An advanced nonlinear switched heat exchanger model for vapor compression cycles using the moving-boundary method

Abstract

Simulation is commonly used to develop control and diagnostic algorithms. Because vapor compression cycles are essentially heat management devices, transient and steady-state heat exchanger performance must be predicted accurately. However, for hardware- and software-in-the-loop purposes, the simulation must run in real-time. To reconcile these competing needs, a new lumped parameter or moving-boundary heat exchanger model was created. Accuracy concerns are addressed by including finned surfaces, nonlinear air temperature distributions, and non-circular passages. In its current form, the model is applicable to single pass, cross-flow heat exchangers. The mathematical basis of the model is given and shown to be consistent with integral forms of the energy and continuity equations. Although known to be more computationally efficient than finite volume models, moving-boundary models become singular and fail under certain conditions. To address this shortcoming, particular attention was focused on algorithms for switching between different representations and rezoning wall temperatures. Robustness to changing flow regimes is demonstrated through simulation test cases, and model application to a chiller system is shown. As such, the model provides improved accuracy, robustness, and operating range while maintaining real-time capability.