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hydraulics
Hydraulics Lab (ECIV 3122)
Islamic University – Gaza (IUG)
Experiment (3): Impact of jet Introduction: Impact of jets apparatus enables experiments to be carried out on the reaction force produced on vanes when a jet of water impacts on to the vane. The study of these reaction forces is an essential step in the subject of mechanics of fluids which can be applied to hydraulic machinery such as the Pelton wheel and the impulse turbine.
Purpose: To investigate investigate the reaction force produced by the i mpact of a jet of water w ater on to various target vanes.
Equipment set up: Set up the apparatus on top of the hydraulics bench with the left hand support feet of the impact of jet apparatus located on the two left hand locating pegs of the hydraulics bench so that the apparatus straddles the weir channel. Connect the feed tube from the hydraulics bench to the boss on the rear of the base of the impact of jet apparatus. Fit the 5mm nozzle and the normal flat target.
Figure 2: Illustrative figure of impact of jet apparatus
Theory: When a jet of water flowing with a steady velocity strikes a solid surface the water is deflected to flow along the surface. If friction is neglected by assuming an inviscid fluid and it is also assumed that there are no losses due to shocks then the magnitude of the water velocity is unchanged. The pressure exerted by the water on the solid surface will everywhere be at right angles to the s urface. Consider a jet of water which impacts on to a target surface causing the direction of the jet to be changed through an angle as shown in figure 2 below. In the absence of friction the magnitude of the velocity across the surface is equal to the incident velocity V i. The impulse force exerted on the target will be equal and opposite to the force which acts on the water to impart the change in direction. 2
Instructors : Dr. Khalil M. Alastal
Eng. Mohammed Y. Mousa
Hydraulics Lab (ECIV 3122)
Islamic University – Gaza (IUG)
Applying Newton’s second law in the direction of the incident jet
This is the resultant force acting on the fluid in the direction of motion. This force is made up of three components:
Force exerted in the given direction on the fluid by any solid body touching the control volume.
Force exerted in the given direction on the fluid by body force (e.g. gravity).
Force exerted in the given direction on the fluid by fl uid pressure outside the control volume.
By Newton’s third law, the fluid will exert an equal and opposite reaction on its surroundings. The force exerted by the fluid on the solid body touching the control volume is equal and opposite to FR . So the reaction force R is given by:
Application to impact of jet apparatus In each case it is assumed that there is no splashing or rebound of the water from the surface so that the exist angle is parallel to the exit angle of the target. The jet velocity can be calculated from the measured flow rate and the nozzle exit area.
However, as the nozzle is below the target the impact velocity will be less than the nozzle velocity due to interchanges between potential energy and kinetic energy so that :
where is the height of target above the nozzle exit. 1. Impact on normal plane target For the normal plane target = 90
°
Therefore
2. Impact on conical target °
The cone semi-angle = 45 Therefore
Position the weight carrier on the weight platform and add weights until the top of the target is clear of the stop and the weight platform is floating in mid position. Move the pointer so that it is aligned with the weight platform. Record the value of weights on the weight carrier.
2.
Start the pump and establish the water flow by steadily opening the bench regulating valve until it is fully open.
3.
The vane will now be deflected by the impact of the jet. Place additional weights onto the weight carrier until the weight platform is again floating in mid position. Measure the flow rate and record the result on the test sheet, together with the corresponding value of weight on the tray. Observe the form of the deflected jet and note its shape.
4.
Reduce the weight on the weight carrier in steps and maintain balance of the weight platform by regulating the flow rate in about three steps, each time recording the value of the flow rate and weights on the weight carrier.
5.
Close the control valve and switch off the pump. Allow the apparatus to drain.
6.
Replace the 5mm nozzle with the 8mm diameter nozzle and repeat the tests.
7.
Replace the normal vane with the 45° conical vane and repeat the test with both the 5mm and 8mm nozzles.
8.
Replace the 45° conical vane with the hemispherical vane and repeat the tests with both the 5mm and 8mm nozzles.
5
Instructors : Dr. Khalil M. Alastal
Eng. Mohammed Y. Mousa
Hydraulics Lab (ECIV 3122)
Islamic University – Gaza (IUG)
Results: 1.
Record the results on a copy of the results sheet provided.
2.
Calculate for each result the flow rate and the nozzle exit velocity. Correct the nozzle velocity for the height of the target above the nozzle to obtain the impact velocity.
3.
Calculate the impact momentum and plot graphs of impact force against impact momentum
and determine the slope of the graphs for each target. Compare with the theoretical values of 1, 0.2929 and 1.7071 for the normal plane target, conical target and hemispherical target respectively. Target
