Supercritical carbon dioxide (S-CO2) Brayton cycle system is a promising closed-loop energy conversion system frequently mentioned in the automotive and power generation field in recent years. To develop a suitable design methodology for S-CO2 turbines with better performance, an understanding of the vortex flow patterns and associated aerodynamic loss inside a S-CO2 turbine is essential. In this paper, a hundred-kilowatt level S-CO2 axial turbine is designed and investigated using a three-dimensional transient viscous flow simulation. The NIST Span and Wagner equation of state model that considers the real gas effects is utilized to estimate the thermodynamic properties of the supercritical fluid. The numerical methods are experimentally validated. The results indicates that the aspect ratio and tip-to-hub ratio are different in the S-CO2 turbine from that in the gas turbine, and the vortex flow patterns are influenced notably by these geometrical parameters. Both the vortex structure and moving tracks of passage vortices are changed as a result of large centrifugal force. An interaction between tip leakage vortex and hub passage vortex is observed in the impeller passage and its formation and development mechanism are revealed. To further explore the aerodynamic loss mechanism caused by vortex interaction, the energy loss in the impeller passage is analyzed with the enstrophy dissipation method, which can not only accurately calculate the energy loss but also estimate how the vortical motions occur. It is found that the enstrophy and energy loss can be effectively reduced by vortex interaction between tip leakage vortex and hub passage vortex. The results in this study would increase the knowledge of vortex flow patterns in S-CO2 turbine and the proposed enstrophy production method can be used intuitively to provide a reference for flow vortical motion study in turbines.

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Closed regenerative Brayton cycle utilizing supercritical CO2 as working fluid has attracted more concern recently as it offers high cycle thermal efficiency and decreases size of turbomachinery. The performance testing of the CO2 turbine is very important for the supercritical CO2 cycle development. However, testing the turbine performance in a closed supercritical CO2 cycle is of difficulty and complication. This paper proposes a novel experimental method on CO2 turbine testing using high pressurized air as surrogate fluid. The performance similarity laws for the experimental method are studied, because traditional similarity criteria are not applicable since supercritical CO2 and high pressurized air are both non-ideal gases. A CO2 turbine is simulated by the CFD method under similar operating conditions with CO2 and high pressurized air, and turbine performance and flow field is analyzed. CFD analysis results show that turbine’s overall performance matches up well, and difference of parameter in flow field is small. The proposed experimental method can be used for CO2 turbine performance testing without setting up the complex closed supercritical CO2 cycle test bench.

Proton Exchange Membrane Fuel Cell (PEMFC) is a very attractive power source to meet high efficiency and low emission. For mobility applications, PEMFC needs to have a larger power density and it can be achieved with an air compressor to intake more air for chemical reaction. Different from a turbocharger, the compressor for PEMFC is not driven by a turbine, but by an electric motor as well. Due the limitation of motor speed and compact system size, the air compressor must be in small size and operate with low rotational speed. In compressor aerodynamic study, low specific speed and small size is believed to have large loss and it needs to be further investigated and improved. In this paper, a centrifugal compressor combined with an air bearing is specially developed, with rotational speed as 120k RPM and pressure ratio as 3.5. The compressor impeller, diffuser and volute are designed by mean-line method followed by 3D detailed design. Computational fluid dynamics method is employed to predict compressor performance as well as analyze compressor internal flow field and loss mechanism. Simulation results indicate that major losses including leakage flow loss in impeller and loss in diffuser. As a result, corresponding optimization design method is proposed, the total-to-total aerodynamic efficiency of the redesigned compressor has increased 5% at design point.

Facing the growing traffic fleet in the cities nowadays, it is believed that three-dimensional urban transportation could be a solution and will be introduced in the near future. Vertical take-off and landing flying platforms powered by ducted fans will attract increasingly attention because it has advantages on high propulsion efficiency, low noise, and better safety. However, unlike traditional open-blade multi-rotor drones, ducted fan drones lack a systematic design approach that comprehensively considers the overall system performance and the power unit efficiency. Current design procedure leads to insufficient load capacity and low efficiency systems. This paper proposes an overall design method for a ducted fan-type vertical take-off and landing flight platform. The ducted fan and motor of the core power unit are designed and selected aiming at improving aerodynamic efficiency and structural utilization of the system. A heavy-load vertical take-off and landing Unmanned Aerial Vehicle (UAV) powered by ducted fans with a take-off weight of 450kg is designed based on this method. CFD simulation is utilized to calculate the performance of the designed Unmanned Aerial Vehicle, and finite element analysis is carried out to examine the overall strength safety. The final design results show that the overall design method plays a great role in the development of ducted fan UAV.