Finite Time Thermodynamic Analysis of Modified Brayton Cycle

Abstract

It is well known that reheating in gas turbine engines limits the extent to which an isothermal heat addition is approached. With respect to a simple heat addition, when a compressible gas with subsonic velocity flows through a frictionless constant area duct with heat addition, the temperature of the gas increases along the duct. Also with respect to simple area change, when a compressible fluid/gas with subsonic velocity flows through a frictionless adiabatic duct with decreasing area, the temperature of the gas decreases along the duct. The idealized isothermal process consists of a compressible gas with subsonic velocity flowing through a frictionless converging duct, such that while heated all along the duct, any infinitesimal decrease in temperature due to simple area change is exactly compensated by the simple heat addition. It is noted that, since temperature of the gas is constant during the isothermal heat addition, the kinetic energy of the gas and hence the Mach number must increase in order to satisfy the conservation of energy. The appropriate application of the idealized isothermal process is to gas turbine engines operating with air. It is equally desirable that the Brayton cycle is modified by the isothermal heat addition process.