A capacitance-based void fraction sensor has been developed for the rocket or airbreathing engines, which is simple and do not disturb the flow. Typical conventional sensors usually have two concave electrodes mounted on the outer wall of the dielectric tube. They are relatively low accuracy if they have a noise shield; the maximum measurement error is over 30% in our research. The aim of this study is to improve the measurement accuracy while keeping the advantage of simplicity, mountability and non-intrusive characteristics. A theoretical formulae and electromagnetic field analysis, EFA, are used to design the sensors and are compared to an experiment using air/silicon-oil mixture flow. As the result, a newly developed asymmetrical type sensor which consists of asymmetric flat electrodes with side walls shows good performance; the inaccuracy between true void fraction and measured void fraction is 6% for the stratified flow.
This study experimentally investigates the unstable flow in a five-stage centrifugal blower with inlet guide vanes (IGVs) upstream of the first stage. Performance characteristics of the blower were obtained under three IGVs opening conditions. Measurements of the unsteady pressure were performed using six high-response pressure transducers. To distinguish rotating stall from surge, three of these transducers were located thirty degrees apart in the circumferential direction at the vaneless space upstream of the diffuser vane. Three types of pressure fluctuation in the low flowrate region were confirmed. The first one occurred in the positive slope region of the total-system head curve with the largest amplitude and the lowest frequency under the full opening condition of IGVs. The second one was the spike-like fluctuation propagating in the circumferential direction in the vaneless space. The last one appeared under the partial opening condition of IGVs with smaller amplitude and higher frequency than those of the first one. It was confirmed that this pressure fluctuation oscillated at the same phase in the vaneless space and occurred even in the steeply negative slope region of the total-system head curve under the minimum opening condition of IGVs. It was considered that the first one was deep surge and the second one was rotating stall. The last one was considered to be mild surge. Under the minimum opening condition of IGVs, the slope of the head curve of the first stage becomes steeply negative by prewhirl. This negative slope stabilizes the total system of the blower. By contrast, the slope of the head curve of the other stages becomes positive in the low flowrate region. This positive slope destabilizes the total system of the blower. Therefore, even if the total-system head curve keeps a steeply negative slope, the system operating point is considered to oscillate slightly, corresponding to mild surge situation, due to these stability balances.
To reduce costs involved in manufacturing small wind turbines, an aluminum circular-blade butterfly wind turbine (ACBBWT) has been developed, in which four blades of the turbine were extruded and bent to shape then attached directly to a rotating flange. The ACBBWT is a vertical axis wind turbine (VAWT) and the rotor diameter of the prototype is 2.06 m. Experiments to obtain the output performance were conducted outdoors using an axial blower; however, the data obtained were rather scattered due to the effects of natural wind. Therefore, performance curves in the high wind speed range are predicted by fitting theoretical curves based on the Blade Element Momentum (BEM) theory, in which modification of virtual incidence due to flow curvature effects is included. Three-dimensional computational fluid dynamics (CFD) analysis of a circular-blade wind turbine model (dia. 2 m) with a shape almost identical to that of the experimental rotor is performed. The results assuming an energy-conversion efficiency of 0.8 agree well with the experimental results at 7 m/s. CFD analysis shows that tip vortices are shed from the top and bottom parts of a circular blade, as with straight-blade VAWTs. However, vorticity in the circular-blade case is lower than that in the straight-blade case, and the cross-section of each tip vortex shed from circular blades appears to be in the shape of a deformed ellipse. In cases of small tip speed ratios, vortex shedding caused by the dynamic stall phenomena is observed around the equator plane in both the downstream and upstream regions, and the vortex shed in the downstream region by a circular blade forms a looped shape. Since distributions of surface pressure and skin friction obtained by 3D-CFD have a similar pattern in both the upstream and downstream regions, which is related to vortex shedding, it is considered that the vortex in the upstream region is likely to also have a looped shape.
An optimum aerodynamic design method has been developed for the wind-lens turbine. The wind-lens turbine has a brimmed diffuser around a turbine rotor, which is referred to as wind-lens. The wind-lens can achieve the wind concentration on the turbine rotor, resulting in the significant enhancement of the turbine output. The present design method is based on a quasi-three-dimensional aerodynamic design method and a genetic algorithm. The quasi-three-dimensional design consists of two parts: a meridional viscous flow analysis and two-dimensional blade element designs. In the meridional viscous flow analysis, the axisymmetric Reynolds-averaged Navier-Stokes equations are numerically solved on a meridional plane to determine the wind flow rate through the wind-lens and the spanwise flow distribution at the rotor. The turbine rotor blade geometry is determined by the two-dimensional blade element theory based on the momentum theorem of the ducted turbine. The turbine rotor and wind-lens are simultaneously optimized by the present design method. Aerodynamic performance and flow fields in the optimum and conventional design cases have been investigated by wind tunnel tests and three-dimensional Reynolds-averaged Navier-Stokes simulations, in order to verify the effectiveness of the present design method. It is found that the optimum design case achieves the significant improvement in the output power coefficient, so that its numerical and experimental results of the output power coefficient exceed the Betz limit, which is the theoretical maximum output power coefficient for bare wind turbines. It is revealed that the aerodynamic matching between the turbine rotor and the wind-lens is essential to the performance enhancement of the wind-lens turbine.
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