Modelling, simulation, and experimental evaluation of a crossflow heat exchanger for an aircraft environmental control system

Abstract

Crossflow heat exchangers play a significant role in the operation of an aircraft's environmental control system (ECS). The bleed air supplied by an aircraft engine, at high pressure and high temperature, requires regulation and control in order to be used for various pneumatic services. In the present investigation, the transient temperature response of crossflow plate-and-fin ECS heat exchangers, having a large core capacity with both fluids unmixed, is investigated numerically and experimentally for perturbations experienced in temperature. A non-linear lumped model of crossflow heat exchangers with a state-space solution valid for equal fluid velocities is derived and evaluated, in terms of fluid placement and number of lumps (sections) required. Dependency of the heat transfer coefficient on flowrates is incorporated in the dynamic modelling of the heat exchanger. Two models are derived, and the variation of the mean exit temperatures of both fluids with time is compared for the two alternative models, with consideration of the number of transfer units and heat capacitance rate ratios. One model requires axial lumping of the primary surface alone, as done in most of the existing models, and the second involves incorporating the effect of secondary surfaces (fins) on the heat exchanger transient performance. To quantify the importance of modelling the fins, a comparison of both simulation models with experimentally obtained data from a physical model is presented. By including fins and complex non-linearities in modelling of the ECS heat exchangers, a precise representation of the heat exchanger dynamics and accurate temperature responses are predicted. The model developments reported in this article can lead to improved aircraft ECS design and optimization.