Architecture from exergy-based global optimization: Tree-shaped flows and energy systems for aircraft
This paper draws attention to an emerging body of work that relies on exergy analysis and thermodynamic optimization in the pursuit of flow system architecture. Exergy analysis establishes the theoretical performance limit. Thermodynamic optimization (or entropy generation minimization) brings the design as closely as permissible to the theoretical limit. The design is destined to remain imperfect because of constraints (finite sizes, times, and costs). Improvements are registered by spreading the imperfection (e.g., flow resistances) through the system. Resistances compete against each other and must be optimized together. Optimal spreading means spatial distribution, geometric form, topology, and geography. System architecture springs out of constrained global optimization. The principle is illustrated by two classes of applications. In flows that connect a volume (or area) with one point, the resulting structure is a tree of low-resistance interstices. These structures are robust and diverse and are found everywhere (e.g., rivers, lungs, networks for water and electricity, traffic patterns, etc.). In energy systems for aircraft, a key problem is the extrction of maximum exergy from a hot-gas stream that is cooled and discharged into the ambient. The optimal configuration consists of a heat transfer surface with a temperature that decays exponentially in the flow direction. This configuration can be achieved in a counterflow heat exchanger with an optimal imbalance of flow capacity rates. Similar opportunities for optimally matching components and streams exist in considerably more complex systems for power and refrigeration. Examples show that the complete structure of a heat exchanger for an environmental control system can be derived based on this method. © 2000 by Adrian Bejan.
