This study presents an automated shape optimization method for heat sinks. The computational framework has been developed by combining a conjugate heat transfer solver with adjoint capabilities, a CAD parametrization tool, and a gradient-based optimizer. The test case considers the design optimization of a water-cooled heat sink with pin fins, with the goal of concurrently enhancing heat transfer and minimizing pressure losses. Results show that the optimized fin geometry leads to an improvement of the average heat transfer coefficient by 24% while the pressure drop is lowered by 19%. The optimal fin array features an unconventional shape with an enlarged cross-section at the hub and the top with respect to mid-span, and a variation of the pin profile in the streamwise direction. The net effect is a reduction in flow blockage, an increase in fin efficiency, and a lower and more uniform temperature distribution in the heat sink base plate.

Heat exchangers are key components of thermal energy conversion systems, however, their optimal design is still based on reduced order models relying on semi-empirical heat transfer correlations. CFD-based design optimization emerged as a viable method to provide a significant improvement in performance at an affordable cost. This study presents a framework to optimize multiple heat transfer surfaces concurrently using the adjoint method. The heat transfer surfaces are parametrized using a CAD-based parametrization method, and their performance is evaluated using a RANS solver complemented by its discrete adjoint counterpart for gradient computation. The optimization framework is applied to minimize the pressure drop across a bare-tube heat exchanger while constraining the heat transfer rate. Two variants of the same optimization problem are formulated: in the first one, the sensitivities are averaged and the tubes are constrained to maintain the same shape, while in the second variant, the shape of the tubes can vary, resulting in an optimum solution with non-identical tube shapes. The results show that the optimized geometry reduces the pressure drop by 19% if the tube shapes are identical, and by 25% in the case of non-identical shapes, compared to the baseline. To identify the physical mechanisms contributing to the fluid-dynamic losses, entropy generation along the flow path was investigated. The results reveal that the major loss reduction observed for the case of non-identical tube shapes is due to the better thermo-hydraulic performance of the first and last tubes.

Decarbonizing aviation requires the development of novel propulsion systems that would be powered by renewable energy stored in batteries, green hydrogen, and e-SAF (a type of sustainable aviation fuel). To increase the viability of these carbon-neutral solutions, minimizing mission energy consumption will remain the key driver of the design of next-generation aircraft systems and their components. Additionally, increasing importance is placed on thermal energy recovery and thermal management, which necessitates the design of high-performance thermal components, namely heat exchangers and heat sinks.

This dissertation documents research on shape optimization using the discrete adjoint method and CAD-based parametrization for the design of aerospace-grade heat exchangers. The main outcome of this work is the development of the optimization framework to concurrently optimize multiple heat transfer surfaces parametrized using a CAD method based on Non-Uniform Rational Basis Splines (NURBS) and the discrete adjoint method available in the open-source computational fluid dynamics (CFD) software SU2. The application of the design method is demonstrated for two-dimensional and three-dimensional heat transfer surfaces in configurations representative of aircraft condensers and evaporators, as well as heat sinks for thermal management. In this regard, two formulations of surface sensitivity are proposed such that the resulting optimal solutions can feature identical shapes using averaged sensitivities or non-identical shapes when optimized concurrently, albeit independently. Additionally, the feasibility of integrating the CFD-based method in system-level design and its potential for enhancing system performance are investigated.

The results obtained using the design method show that the application of this framework can achieve geometries of thermal components with reduced pressure drop and enhanced heat transfer coefficient compared to conventional designs. The automated design chain applied to a two-dimensional configuration representing tubular heat exchangers reduced the pressure drop significantly while constraining the heat transfer rate. Using three-dimensional shape optimization of pin-fins with conjugate heat transfer resulted in an unconventional fin shape that led to a simultaneous reduction in total pressure losses and an increase in heat transfer coefficient. These performance improvements of about 20% corresponding to optimal geometries obtained from shape optimization can lead to significant gains in the performance of the system, as demonstrated by its application in the early phase of system-level design reported in this work. Future developments on such a design method have the potential to conceive designs of the next-generation heat exchangers that could be deployed in propulsion systems, enabling carbon-neutral aviation.

This study presents a Computational Fluid Dynamics (CFD)-based optimization framework to enhance the performance of bare-tube heat exchangers. The framework comprises a CFD solver, an adjoint solver, and a CAD-based parametrization tool. This framework simultaneously optimizes the tube shape and layout to obtain an optimum heat exchanger configuration with a higher heat transfer rate and lower pressure drop. Firstly, a parametric study of the longitudinal pitch is performed that shows that increasing the longitudinal pitch results in an increase in pressure drop and heat transfer rate. Furthermore, the proposed framework was applied to optimize an in-line elliptical tube configuration. The optimum geometry showed a performance improvement of 29% as compared to the baseline geometry while satisfying the constraint on the heat transfer rate.