Brayton cycle

 

The Brayton cycle is a constant-pressure cycle named after George Brayton (1830–1892), the American engineer who developed it. It is also sometimes known as the Joule cycle. It was originally proposed by Barber in 1791. The Ericsson cycle is also similar but uses external heat and incorporates the use of a regenerator. See Ericsson Cycle.

History

1500 - Leonardo da Vinci drew a sketch of a device, the chimney jack, that rotated due to the effect of hot gases flowing up a chimney. It looked like a device that used hot air to rotate a spit. The hot air came from the fire and rose upward to pass through a series of fan like blades that turned the roasting spit.

1791 - John Barber received the first patent for a heat engine in which a bellows (compressor) and a turbine (expander) were connected to a common shaft. The bellows compressed the ambient air, then the air was passed through a fire and heated, finally the hot air was expanded through a turbine wheel where work was realized. Technically Barber was the inventor of the first gas turbine. His design was planned to use as a method of propelling the 'horseless carriage'.

1872 - Dr. F. Stolze designed a gas turbine engine that used a multistage turbine section and a flow compressor. This engine never ran under its own power. This same year George Brayton 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. Today the term Brayton cycle is generally assciated with the gas turbine even though Brayton never built anything other than piston engines.

1903 - Ægidius Elling of Norway built the first successful gas turbine using both rotary compressors and turbines - the first gas turbine with excess power.

1918 - General Electric company started a gas turbine division. Dr. Stanford A. Moss developed the GE turbosupercharger engine during W.W.I. It used hot exhaust gases from a reciprocating engine to drive a turbine wheel that in turn drove a centrifugal compressor used for supercharging.

1920 - Dr. Alan Arnold Griffith developed a theory of turbine design based on gas flow past airfoils rather than through passages.

1930 - Sir Frank Whittle in England patented a design for a gas turbine for jet propulsion. The first successful use of this engine was in April, 1937. His early work on the theory of gas propulsion was based on the contributions of most of the earlier pioneers of this field.

The specifications of the first jet engine were:

   Airflow = 25 lb/s
   Fuel Consumption = 200 gal/hr or 1300 lb/hr
   Thrust = 1000 lb
   Specific Fuel consumption = 1.3 lb/hr/lb 

1936 - At the same time as Frank Whittle was working in Great Britain, Hans von Ohain and Max Hahn, students in Germany developed and patented their own engine design.

1939 (August) - The aircraft company Heinkel flew the first flight of a gas turbine jet, the Heinkel He 178.

1941 - Sir Frank Whittle designed the first successful turbojet airplane, the Gloster Meteor. Whittle improved his jet engine during the war, and in 1942 he shipped an engine prototype to General Electric in the United States. America's first jet plane was built the following year.

1942 - Dr. Anselm Franz developed the axial-flow turbojet, Junkers Jumo 004, used in the Messerschmitt Me 262, the worlds first operational jet fighter. After W.W.II, the development of jet engines was directed by a number of commercial companies. Jet engines soon became the most popular method of powering airplanes.

Like other internal combustion power cycles, The Brayton cycle is an open system, though for thermodynamic analysis it is conventionally assumed that the exhaust gases are reused in the intake, enabling analysis as a closed system.

Model

A Brayton-type engine consists of three components:

• A gas compressor
• A mixing chamber
• An expander

In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process. The compressed air then runs through a mixing chamber where fuel is added, a constant-pressure isobaric process. 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 theoretically isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement.[

[1]

The term Brayton cycle has more recently been given to the gas turbine engine. This also has three components:

• A gas compressor
• A burner (or combustion chamber)
• An expansion turbine

Ideal Brayton cycle:

• isentropic process - Ambient air is drawn into the compressor, where it is pressurized.
• isobaric process - 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 - 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 is the most direct way to increase the overall power output of a Brayton system. ^[1]

Here are two plots, Figure 1 and Figure 2, for the ideal Brayton cycle. One plot indicates how the cycle efficiency changes with an increase in pressure ratio, while the other 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.

Methods to improve efficiency

The efficiency of a Brayton engine can be improved in the following manners:

• Reheat, wherein the working fluid—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 constraints. (Although use of an afterburner can also be referred to as reheat, it is a different process that increases power while markedly decreasing efficiency.)

• Intercooling, wherein the working fluid passes through a first stage of compressors, then a cooler, then a second stage of compressors before entering the combustion chamber. While this requires an increase in the fuel consumption of the combustion chamber, this allows for a reduction in the specific heat of the fluid entering the second stage of compressors, with an attendant decrease in the amount of work needed for the compression stage overall.

• Regeneration, wherein the still-warm post-turbine fluid is passed through a heat exchanger to pre-heat the fluid just entering the combustion chamber. This allows for lower fuel consumption and less power lost as waste heat.

• A Brayton engine also forms half of the combined cycle system, which combines with a Rankine engine to further increase overall efficiency.

• Cogeneration 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 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.

References

1. ^ Lester C. Lichty, Combustion Engine Processes, 1967, McGraw-Hill, Inc., Lib.of Congress 67-10876

See also

• Gas turbine
• jet engine
• Heat engines
• thermodynamics
• power
• HVAC
• engineering

Thermodynamic cycles
Cycles normally with external combustion
Gas cycles without phasechange - hot air engine cycles
Bell Coleman cycle · Brayton/Joule cycle; (Externally heated) · Carnot cycle  · Stirling cycle  · Pseudo Stirling cycle is same as Adiabatic Stirling cycle [2] [3]  · Ericsson cycle · Stoddard cycle · Ported constant volume cycle [4]  · Vuilleumier cycle
Cycles with phasechange
Kalina cycle · Rankine cycle · Regenerative cycle · Two phased Stirling cycle [5]
Cycles normally with internal combustion
Atkinson cycle · Brayton/Joule cycle · Diesel cycle · Otto cycle · Lenoir cycle · Miller cycle
Cycle mixing
Combined cycle · HEHC cycle [6][7] · Mixed/Dual Cycle
Not categorized
Claude cycle [8] · Fickett-Jacobs cycle · Gifford-McMahon cycle [9] · Hirn cycle  · Humphrey cycle · Linde-Hampson cycle

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

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