Thermodynamic analysis of the Brayton-cycle gas turbine under equilibrium chemistry assumptions

A design-point thermodynamic model of the Brayton-cycle gas-turbine under assumptions of perfect chemical equilibrium is described. This approach is novel to the best knowledge of the author. The model uniquely derives an optimum work balance between power turbine and nozzle as a function of flight conditions and propulsor efficiency. The model may easily be expanded to allow analysis and comparison of arbitrary cycles using any combination of fuel and oxidizer. The model allows the consideration of engines under a variety of conditions, from sea level/static to >20 km altitude and flight Mach numbers greater than 4. Isentropic or polytropic turbomachinery component efficiency standards may be used independently for compressor, gas generator turbine and power turbine. With a methodology based on the paper by M.V. Casey, “Accounting for losses” (2007), and using Bridgman’s partial differentials , the model uniquely describes the properties of a gas turbine solely by reference to the properties of the gas mixture passing through the engine. Turbine cooling is modelled using a method put forward by Kurzke. Turboshaft, turboprop, separate exhaust turbofan and turbojet engines may be modelled. Where applicable, optimisation of the power turbine and exhaust nozzle work split for flight conditions and component performances is automatically undertaken. The model is implemented via a VB.net code, which calculates thermodynamic states and controls the NASA CEA code for the calculation of thermodynamic properties at those states. Microsoft Excel® is used as a graphical user interface. It is explained that comprehensive design-point cycle analysis may allow novel approaches to off-design analysis, including engine health management, and that further development may allow the automation of cycle design, possibly leading to the discovery of opportunities for novel cycles.