Brayton cycle
Overview
 
The Brayton cycle is a thermodynamic cycle
Thermodynamic cycle
A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...

 that describes the workings of the gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 engine, basis of the airbreathing jet engine
Airbreathing jet engine
An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....

 and others. It is named after George Brayton
George Brayton
George Brayton was born in Rhode Island, son of William H. and Minerva Brayton. He was an American mechanical engineer who lived with his family in Boston, and who is noted for introducing the continuous combustion process that is the basis for the gas turbine, and which is now referred to as...

 (1830–1892), the American engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...

 who developed it, although it was originally proposed and patented by Englishman John Barber
John Barber (engineer)
John Barber was an English coalmaster and inventor. He was born in Nottinghamshire, but moved to Warwickshire in the 1760s to manage collieries in the Nuneaton area. For a time he lived in Camp Hill House, between Hartshill and Nuneaton, and later lived in Attleborough...

 in 1791. It is also sometimes known as the Joule
James Prescott Joule
James Prescott Joule FRS was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work . This led to the theory of conservation of energy, which led to the development of the first law of thermodynamics. The...

 cycle
. The Ericsson cycle
Ericsson cycle
The Ericsson cycle is named after inventor John Ericsson, who designed and built many unique heat engines based on various thermodynamic cycles. He is credited with inventing two unique heat engine cycles and developing practical engines based on these cycles...

 is similar but uses external heat and incorporates the use of a regenerator.
In 1872, George Brayton
George Brayton
George Brayton was born in Rhode Island, son of William H. and Minerva Brayton. He was an American mechanical engineer who lived with his family in Boston, and who is noted for introducing the continuous combustion process that is the basis for the gas turbine, and which is now referred to as...

 applied for a patent for his Ready Motor.
Encyclopedia
The Brayton cycle is a thermodynamic cycle
Thermodynamic cycle
A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...

 that describes the workings of the gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 engine, basis of the airbreathing jet engine
Airbreathing jet engine
An airbreathing jet engine is a jet engine propelled by a jet of hot exhaust gases formed from air that is drawn into the engine via an inlet duct....

 and others. It is named after George Brayton
George Brayton
George Brayton was born in Rhode Island, son of William H. and Minerva Brayton. He was an American mechanical engineer who lived with his family in Boston, and who is noted for introducing the continuous combustion process that is the basis for the gas turbine, and which is now referred to as...

 (1830–1892), the American engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...

 who developed it, although it was originally proposed and patented by Englishman John Barber
John Barber (engineer)
John Barber was an English coalmaster and inventor. He was born in Nottinghamshire, but moved to Warwickshire in the 1760s to manage collieries in the Nuneaton area. For a time he lived in Camp Hill House, between Hartshill and Nuneaton, and later lived in Attleborough...

 in 1791. It is also sometimes known as the Joule
James Prescott Joule
James Prescott Joule FRS was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work . This led to the theory of conservation of energy, which led to the development of the first law of thermodynamics. The...

 cycle
. The Ericsson cycle
Ericsson cycle
The Ericsson cycle is named after inventor John Ericsson, who designed and built many unique heat engines based on various thermodynamic cycles. He is credited with inventing two unique heat engine cycles and developing practical engines based on these cycles...

 is similar but uses external heat and incorporates the use of a regenerator.

History

In 1872, George Brayton
George Brayton
George Brayton was born in Rhode Island, son of William H. and Minerva Brayton. He was an American mechanical engineer who lived with his family in Boston, and who is noted for introducing the continuous combustion process that is the basis for the gas turbine, and which is now referred to as...

 applied for a patent for his Ready Motor. The engine used a separate piston compressor and expander. The compressed air was heated by internal fire as it entered the expander cylinder. Brayton produced and sold "Ready Motors" to perform a variety of tasks like water pumping, mill operation, even marine propulsion.

Critics of the day claimed the engines ran smoothly and had an efficiency of about 17%.

Today the term Brayton cycle is generally associated with the gas turbine, even though Brayton only built piston engines.

The Brayton cycle is a cycle which can be used in both internal combustion engines (such as jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

s) and for external combustion engines.

