Brayton Cycle

The Brayton cycle is the ideal thermodynamic cycle used to model gas-turbine and jet-engine power systems. In the simple open-cycle representation, air is compressed approximately adiabatically in the compressor, heat is added at approximately constant pressure in the combustor, the high-temperature gas expands through the turbine, and the exhaust rejects heat to the surroundings. The ideal closed-cycle representation consists of four processes: 1→2 isentropic compression, 2→3 constant-pressure heat addition, 3→4 isentropic expansion, and 4→1 constant-pressure heat rejection. The pressure ratio r_p = P_2/P_1 is a key design parameter. For an ideal gas with constant specific heats, the isentropic temperature relation is T_2/T_1 = r_p^((k-1)/k), while expansion gives T_3/T_4 = r_p^((k-1)/k). The compressor specific work is w_c = c_p(T_2 - T_1), and the turbine specific work is w_t = c_p(T_3 - T_4). The net specific work is w_net = w_t - w_c. For the ideal simple Brayton cycle, thermal efficiency is eta_th = 1 - 1/r_p^((k-1)/k). Thus, increasing pressure ratio increases ideal thermal efficiency, although the net-work optimum and practical engine performance also depend on turbine inlet temperature, component efficiencies, pressure losses, and material temperature limits. In a real gas-turbine cycle, compressor and turbine processes are not perfectly isentropic, combustion occurs with pressure losses, and mechanical and cooling requirements consume part of the produced work. Compressor and turbine isentropic efficiencies are therefore required to estimate actual performance. Regeneration can improve efficiency when the turbine exhaust temperature is sufficiently higher than the compressor exit temperature, while intercooling and reheating can alter the balance between efficiency and specific work.