Optimal Second-Law Efficiency for a Brayton Cycle With an Internal Heat Source

Abstract

Net work of an endoreversible Brayton cycle and its second-law efficiency are examined for a heat source (hot air) that is initially at a temperature typical of adiabatic and stoichiometric combustion. That temperature is taken to be well above maximum cycle temperature. When heat is transferred to the cycle via a heat exchanger, maximum work per unit of heat source mass and per unit of heat source availability (second-law efficiency) are found to occur at a cycle pressure ratio that differs from the pressure ratio for maximum cycle first-law efficiency. When the heat source is internal, i.e., a fraction of the cycle working fluid, maximum work per unit of heat source mass is found to occur at the pressure ratio found for the external source; on the other hand, maximum work per unit of heat source availability is found to occur at the highest possible cycle pressure ratio, which is the same point at which cycle first-law efficiency is a maximum.