IMPACT OF JETS

1. Introduction

When a jet of fluid comes out from a nozzle and strikes a stationary or moving surface, the momentum of the fluid changes. According to Newton’s second law, this change in momentum produces a force on the surface. This force is known as the impact of jet.

It is mainly used to determine:

• Force exerted by fluid.
• Work done by the jet.
• Efficiency of hydraulic machines.

2. Basic Principle

The impact of a jet is based on:

• Newton’s second law of motion
• Conservation of momentum

$$Force = \text{Rate of change of momentum}$$If:

• $m$m = mass flow rate
• $V$V = velocity of jet

Then:$$Force = m (V_{initial} – V_{final})$$

3. Types of Jets

Jets can be classified based on direction and surface type:

1. Jet Striking a Stationary Plate

• Flat plate
• Inclined plate
• Curved plate

2. Jet Striking a Moving Plate

• Moving flat plate
• Moving curved vane

4. Force Due to Jet on a Flat Plate (Normal to Jet)

Consider a jet of:

• Velocity = $V$V
• Area = $A$A
• Density = $\rho$ρ

Mass flow rate:$$\dot{m} = \rho A V$$Force exerted:$$F = \rho A V^2$$ Here the jet comes to rest after striking the plate.

5. Jet Striking an Inclined Flat Plate

If the plate is inclined at angle $\theta$:

Force in direction of jet:$$F = \rho A V^2 \cos \theta$$The velocity component normal to the plate is reduced.

6. Jet Striking a Curved Vane

When a jet strikes a curved vane, the fluid is deflected, and both magnitude and direction change.

Force:$$F = \dot{m} (V_{w1} – V_{w2})$$Where:

• $V_{w1}$Vw1​ = initial velocity component
• $V_{w2}$Vw2​ = final velocity component

This is important in hydraulic turbines.

7. Jet on a Moving Plate

If the plate moves with velocity $u$u:

Relative velocity:$$V_r = V – u$$Mass flow rate:$$\dot{m} = \rho A (V – u)$$Force:$$F = \rho A (V – u)^2$$Work done:$$Work = Force \times velocity$$Efficiency:$$\eta = \frac{Work\ done}{Kinetic\ energy\ of\ jet}$$Maximum efficiency occurs when:$$u = \frac{V}{2}$$8. Assumptions in Impact of Jets

1. Flow is steady.
2. Jet velocity is uniform.
3. No frictional losses.
4. Plate is smooth.
5. Fluid is incompressible.

9. Applications

9.1. Hydraulic Turbines

• Pelton turbine
• Impulse turbines.

2. Industrial Cleaning

• High-pressure jet cleaning.

3. Cutting and Drilling

• Water jet cutting.

4. Fire Fighting

• High-speed water jets.

5. Power Generation

• Conversion of kinetic energy into mechanical energy.

10. Experimental Setup

In laboratories, the impact of jet apparatus is used to measure force and compare theoretical and practical results.

Components:

• Water tank.
• Pump.
• Nozzle.
• Target plate.
• Force measuring device.

11. Advantages

• High efficiency in turbines.
• Simple design.
• Useful in power generation.

12. Limitations

• Losses due to friction.
• Splashing and energy loss.
• Not suitable for low-head systems.