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Brayton Cycle

Last Update:

03/06/2016

Introduction

Gas turbines work on a thermodynamic cycle termed Brayton cycle after George Brayton. This fundamental cycle now powers jet engines used in air crafts, helicopters and submarines and turbines used in power generation.

History

  • George Brayton, an American Mechanical Engineer, is credited with the development of the Brayton cycle engine.
  • He first patented a constant pressure engine in 1872 he called the ‘Ready Motor’. This was for a rudimentary engine using two cylinders arranged to allow continuous burning of the fuel-air mixture.
  • It was followed by a patent in 1874 for a liquid fuel injection system that created a constant pressure cycle. The design also overcame issues of ignition since the continuous burning maintained a pilot flame.
  • Most of the Ready Motors were used for stationary purposes like water pumping and for propulsion of marine vessels.
  • His designs were used by John Philip Holland in 1884 to power Fienian Ram, a submarine.

While Brayton implemented the theory in practical, workable designs, the idea was not new since John Barber, an Englishman, patented a design as far back as 1791.

 

Types

  • The original Brayton engine design has a compressor, a mixing chamber and an expander. Air is drawn in by a piston compressor with the compressed air forced into the mixing chamber where it mixes with the fuel. The pressurized fuel-air mixture is ignited in an expansion chamber to drive a piston in a cylinder that also transfers part power to the compression piston.
  • The piston-cylinder engine had inherent drawbacks but the same principle was modified and was successful in a gas turbine arrangement comprised of a gas compressor, a burner and an expansion turbine. The Brayton cycle based gas turbine may be of two types.

Open Gas Turbine Cycle Engine:  In which the exhaust gases are expelled into the atmosphere.

Closed Loop Brayton Cycle: This is also known as the ideal Brayton cycle that is achieved with a closed loop configuration in which, the exhaust gases are partly recycled through a heat-exchanger and is fed back to the compressor. This type of set up is common in power generation stations and pumping stations.

Reversed Brayton Cycle:  This engine is driven in reverse through a network to expel the cool air. This principle works in air conditioners and air refrigeration cycle that works to remove heat and cool the atmosphere. 

 

Parts

The main parts of gas turbines operating on the Brayton cycle comprise of:

  • A compressor at the front to progressively compress air to over 8 atmospheric pressures
  • A mixing or a combustion chamber where fuel is sprayed into the compressed air and the compressed air-fuel is ignited by the pilot flame
  • An expander with multi-bladed turbine.
  • A common rail shaft to which the turbine and compressor are connected

The nozzle through which exhaust gases flow out and provide thrust to the aircraft.

 

How It Works

The Brayton cycle works in four stages in a typical jet engine operating on this principle:

  • Air is drawn into the compressor from the atmosphere and it is compressed in an adiabatic process to achieve a pressure of over 8 atmospheres. This pressured air is then pushed to the fuel mixing and combustion chamber.
  • The compressed fuel-air mixture is ignited by a pilot flame in the combustion chamber at constant pressure. This is the isobaric process of heat addition.
  • The resulting burning of fuel-air results in exhaust gas that is at a far higher temperature and higher pressure than the inlet. This is the adiabatic process of expansion.
  • The final stage is that of heat rejection, which is also known as isobaric process. The burnt gases pass through the exhaust and turn a multi-bladed turbine. It is usually connected to the compressor through a common shaft that partially transfers the energy to turn the compressor fans while the exhaust gases pass through the nozzle at high pressure, creating a thrust to propel the aircraft.
  • The energy may also be used for liquid jet propulsion in marine vessels or to drive a shaft connected to a dynamo in power generation equipment.

The combustion process is continuous in the brayton cycle, unlike the piston based Otto-cycle engines. In an open cycle Brayton process, air is taken in and expelled from the nozzle and expanded to ambient conditions. In a closed cycle the fluid or gas is recirculated in a loop through a heat exchanger and sent back to the compressor.

Effectiveness and Challenges

  • The Brayton cycle is a continuous process, delivering a far more consistent power output in a small size package than a similarly sized piston engine with a significantly higher power to weight ratio.
  • Gas turbine powered aircraft working the Brayton cycle can operate at higher altitudes and can fly faster.
  • There are fewer moving parts in a Brayton cycle engine and combustion efficiency is also higher.
  • The Brayton cycle can use a variety of fuels like Kerosene, oil, petrol and alcohol.
  • In jet aircrafts the air from the engine compressor can be used to power air conditioners in a reverse Brayton cycle. The Brayton cycle also works in ramjets where combustion does not depend on atmospheric oxygen.

The challenges of the Brayton Cycle Engines:-

  • The drawback is that such engines produced a high pitched, high decibel whine
  • All parts, especially the turbine, must be made of special, high grade, expensive metal alloys that increases cost of the engine. Replacing parts is also an expensive affair.
  • There is a limit to maximum temperatures and pressures that can be generated, dictated by the ability of the turbine blades to withstand the stresses. The challenge is in the compressor and the turbine design to allow for greater efficiencies and also the combustion process to realize fuel economies regardless of engine RPM.

 

Maintenance

These Engines are high-grade equipments which are inspected regularly for maintenance requirement that is performed only by an authorized personnel. Some of the usual wear and tear are -

  • Air compressor blades can develop faults and wear out, resulting in reduced compression efficiency and pressure and need to be checked and replaced.
  • Turbine blades are subject to high pressure and high temperature and are susceptible to stress related failures. The shaft is another area that requires regular checks.

 

 

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