IMPULSE PRINCIPLE

Figure 5-2.—Impulse turbine. Figure 5-3.—Simple impulse turbine principle. IMPULSE PRINCIPLE 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 from the impact of the steam on the blades. The toy pinwheel (fig. 5-3) 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. 5-2