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Basic concept of gas turbine


Gas turbine is a kind of internal combustion engine that extracts power directly from the combusted hot gas. The main difference from a steam turbine is that the power is directly derived from the combusted gas, not from the steam boiled by the combusted gas.
Diesel engines and gasoline engines are well known, commonly used around us and gas turbine engines may not be familiar to us. But a well known jet engine is a kind of gas turbine and most helicopters and propeller planes are powered by gas turbines except for some small planes. Other vehicles such as Jetfoils, high speed ships,  most worships such as destroyers and cruisers are propelled by gas turbine engines. In the power generation, a gas turbine is a main power source especially in the emergency purpose and the large power plant. Recently cogeneration systems to supply heat and power are spreading in convenience stores and hospitals and gas turbines are becoming more and more familiar to  us.

How it works

A gas turbine requires the very precise manufacturing process, but its basic component is simple.

 The schema above shows three major componets, the compressor, the combustor and the turbine.  The compressor is usually consists of the core shaped cylinder with many fan blades attached in rows The high speed rotation of these fan blades compresses the air and send it to the combustor. In the combustor, fuel is injected in to the compressed air and  ignited. Then combusted high pressure hot gases enter into the turbine section, spin the turbine and the exhausted gases are released into the atmosphere. The compressor and the turbine are directly coupled and turn as the single unit.
The most part of the power derived from the turbine is used to spin the compressor and the residual power can be used to drive a generator or a pump. In a jet engine, residual power is used as a thrust .
As this gas turbine has single shaft in one engine, it is called "single shaft gas turbine engine". This type of gas turbine cannot spin at the low speed, because the low speed rotation of the compressor cannot produce the high pressure compressed air, resulting in the combustion failure and the engine stall.  So single shaft gas turbine is usually applied to the power generation, where the variable speed is not required.

In the next schema, a turbine is separated into two part, each can spin independently.  The compressor and the left turbine turn as a single unit and the right turbine turns freely.

Therefore, the output shaft can stop while the engine is running and can start at the zero speed, can turn at any speed. This gas turbine has two separated shafts and then called "two shaft gas turbine" or "free shaft gas turbine". Even if the output shaft is stopped, the gas generator (compressor-turbine unit) can spin at full speed, high pressure gases are continuously supplied and the high torque is produced. This type of gas turbine has high torque at low speed and comparably high efficiency at low speed, so it is suitable for propelling cars and trains.