Theory Of Gas Turbine Engines

1. GENERAL

The laws of physics and fundamentals pertaining to the theory of jet propulsion. The gas turbine engines used to power Army aircraft are turboshaft powerplants. The energy produced drives the power shaft. Energy is generated by burning the fuel-air mixture in the engine and accelerating the gas tremendously. These high-velocity gases are directed through turbine wheels which convert the axial movement of the gas to a rotary motion. This rotary power is used to drive a powershaft, which drives a propeller or a rotor transmission.

2. LAWS OF MOTION

The theory of gas turbine engines is based on the laws and principles of physics discussed in the subparagraphs that follow.

Newton‘s First Law of Motion. The first law states that a body in a state of rest remains at rest, and a body in motion tends to remain in motion at a constant speed and in a straight line, unless acted upon by some external force.

Newton‘s Second Law of Motion. The second law states that an imbalance of forces on a body produces or tends to produce an acceleration in the direction of the greater force, and the acceleration is directly proportional to the force and inversely proportional to the mass of the body.

Newton‘s Third Law of Motion. The third law states that for every action there is an equal and opposite reaction, and the two are directed along the same straight line.

Bernoulli’s Principle. This principle states that if the velocity of a gas or liquid is increased its pressure will decrease. The opposite is also true. If the velocity of a gas or liquid is decreased its pressure will increase. This fact relates directly to the law of conservation of energy.

Einstein’s Law of Conservation of Energy. This law states that the amount of energy in the universe remains constant. It is not possible to create or destroy energy; however, it may be transformed.

Boyle’s Law. This law states that if the temperature of a confined gas is not changed, the pressure will increase in direct relationship to a decrease in volume. The opposite is also true — the pressure will decrease as the volume is increased. A simple demonstration of how this works may be made with a toy balloon. If you squeeze the balloon, its volume is reduced, and the pressure of air inside the balloon is increased. If you squeeze hard enough, the pressure will burst the balloon.

Charles’ Law. This law states that if a gas under constant pressure is so confined that it may expand, an increase in the temperature will cause an increase in volume. If you hold the inflated balloon over a stove, the increase in temperature will cause the air to expand and, if the heat is sufficiently great, the balloon will burst. Thus, the heat of combustion expands the air available within the combustion chamber of a gas turbine engine.

Pressure and Velocity. Air is normally thought of in relation to its temperature, pressure, and volume. Within a gas turbine engine the air is put into motion so now another factor must be considered, velocity. Consider a constant airflow through a duct. As long as the duct cross-sectional area remains unchanged, air will continue to flow at the same rate (disregard frictional loss). If the cross-sectional area of the duct should become smaller (convergent area), the airflow must increase velocity if it is to continue to flow the same number of pounds per second of airflow (Bernoulli’s Principle). In order to obtain the necessary velocity energy to accomplish this, the air must give up some pressure and temperature energy (law of conservation of energy). The net result of flow through this restriction would be a decrease in pressure and temperature and an increase in velocity. The opposite would be true if air were to flow from a smaller into a larger duct (divergent area); velocity would then decrease, and pressure and temperature would increase. The throat of an automobile carburetor is a good example of the effect of airflow through a restriction (venturi); even on the hottest day the center portion of the carburetor feels cool. Convergent and divergent areas are used throughout a gas turbine engine to control pressure and velocity of the air-gas stream as it flows through the engine.

3. THEORY OF JET PROPULSION

The principle of jet propulsion can be illustrated by a toy balloon. When inflated and the stem is sealed, the pressure is exerted equally on all internal surfaces. Since the force of this internal pressure is balanced there will be no tendency for the balloon to move.

If the stem is released the balloon will move in a direction away from the escaping jet of air. Although the flight of the balloon may appear erratic, it is at all times moving in a direction away from the open stem.

The balloon moves because of an unbalanced condition existing within it. The jet of air does not have to push against the outside atmosphere; it would function better in a vacuum. When the stem area of the balloon is released, a convergent nozzle is created. As the air flows through this area, velocity is increased accompanied by a decrease in air pressure. In addition, an area of skin against which the internal forces had been pushing is removed. On the opposite internal surface of the balloon, an equal area of skin still remains. The higher internal pressure acting on this area moves the balloon in a direction away from the open stem. The flight of the balloon will be of short duration, though, because the air in the balloon is soon gone. If a source of pressurized air were provided, it would be possible to sustain flight of the balloon.

4. THEORY OF GAS TURBINE ENGINES

If the balloon were converted into a length of pipe, and at the forward end an air compressor designed with blades somewhat like a fan were installed, this could provide a means to replenish the air supply within the balloon.

A source of power is now required to turn the compressor. To extend the volume of air, fuel and ignition are introduced and combustion takes place. This greatly expands the volume of gas available.

In the path of the now rapidly expanding gases, another fan or turbine can be placed. As the gases pass through the blades of the turbine, they cause it to rotate at high speed. By connecting the turbine to the compressor, we have a mechanical means to rotate the compressor to replenish the air supply. The gases still possessing energy are discharged to the atmosphere through a nozzle that accelerates the gas stream. The reaction is thrust or movement of the tube away from the escaping gas stream. We now have a simple turbojet engine.

