International Journal of Turbomachinery, Propulsion and Power

A major challenge for aircraft fuel cell propulsion systems is to ensure that the air properties on the cathode side remain within a narrow, suitable envelope throughout the flight. The components must maintain almost constant temperature, pressure and humidity levels under widely varying ambient conditions. The choice of components must take into account the aviation-specific requirements for weight and waste heat. In this numerical study, we investigate a novel cathode air supply system for a hydrogen fuel cell propulsion system which replaces the state-of-the-art electrical components used to drive the compressor in the cathode air supply system with a hydrogen-fuelled micro gas turbine. Previous studies have shown the potential of waste heat and overall cathode gas path size reduction but the off-design performance of such system is yet to be investigated. Hence, based on realistic regional aircraft flight missions and realistic atmospheric conditions, we investigate the off-design performance of the propulsion system. Therefore, a constant mass flow algorithm along cathode and gas turbine gas paths is developed and presented. Next, earth observation data are used to determine realistic boundary conditions and air contamination. Based on these data, the possible contaminant ingestion of the fuel cell is evaluated to allow for future sizing of filters for robust operation. Furthermore, the effects of realistic ambient conditions on the thermodynamic cycle yield important information about necessary revisions of the cycle design point.

Boundary Layer Ingestion propulsors operate in an adverse aerodynamic environment with high levels of distortion. With the purpose of extending the operating range of transonic fan rotors for BLI applications, in this paper we present an optimisation study focused on blade profiles design under different working conditions. Quasi-2D blade sections are optimised using a genetic algorithm and numerical simulations, by varying the camberline and thickness distribution. A method to efficiently achieve a combination of total pressure ratio at a given relative inlet Mach number is devised. The isentropic efficiency is optimised at the design point, concurrently with the stall total pressure ratio at a lower inlet Mach number, in a multi-objective fashion. Pareto-optimal profiles exhibit a moderate leading edge concavity for high efficiency and a straighter fore part with increased trailing edge deflection for higher compression at stall. Optimised airfoils are used in a preliminary three-dimensional evaluation with a realistic BLI inflow, in which the unsteady full-annulus analysis corroborates the approach of the sectional optimisation, also showing the possibility of estimating the integral performance of the machine with a simplified approach based on a single-passage simulation with a circumferential-averaged inflow distribution.

A common solution for reducing the tonal noise annoyance caused by fans is to change the circumferential blade spacing from even to uneven. However, this technique requires predictive tools to simulate and assess their acoustic performance at a lower cost compared to experimental tests, which remain very costly. In this study, a hybrid analytic/numeric (HAN) approach for predicting the tonal noise of fans is proposed. It is based on the acoustic interference law, which is applied to the sound pressure generated by each blade, and Computational Aeroacoustics (CAA). This model allows for the analytical construction of a fan’s acoustic pressure spectrum from the numerically computed response of a single blade, significantly reducing computation time. An optimization procedure is then implemented to minimize the prominence of tonal noise peaks, where the decision variables are the blades’ angular positions and the constraints are rotor balance and the minimum angular distance between adjacent blades. The results show that the developed method may help designers reduce tonal noise annoyance by optimizing blade spacing.

Transonic compressors encounter significant challenges from shock formations due to high-speed supersonic blade tips, particularly at high altitudes where lower Reynolds numbers result in laminar boundary layer separation and increased mixing losses. Understanding shock wave–boundary layer interaction (SBLI) is essential for improving compressor performance. This study examines SBLI under varying Reynolds numbers, simulating higher altitude conditions in a transonic blow-down wind tunnel. Using an inlet valve setup to control inflow total pressure and Reynolds numbers, this study also reveals an increase in turbulence. The findings indicate that laminar-to-turbulent transition occurs upstream of the shock wave, resulting in interaction with a turbulent boundary layer, even at lower Reynolds numbers.

Large-diameter axial-flow fans are predominantly used for cooling purposes, such as in air-cooled heat exchangers. Since it is difficult to experimentally test large-scale fans in the controlled environments provided by fan test facilities, smaller scaled-down versions of the fans are tested instead. Scaling laws, also called affinity laws, are then used to determine the performance characteristics of the large-scale fan. The size difference between the two scaled fans means that it is not possible to match their Reynolds numbers when testing with the same test fluid. A comparison is conducted using experimental results and four numerical models for two different fans, which are scaled to different fan sizes: 0.63 m, 1.542 m, 3.658 m and 7.315 m, to determine the effect of Reynolds number on the performance characteristics of an axial-flow fan. The numerical geometries are based on the M- and B2a-fans, and are tested in the A-type experimental setup fan test facility at Stellenbosch University, which is used to obtain the experimental results. It was found that the numerical approach discussed within this paper, namely a Reynolds-Averaged Navier–Stokes (RANS) approach, can predict the performance of multiple fan sizes without relying on turbomachinery or blade-specific empirical correlations. This approach accelerates the evaluation of fan performance while enabling the parameterization of fan configurations.