The Brayton cycle was first proposed by George Brayton for use in the recip-rocating oil-burning engine that he developed around 1870. Today, it is used

for gas turbines only where both the compression and expansion processes

take place in rotating machinery. Gas turbines usually operate on an open cy-cle,as shown in Fig. 8–25. Fresh air at ambient conditions is drawn into the

compressor, where its temperature and pressure are raised. The high-pressure

air proceeds into the combustion chamber, where the fuel is burned at constant

pressure. The resulting high-temperature gases then enter the turbine, where

they expand to the atmospheric pressure, thus producing power. The exhaust

gases leaving the turbine are thrown out (not recirculated), causing the cycle

to be classified as an open cycle.

The open gas-turbine cycle just described can be modeled as a closed cycle,

as shown in Fig. 8–26, by utilizing the air-standard assumptions. Here thecompression and expansion processes remain the same, but the combustion

process is replaced by a constant-pressure heat-addition process from an ex-ternal source, and the exhaust process is replaced by a constant-pressure heat-rejection process to the ambient air. The ideal cycle that the working fluid

undergoes in this closed loop is the Brayton cycle,which is made up of four

internally reversible processes:

1-2 Isentropic compression (in a compressor)

2-3 Constant-pressure heat addition

3-4 Isentropic expansion (in a turbine)

4-1 Constant-pressure heat rejection

The T-sand P-υdiagrams of an ideal Brayton cycle are shown in Fig. 8–27.

Notice that all four processes of the Brayton cycle are executed in steady-flow

devices; thus, they should be analyzed as steady-flow processes. When the

changes in kinetic and potential energies are neglected, the energy balance for

a steady-flow process can be expressed, on a unit-mass basis, as

(qin-qout)+(win-wout)=(hexit-hinlet)

The highest temperature in the cycle occurs at the end of the combustion

process (state 3), and it is limited by the maximum temperature that the turbine

blades can withstand. This also limits the pressure ratios that can be used in the

cycle. For a fixed turbine inlet temperature T3

, the net work output per cycle in-creases with the pressure ratio, reaches a maximum, and then starts to decrease,

as shown in Fig. 8–29. Therefore, there should be a compromise between the

pressure ratio (thus the thermal efficiency) and the net work output. With less

work output per cycle, a larger mass flow rate (thus a larger system) is needed

to maintain the same power output which may not be economical. In most

common designs, the pressure ratio of gas turbines ranges from about 11 to 16.

The air in gas turbines performs two important functions: It supplies the

necessary oxidant for the combustion of the fuel, and it serves as a coolant to

keep the temperature of various components within safe limits. The second

function is accomplished by drawing in more air than is needed for the com-plete combustion of the fuel. In gas turbines, an air–fuel mass ratio of 50 or

above is not uncommon. Therefore, in a cycle analysis, treating the combus-tion gases as air will not cause any appreciable error. Also, the mass flow rate

through the turbine will be greater than that through the compressor, the dif-ference being equal to the mass flow rate of the fuel. Thus, assuming a con-stant mass flow rate throughout the cycle will yield conservative results for

open-loop gas-turbine engines.

The two major application areas of gas-turbine engines are aircraft propul-sionand electric power generation.When it is used for aircraft propulsion, the

gas turbine produces just enough power to drive the compressor and a small

generator to power the auxiliary equipment. The high-velocity exhaust gases

are responsible for producing the necessary thrust to propel the aircraft. Gas tur-bines are also used as stationary power plants to generate electricity as stand-alone units or in conjunction with steam power plants on the high-temperature

side. In these plants, the exhaust gases of the gas turbine serve as the heat

source for the steam. The gas-turbine cycle can also be executed as a closed cy-cle for use in nuclear power plants. This time the working fluid is not limited to

air, and a gas with more desirable characteristics (such as helium) can be used.

The majority of the Western world’s naval fleets already use gas-turbine

engines for propulsion and electric power generation. The General Electric

LM2500 gas turbines used to power ships have a simple-cycle thermal

efficiency of 37 percent. The General Electric WR-21 gas turbines equipped

with intercooling and regeneration have a thermal efficiency of 43 percent and

produce 21.6 MW (29,040 hp). The regeneration also reduces the exhaust tem-perature from 600˚C (1100˚F) to 350˚C (650˚F). Air is compressed to 3 atm be-fore it enters the intercooler. Compared to steam-turbine and diesel-propulsion

systems, the gas turbine offers greater power for a given size and weight, high

reliability, long life, and more convenient operation. The engine start-up time

has been reduced from 4 h required for a typical steam-propulsion system to

less than 2 min for a gas turbine. Many modern marine propulsion systems use

gas turbines together with diesel engines because of the high fuel consumption

of simple-cycle gas-turbine engines. In combined diesel and gas-turbine sys-tems, diesel is used to provide for efficient low-power and cruise operation,

and gas turbine is used when high speeds are needed

In gas-turbine power plants, the ratio of the compressor work to the turbine

work, called the back work ratio,is very high (Fig. 8–30). Usually more than one-half of the turbine work output is used to drive the compressor. The situ-ation is even worse when the isentropic efficiencies of the compressor and the

turbine are low. This is quite in contrast to steam power plants, where the back

work ratio is only a few percent. This is not surprising, however, since a liq-uid is compressed in steam power plants instead of a gas, and the reversible

steady-flow work is proportional to the specific volume of the working fluid.