The turbojet engine is a high-speed, high-altitude powerplant. The Army, at present, has no requirement for this type of engine. Because it is simple and easy to operate and maintain, however, the Army does use the gas turbine engine. The simple turbojet engine has primarily one rotating unit, the compressor/turbine assembly. The turbine extracts from the gas stream the energy necessary to rotate the compressor. This furnishes the pressurized air to maintain the engine cycle. Burning the fuel-air mixture provides the stream of hot expanding gas from which approximately 60 percent of the energy is extracted to maintain the engine cycle. Of the total energy development, approximately 40 percent is available to develop useful thrust directly.

If we had ten automobile engines that would equal the total shaft horsepower of a turbine engine, it would take six of these engines to turn the compressor, and the other four would supply the power to propel the aircraft. The amount of energy required to rotate the compressor may at first seem too large; however, it should be remembered that the compressor is accelerating a heavy mass (weight) of air towards the rear of the engine. In order to produce the gas stream, it was necessary to deliver compressed air by a mechanical means to a burner zone. The compressor, being the first rotating unit, is referred to as the N1 system.

With a requirement for an engine that delivers rotational shaft power, the next step is to harness the remaining gas stream energy with another turbine (free turbine). By connecting the turbine to a shaft, rotational power can be delivered to drive an aircraft propeller, a helicopter rotor system, a generator, a tank, an air cushion vehicle (ACV), or whatever is needed. The power shaft can extend from the front, back, or from an external gearbox. All of these locations are in use on various types of Army engines at present.

The following sketch shows a turboshaft engine with the power shaft extended out the front. The bottom sketch shows the same engine with the power shaft extending out the back.

The basic portion of the turbine engine, the gas producer, extracts approximately 60 percent of the gas stream energy (temperature/pressure) to sustain the engine cycle. To develop rotational shaft power, the remaining gas stream energy must drive another turbine. In Army engines today, a power turbine that is free and independent of the gas producer system accomplishes this task. The power turbine and shaft (N2 system) are not mechanically connected to the gas producer (N1 system). It is a free turbine. The gas stream passing across the turbines is the only link between these two systems. The free-turbine engine can operate over wide power ranges with a constant output-shaft speed.

In operation, the gas producer (N1) system automatically varies its speed, thereby controlling the intensity of the gas stream in relation to the load applied to the power (N2) shaft. This is accomplished by a fuel metering system that senses engine requirements. The free turbine design has revolutionized the methods of application of shaft turbine engines. Why a shaft turbine? Why is a perfectly good jet engine used to drive a propeller? Because in the speed range that Army aircraft operate, the propeller or helicopter rotor is more efficient. With a turbojet engine, power (thrust) produced is roughly the difference between the velocity of the air entering the engine and the velocity of the air exiting from the engine. Efficiency of the engine (power producer versus fuel consumed) increases with speed until it is 100 percent efficient when the forward speed of the engine is equal to the rearward speed of the jet. It is this low efficiency at takeoff and at low cruising speed (i.e., 400 mph) that makes the turbojet engine unsuitable for use in Army aircraft. The propeller does not lack efficiency at low speed; the reverse is true, in that efficiency falls off at high speed. The result is to harness the jet engine’s gas stream energy to drive a propeller or helicopter rotor system, thereby taking advantage of the best features of both.

Aircraft reciprocating engines operate on the four-stroke, five-event principle. Four strokes of the piston, two up and two down, are required to provide one power impulse to the crankshaft. Five events take place during these four strokes: the intake, compression, ignition, power, and exhaust events. These events must take place in the cylinder in the sequence given for the engine to operate.

Although the gas turbine engine differs radically in construction from the conventional four-stroke, five-event cycle reciprocating engine, both involve the same basic principle of operation. In the piston (reciprocating) engine, the functions of intake, compression, ignition, combustion, and exhaust all take place in the same cylinder and, therefore, each must completely occupy the chamber during its respective part of the combustion cycle. In the gas turbine engine, a separate section is devoted to each function, and all functions are performed at the same time without interruption.

5. SUMMARY

The theory of gas turbine engine operation is based on the laws or principles of physics. The principle of jet propulsion can be illustrated by a toy balloon. When the balloon is inflated and the stem is unsealed the balloon will move in a direction away from the escaping jet of air. If the balloon is converted into a length of pipe, and at the forward end an air compressor is installed to supply air for combustion, and to expand the volume of air, fuel and ignition are introduced and combustion takes place. Then, in the path of the expanding gases a turbine rotor is installed. As the gases pass through the turbine blades, the turbine rotor is rotated at high speed. This turbine rotor is connected to the compressor shaft, and we now have a means to rotate the compressor to replenish the air supply. The remaining gases are discharged to the atmosphere. The reaction of these gases is thrust, or movement of the tube away from the escaping gases. This is a simple turbojet engine.

In the turbojet engine, approximately 60 percent of the energy is extracted to rotate the compressor, while the remaining 40 percent is used to develop thrust. In the turboshaft engine, the remaining energy is used to drive a turbine rotor attached to a transmission or propeller. On a free-turbine engine, the gas stream passing across the turbines is the only link between the two turbine rotors. One turbine drives the compressor and the other turbine propels the aircraft. The free-turbine engine is used in Army aircraft.

The gas turbine engine differs radically in construction from the reciprocating engine in that the turbine engine has a separate section for each function, while in the reciprocating engine all functions are performed in the same cylinder.

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