Brayton Cycle( Joule s cycle

Brayton Cycle( Joule s cycle):

The Ideal Cycle for Gas Turbine Engines

• In reality, gas turbines operate on an open cycle
• Fresh air is continuously drawn into the compressor and exhaust gases are thrown out
• The open gas-turbine cycle can be modeled as a closed cycle
• The combustion process is replaced by a constant-pressure heat-addition process and the exhaust process is replaced by a constant-pressure heat-rejection process
GAS TURBINE COMBUSTORS:
The heat is added to the air flowing through the gas turbine in the combustors.1 The air leaving the compressor enters the combustors. Its temperature increases while the pressure drops slightly across the combustors. Thus, combustors are direct-fired air heaters. The fuel is burned almost stoichiometrically with 25 to 35 percent of the air entering the combustors. The combustion products mix with the remaining air to arrive at a suitable temperature for the turbine. The three major types of combustors are tubular, tuboannular, and annular. All combustors, despite their design differences, have the following three zones:
1. Recirculation zone
2. Burning zone
3. Dilution zone
The fuel is evaporated and partially burned in the recirculation zone. The remainder of the fuel is burned completely in the burning zone. The dilution air is mixed with the hot gas in the dilution zone. If the combustion is not complete at the end of the burning zone, the addition of dilution air can chill the hot gas. This prevents complete combustion of the fuel. However, there is evidence that some combustion occurs in the dilution zone if the burning zone is run over rich. The fuel-to-air ratio varies during transient conditions. It is high during the acceleration phase and low during the deceleration phase. Thus, the combustor should be able to operate over a wide range of mixtures. The combustor performance is measured by efficiency, pressure drop across the combustor, and evenness of the outlet temperature profile. The combustor efficiency is a measure of combustion completeness. It affects the fuel consumption directly because the unburned fuel is wasted. The combustor efficiency is the ratio of the increase in gas enthalpy and the theoretical heat input of the fuel. It is given by











• The idealized closed loop cycle is the Brayton cycle, which consists of the following four internally reversible processes
1?2 isentropic compression in compressor

2?3 Constant-pressure heat addition

3?4 isentropic expansion in gas turbine.

4?1 Constant-pressure heat rejection