Regenerative Brayton Cycle

Regenerative Brayton Cycle

For the Brayton cycle, the turbine exhaust temperature is greater than the compressor exit temperature. Therefore, a heat exchanger can be placed between the hot gases leaving the turbine and the cooler gases leaving the compressor. This heat exchanger is called a regenerator or recuperator. The sketch of the regenerative Brayton cycle is shown below.







If heat transfer 100% in H.E : T2=T6 & T5=T4
EX: If a heat exchanger is used in the previous exercise ,find the
cycle efficiency?
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