IMPACT OF A JET

IMPACT OF A JET

Introduction The Impact of a Jet experiment shows students the force produced by a jet of water as it strikes a flat plate, curve surface or hemispherical cup. They can then compare this to the momentum flow rate in the jet. Water jet momentum in civil engineering used in water dams (to move the turbines, next generating electricity) and in water pressure test in building piping system (Hydraulic pressure test). Water turbines are widely used throughout the world to generate power. By allowing fluid under pressure to strike the vanes of a turbine wheel, mechanical work can be produced. Rotational motion is then produced by the force generated as the jet strikes the vanes. One of the common types of water turbines is Pelton wheel. In this type of water turbine, one or more water jets are directed tangentially on to a vanes or buckets that are fastened on the rim of the turbine disc. The impact of the water on the vanes generates a torque on the wheel causing it to rotate and to develop power. To predict the output of a Pelton wheel and to determine its optimum rotational speed, understanding on how the deflection of the jet generates a force on the buckets and how the force is related to the rate of momentum flow in the jet are needed. This experiment aims at assessing the different forces exerted by the same water jet on a variety of geometrical different plates. Under this experiment, the force generated by a jet of water striking a deflector is measured and compare the results with the computed momentum flow rate in the jet. To perform experiments, level the apparatus and zero the weigh beam assembly. Set the flow from the hydraulic bench to maximum, and measure the jet force. Then reduce the flow from the hydraulic bench in several increments. At each increment record, record the force of the jet on the plate and the flow rate. Then repeat the experiments for different test plates. All test plates are all easily interchangeable, taking only a few seconds and needing no tools. Compare the results to those calculated from theory, working out graphs of rate of delivery of momentum against force on the plate.

Objectives i. ii.

iii.

To compare the forces obtained from the experiment and the actual force using theoretical formula. To understand correctly how a turbine (a Pelton wheel for example) works, students need to understand how jet deflection produces a force on turbine vanes. To understand how this force affects the rate of momentum flow in the jet.

Theory Mechanical work can be produced by using pressure of moving fluid at high velocity. As an example jet of water from nozzle can produce force when it strikes a plane of the surface of plate. This type of force can produce power to generate a system such as hydropower turbine. The force exerted onto the plate will depend on the density of fluid, discharge and jet velocity. The force is also depends on whether the plate is moving or static condition. Theoretically, the force of water jet striking on a statistic plate can be calculated by using the following equations: Is given by its angle of incidence α:

For a flat surface

the previous equation has the form:

For a curve surface

the equation is:

(

For Hemisphere

)

the equation has the form:

(

)

Equipment/Apparatus      

Hydraulic Bench Impact of jet equipment Chronometer Gauge Stopwatch Weight

Unit Description

The unit is designed to investigate jet forces impacting against stationary deflectors. The impact forces are produced by a water jet. The impact forces are measured using a lever mechanism and loading weights. The impact forces of the water jet are set via the flow rate.

The unit essentially consists of: – Loading weights [1] – Lever mechanism [2] – Deflector [3] – Nozzle [4] – Perspex vessel [5] – Drain connection [6] – Base Plate [7] – Inlet connection [8]

Various deflectors can be fitted at position [3]. –

Plate

–

Hemisphere

–

Slope

–

Cone

Method 1.

Disassemble the top that is placed over the transparent water tank to place the flat surface in the impact place and secure to the vertical rod that is part of the calibration system assembled in the top.

2.

Cover the tank again.

3.

Place the device on the Hydraulic Bench channel, connecting its water inlet to its quick connector.

4.

Balance the set.

5.

Adjust the gauge indication until it is situated at the same level as the signal of the auxiliary platform.

6.

Place on the platform a weight.

7.

Close the flow control valve of the hydraulic bench.

8.

Switch on the pump of the Hydraulic Bench.

9.

Regulate the flow that impacts against the surface to get that the platform signal is at the same height as the gauge indication.

10. In this balance situation, measure the outlet flow through the nozzle and note down the weight value of the weight placed on the platform. Repeat the previous steps increasing. 11. Repeat procedure 1 to 10 using curve surface of 120 and semishherical surface.

