Grant Ingram

Darrieus turbines: The physics of self-starting

Abstract As the demand for renewable energy grows, the use of small wind turbines becomes increasingly attractive. Turbines using vertical-axis geometries are particularly suited to the urban environment by virtue of their operation being independent of wind direction. However, such designs have received much less attention than the more common ‘propeller type’ designs and the understanding of some aspects of their operation remains weak. This is particularly true of their starting characteristics. Indeed, some authors maintain that they cannot start without external assistance. In this investigation a numerical model is used to simulate the starting of an H-rotor Darrieus turbine under steady wind conditions. Experimental wind-tunnel data for a small prototype is presented, demonstrating unaided start-up of a three-bladed Darrieus in a steady wind. Discrepancy between the modelled and experimental results demonstrate that modelling remains constrained by the quality of data on aerofoil characteristics.

A Comparison of Real-Time Thermal Rating Systems in the U.S. and the U.K.

Real-time thermal rating is a smart-grid technology that allows the rating of electrical conductors to be increased based on local weather conditions. Overhead lines are conventionally given a conservative, constant seasonal rating based on seasonal and regional worst case scenarios rather than actual, say, local hourly weather predictions. This paper provides a report of two pioneering schemes-one in the U.S. and one in the U.K.-where real-time thermal ratings have been applied. Thereby, we demonstrate that observing the local weather conditions in real time leads to additional capacity and safer operation. Second, we critically compare both approaches and discuss their limitations. In doing so, we arrive at novel insights which will inform and improve future real-time thermal rating projects.

2-D lumped-parameter thermal modelling of axial flux permanent magnet generators

Results from lumped parameter thermal modeling of an axial flux permanent magnet generator based on the application of the 2-D equivalent thermal circuit are presented. The components of the generator and the internal air-flow domain are split into a system of connected and interacting control volumes. Energy and mass conservation equations are then solved for each of volume to determine its thermal state. This method takes into account heat transfer due to both conduction and convection. Two case studies have been performed to validate the accuracy of the 2-D equivalent thermal circuit model by comparison with CFD results.

The Effect of End-Wall Profiling on Secondary Flow and Loss Development in a Turbine Cascade

The potential for loss reduction by using non-axisymmetric end-wall profiling has been demonstrated in the so called “Durham” cascade (Hartland et al [1]) and in a turbine representative rig (Brennan et al [2] and Rose et al [3]). This paper aim to enhance the understanding of end-wall profiling. It describes detailed measurements from upstream to downstream through the Durham cascade. The measurements cover the profiled end-wall used by Hartland, a second generation end-wall design (Gregory-Smith et. al. [4]) and the planar reference case. Considerable effort has gone into refining the measurement technique used in the cascade and new results are presented for traverses downstream which capture more accurately the flow near the end-wall. These measurements show the development of loss and secondary flow throughout the blade row. It is shown that end-wall profiling has a dramatic effect on the flow patterns in the early part of the blade row which then translates to a loss reduction later in the blade row. Comparison with CFD results aids the understanding of the role of the reduced horseshoe vortex in this process.Copyright

Investigation of a novel secondary flow feature in a turbine cascade with end wall profiling

A novel secondary flow feature, previously unreported for turbine blading as far as the authors are aware, has been discovered. It has been found that it is possible to separate part of the inlet boundary layer on the blade row end wall as it is being over-turned and rolled up into the passage vortex. This flow feature has been discovered during a continuing investigation into the aerodynamic effects of non-axisymmetric end wall profiling. Previous work, using the low speed linear cascade at Durham University, has shown the potential of end wall profiling for reducing secondary losses. The latest study, the results of which are described here, was undertaken to determine the limits of what end wall profiling can achieve. The flow has been investigated in detail with pressure probe traversing and surface flow visualization. This has found that the inlet boundary locally separates, on the early suction side of the passage, generating significant extra loss which feeds directly into the core of the passage vortex. The presence of this new feature gives rise to the unexpected result that the secondary flow, as determined by the exit flow angle deviations and levels of secondary kinetic energy, can be reduced while at the same time the loss is increased. CFD was found to calculate the secondary flows moderately well compared with measurements. However, CFD did not predict this new feature, nor the increase in loss it caused. It is concluded that the application of non-axisymmetric end wall profiling, although it has been shewn to be highly beneficial, can give rise to adverse features that current CFD tools are unable to predict. Improvements to CFD capability are required in order to be able to avoid such features, and obtain the full potential of end wall profiling.

