Graphical Value

Slope = cm3/2/sec

ie. Cd x (A1xA2)x (2g)1/2 =

(A12-A22)1/2 Therefore, Cd = The expression for discharge is

ResultAverage value of

Analytically = _________________ .

Graphically = __________________.Precautions1. Make sure that there are no air bubbles in the manometer.

2. Take the observations after the flow becomes steady.

3. The head in the supply tank should be kept constant.DiscussionThe major experimental error encountered can be attributed to many variables. One of these is the angle of the diverging cone, which if more than 7 can cause a major loss of head. Also, as we use a mercury manometer, any error already present is magnified by the high specific gravity having mercury.

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2. venturimeter

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