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IMPACT OF JET

Introduction The fluid stream coming out from nozzle with high velocity & hence a high kinetic energy is known as fluid jet. Therefore impact of jet means the force exerted by the fluid jet on a plate. As we know Newton's second law of motion “The rate of change of momentum is directly proportional to the impressed force & takes place in the same direction in which the force acts.”

Impact of jet on vertical fixed plate Consider a jet of water impinging normally on the fixed plate held perpendicular to flow direction of jet as shown. The jet after striking the plate, jet moves along the Plate. Therefore final velocity of fluid in direction of jet after striking plate will be zero.

Impact of jet on vertical fixed plate Let, V=velocity of jet d=diameter of jet =density of fluid(water) A=cross section area of jet= m=mass flow rate of fluid, kg/s=AV We know that, Impact of jet(force exerted by jet on plate) =Rate change of momentum in the direction of force =Initial moment – Final moment time =|(Mass  Initial velocity)-(Mass  Final velocity)|

Impact of jet on vertical fixed plate =Mass[Velocity of jet before striking – velocity of jet time after striking]

Impact of jet on fixed inclined flat plate Let, V=velocity of jet, A=cross section area of jet, =inclination of plate with the jet Fn=force exerted by the jet on the plate in the direction normal to the plate Fx=force exerted by the jet on the plate in the direction to the jet Fy=force exerted by the jet on the plate in the direction perpendicular to direction of jet

Impact of jet on fixed inclined flat plate From fig., and (2.6) And, (2.7) If =90 then the plate becomes vertical and equations (2.6) & (2.7) becomes (2.5)

Impact of jet on fixed inclined flat plate Also, when jet strikes the plate, fluid mass divided into two portion as Q1 and Q2 as shown in fig. Since, Q=AV, and Q1+Q2=Q. If there is no loss then, Initial momentum along axis of plate = Final momentum along the axis of plate But Q1+Q2=Q

Impact of jet on fixed inclined flat plate And Ratio of discharge, (2.8)

Impact of jet on a hinged plate Let, V=velocity of jet, h=height of plate, W=weight of plate acting at C.G. of plate, x=distance of the centre of jet from hinge O. When plate swings, point A is shifted to A’ after the swing of the plate. But OA=OA’=x=h/2.When the plate is in equilibrium after the jet strikes the plate, the moments of all force on the plate about the hinge O must be zero.

Impact of jet on a hinged plate (2.9) But force exerted normal to the inclined plate is given by where =angle between plate & jet where =angle between plate & vertical axis. Equation (2.9) becomes and

Impact of jet on a moving plate Mass of water striking the plate per second =Avelocity with which jet strikes the plate = A(V-u) Now, impact of water jet on moving plate in the direction of jet, Fx=(Mass of jet strikes/sec) [Initial relative velocity of jet-Final relative velocity of jet in the direction of jet] (2.12)

Impact of jet on a moving plate Work done by jet on the plate W=Force  distance travelled by the plate time = Fxvelocity of plate in direction of jet If plate is moving towards the jet, and (2.13)

When moving plate is inclined Mass of water striking the plate per second =A  velocity with which jet strikes the plate = A(V-u) Fn=(Mass of jet strikes/sec) [Initial relative velocity of jet in the direction normal to plate-Final relative velocity of jet in the direction normal to plate]

When moving plate is inclined Force Fx in the direction of jet, Force Fy perpendicular to the direction of jet, Work done per second by the water jet on the plate, (2.14) (2.15)

Reference Fluid Power Engineering Sixth edition 2015 By J.P. Hadiya -Books India Publication