Although the Brayton cycle is usually run as an open system (and indeed must be run as such if internal combustion is used), it is conventionally assumed for the purposes of thermodynamic
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 analysis that the exhaust gases are reused in the intake, enabling analysis as a closed system.

Another interesting piece of Brayton cycle history was its use in the Selden patent. In the early days of the automobile, a creative attorney "Selden" claimed to have a patent for the internal combustion powered version. The patent drawings showed the use of Brayton cycle engine. Instead of paying royalties, Henry Ford
Henry Ford
Henry Ford was an American industrialist, the founder of the Ford Motor Company, and sponsor of the development of the assembly line technique of mass production. His introduction of the Model T automobile revolutionized transportation and American industry...

 fought the Selden patent. Ford argued his cars used the four-stroke Otto cycle
Otto cycle
An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....

 and not the Brayton engine shown used in the Selden auto. Ford won the appeal of the original case.

Model

A Brayton-type engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 consists of three components:
  • A gas compressor
    Gas compressor
    A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

  • A mixing chamber
  • An expander
    Turboexpander
    A turboexpander, also referred to as a turbo-expander or an expansion turbine, is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce work that is often used to drive a compressor....



In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process
Isentropic process
In thermodynamics, an isentropic process or isoentropic process is one in which for purposes of engineering analysis and calculation, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the...

. The compressed air then runs through a mixing chamber where fuel is added, an isobaric process
Isobaric process
An isobaric process is a thermodynamic process in which the pressure stays constant. The term derives from the Greek isos, , and barus,...

. The heated (by compression), pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder; another ideally isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement.

The term Brayton cycle has more recently been given to the gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 engine. This also has three components:
  • A gas compressor
  • A burner (or combustion
    Combustion
    Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

     chamber)
  • An expansion turbine


Ideal Brayton cycle:
  • isentropic process
    Isentropic process
    In thermodynamics, an isentropic process or isoentropic process is one in which for purposes of engineering analysis and calculation, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the...

     - Ambient air is drawn into the compressor, where it is pressurized.
  • isobaric process
    Isobaric process
    An isobaric process is a thermodynamic process in which the pressure stays constant. The term derives from the Greek isos, , and barus,...

     - The compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out.
  • isentropic process - The heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor.
  • isobaric process - Heat rejection (in the atmosphere).


Actual Brayton cycle:
  • adiabatic process
    Adiabatic process
    In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...

     - Compression.
  • isobaric process - Heat addition.
  • adiabatic process - Expansion.
  • isobaric process - Heat rejection.


Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable working inefficiencies. In general, increasing the compression ratio
Compression ratio
The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...

 is the most direct way to increase the overall power output of a Brayton system.

The efficiency of the ideal Brayton cycle is , where is the heat capacity ratio
Heat capacity ratio
The heat capacity ratio or adiabatic index or ratio of specific heats, is the ratio of the heat capacity at constant pressure to heat capacity at constant volume . It is sometimes also known as the isentropic expansion factor and is denoted by \gamma or \kappa . The latter symbol kappa is...

. Figure 1 indicates how the cycle efficiency changes with an increase in pressure ratio. Figure 2 indicates how the specific power output changes with an increase in the gas turbine inlet temperature for two different pressure ratio values.

In 2002 a hybrid open solar Brayton cycle was operated for the first time consistently and effectively with relevant papers published, in the frame of the EU SOLGATE program.
The air was heated from 570 K to over 1000 K into the combustor chamber.
Further hybridization was achieved during the EU Solhyco project running a hybridized Brayton cycle with solar energy and Biodiesel only.

Methods to increase power

The power output of a Brayton engine can be improved in the following manners:
  • Reheat, wherein the working fluid
    Working fluid
    A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder...

