Impact of Jet
C IMPACT OF JET EXPERIMENT 1 Aim To find the impact force caused by a fluid jet on a horizontal plate and a hemispherical shell and compare it with the expected theoretical force. 2 Theory When a rubber ball strikes a solid surface it nicely rebounds from the solid surface. The amount of rebound would depend on the coefficient of restitution. However, when a steady fluid jet strikes a solid surface, it does not rebound. But, a stream of fluid is formed which moves over the surface and the fluid leaves the surface tangentially. If a fluid jet is impacting a horizontal plate, at an angle ? the rate of change of momentum can be equated to the force exerted by the jet on the plate. When a vertical fluid jet strikes a horizontally positioned plate, the value of ? will be 90°. A control volume analysis can be performed to give the force F = ? QV, (1) where ? is the density of the water, Q is the jet flow rate and V is the velocity of the jet. For the case of a hemispherical shell when the stream of water is turned back by 180o, the force experienced by the shell will be F = 2? QV, (2) two times the force experienced by the flat plate. Experimental Set-up The set up consists of a closed cylindrical container housing the horizontal plate/hemispherical shell to receive the impact of the jet. The vertical water jet is produced by a pump forcing water through a nozzle of diameter 6mm for the flat plate and of diameter 8mm for the hemispherical set up. The flow rate through the nozzle is measured by a rotameter. The velocity of jet could be obtained by knowing the flow rate. 4 Experimental Procedure Switch on the motor and using the two gate valves control the flow rate.
Place suitable weights to counter balance the impact force of the liquid jet. When the load is higher than the impact load of the jet, the jet will be blocked by the plate. When the load is low the load arm will rise up and touch the top part of the housing. Adjust the flow rate for any selected weight so that the load arm stays in the middle of the distance from the nozzle tip to the top part of the housing. Ensure that the steady flow rate is maintained and take at least six independent measurements by choosing six different flow rates that would balance the weights.
Important steps 1 Allow the float to be steady before you start noting down the readings. 2 Ensure that the fluid jet nicely counter balances the weight it is carrying. Flat plate S. No. Flow Rate Q (m3/s) Velocity V (m/s) Force (Experimental) Fex (N) Force (Theoretical) Fth (N) S. No. Flow Rate Q (m3/s) Hemispherical shell Velocity V (m/s) Force (Experimental) Fex (N) Force (Theoretical) Fth (N) 5 Results and Discussion Plot the following graphs. Fex vs Fth 2. Fex/(1/2 ? V2 A) vs Re=Vd/? , where A is the area of cross section of the jet and Re is the Reynolds number of the jet 1.
The graphs for the flat plate and the hemispherical shell should be plotted on the same graph. Based on these plots answer the following questions 1. Do a control volume formulation to derive (1) and (2). 2. Why is the force independent of the area of the plate/shell? 3. What is the slope of the Fex vs F th plot. What should the ideal value of this slope be? If the slope is different from the ideal value, why is it different. 4. What non dimensional number is the y axis of the second plot? What is the significance of the variation this non dimensional number that you observe from your experiments?
