A gas turbine engine is an internal-combustion engine that uses gas as the working fluid to turn a turbine. Their efficiency and operational supremacy make them a very popular option for jet aircraft. There are two types of gas turbine engines, classified by two factors: the type of compressor they use, and how the power is utilized. This blog will explain both types of gas turbine engines and their basic operation.
The first type of gas turbine engine is the centrifugal compressor type. In this configuration, the centrifugal compressor draws in air at the center or the eye of the impeller, subsequently accelerating it around and outward. This configuration comprises an impeller, diffuser, and compressor manifold. The diffuser is connected to the manifold and, though they are technically separate parts, this assembly is usually referred to solely as the diffuser. The impeller can be either single or dual entry. The key differences between the two are the size of the impeller and ducting arranged. A single-entry impeller permits ducting directly to the inducer vanes, whereas the dual-entry type needs a far more complicated ducting setup to reach the rear of the entry. Although they are more efficient, single entry impellers must be of greater diameter to intake enough air.
Dual-entry impellers have smaller diameters but rotate at higher speeds to provide sufficient airflow. Most modern gas turbine engines use dual-entry configurations due to the advantages provided by its smaller diameter. A plenum chamber (an enclosure in which air is a pressure greater than outside the enclosure) is also required for dual-entry air compressors, because the air must enter the engine at nearly right angles to the engine axis. In order to provide positive flow, the air must surround the compressor at positive pressure.
The other type of gas turbine engine is the axial-flow compressor type. In this configuration, the air is compressed while it continues in the original direction of flow parallel to the axis of the compressor rotor. The compressor is located at the very front of the engine, where its purpose is to take in ambient air, increase the speed and pressure, and discharge the air through the diffuser into the combustion chamber. The main elements of the axial-flow compressor are the rotor and stator. The rotor is the rotational element of the compressor, while the stator is the fixed element. Both the rotor and stator are enclosed in the compressor case.
The rotor features fixed blades that force the air backward, similar to the operation of an aircraft propeller. In front of the first rotor stage are the inlet guide vanes (IGVs), which direct the intake air toward the first set of rotor blades. Behind each stage of rotors is a stator directing the air back to the subsequent rotor stage. Each pair of rotor and stator blades constitutes a pressure stage. The action of the rotor increases air compression at each stage, accelerating the air further backward. Through this increased velocity, energy is transferred from the compressor to the air in the form of velocity. The number of stages is dependent on the pressure and sheer amount of air needed. Most modern engines feature eight to sixteen engines.
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