    —in most cases air—expands through a series of turbines, then is passed through a second combustion chamber before expanding to ambient pressure through a final set of turbines. This has the advantage of increasing the power output possible for a given compression ratio without exceeding any metallurgical
    Metallurgy
    Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...

     constraints (typically about 1000 °C). The use of an afterburner
    AfterBurner
    The AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs.Originally, portablemonopoly.net was a website created to petition Nintendo to put some kind of light in their Game Boy Advance system...

     for jet aircraft engines can also be referred to as "reheat"; it is a different process in that the reheated air is expanded through a thrust nozzle rather than a turbine. The metallurgical constraints are somewhat alleviated, enabling much higher reheat temperatures (about 2000 °C). Reheat is most often used to improve the specific power (per throughput of air), and is usually associated with a reduction in efficiency; this is most pronounced with the use of afterburners due to the extreme amounts of extra fuel used.

  • Overspray, wherein, after a first compressor stage, water is injected into the compressor, thus increasing the mass-flow inside the compressor, increasing the turbine output power significantly and reducing compressor outlet temperatures. In a second compressor stage the water gets completely evaporated.

Methods to improve efficiency

The efficiency of a Brayton engine can be improved in the following manners:
  • Increasing pressure ratio - As Figure 1 above shows, increasing the pressure ratio increases the efficiency of the Brayton cycle. This is analogous to the increase of efficiency seen in the Otto cycle when the compression ratio is increased. However, there are practical limits when it comes to increasing the pressure ratio. First of all, increasing the pressure ratio increases the compressor discharge temperature. This can cause the temperature of the gasses leaving the combustor to exceed the metallurgical limits of the turbine. Also, the diameter of the compressor blades becomes progressively smaller in higher pressure stages of the compressor. Because the gap between the blades and the engine casing increases in size as a percentage of the compressor blade height as the blades get smaller in diameter, a greater percentage of the compressed air can leak back past the blades in higher pressure stages. This causes a drop in compressor efficiency, and is most likely to occur in smaller gas turbines (since blades are inherently smaller to begin with). Finally, as can be seen in Figure 1, the efficiency levels off as pressure ratio increases. Hence, there is little to gain by increasing the pressure ratio further if it is already at a high level.

  • Regeneration, wherein the still-warm post-turbine fluid is passed through a heat exchanger to preheat the fluid just entering the combustion chamber. This directly offsets fuel consumption for the same operating conditions, improving efficiency; it also results in less power lost as waste heat. However, at higher pressure ratios, the compressor discharge temperature can exceed the exhaust temperature. Under these conditions, regeneration would be counterproductive. Therefore, regeneration is only an option when the pressure ratio is sufficiently low that the exhaust temperature is higher than the compressor discharge temperature.

This feature is only available if the exhaust heat is not used otherwise, as in cogeneration or combined cycle applications.
  • A Brayton engine also forms half of the combined cycle
    Combined cycle
    In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...

    system, which combines with a Rankine engine
    Rankine cycle
    The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...

     to further increase overall efficiency. However, although this increases overall efficiency, it does not actually increase the efficiency of the Brayton cycle itself.

  • Cogeneration
    Cogeneration
    Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....

    systems make use of the waste heat from Brayton engines, typically for hot water production or space heating.

Reverse Brayton cycle

A Brayton cycle that is driven in reverse, via net work input, and when air is the working fluid, is the air refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 cycle
or Bell Coleman cycle. Its purpose is to move heat, rather than produce work. This air cooling technique is used widely in jet aircraft.

See also

  • Gas turbine
    Gas turbine
    A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

  • Jet engine
    Jet engine
    A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

  • Heat engines
  • Thermodynamics
    Thermodynamics
    Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

  • Power
    Power (physics)
    In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

  • HVAC
    HVAC
    HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

  • Engineering
    Engineering
    Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

  • Gerotor
    Gerotor
    A gerotor is a positive displacement pumping unit. The name gerotor is derived from "Generated Rotor". A gerotor unit consists of an inner and outer rotor. The inner rotor has N teeth, and the outer rotor has N+1 teeth. The inner rotor is located off-center and both rotors rotate...


External links

  • Today in Science article on Brayton Engine
  • http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JSEEDO000126000003000872000001&idtype=cvips&gifs=yes
  • http://elib.dlr.de/46328/
  • http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V50-4GP6WDN-1&_user=10&_coverDate=10%2F31%2F2006&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b7c7869ea69813a7397758263df4667c
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK