Thermal Systems Oriented Two-Phase Heat Exchanger Models. Focus on Numerical Robustness

Abstract

The simulation of complex refrigeration architectures (that usually include vapor compression cycles) provides useful information for design, study and optimization purposes. Such arrangements may include several interconnected systems and a large variety and quantity of components. All components must meet two crucial requirements, namely, low CPU resolution time and high numerical robustness, in order to achieve relatively fast simulations and to prevent solver resolution issues at the architecture level. Among the usual components present, heat exchangers are the most challenging to address considering both the phenomenological and the numerical point of views. A generic heat exchanger model oriented for flexible purposes and meeting the aforementioned requirements has been developed under the Modelica programming language. The model can handle both single-phase and two-phase flows based on a simplified approach that considers three different zones for the refrigerant phase. Its numerical robustness has been extensively tested focusing on different boundary characteristics (definition, values, and signal types) and on demanding operating conditions (null mass flow rate, reversed flow, and reversed heat direction). This document presents the main characteristics of the model and a complete assessment of its numerical behaviour in terms of robustness and CPU time consumption.