Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 84

The analysis described in all prior sections assumes that the rotor has an infinite number of blades of infinitesimal chord so that every fluid particle passing through the rotor disc passes close to a blade through a region of strong interaction, i.e. that the loss of momentum in any annulus is uniform with respect to azimuth angle θ. With a finite (usually small, two or three) number of blades, some fluid particles will interact more strongly with the blades and some less strongly. The immediate loss of (kinetic) momentum by a particle will depend on the distance between its streamline and the blade as the particle passes through the rotor disc. These differences are subsequently reduced but not eliminated by the action of pressure forces between the adjacent curved streamlines and eventually by mixing. The axial induced velocity will therefore vary around the disc, the average value determining the overall axial momentum of the flow. What is also relevant is the incident velocity (relative angle and speed) that each blade section senses, i.e. to which it responds, as it rotates. When as here the incident flow is not uniform, a blade section senses a weighted average of the flow induced in the region occupied by the section in the absence of its own self‐generated flow field. This is usually evaluated as the velocity at the quarter chord of the section (cf. lifting line theory). For a rotor for which the product σλ of solidity and tip speed ratio is not too small, as is usual, it is found that the combination of incident wind, average axial induced velocity, and blade rotation speed gives a very good approximation to this incident velocity except close to the tip and root ends of the blade, where the sectional approximation breaks down.