Computations on heat transfer in axial flux permanent magnet machines

Effective cooling is of paramount importance for axial flux permanent magnet (AFPM) machines due to their high power density. This paper presents a computational investigation on the effect of variation in some geometric parameters, (running clearance, rotor groove depth and rotational speed), on the cooling effectiveness of an AFPM machine. The numerical model used has been validated by comparison with a small test rig and the basic flow pattern inside the generator has been described.

An experimental study of reverse compound lean in a linear turbine cascade

Abstract This paper describes a detailed experimental investigation into the effects of reverse compound lean (RCL) in a highly loaded axial turbine cascade. The geometry was designed using fully three-dimensional viscous CFD calculations to achieve a reduction in secondary flow. Traverses were made upstream and downstream with three-hole and five-hole probes to quantify the effects on the flow and losses produced by the leaned blade compared with a prismatic blade. These measurements were supplemented with blade static pressure measurements and surface flow visualization. It was found that the RCL blade produced higher overturning at the end-wall accompanied by higher secondary loss but this was constrained closer to the end-wall. Near mid-span, the turning was reduced slightly but the overall turning for the row was unaltered. The mid-span showed much less loss, so that overall the loss was reduced by 11 per cent. An understanding of these effects may be gained by consideration of the three-dimensional effects produced by the RCL.

The Application of Non-Axisymmetric Endwall Contouring in a Single Stage, Rotating Turbine

As turbine manufacturers strive to develop machines that are more efficient, one area of focus has been the control of secondary flows. To a large extent these methods have been developed through the use of computational fluid dynamics and detailed measurements in linear and annular cascades and proven in full scale engine tests. This study utilises 5-hole probe measurements in a low speed, model turbine in conjunction with computational fluid dynamics to gain a more detailed understanding of the influence of a generic endwall design on the structure of secondary flows within the rotor. This work is aimed at understanding the influence of such endwalls on the structure of secondary flows in the presence of inlet skew, unsteadiness and rotational forces. Results indicate a 0.4% improvement in rotor efficiency as a result of the application of the generic non-axisymmetric endwall contouring. CFD results indicate a clear weakening of the cross passage pressure gradient, but there are also indications that custom endwalls could further improve the gains. Evidence of the influence of endwall contouring on tip clearance flows is also presented.Copyright

A low-Reynolds-number, high-angle-of-attack investigation of wind turbine aerofoils:

This article describes an experimental, aerodynamic investigation of four aerofoils intended for small wind turbine applications. The aerofoils of these small machines (both horizontal and vertical axes) normally experience conditions that are quite different from large-scale machines due to smaller chord length and lower wind speed, resulting in significantly lower Reynolds numbers. They also operate with an unusually wide range of incidence angles (0° to 90° for horizontal axis and 0° to 360° for vertical axis). Four appropriate aerofoils were chosen for testing at three Reynolds numbers (65 000, 90 000, and 150 000) through 360° incidence to cover almost all possible conditions that might be encountered by both types of turbines. The investigations were conducted in terms of lift, drag, and surface static pressure coefficients. The experimental results show that both geometry and Reynolds number had significant effects on aerodynamic lift, not only when unstalled but particularly in the post-stall region from 20° to 50° incidence. These effects were also seen at other incidences but to a lesser extent. By contrast, the drag characteristics were similar for all blade geometries. Static pressure measurement revealed that, at these low Reynolds numbers, separation bubbles always form near the leading edge of the suction surface at moderate incident angles and increase in size with decreasing Reynolds number. Comparisons of force and static pressure measurements showed that the aerofoil stalling behaviour is closely related to the presence of a separation bubble at the leading edge of the suction surface. Discrepancies between the experiments and predictions using the AERODAS model confirm the continued need for accurate wind tunnel testing.

Experiments and Computations on Large Tip Clearance Effects in a Linear Cascade

Large tip clearances typically in the region of 6% exist in the high pressure (HP) stages of compressors of industrial gas turbines. Due to the relatively short annulus height and significant blockage, the tip clearance flow accounts for the largest proportion of loss in the HP. Therefore increasing the understanding of such flows will allow for improvements in design of such compressors, increasing efficiency, stability, and the operating range. Experimental and computational techniques have been used to increase the physical understanding of the tip clearance flows through varying clearances in a linear cascade of controlled-diffusion blades. This paper shows two unexpected results. First the loss does not increase with clearances greater than 4% and second there is an increase in blade loading toward the tip above 2% clearance. It appears that the loss production mechanisms of the pressure driven tip clearance jet do not increase as the clearance is increased to large values. The increase in blade force is attributed to the effect of the strong tip clearance vortex, which does not move across the blade passage to the pressure surface, as is often observed for high stagger blading. These results may be significant for the design of HP compressors for industrial gas turbines.