Integration of Functional Mock-up Units into Digital Twins of Aircraft Thermal Management Systems

Kurzfassung

As part of the EU Clean Aviation initiative, the project TheMa4HERA (Thermal Management for Hybrid Electric Regional Aircraft) addresses the development of innovative thermal management solutions for future regional aircraft with hybrid-electric propulsion systems. Within this project, the German Aerospace Center (DLR) is tasked to develop a detailed digital twin of the overall Thermal Management System (TMS). This digital twin aims to simulate critical operational scenarios prior to hardware testing and to virtually demonstrate system-level behavior throughout complete flight missions. This approach is used to enable early detection of critical conditions and design shortcomings. Furthermore, it contributes to improved system designs and supporting certification processes and flight demonstrations by providing validated predictive insights at an early development stage. The foundation of the TMS digital twin builds on the object-oriented modeling language Modelica and the open-source ThermoFluid Stream library (TFS), developed internally at the DLR. The TFS offers robust numerical solutions particularly suited for the large-scale models, characteristic of complex thermal management systems. The modeled TMS includes three primary subsystems: the fuselage environmental control system (ECS), the battery- and power electronics cooling system, and the fuel cell cooling system. Each subsystem introduces specific thermal management challenges, requiring detailed and interconnected modeling strategies to ensure an accurate representation of the physical behavior across varying flight and mission profiles. These detailed dynamic models are shared by the partners inside the project as Functional Mockup Units (FMUs). The FMUs are based on the Functional Mock-up Interface (FMI), that specifies a standardized interface for model exchange between different simulation software packages, additionally ensuring the protection of intellectual property. The integration of FMUs inside large and complex thermofluid models have proven to be difficult. In previously published research, a method based on integration using an controller was developed and tested to integrate an FMU of a detailed outflow valve model – with a single significant control output such as mass flow -inside the Digital Twin model. The current study advances the integration methodology to more complex subsystem modeling. Specifically, attention is focused on vapor cycle system (VCS) modeled both as an open Modelica model using the TFS and as an FMU. It features multiple critical input and output parameters, including temperature, pressure, and mass fraction, thus increasing the complexity of the integration task significantly. This study analyzes and contrasts the method proposed in previous research with established FMU integration approaches, such as the use of adaptors. The comparison focuses on their respective positions within the simulation architecture, the mechanisms by which they are coupled to the native TMS model, and their impact on overall system-level performance and numerical robustness. Considering the computational demands associated with simulating such large-scale thermal management systems, achieving efficient simulation performance without compromising model fidelity for multiple different conditions is of significant importance. Therefore, the various model integration methods are benchmarked against key performance indicators such as simulation time, numerical stability, model accuracy, and robustness. The study systematically evaluates the different integration methods by the key indicators and suggests a preferred way of integrating FMUs into the simulation environment by the example of the VCS. Since FMUs are of significant importance for model sharing and virtual testing, the study aims to support future developments in simulation methodology, and certification support within the broader context of thermal aviation technologies.