Also known as a combustion turbine, Gas turbines are a type of continuous internal combustion engine that is constantly rotating to compress natural gas into electrical and mechanical energy. Gas turbines have been in the face of the world since 1995 when Siemens became the first manufacturer of large electricity producing gas turbines. In 2011 Mitsubishi tested their first combined cycle gas with more than 60% efficiency.
In 2020, the global gas turbine market size was valued at $20.38 billion and by the year 2028, the compound annual growth rate or CAGR is expected to be around 6.8%. Before we learn more about the types of gas turbines, let us understand how it works.
How does a gas turbine function?
In theory, ideally, gas turbines undergo four processes of thermodynamics, they are:
- Isentropic compression
- Isobaric or constant pressure combustion,
- Isentropic expansion
- Heat rejection
When these are combined together, they form the Brayton cycle.
However, realistically, in a gas turbine, the mechanical energy is irreversibly changed into thermal energy and pressure due to the compression of gas. The combustion chamber is then heated and the specific volume of the gas increases and there is a loss of pressure. This process is repeated and fresh air is provided instead of heat rejection.
When the air is compressed the temperature rises and so does the pressure. The fuel is injected and due to the heat and pressure, it starts to burn. When it does, it produces chemical energy. The hot gas now comes in contact with the blades of the turbine and thus it starts to rotate at a very high speed and hence produces mechanical energy. After this, the mechanical energy is then converted to electrical energy by passing it through the shaft of the generator through the gearbox. This generator consists of a large magnet with coils of copper wire surrounded around it. When this magnet starts rotating at a great speed, a magnetic field is created that causes movement in electrons which causes electricity to be produced. The energy that is formed as a by-product can be used to run a steam turbine.
Types of gas turbines
There are different types of gas turbines with different uses. Each gas turbine works on the same principle of the Brayton Cycle and converts chemical energy into mechanical and electrical energy. A detailed description of their types and the working principle is explained below.
Turbojet – Developed by the Germans during the Second World War, turbojet is the simplest of all gas turbines. With engines having limited range and endurance, the air is passed at a high speed into the combustion chamber which has the fuel inlet and igniters. However, it has high fuel consumption. Nevertheless, the turbine expands air and that causes thrusts from the accelerated exhaust gases. These types of engines require regular maintenance.
Turboprop– These gas turbines are the most commonly used in small aircraft, cargo planes, and agriculture. This is because they do not require much fuel to operate. These engines perform through reduction gear which provides propeller performance at its optimum at a slower rpm speed. These propellers are less efficient with increasing speed thus it is ideal for agricultural use since those planes maintain consistency in speed.
Turbofan – Combining the features of turbojet and turboprop, turbofan is considered ideal. The turbofan is a turbine used in aircraft that diverts a flow of air for the second time around the combustion chamber and thus it creates an additional thrust. This is the latest version of the aircraft turbine and can be used to manure high-speed aircraft like commercial transportation and fighter jets. Turbofan engines can be attached to turbojet engines with a duct fan which in turn helps reduce the engine output noise and give it an extra push and also helps to cool the engine
Afterburning turbojet – Normally used in fighter jets and planes which require a lot of high power and speed. After afterburning turbojets use an afterburner in the core of the turbojet thus providing some of the energy from the exhaust that will help the turbine to turn. When the afterburner is turned on, an additional amount of fuel is injected into the exhaust streams which produces additional thrust and helps provide support when the engine needs to increase its speed. One drawback of this kind of engine is that it burns more fuel.
In this article, we learnt the different types of gas turbines and their uses when it comes to aircraft. Gas turbines have been providing us with the best service ever since their invention and it does not seem to let us down no matter much, we increase the speed. The market for gas turbines is increasing day by day and that is because of its potential to help in transportation and agriculture in various ways.