How Gas Turbine Engines Work
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Definition
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A gas turbine engine ejects air at a high velocity through a nozzle to generate thrust. To obtain that high velocity, the air typically passes through a diffuser, a compressor, a combustion chamber and a turbine to add energy to the flow before entering the nozzle. Although the use of a gas turbine engine is not limited to aerospace applications, the turbojet and turbofan engines are the most common types of gas turbine engine.
Diffuser
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A diffuser is a device designed to reduce airflow velocity. The cross-sectional area of the diffuser at its exit is larger than the cross-sectional area at the entrance. Because mass must be conserved, the mass flow rate, defined as density multiplied by area multiplied by velocity, must be constant throughout the diffuser. This means cross sectional area has an inverse proportional relationship with velocity. As area increases, velocity decreases. This decrease in velocity lowers the dynamic pressure, and since Bernoulli's equation states that the total pressure--the sum of dynamic and static pressure--is constant, the static pressure is increased.
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Compressor
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The diffuser opens directly into the compressor, which increases the total pressure of the flow. Usually, this will be a multistage axial compressor, where the blades of the compressor rotate around a central axis. Some smaller or older designs, however, use a centrifugal compressor, which must turn the airflow up to 90 degrees. Compression efficiency is a large part of gas turbine design because the compressor requires the most energy to operate.
Combustion Chamber
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After the air is compressed, the combustion chamber raises the total pressure by raising the temperature of the flow dramatically. According to the perfect gas equation, pressure is proportional to temperature. Fuel flows into the airflow and ignites, which raises the temperature to values around 1,940 degrees F (1,060 degrees C).
Turbine
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The flow loses energy after it leaves the combustion chamber. The next, which looks much like an axial compressor with fewer sets of blades, is the turbine. The blades take energy from the flow and transmit it to a shaft that drives the compressor. This is why compression efficiency is so important. The more energy it takes to drive the compressor, the less energy the flow will have to generate thrust.
Nozzle
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A nozzle is the opposite of a diffuser. As the flow approaches the outlet, the cross sectional area decreases, which creates an increase in velocity. This velocity, along with the accompanying pressure change, is what creates thrust in a gas turbine engine.
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