Development of additively manufactured cryogenic heat exchangers for hydrogen-electric aircraft propulsion

Storage of liquid hydrogen at low pressures and temperatures is advantageous in terms of overall system weight compared to high-pressure gas storage at room temperature [1]. The weight penalty for insulation of cryogenic media is typically outperforming the combined weight and volume penalty of high-pressure storage [2] [3]. For aviation applications, where light-weight and compact design is key, it is, therefore, preferred to store the propellant as liquid. In the context of hydrogen, this implies storage of the liquid between 20K and 30K, depending on tank pressure. As the fuel cell, which converts chemical energy to electrical energy, requires the hydrogen to be provided at temperatures well above the freezing level of water, hydrogen extracted from the tank must be thermally conditioned, to comply with applicable requirements [4] [5]. In the presented work, a liquid hydrogen and fuel cell based electrical propulsion system for aircraft is developed, which incorporates several novel technologies and non-trivial operating conditions [6]. One major focus is the utilization of additive manufacturing enabling advanced geometric designs to enhance performance and reduce weight. However, the influence of hydrogen and cryogenic operating conditions on thermal and mechanical properties of additively manufactured materials must be assessed and accounted for in the design process. The underlying architecture of the investigated hydrogen-electric aircraft powertrain incorporates several heat exchangers with different purposes. One of the heat exchangers is tasked with transfer of heat from a liquid coolant cycle to the gaseous hydrogen in order to rise its temperature to the working level of the fuel cell (>300K). The design of the heat exchanger is based on empirical formulae for cylindrical tube bundles (pins), which enhance heat transfer on the gas side. Alternating flow planes of gaseous hydrogen and liquid coolant media are stacked to reach a number of units, which generate the desired heat transfer rates and keep the pressure loss due to friction below the defined limit. The particular arrangement of rows, columns, pin diameter or distances between individual pins is optimized to arrive at the minimum structural mass, while satisfying heat transfer and pressure drop requirements. Boundaries for this parameter optimization may originate from manufacturing limitations, like the minimum thickness of fine structures impacting the reliability of the manufacturing process. Therefore, accompanying manufacturing tests are required, to feed the optimizer with meaningful limits. After the initial optimization, computational fluid dynamics (CFD) analysis confirms the performance of the entire heat exchanger, including the intake and outflow manifold. Lastly, the lower limits of wall thicknesses may be defined by either the manufacturability in the 3d-printer or the differential pressure across the wall causing material stress. Therefore, mechanical analysis is performed, to confirm that the manufacturing limit is the actual driving limit for the minimum wall thickness. Comparison of results of the analytical method with CFD show very good agreement with deviations of average media temperature at the heat exchanger exit of less than 3% for relevant conditions. The pressure drop results deviate predictably in the order of 50%, which can be accounted for with a correction factor. CFD analysis of the entire heat exchanger shows good agreement of the flow properties in the heat exchanger core, but also demonstrate that the intake (from pipe flow into heat exchanger core) and outflow plena can contribute significantly to the overall pressure drop, depending on flow conditions and geometry. This presentation summarizes the entire development process of a heat exchanger starting with the requirements derivation based on a system analysis, via the initial design optimization based on empirical coupled thermal and fluid mechanical formulae to CFD and mechanical strength analysis of the detailed design. Close attention is paid on the additional degrees of freedom and restrictions in design gained by additive manufacturing as well as the impact of the manufacturing method on material properties especially in the context of cryogenic hydrogen.