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Gas Turbines

Last Update:

09/06/2016

Introduction

Gas turbines have changed the world. From powering aircraft for fast travel halfway across the world to providing power that lights up the world, gas turbines are simply indispensable.

 

History

  • The basic principle of gas turbines has been known to man ever since centuries. The Chinese used it as far back as the 10th century to project an image of a horse- riding figures. Leonardo da Vinci, sometime around the 16th century prepared a drawing of a rudimentary gas turbine.
  • Around 1678 Ferdinand Verbiest used steam power jet to draw a carriage. John Barber patented a forerunner of the gas turbine in 1791.
  • Sir Charles Parsons built a steam turbine powered ship, The Turbinia in 1894, which were installed in Cambridge power station and Aegidius Elling, a Norwegian, built a gas turbine version that gave an output of 11 HP in 1903. However, all these never really got off the ground.
  • It was left to Sir Frank Whittle, an RAF officer and pilot as well as an engineer, to design and build a centrifugal gas turbine used for Jet propulsion in 1930, which was patented and successfully deployed first in 1937.
  • A Gloster E28 powered by the Whittle turbojet took to the skies in 1941. However, his designs were copied in USA and Germany, where Hans Von Ohain beat him in the race to build the first jet powered aircraft, the Heinkel HE 178 that flew in 1939.
  • WWII gave an impetus to further develop the gas turbines for the aircraft and since then different types were developed for use in aircraft even as their use spread in power generation.

 

Types

Gas turbines can be broadly categorized into engines that are used to provide locomotive power and engines that are used for power generation and mechanical drive.

There are four main types of gas turbines used in aviation.

  • TurboJet Engine: One is the turbojet in which air is sucked in by a compressor and drawn into the combustion chamber where fuel is injected and the mixture is ignited. The resulting pressure of exhaust gases causes thrust to propel an aircraft forward.
  • AfterburningTurbojet: The afterburning turbojet has an afterburner located at the exhaust side where more fuel is injected into the exhaust flow that, on burning, provides more thrust and power at the expense of fuel economy. This type of engine is used on military aircraft.
  • TurboProp Engine: The turboprop uses the same basic configuration, but the engine drives the propellers at the front to provide lift and forward momentum to aircraft. This type of engine is generally used in smaller craft traveling at medium speeds where fuel efficiency is important.
  • Turbofan: Turbofan gas turbines have a large fan at the front that blows in air, part of which goes to the combustion chamber and part of it over and around the chamber directly to the exhaust to provide more thrust. This type of engine is in widespread use in general commercial aircrafts.

Other than these four there are also aeroderivative gas turbines used in electrical power generation. There are also different Industrial gas turbines that assist in power generation and mechanical drives. Apart from these, Turboshaft engines that power pumps used in petroleum drilling which also works on a similar principle.

Main parts

The basic gas turbine engine is simple in construction:

  • At the front is a cone shaped compressor with fan blades arranged in rows to progressively increase pressure as air is drawn into the combustion chamber. This is sequenced as a collection of stages, each with its own set of rotating blades and stationary blades to bring about desired compression.
  • The combustion chamber where air is sucked in and mixed with fuel spray to cause ignition.
  • The combustion section has a casing, a flame tube and a fuel injection system.
  • The turbine section at the rear consisting of multiple vanes that rotate as the force of the exhausting gases are applied while exiting through the nozzle at the rear.
  • The entire assembly of a gas turbine and compressor is mounted on a single shaft.
  • A variation of this design separates both, the turbine and the compressor  from the shaft.

 

How It Works

The compressor draws in air and compresses it to about 8 atmospheric pressures. This compressed air is fed into the combustion chamber.

  • Fuel is sprayed into the combustion chamber. The flame tube provides ignition and the fuel-air mixture ignites. The resulting expanded hot gases pass over the turbine rotors.
  • Turbine blades rotate due to the force of the exhaust gases. The pressure is several times that of the inlet pressure. The turbine rotates the shaft that also drives the compressor to continue the cycle.
  • Exhaust gases are expelled at much higher pressure and this pressure generates thrust to propel an aircraft forwards.

Effectiveness and challenges

  • Gas turbine engines are relatively simpler in comparison to piston engines and have fewer moving parts.
  • They have a high power to weight ratio that translates to higher speeds in aircrafts or higher force in power generation systems. Hence the engine efficiency is better as even at lower engine speed of 40% about 60% torque is maintained.
  • A gas turbine can process a higher amount of air and fuel mixtures than a piston-cylinder combination in the same size.
  • Piston engines use only petrol or diesel, whereas the gas turbines can use a variety of fuel sources such as aviation kerosene, petrol, light oil, natural gas, hydrogen or a mix of alcohol and petrol. It consumes comparatively lesser lubricating oil and there is no need for a large cooling system.
  • Gas turbines also have lower vibrations and are also easier to start in cold conditions.

Challenges:

  • The disadvantages are that gas turbines consume a large amount of fuel required to power huge vehicles. The same is quite visible at lower loads and while idling.
  • Due to specific kind of materials used during construction, gas turbines are expensive to manufacture.

Gas turbines also produce a high pitched, high decibel whine that can be damaging at close quarters.

Maintenance

  • Since gas turbines used in aircraft are critical to maintain, they routinely undergo tests every flight cycle for lubrication level and engine conditions, which is perfomed by only authorized personnel.
  • Visual inspection and sound checks indicate any abnormalities in the engine.
  • Gas turbines used in stationary power generation equipment or for pumping need to have their air filters checked and cleaned at regular intervals.
  • Surface tests as well as penetrant tests are carried out to check for cracks or defects.

 

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