GB2495285A - Axial flow helical water or wind turbine - Google Patents

Abstract

An axial flow water turbine has helical blades and may be used in ocean and river current turbines, either horizontally, or at an angle (inclined) to the surface so that the generator and gearbox are above the surface. The blades may be formed from a sheet of material, e.g. in a cardioid shape (see figure 1). The ends of the blades are then attached to hubs to give the required overall shape. The rotor may be fully submerged, or only partially submerged. The rotor may be used in either direction, and may also be used as a wind turbine, pump impeller, marine propeller or mixer.

Description

Axial Flow Turbine ImpeDer.

1.1 Introduction

This invention relates to the design of an impeller for use in current flow axial turbines.

However, it can also be used as a marine propeller, a pump impeller, in mixing and blending applications, and wind turbines.

Existing axial flow turbine impellers/rotors are either multi-vane or types of propeller.

Open turbines extract energy from the fluid by reducing the velocity with virtually no pressure reduction as the fluid passes through the impeller. The stream lines must expand to maintain continuity, but they cannot expand indefinitely. Thus, there is a theoretical limit to the kinetic energy that can be extracted. This, the Coefficient of Power or Energy efficiency Cp, has been shown to have a maximum of 59.35% by Betz for a single actuator disc -the surface from which energy is extracted as the flow passes through it. Other researchers have even suggested a lower figure of 30.1% but all these were in relation to wind turbines. It still has to be proved that it applies to full bladed rotor water turbines, where there is some thought that more energy could come from the inertia of the incompressible water pushing behind the water going through the turbine.

However, existing unshrouded axial flow water turbines currently in use, or being tested, only show a Cp of between 25 and 35%.

The invented impeller has depth, making it in essence a continuous series of surfaces (actuator discs) from which energy is extracted and therefore a more efficient energy convertor.

The present invention shows Cp values of up to 39% with theoretical calculations of up to 55% 1.2 Design Concept.

The new helicoidal impeller is an axial flow device and the principal, of operation is analogous to that of other helicoidal axial marine current turbines (HAMCT's).

The turbine consists of three blades mounted on a single axis with the geometry of the blades generated from the cardloid pattern (Fig 1). The two ends A and B are obtained from the origin of the cardioids as the nose and tail of the blades.These two ends are then extended along the shaft axis and each is mounted on the shaft at a predetermined point depending on the pitch required (Fig 2).The shape is symmetrical along the axis and therefore can work as efficiently in either direction. The typical Pitch to Diameter ratio is 1 but can be as low as 04. The inner surface of the blade is inscribed inside the cardiod shape to give the blade width.

Each blade is calculated from the formula r =2a(1-CosØ) where: r= length of each radian at factor for determining the ultimate size of the cardioid pattern and thus the impeller.

0= the angle of each radian.

Other formulas can be use for generating the cardioid curve.

In this case the value of a' for the prototype was a = 5.1 to give a finished impeller of 28cms diameter (R=0.14m) The cardioid shape in Figi is then drawn to scale as the pattern for each blade.

The width of each blade in the prototype was chosen as 30% of half the width of the cardioid. The cardiod maximum width is win Fig 1 which is the length of the l2Odeg.

radian * sin 60 * 2.

From r=2a(1-CosO), the l2Odeg. radian =13.3 and Thus w = 26.4cms Blade width = (w/2)*30% = 3.96cms. Say 4.0 mis as used in the prototype.

Other widths can be used but this was found to be the most efficient.

The table below gives the factor a'; Swept radius R; and Blade width.

Factor a' Swept Blade Radius R Width metres metres 0. 14 0. 03 96 36.4 1.0 0.282 .50.423 109.2 3.0 0.846 5.01.410 1.3 Building the Impeller The thickness of each blade was 0.914mm (2Ogg) for the prototype.

The three blades are mounted round the shaft at equal intervals of lZOdeg.

Left and right handed turning impellers can be made depending on which end (A or B) is mounted first.

The overall length is equivalent to a propellers pitch. The prototype model had a pitch to diameter ratio P/D of approximately 1 The first prototype was made with a 6mm shaft and 3omms. mounting spheres fixed to it so that the blade ends are 28cms.apart. Each sphere has three grooves cut into it at 4sdeg. equally spaced round the circumference.

Mount one end of each blade in one sphere and extend the blade to fit into the other sphere. Pin for security.

Later models were made with a 30mm steel shaft, and grooves cut into it equafly spaced round it at the appropriate angle for the pitch. The blades in this case were also of steel which can then be brazed to the shaft. (Figs. 3 & 4) A model was made with a P/v of approximately 0.5 (Fig 7)where it was found that the value of Cp was greatest. In this case the mounting grooves were at 70 degrees to the shaft axis Other shapes can be used for the blade template e.g. a circle, various spirals etc., but the carclioid was found to be the most perfect, symmetrical and efficient when formed into the blade.

This claim includes all other shapes to create a spiral design including those with 1,2,3,4,5 or more blades, (The more blades give increasing problems of parts being hidden from the water flow) and all P/D ratios The blades can also be made from fibreglass, or the whole impeller, including shaft, as a solid brass or other material casting, where the dimensions can be created from an XYZ set of co-ordinates or by using CAD programmes.

Blades made to shape only need fitting into suitable mountings on an appropriate shaft.

Blades can also be profiled, but the advantages over the simplicity of a flat surface has yet to be proved and this claim covers impellers made with profiled blades and non profiled blades.

It should be noted that this impeller design also works when placed at an angle to the water flow,( Figs) with the generator and any gearbox out of the water. And also with the shaft horizontal, just on the water surface.(Fig 6