Abstract:
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.
