Coupled Design Optimization of Compact Heat Exchangers in Aviation. A design study using a generalized heat exchanger model, computational fluid dynamics and Bayesian optimization

Abstract

The transition to hydrogen-fuelled aviation presents significant challenges in thermal management, particularly in the integration of high-performance, compact heat exchangers. As the European Union aims to achieve net-zero emissions by 2050, hydrogen-powered gas turbine engines are a promising solution. Future civil and military engines will face increased thermal loads, necessitating the integration of megawatt-class heat exchangers while maintaining aerodynamic efficiency.

This master’s thesis addresses this problem through the optimization of duct geometries with integrated finned heat exchangers. The primary focus of the thesis is to investigate how performance varies with heat exchanger inlet area and total duct length. To enable an objective comparison of losses across different designs, the internal heat exchanger geometry was updated between each CFD iteration to converge the solution to a specified performance target.

The results of this study provide new insights into the underlying trade-offs. Heat exchangers with a larger area result in lower losses over the heat exchanger matrix but incur increased losses in the ducts. Additionally, shorter ducts lead to higher losses over the heat exchanger due to the reduced diffusive capacity, while duct losses remain largely unchanged. Another notable trend in the optimized designs is the presence of significant recirculation regions in all duct geometries, highlighting the strong diffusive capacity of the heat exchanger.

The study also includes a limited investigation into the effects of the transversal fins of the heat exchanger by selecting a fixed geometry from the previous optimization trials and removing the sink term associated with the fins. This modification increases the normalized losses from 1.35 to 1.37, indicating that the pressure drop across the heat exchanger matrix is the primary driver of diffusion, rather than the finned structure itself.