On Exergy and Aero Engine Applications

Licentiate thesis, 2016

Aero engine performance analysis is highly multidimensional using various measures of component performance such as turbomachinery and mechanical efficiencies, and pressure loss coefficients. Using conventional performance analysis, relying on only the first law of thermodynamics, it is possible to understand how the performance parameters affect the component performance, but not how the component performance relates to the system performance. A comprehensive framework has been detailed to analyze an aero engine in one common currency by complementing the analysis with the second law of thermodynamics. As it yields a measure of the lost work potential in every component it is used to relate the component performance to the system performance. The theory includes a more detailed layout of all the terms that apply to a propulsion unit than presented before and is here adopted to real gases to be used in state of the art performance codes. The theory is also extended upon by presenting the installed rational efficiency, a true measure of the propulsion subsystem performance, including the installation effects of the propulsion subsystem as it adds weight and drag that needs to be compensated for in the performance assessment. The exergy methodology is applied to a modern direct-drive two-spool turbofan, chosen for its dominating market share in modern commercial aviation. The loss sources during an aircraft mission are then assessed and yield the major contributors in the entropy generated during combustion, the thermal energy leaving the nozzle and the exhaust nozzle kinetic energy that is not contributing to the thrust. Radical technology that can be utilized to address each specific loss are thereafter detailed. This includes intercooled and recuperated cycles, reheated cycles, bottoming Rankine cycles, pulse detonation combustion, piston topped composite cycles, nutating disc combustion, and open rotor and other ultra high bypass architectures.