Gas Turbines

A gas turbine is a continuous-flow internal combustion engine that produces shaft work by compressing air, adding heat in a combustor, and expanding the hot combustion gases through a turbine. The thermodynamic cycle is the Brayton cycle, which in its ideal form consists of four internally reversible processes: isentropic compression (1-2), constant-pressure heat addition (2-3), isentropic expansion (3-4), and constant-pressure heat rejection (4-1) in an open cycle to the atmosphere. The compressor, usually axial or centrifugal, consumes a large fraction of the turbine work; the net work is the difference between turbine and compressor work. The compressor pressure ratio r_p = P_2 / P_1 strongly influences both specific work and thermal efficiency. For an ideal Brayton cycle with air-standard assumptions, the thermal efficiency depends only on the pressure ratio: eta_th = 1 - 1 / r_p^((gamma-1)/gamma). In real machines, component isentropic efficiencies (eta_c for the compressor and eta_t for the turbine) reduce net work, and pressure losses in the combustor and ducts further degrade performance. The combustor adds heat at nearly constant pressure, and turbine inlet temperature (TIT) is limited by blade material, cooling technology, and coatings. Higher TIT raises specific work and efficiency but demands advanced cooling and thermal barrier coatings. Practical performance is described by specific fuel consumption (SFC), heat rate, and thermal efficiency. Gas turbines are used in aircraft propulsion (turbojet, turbofan, turboprop), power generation (simple cycle, combined cycle, and CHP), and mechanical drives. In combined-cycle plants, the turbine exhaust is used in a heat recovery steam generator (HRSG) to drive a steam bottoming cycle, lifting overall efficiency above 60%.