Impact of jet

TITLE

Impact of Jet

OBJECTIVE

·         To demonstrate that the force on a vane is proportional to the rate of delivery off momentum.

·         To show that you can predict the force on a vane from a combination of its surface shape and the properties of the jet directed at it.

INTRODUCTION

One way of producing mechanical work form fluid under pressure is to use the pressure to accelerate the fluid to a high velocity in a jet. When directed on to the vanes of a turbine wheel, the force of the jet rotates the turbine. The force generated is due to the momentum change or ‘impulse’ that takes place as the jet strikes the vanes. Water turbines working on this impulse principle have been constructed with outputs of the order of 100,000kW and with efficiencies greater than 90%

The TecQuipment Impact of a jet (H8) fits onto either of the TecQuipment’s Hydraulic Benches. It allows students to experiment with the force generated by a jet of water as it strikes a vane in the shape of a flat plate or hemispherical cup, and to compare it with the momentum flow rate in the jet. Also available from TecQuipment are a 120 Degree Conical Plate and 30 Degree Angled Plate (H8a).

   

APPARATUS

1. Impact of jet apparatus (Figure 1)

2. Hydraulic bench

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.

Technical Details

Item	Details
Dimension and weigh (assembled)	720mm high x 520mm x 470mm and 6kg
Weigh Beam scale	250mm in 1mm divisions
Nominal nozzle diameter and area	10mm and 78.54mm2 or 0.00007854m2
Nominal distance from nozzle tip to impact point on vane	35mm or 0.035m
Jockey weight	600g or 0.6kg
Standard vanes (supplied)	Flat plate 80mm long with a 75mm diameter flat face at right angles to the jet. Hemispherical vane 110mm long with a 330mm internal radius hemisphere.
Optional plates (H8a)	120degree conical plate: 100mm long with 75mm diameter 120 degree conical face. 30degree angled plate: 100mm long with a 75mm diameter plate at 30 degrees to the jet.

Noise Levels

The noise levels recorded at this apparatus are lower than 70dB (A)

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 surface.

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 Vi. 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.

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:

·         F₁ = F_R = Force exerted in the given direction on the fluid by any solid body touching the control volume.

·         F₂ = F_B = Force exerted in the given direction on the fluid by body force (e.g. gravity).

·         F₃ = F_P = Force exerted in the given direction on the fluid by fluid 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 F_R. So the reaction force R is given by:

Fig 2: Impact of a jet

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 h is the height of target above the nozzle exit.

1. Impact on normal plane target

For the Normal plane target θ=90^o

Therefore cos θ=0

2. Impact on conical target

The cone semi-angle θ=45^o

Therefore cos θ=0.7071

3. Impact on semi-spherical target

The target exit angle θ=135^o

Therefore cos θ=-0.7071

PROCEDURES

1.      Make sure the weigh beam is at balance with the jockey weight at the zero position.

2.      Start the hydraulic bench and set to maximum flow.

3.      Move the jockey weight until the beam balances again. Note the distance y from the zero position.

4.      Record the flow rate using your hydraulic bench.

5.      Reduce the hydraulic bench flow in steps to give at least 8 more readings of distance y and flow in relatively equal increments.

6.      Repeat for the other vanes (plates) that you need to test.

GRAPH

Table 1: Flat Vane Graph

Table 2: Hemispherical Vane Graph

          

DISCUSSION

There were a few errors that might affect the data collected during the experiment. Firstly the condition of the instrument, hydraulic water pump bench. Poorly maintenance of the hydraulic water bench surface of the water reservoir effect the stream off the water causing the irregular flow of the water.  Presence of impurities in the reservoir and fluid like dust disrupt the flow of water through both weirs. The pump also doesn’t show a consistent flow rate as it increases causing the difficulty to maintain a steady flow in order to measure the height as a steady flow of water. The condition of the pump which produce a vibration throughout the bench also cause the water to have an inconsistent flow throughout the experiment. The presence of leakage at the bench also affect the water even though the leakage is minimal it still effects the result of the experiment over time due to the loss of water.

Environmental factors that affect the result of the experiment are running the experiment in a fan condition room cause the water surface to ripple due to the air movement surrounding the environment. An air conditioned room is highly advised to run this experiment.

From the result, we assumed the errors occurred during the experiment. Then, some of precautions should be taken, for example:

·         Avoid shaking the water bench, so that the water level is set to a balanced position.

·         During the experiment the measurement taken using eye that can make errors while reading the measurement that needs to avoided.

With the measurement taken, it can lower to the minimum error and get accurate reading to avoid the great difference of gradient of slope value. However, in this experiment we have success to prove that when the greater surface area of vane plate, the larger the forces that can withstand by the plate.

CONCLUSION

·         As the conclusion, the experiment that have been carried out were successful, even though the data collected are a little bit difference compared to the theoretical value.

·         The difference between the theoretical value and the actual value may mainly due to human and servicing factors such as parallax error. This error occur during observer captured the value of the water level.

·         This error may occur because the water valve was not completely close during collecting the water. This may affect the time taken for the water to be collected.

·         The recommendation to overcome the error is ensure that the position of the observer’s eye must be 90^o perpendicular to the reading or the position. Then, ensure that the apparatus functioning perfectly in order to get an accurate result.