EXPERIMENT – 09

ENERGY LOSSES IN PIPES

OBJECTIVE: To investigate the head loss due to friction in the flow of water through a pipe and to determine the associated friction factor. Both variables are to be determined over a range of flow rates and their characteristics identified for both laminar and turbulent flows. METHOD: By measurement of pressure difference between two fixed points in a long (length= many diameters) straight tube of circular cross-section for steady flows. The range of flow rates will cover both laminar and turbulent flow regimes. UTILITIES REQUIRED:

i) The F1-10 Hydraulic Bench. ii) The F1-18 Pipe Friction Apparatus. iii) A stopwatch to determine the flow rate of water. iv) A thermometer. v) A spirit level for setting up the equipment. vi) A measuring cylinder for measuring very low flow

rates.

DATA GIVEN: Length of test pipe, L = 0.500 m. Diameter of test pipe, L = 0.003 m. THEORY: A basic momentum analysis of fully developed flow in a straight tube of uniform cross-section shows that the pressure difference 1 2(p p )− between two points in the tube is due to the effects of viscosity (fluid friction). The head-loss Δh is directly proportional to the pressure difference (loss) and is given by:

1 2(p p )h=ρg−

Δ

and the friction factor, f, is related to the head loss by the equation:

2fLvh=2gd

Δ , where d is the pipe diameter and, in this

experiment, Δh is measured directly by a manometer which connects to two pressure tapings a distance L apart; v is the mean velocity given in terms of the volume flow rate Qt.

t

2

4Qv=πd .

The theoretical result for laminar flow is

64f=Re

, and ν is the kinematic viscosity.

For turbulent flow in a smooth pipe, a well-known curve fit to experimental data is given by -0.25f=0.316Re

PROCEURE: (EQUIPMENT SETUP) 1. Mount the test rig on the hydraulic bench and, with a spirit level, adjust the feet to ensure

that base plate is horizontal and, hence, the manometers are vertical. 2. Check the mercury (Hg.) manometer is correctly filled; Attach a Hoffman clamp to each of

the two manometer connecting tubes and close them off. 3. Setting up for high flow rates:

a. The test rig outlet tube must be held by a clamp to ensure that the outflow point is firmly fixed. This should be above the bench collection tank and should allow enough space for insertion of the, measuring cylinder.

b. Join the test rig inlet pipe to the hydraulic bench flow connector with the pump turned off.

c. Close the bench gate-valve, open the test rig flow control valve fully and start the pump. Now open the gate valve progressively and turn the system until air is purged.

d. Open the Hoffman clamps and purge any air from the two-bleed points at the top of the Hg. manometer.

4. Setting up for low flow rates (using the header tank): a. Attach a Hoffman clamp to each of the two manometer connecting tubes and close

them off. b. With the system fully purged of air, close the bench valve, stop the pump, close the

out flow valve and remove Hoffman clamps from the water manometer connection. c. Disconnect test section supply tube and hold high to keep it liquid filled. d. Connect bench supply tube to header tank inflow, run pump and open bench valve to

allow flow. When outflow occurs from header tank snap connector, attach test section supply tube to it, ensuring no air entrapped.

e. When outflow occurs from header tank overflow, fully open the outflow control valve.

f. Slowly open air vents at top of water manometer and allow air to enter until manometer levels reach convenient height, then close air vent. If required, further control of levels can be achieved by use of hand-pump to raise manometer air pressure.

PROCEDURE- TAKING A SET OF RESULTS. • Running high flow rate tests:

a. Apply a Hoffman clamp to each of the water manometer connection tubes (essential to prevent a flow path parallel to the test section).

b. Close the test rig flow control valve and take a zero flow reading from the Hg. manometer, (may not be zero because of contamination of Hg. and/or tube wall).

c. With flow control fully open, measure the head loss ‘h’ Shown by the manometer. d. Determine the flow rate by timed collection and measure the temperature of the

collected fluid. The kinematic viscosity of water at Atmospheric Pressure can then be determined from the table provided in this manual.

e. Repeat this procedure to give at least nine flow rates; the lowest to give ‘h’ = 30mmHg. Approximately.

• Running low flow rate tests: a. Repeat procedure given above but using water manometer throughout. b. With the flow control valve fully open, measure the head loss ‘h’ shown by the

manometer. c. Determine the flow rate by timed collection and measure the temperature of the collected fluid. The kinematic viscosity of water at Atmospheric Pressure can then be determined from the table provided in this manual. d. Obtain data for at least eight flow rates, the lowest to give ‘h’ = 30mm approximately.

OBSERVATION AND RESULTS: Table: 1

Test Pipe

Length. L (m).

Test Pipe Dia.

d (m).

Volume V m3

Time to

Collect T

(sec.).

Temp of

water 0C.

Kin. Vis ν

m2/sec.

Man.h1

(m)

Man.h2

(m)

HeadLoss Δh

Flow Rate Qt

(m3/sec.)

Vel. V

(m/sec.)

FrictionFactor

f.

Table: 2

Reynolds number Re ln f ln Re ln h ln V

Plot of Graphs: ln (friction factor) vs. ln (Reynolds’s no.) and ln ( head loss) vs. ( velocity) EXPERIMENTAL RESULTS:

1. Identify the laminar and turbulent flow regimes. What is the critical Reynolds Number. 2. Assuming a relationship of the form f = K Ren calculate these values from the graphs you

have plotted and compare these with the accepted values shown in the theory section. 3. What is the cumulative effect of experimental errors on the values of K and n? 4. What is dependence of head loss upon flow rate in the laminar and turbulent regions of

flow? 5. What is the significance of changes in temperature to the head loss?

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