Brayton cycle

The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the jet engine and others. It is named after George Brayton (1830–1892), the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber in 1791.^[1] It is also sometimes known as the Joule cycle. The Ericsson cycle is similar but uses external heat and incorporates the use of a regenerator.

History

In 1872 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 associated with the gas turbine even though Brayton never built anything other than piston engines.

The Brayton cycle is the only thermodynamic cycle which can be used in both internal combustion engines (such as jet engines) 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 analysis 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 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 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. ^[2]

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.

Figure 1: Brayton cycle efficiency

Figure 2: Brayton cycle specific power output

Methods to increase power

The power output 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 (typically about 1000°C). The use of an afterburner 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). The use of 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.

Methods to improve efficiency

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

• 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 volume of the fluid entering the second stage of compressors, with an attendant decrease in the amount of work needed for the compression stage overall. There is also an increase in the maximum feasible pressure ratio due to reduced compressor discharge temperature for a given amount of compression, improving overall efficiency.

• 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 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.

• 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.

See also

Wikimedia Commons has media related to: Brayton cycle

• Gas turbine
• Jet engine
• Heat engines
• Thermodynamics
• Power
• HVAC
• Engineering
• Gerotor

References

1. ^ according to Gas Turbine History
2. ^ Lester C. Lichty, Combustion Engine Processes, 1967, McGraw-Hill, Inc., Lib.of Congress 67-10876

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