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q C T ( k ) 1 k 1 1 1
1 2 1 vm 1 1 (4.7)
th k 1 k 1
q C T [( ) 1 k ( ) 1 1 k ( ) 1
1 vm 1
As can be seen, increasing the compression ratio and the pressure ratio will improve
thermal efficiency. However, decreasing the cutoff ratio will decrease thermal
efficiency.
Increasing the compression ratio causes the peak temperature to go up, which may
cause spontaneous, uncontrolled ignition of the fuel, which leads to a shock wave
traveling through the cylinder, and is called knocking.
4.6 Gas Turbine Cycle (or Joule-Brayton Cycle)
Gas turbines are rotary internal combustion engines Fig. 4.7. Gas turbines usually
operate on an open cycle, as shown in Fig. 4.8a. Fresh air at ambient conditions is
drawn into the compressor 1, where its temperature and pressure are raised. The high-
pressure air proceeds into the combustion chamber 2, where the fuel is burned at
constant pressure. The resulting high-temperature gases then enter the turbine 3, where
they expand to the atmospheric pressure while producing power. The exhaust gases
leaving the turbine are thrown out (not recirculated), causing the cycle to be classified
as an open cycle.
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 keep the temperature of
various components within safe limits. The
second function is accomplished by drawing in
more air than is needed for the complete
Fig. 4.7 - Gas turbines 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 combustion gases as air does not cause any
appreciable error. Also, the mass flow rate through the turbine is greater than that
through the compressor, the difference being equal to the mass flow rate of the fuel.
Thus, assuming a constant mass flow rate throughout the cycle yields conservative
results for open-loop gas-turbine engines.
The two major application areas of gas-turbine engines are aircraft propulsion and
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 turbines 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
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