The impulse turbine (fig 5-2) consists basically of a rotor mounted on a shaft that is free to rotate in a set of bearings. The outer rim of the rotor carries a set of curved blades, and the whole assembly is enclosed in an airtight case. Nozzles direct steam against the blades and turn the rotor.

The energy to rotate an impulse turbine is derived from the kinetic energy of the steam flowing through the nozzles. The term impulse means that the force that turns the turbine comes

NOZZLE NO. 4

from the impact of the steam on the blades. The toy pinwheel can be used to study some of the basic principles of turbines. When you blow on the rim of the wheel, it spins rapidly. The harder you blow, the faster it turns. The steam turbine operates on the same principle, except it uses the kinetic energy from the steam as it leaves a steam nozzle rather than air.

Steam nozzles (hereafter referred to as nozzles or stationary blades) are located at the turbine inlet. As the steam passes through a nozzle, potential energy is converted to kinetic energy. This steam is directed toward the turbine blades and turns the rotor. The velocity of the steam is reduced in passing over the blades. Some of its kinetic energy has been transferred to the blades to turn the rotor.

Impulse turbines may be used to drive forced draft blowers, pumps, and main propulsion turbines.

Figure 5-2 shows an impulse turbine as steam passes through the nozzles.

REACTION PRINCIPLE

The ancient turbine built by Hero operated on the reaction principle. Hero's turbine was invented long before Newton's time, but it was a working model of Newton's third law of motion, which states: "For every action there must bean equal and opposite reaction."

If you set an electric fan on a roller skate, the roller skate will take off across the room fig.5-4 . The fan pushes the air forward and sets up a breeze (velocity). The air is also pushing backward on the fan with an equal force, but in an opposite direction.

If you try to push a car, you will push back with your feet as hard as you would push forward with your hands. Try it sometime when you are standing on an icy road. You will not be able to move the car unless you can dig in with your feet to exert the backward force. With some thought on your part, you could come up with examples to prove to yourself that Newton's third law of motion holds true under all circumstances.

Figure 5-4.-Demonstration of the velocity of the reaction principle.

Figure 5-5.-Demonstration of the kickback of the reaction principle.

The reaction turbine uses the reaction of a steam jet to drive the rotor. You learned that an impulse turbine increases the velocity of steam and transforms that potential energy under pressure into kinetic energy in a steam jet through nozzles. A forward force is applied to the steam to increase its velocity as it passes through the nozzle. From Newton's third law of motion, you see that the steam jet exerts a force on the nozzle and an equal reactive force on the turbine blades in the opposite direction. THIS IS THE FORCE THAT DRIVES THE TURBINE.

In the reaction turbine, stationary blades attached to the turbine casing act as nozzles and direct the steam to the moving blades. The moving blades mounted on the rotor act as nozzles. Most reaction turbines have several alternating rows of stationary and moving nozzle blades.

You can use a balloon to demonstrate the kickback or reaction force generated by the nozzle blades fig.5-5. Blow up the balloon and release it. The air will rush out through the opening and the balloon will shoot off in the opposite direction.

When the balloon is filled with air, you have potential energy stored in the increased air pressure inside. When you let the air escape, it passes through the small opening. This represents a transformation from potential energy to kinetic energy. The force applied to the air to speed up the balloon is acted upon by a reaction in the opposite direction. This reactive force propels the balloon forward through the air.

You may think that the force that makes the balloon move forward comes from the jet of air blowing against the air in the room, not so. It is the reaction of the force of the air as it passes through the opening that causes the balloon to move forward.

The reaction turbine has all the advantages of the impulse-type turbine, plus a slower operating speed and greater efficiency. The alternating rows of fixed and moving blades transfers the heat energy of the steam to kinetic energy, then to mechanical energy.

We have discussed the simple impulse and reaction turbines. Practical applications require various power outputs. Turbines are constructed with one or more simple turbines made as one. This is done in much the same way that the varying cylinder size of a car engine varies power. Figures 5-6 and 5-7 show typical naval turbines.