Condensation phenomena in fuel cell systems for aviation - KONBREFF

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A major component of polymer electrolyte fuel cell systems is the cathode system. It provides ambient air at an optimal pressure level to the fuel cell stack. The central component of the cathode system is a turbocharger with compressor, electric drive and a turbine for utilizing the enthalpy of the fuel cell outflow. The fuel cell outflow and thus also the turbine inflow are almost or completely saturated with water vapor. Expansion in the turbine initially leads to supersaturation of the water vapor and then to condensation of water droplets. This process, and in particular the condensation enthalpy released in the process, have a significant influence on the aerodynamics and performance parameters of the turbine.In a current research project at the Institute of Jet Propulsion and Turbomachinery, a high-fidelity Euler-Lagrange approach for the numerical modeling of multiphase flow in the turbine of an automotive fuel cell turbocharger could be developed. The results show that condensation can lead to significant thermal throttling of the turbine, as well as efficiency losses. However, the simultaneous increase of the turbine outlet temperature by up to 50 K provides a significant performance potential for downstream turbine stages. This is particularly interesting for aerospace applications, where multi-stage turbines have to be used due to the higher pressure ratios. However, turbines with condensation are so far only known from stationary applications with low power density requirements. The question to be addressed in this project is therefore:What are the fundamental effects of condensation on the performance, efficiency and optimum operating point of the turbine stages of an aerospace fuel cell turbocharger with very high gravimetric and volumetric power density requirements? What are the dominant mechanisms and how can they be accounted for as reduced-order models in turbine and system design?To answer these research questions, numerical investigations are to be carried out on the basis of the already validated and tested Euler-Lagrange approach. First of all, a turbine with two radial stages will be designed for an aviation reference system. The focus is on a design that is as small and light as possible. Subsequently, the optimal operating points for exemplary flight altitudes are considered and investigated with respect to their condensation phenomena. The results and findings of the proposed project form an important basis for the future development and design of fuel cell turbochargers suitable for aviation and for the calculation of their system behavior.