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Of gas turbines and engines
The idea of the gas turbine was born in 150 BC. This was when an Egyptian called “Hero” invented a rotating toy. When the toy was placed on a boiling pan, the emitting gas or steam caused the spinning effect. Hero named his invention “aeolipile”.
But the first real concept of the steam turbine came from Italy in the 16th century. Leonardo da Vinci sketched the device whose object was to rotate the grilling meat. The device was supposed to get power from hot gases steaming up the chimney.
However, it was Giovanni Branca, another Italian, who actually invented steam turbine in 1629. He devised a system that utilised jets of steam to give rotation to a turbine, which was to make any machinery operative.
In recent times, however, the interest of scientists towards jet engines is more than 150 years old. The year 1930 is very rightly commemorated as a milestone in this regard: this year Sir Frank Whittle, an officer of the Royal Air Force, was awarded a patent for his jet engine.
A gas turbine is a sort of jet engine. A jet engine is a machine that gets power by converting chemical energy into kinetic energy. Jet engines in their fundamental forms (such as a rocket) are less efficient and are only useful for a short-term use. For greater efficiencies, they need to be utilised at higher speeds.
Reaching higher speeds is a problem, though. Therefore, the key factor that caused the development of gas turbines was to make jet engines more efficient.
Types of gas turbines
Though the gas turbine engines have several types, they are classified into two emblematic categories: aero or industrial. Aero are the engines which are developed for planes, with the inclination to keep them light in weight. But at the same time these engines are less rugged. Industrial engines, on the other hand, are motors that are exclusively built to serve in the industry. They are cheaper and hence more popular for industrial use.
Largely, the aircraft engines lack a means of starting on their own. The industrial engines are designed with the consideration to make them heavier, more powerful and robust. They usually have a built-in self-starting capacity.
Applications
Gas turbine engines may fall into two distinctive categories, according to their handling. In the aircraft engines as a primary application, the jet is utilised directly to propel the craft along, with the ejection of high-velocity gases, or in an indirect mode it uses the power produced to drive a propeller, as we see in a turbo-prop plane. Gas turbine engines, by producing power supply, are substantiating sustenance to the industry.
A gas turbine, which is also designated as a combustion turbine, is actually a rotary engine. It may also be identified as the turbine element. This contraption extorts energy from a current of burning gas and is equipped with an upstream compressor, besides being attached to a downstream turbine. A combustion chamber finds space between the two parts.
In the combustor (combustion chamber and its injectors, igniters, and other associated devices in a jet engine or gas turbine) energy is added to gas stream which gets mixed with fuel for ignition reasons. Combustion results in amplifying the temperature, volume and velocity of gas flow.
This emission of gases is directed towards the turbine blades, by using a nozzle. When the gases strike the turbine blades, causing the turbine to rotate, it ultimately powers up the compressor. The energy that results due to this process may be in any shape like shaft power, compressed air, or thrust, and employed to power tanks, aircraft, ships and generators.
Air intake
This provides air to the turbine. The air is cleaned on its way to turbine blades, to protect them from dust and impurities. This is essential since accumulation of dust and impurities on the blade would cause the speed to reduce and will ultimately result in power losses.
To get the maximum efficiency out of a turbine, a compressor is required to compress the air. In spite of compression, the maximum efficiency that can be realised from the turbine is a mere 35 per cent — a 5 per cent loss is due to friction and the remaining 60 per cent is consumed by the compressor itself.
The air compressors are of two types: centrifugal flow compressors and axial flow compressors. The centrifugal flow compressors have an impeller to accelerate the air, whereas the axial flow compressors have alternating rows of rotating (rotor) blades, which propel the air. They also have stationary (stator) vanes, which cause a diffusion of the air, thus causing an increase in the pressure until it gets to the desired pressure level. Therefore, the centrifugal compressors are simpler and more powerful. However, the axial flow compressors are more common since they can generate greater pressures.
The air from the compressor is directed towards the combustion chambers for ignition purposes. To decrease the running overheads and to minimise pollution, the fuel needs to be burnt as proficiently as possible.
With the intention of achieving requisite results, the fuel is atomised (reducing the fuel to a fine spray so that each molecule can come in contact with oxygen) and burnt at as high a pressure and temperature as is practically convenient. Since too much pressure can extinguish the flame, an upper limit is made mandatory.
All this warrants extra care as excessive heat may damage the turbine. The maximum temperature of the exhaust gases should be somewhere between 450 to 500 C and the operator needs to make sure that burning is taking place in all the combustion chambers.
The exhaust gases, leaving the combustion chambers, drive the turbine blades. The turbine blades serve two objectives. The primary reason is to power the compressor and the secondary one is to generate power. Aero-engines are contrived to have the blades set so that power generation is much smaller.
Actually, a large part of the useful power output is used to give thrust and a very small share of it is used to produce other forms of energy. Contrary to this, the industrial turbines have both sets of blades of the same size, plus an option of utilising power to run other types of machinery.
The writer cocographer@yahoo.com is a senior instructor at a technical college
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