Aerospace system efficiency improvement and capacity growth has fueled demand for innovative, affordable and scalable thermal management technologies. Recent advancements in additive manufacturing (AM) and materials has extended the thermal design space for heat exchangers, cold plates, heat sinks, and heat pipes. Novel heat transfer enhancement techniques, along with design and system interface innovations, offer attractive cooling solutions for use in numerous aircraft systems. These advances are becoming increasingly relevant in aircraft systems as customers are demanding the use of air-cooling instead of liquid-cooling with minimal impact on overall energy conversion efficiency, installed volume and weight. This paper provides an overview of Boeing-led advances in analysis, design, fabrication and testing of next generation heat transfer devices. A case study is presented to provide insight into a methodology for selection of heat transfer surfaces and design optimization for an air-to-air heat exchanger. Design considerations are presented for additive manufacturing of the thermal management devices using a range of high performance materials including aluminum, titanium, stainless steel, and conductive polymer composites.

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Phase change materials (PCM), especially salt hydrates possess high volumetric energy storage capacity in their transition temperature range. These materials are used in applications where it is necessary to store thermal energy due to temporary load shift between demand and availability. Thus, possible applications are HVAC, recovery of waste heat, and defense thermal management. In spite of salt hydrates potential, the practical feasibility of latent heat storage with salt hydrates is limited due to low power rating, supercooling, phase segregation, and long- term stability. Its low power rating and long-term stability limits its application in most applications. This work experimentally validates, the stability and thermal performance of a compact heat exchanger charged with salt hydrate during melting and freezing. The compact heat exchanger was designed with fins on both the Heat Transfer Fluid (HTF) and salt hydrate PCM side. The thermal performance of the Latent Heat Thermal Energy Storage System (LHTESS) were evaluated for various operating conditions. The results show that LHTESS could achieve an average heat transfer coefficient of 124 and 87 W/m2 - K during melting and solidification respectively. The stability of the system in suppressing supercooling were validated over 800 cycles with nucleating agent and active homogenous nucleation techniques. The supercooling was reduced to 3 °C with zinc hydroxyl nitrate as nucleating agent and less than 1 °C with active homogenous nucleation technique. The LHTESS showed less than 5% degradation in energy storage capacity over 800 cycles.

Although model predictions of thermal energy storage (TES) performance have been explored in previous investigations, relevant test data that enables experimental validation of performance models has been limited. This is particularly true for high-performance TES designs that facilitate fast input and extraction of energy. In this paper, we present a summary of experimental tests of a high-performance TES unit using lithium nitrate trihydrate phase change material (PCM) as a storage medium. Performance data is presented for complete dual-mode cycles consisting of extraction (melting) followed by charging (freezing). These tests simulate the cyclic operation of a TES unit for asynchronous cooling in a variety of applications. The model analysis is found to agree reasonably well, within 10%, with the experimental data except for conditions very near the initiation of freezing, a consequence of subcooling that is required to initiate solidification.

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