A Work Availability Perspective of Turbofan Engine Performance

This paper presents a work availability perspective on the thermodynamic performance of the turbofan engine and contrasts this with the classic presentation, which describes performance based primarily on cycle efficiency. It is shown that the availability perspective leads to a more fundamental understanding of the basic problem, this being to maximize the conversion of work potential stored in the fuel into useful work output. The discussion specifically addresses the impact of primary turbofan cycle parameters on usage and loss of work potential. It is shown that cycle pressure ratio governs exhaust heat losses, turbine inlet temperature governs non-equilibrium combustion losses, and fan pressure ratio governs loss due to residual exhaust kinetic energy. Finally, simplified loss calculation methods applicable to any turbofan engine are presented and the method is applied to the analysis of cycle losses in the Northrop F-5E propulsion system. Introduction Ever since the inception of the heat engine, the thermodynamic objectives guiding the design of primemovers has always been clear: to build machines with the highest possible efficiency and specific power output. It is accurate to say that this objective continues to be the primary preoccupation in the thermodynamic design and optimization of modern gas turbine engines. As such, it is of obvious importance to have a complete understanding of the underlying principles impacting the performance of these machines. Modern textbook-type presentations of gas turbine thermodynamic performance typically focus on the use of cycle efficiency as an overall figure of merit. This approach has proven quite useful in understanding the relationship between cycle parameters and thermodynamic performance, particularly when used in conjunction with temperature-entropy diagrams. However, the drawback to this approach is that it is based purely on conservation of energy and concentrates on accounting for the transfer of heat * Research Engineer, Aerospace Systems Design Laboratory (ASDL), Georgia Tech, Member, AIAA. † Boeing Chair for Advanced Design, School of Aerospace, Director, ASDL, Senior Member, AIAA. Copyright © 2000 by Roth and Mavris. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. energy into useful work. Therefore, it gives no insight as to the efficiency in transferring work potential into useful work, and it is the transfer of work potential that is truly the crux of thermodynamic design. Consequently, there is a need for some means of quantifying thermodynamic performance in terms of work potential transfer, as opposed to energy transfer. Fortunately, such a work potential figure of merit already exists and has in fact been the subject of research and theoretical development starting with the work of Carnot, and later, J.W. Gibbs. This figure of merit is presently known as availability or exergy, the theoretical underpinnings of which are today quite well developed. Exergy can be thought of as the maximum work that can be obtained in taking a substance from a given temperature, pressure, and chemical composition into a state of thermal, mechanical, and chemical equilibrium with the environment. Therefore, exergy is a measure of work potential, and exergy transfer is the metric for transfer of work potential alluded to previously. This concept is the key to enabling a description of cycle performance based on work potential, thus providing another point of view to augment the efficiency-based presentations of turbofan engine performance so prevalent today. The presentation of turbofan engine performance in terms of efficiency is well-known, as evidenced by the voluminous literature available on this topic (notably the texts by Bathie, Hill and Peterson, and Whittle). This discussion will attempt to add to current understanding by framing turbofan engine performance in terms of obtaining the maximum possible work from the work potential stored in the fuel. This description is then examined vis Æ vis the established efficiency-based presentation. The impact of each cycle parameter on transfer of work potential is considered separately, and both the efficiency-based and work potential-based viewpoints are discussed for each parameter. Finally, simplified methods for estimation of losses due to engine cycle impacts are presented to illustrate the ideas suggested in each section and these methods are applied to the analysis of the Northrop F-5E propulsion system as a case study. Classic Presentation of Turbofan Performance The factors that impact specific power output and efficiency of a engine more than all others are the basic cycle parameters. Consequently, an understanding of the relationship between cycle parameters and AIAA2001-0391