Result / Data & Calculation

PLANE IMPACT SURFACE (α = 90°) Measured Force

Calculated Force

Volume

50

0.4905

0.006

35.7

1.680 x 10-4

2.824 x 10-8

0.561

100

0.9810

0.006

26.0

2.31x 10-4

5.34x 10-8

1.062

150

1.4715

0.006

21.4

2.80x 10-4

7.84 x 10-8

1.554

200

1.9620

0.006

18.4

3.26x 10-4

1.06x 10-7

2.109

CURVE IMPACT SURFACE (α = 120°)

calculated Force

Measured Force

50

0.4905

0.006

43.2

1.389x 10-4

1.929x 10-8

0.575

100

0.9810

0.006

32.5

1.845x 10-4

3.411x 10-08

1.018

150

1.4715

0.006

27.4

2.120x 10-4

4.790 x 10-8

1.423

200

1.9620

0.006

23.8

2.520x 10-4

6.360 x 10-8

1.898

HEMISPHERICAL IMPACT SURFACE (α = 180°) Measured force

Calculated Force

50

0.4905

0.006

52.3

1.538 x 10-4

2.367 x 10-8

0.038

100

0.9810

0.006

36.5

1.875 x 10-4

3.516 x 10-8

0.056

150

1.4715

0.006

29.2

2.308 x 10-4

5.327 x 10-8

0.085

200

1.9620

0.006

25.7n

2.5 x 10-4

6.25 x 10-8

0.099

Sample calculation For Flat Impact Surface (Experimental)

Theoretical Way Values (Calculated)

⁄

 

Percentage of different between the Experimental Way Values and Theoretical Way Values

For Curve Impact Surface (Experimental)

Theoretical Way Values (Calculated)

⁄

 

Percentage of different between the Measured Force and Calculated Force

For Hemispherical Impact Surface Experimental Way Values

Theoretical Way Values (Calculated)

⁄

 

Percentage of different between the Experimental Way Values and Theoretical Way Values

Discussion & Recommendation: For this experiment, we used 6 litre of water and a nozzle of diameter 8mm. Based on the time recorded and the information above, we are able to calculate the calculated force, Fy and the percentage of error. First, we convert the unit of the flow rate, Q from L/min to m3/s for the easier of calculation. For the calculated force, F y and percentage of error, they are calculated according to the formulas below. Formulas used to calculate the calculated force, : Plate: Curve: Hemisphere: Formula used to calculate the percentage of error:Percentage of Error =

–

×100

In this experiment, when the measured increased, the time needed for the level in the volumetric tank to rise from 20 to 30 litres decrease. This results in a decrease in the flow rate. When comparing the three types of deflectors, the flow rate for the hemisphere is found to be the lowest. The percentage of error ranged from around 6.7% to around 14.8%. Some of the percentages of error are large due to several errors made during the experiment. One of the errors may be the error made during setting the pointer; the pointer may not precisely set to zero due to parallax error. Another possible error is the error made when recording the time. Possible factors influencing the results of experiment: a) The factor which influenced the result is that the water flow velocity, where if the velocity is high when the rate of flow was rapid. b) Besides that, weight of load also effects to the result because if the weight of load is high, the distance between of nozzle and conical plate will be near and the rate of water flow hence will be high. c) Readjustments were not properly made for the datum each time weight is added to the device, hence increasing errors to the data.

Uses of water jet momentum in civil engineering: a) Usage in water dams (to move the turbines, next generating electricity) b) Water pressure test in building piping system

Conclusion As a conclusion, the calculated force is correlated with the measured force. Both of the forces will have directly proportional relation. Theoretically, the calculated force should be the same as the measured force. However, this cannot be achieved experimentally due to the errors made during the experiment. This experiment also conducted in order to understand correctly how a turbine (a Pelton wheel for example) works, where students need to understand how jet deflection produces a force on turbine vanes. It is also understood how this force affects the rate of momentum flow in the jet based on the experiment. From this experiment, the flow rate for the curve impact surface is found to be the lowest and thus require a longer time for the volumetric tank to rise from 0 to 6 litres.