Dynamic Modeling of Vapor Compression Cycles Using a Novel Lagrangian Approach

Vapor compression cycles-based systems (VCS) are being adapted for thermal management of modern aircraft. Predicting dynamic behavior is critical to the design and control of these systems, which are likely to experience large dynamic changes in heat load. To meet this demand, a novel Lagrangian method to model the dynamic behavior of vapor compression cycles has been developed. The approach described in this paper considers the basic VCS as 4 fluid sides: one high pressure and one low pressure refrigerant side and external fluids which interact with each respectively. Sides are further divided into some number of material volumes. The model simulates compressible, unsteady flow by allowing each volume to translate and displace other elements, expanding and contracting in response to changes in mass, enthalpy and pressure. At every timestep, heat transfer to each mass element is determined and corresponding changes in thermodynamic properties are evaluated. The model predicts transient system response during normal operation as well as startup mode. Results from a dynamic evaporator simulation are presented and discussed. These results show the low-pressure refrigerant and external fluid response to changes in valve position, external fluid inlet temperature, and refrigerant inlet enthalpy. The conclusion drawn from these results is that the modeling framework described in this paper can reproduce the basic dynamics of a two-phase heat exchanger at a rate less than real time.