A power plant with a high back work ratio requires a larger turbine to pro-vide the additional power requirements of the compressor. Therefore, the tur-bines used in gas-turbine power plants are larger than those used in steam

power plants of the same net power output.

Development of Gas Turbines

The gas turbine has experienced phenomenal progress and growth since its

first successful development in the 1930s. The early gas turbines built in the

1940s and even 1950s had simple-cycle efficiencies of about 17 percent be-cause of the low compressor and turbine efficiencies and low turbine inlet

temperatures due to metallurgical limitations of those times. Therefore, gas

turbines found only limited use despite their versatility and their ability to

burn a variety of fuels. The efforts to improve the cycle efficiency was con-centrated in three areas:

1.Increasing the turbine inlet (or firing) temperatures This has been

the primary approach taken to improve gas-turbine efficiency. The turbine inlet

temperatures have increased steadily from about 540˚C (1000˚F) in the 1940s

to 1425˚C (2600˚F) today. These increases were made possible by the devel-opment of new materials and the innovative cooling techniques for the critical

components such as coating the turbine blades with ceramic layers and cooling

the blades with the discharge air from the compressor. Maintaining high tur-bine inlet temperatures with air-cooling technique requires the combustion

temperature to be higher to compensate for the cooling effect of the cooling air.

However, higher combustion temperatures increase the amount of nitrogen

oxides (NOx

), which are responsible for the formation of ozone at ground level

and smog. Using steam as the coolant allowed an increase in the turbine inlet

temperatures by 200˚F without an increase in the combustion temperature.

Steam is also a much more effective heat transfer medium than air.

2.Increasing the efficiencies of turbomachinery components The

performance of early turbines suffered greatly from the inefficiencies of tur-bines and compressors. However, the advent of computers and advanced tech-niques for computer-aided design made it possible to design these components

aerodynamically with minimal losses. The increased efficiencies of the tur-bines and compressors resulted in a significant increase in the cycle efficiency.

3.Adding modifications to the basic cycle The simple-cycle efficien-cies of early gas turbines were practically doubled by incorporating intercool-ing, regeneration (or recuperation), and reheating, discussed in Sections 8–8

and 8–9. These improvements, of course, come at the expense of increased ini-tial and operation costs, and they cannot be justified unless the decrease in fuel

costs offsets the increase in other costs. The relatively low fuel prices, the gen-eral desire in the industry to minimize installation costs, and the tremendous

increase in the simple-cycle efficiency to about 40 percent left little desire for

opting for these modifications.

The first gas turbine for an electric utility was installed in 1949 in Okla-homa as part of a combined-cycle power plant. It was built by General Elec-tric and produced 3.5 MW of power. Gas turbines installed until the

mid-1970s suffered from low efficiency and poor reliability. In the past, the

base-load electric power generation was dominated by large coal and nuclear

power plants. However, there has been a historic shift toward natural gas–

fired gas turbines because of their higher efficiencies, lower capital costs,

shorter installation times, and better emission characteristics, and the abun-dance of natural gas supplies, and more and more electric utilities are using

gas turbines for base-load power production as well as for peaking. The con-struction costs for gas-turbine power plants are roughly half that of compara-ble conventional fossil-fuel steam power plants, which were the primary

base-load power plants until the early 1980s. More than half of all power

plants to be installed in the foreseeable future are forecast to be gas-turbine or

combined gas–steam turbine types.

A gas turbine manufactured by General Electric in the early 1990s had a

pressure ratio of 13.5 and generated 135.7 MW of net power at a thermal effi-ciency of 33 percent in simple-cycle operation. A more recent gas turbine

manufactured by General Electric uses a turbine inlet temperature of 1425˚C

(2600˚F) and produces up to 282 MW while achieving a thermal efficiency of

39.5 percent in the simple-cycle mode. A 1.3-ton small-scale gas turbine la-beled OP-16, built by the Dutch firm Opra Optimal Radial Turbine, can run on

gas or liquid fuel and can replace a 16-ton diesel engine. It has a pressure ra-tio of 6.5 and produces up to 2 MW of power. Its efficiency is 26 percent in

the simple-cycle operation, which rises to 37 percent when equipped with a

regenerator.