On the Definition of Exergy in the Field of Aerodynamics

Abstract

The exergy concept originates from the field of static thermodynamics and expresses the maximum theoretically recoverable mechanical work from a system while it evolves toward its dead thermodynamic state. It accounts for both mechanical and thermal mechanisms, and it allows to separate reversible and irreversible losses in the system’s transformations. The physical insight provided by this concept motivated the development of an exergy-based performance evaluation method in the field of aerodynamics. The resulting formulation has the advantage of being independent of the feasibility of a drag/thrust breakdown (ambiguous for highly integrated engine concepts) and includes thermal effects in the performance metrics. It, however, relies on an adapted definition of exergy, in particular involving a dead state in motion. This adapted definition is not trivial and raises theoretical concerns due to fundamental thermodynamic properties of exergy not being always satisfied. This paper aims at proposing a corrected version of this definition that ensures that the fundamental properties of exergy are respected. First, the exergy concept is presented alongside the concerns raised by its original adaptation, which, to the best of the authors’ knowledge, has been used in all exergy-based flowfield analyses in the field of applied aerodynamics. Then, an unsteady exergy balance is derived in the geocentric reference frame (in which there are no ambiguities in the definition of exergy) and then transformed to a reference frame in translation. The corrected adaptation of the exergy definition for aerodynamics applications is extracted from this transformation and the impact on the exergy balance is analyzed.