System-level optimization of the sizes of organs for heat and fluid flow systems

In this paper we show that the sizes (weights) of heat and fluid flow systems that function on board vehicles such as aircraft can be derived from the maximization of overall (system level) performance. The total weight of the aircraft dictates its fuel requirement. The principle owes its existence to two effects that compete for fuel. Components, power plants and refrigeration plants operate less irreversibly when they are larger. Less irreversibility means less fuel needed for their operation. On the other hand, larger sizes add more to the mass of the aircraft and to the total fuel requirement. This tradeoff pinpoints optimal sizes. The principle is illustrated based on three examples: a power plant the size of which is represented by a heat exchanger, a counterflow heat exchanger without fluid flow irreversibility, and a counterflow heat exchanger with heat transfer and fluid flow irreversibilities. The size optimization principle is applicable to the organs of all flow systems, engineered (e.g., vehicles) and natural (e.g., animals).