Modelling Heat Exchangers with Embedded Phase Change Materials for Aircraft

The growing thermal load in aircraft design poses challenges for the development of thermal management systems (TMS) for future generation aircraft. Current systems are often oversized to manage highly transient thermal loads encountered during flight. Phase change material (PCM)-based heat exchangers offer a promising alternative by mitigating temperature fluctuations, which could lead to an overall system size reduction. PCM are materials that absorb and release thermal energy during phase transitions, typically between solid and liquid states, at nearly constant temperatures. However, existing PCM heat exchanger designs, primarily developed for building applications, require significant geometric modifications to meet the aerospace industry's stringent requirements for high thermal loads and rapid melting cycles. In this work, the integration of PCM in compact heat exchanger is studied on the basis of a plate-fin heat exchanger.

To establish a foundational understanding of PCM melting behaviour and develop an accurate modelling framework, a study of a shell-and-tube heat exchanger from the literature was recreated. Initial two dimensional simulations demonstrated overpredictions in PCM melting due to neglected temperature variations within the fluid channel. This was addressed by incorporating a conjugate heat transfer model, which coupled the PCM and fluid domains. An additional three-dimensional model further highlighted the complex dynamics of axial and lateral convection plumes, emphasizing the need for detailed modelling in future designs. These validations provided critical insights into PCM melting and informed the development of subsequent models.

The melting behaviour of PCM within plate-fin heat exchangers, which are commonly used in thermal management systems, was investigated by using numerical simulations across various extended surface configurations and operating conditions. The incorporation of fins on both the fluid and PCM sides demonstrated significant improvements in the thermal performance, leading to enhanced heat transfer and increased PCM melting efficiency, when compared to the simple rectangular geometries that are commonly found in the literature.

An effectiveness-NTU model was developed based on these findings, providing reliable performance estimates at a fraction of the computational cost of full Computational Fluid Dynamics (CFD) simulations. The model results were compared to the CFD simulations under aerospace-relevant conditions, achieving good agreement for rectangular cavities and capturing the melting dynamics of finned PCM layers. Its flexibility allows for rapid adjustments to design parameters, making it a practical tool for sizing and optimizing compact heat exchangers. This work advances the understanding and modelling of PCM-based heat exchangers, offering a foundation for efficient, lightweight thermal management solutions tailored to high-performance aircraft applications.