Dissipation d'énergie dans un puits à vortex

Energy dissiption in vortex shaft

Electricité de France.

Résumé

Certain types of power plant operation require a bypass system across one or more power units, with suitable energydissipation arrangements. Except in the low turbine head range, 'vortex shafts' frequently provide the answer to the problem, especially in the case of underground power stations. There three main parts of a "vortex shaft" are as follows: 1. A spiral chamber and its supply canal ; 2. A vertical shaft ; 3. An elbow at the bottom of the shaft. The spiral chamber imparts swirling motion to the flow, so that it 'hugs' the shaft walls on its way down. Though this swirling motion is very stable, care is nevertheless required in designing the spiral chamher to ensure an even water distribution around the shaft. Inflow conditions to a flat-bottomed spiral chamber should he of the streaming flow type. Chamber water level variation is linear with rate of flow. Assuming negligible loss of head in the spiral chamber and constant atmospheric pressure within the fluid vein, the kinetic energy along the shaft is found by application of the incompressible fluid flow and universal head loss formulae : Jds = λ / a Rh V2 / 2g ds The kinetic energy V2/2 g at level z is given by the following relationship : z = v2/2g 0 d (V2/2 g) / 1 - (λπD√2g/4Q) (V2/2g)3/2 The Reynolds number is constant, and hence also λ for hydraulically smooth flow. These relationships show that the theoretical velocity limit for a shaft of infinite depth is as fol1ows : V∞2/2g = 4e/λ As the energy of rotation is much smaller than the potential energy, it can be considered that the only effect of the swirling motion is to stabilise the flow along the shaft walls, and that its effect on the loss of energy is negligible. These various relationships have been checked on a model shaft 25 centimetres in diameter, with varying wall roughness. Residual energy in the flow is absorbed at the foot of the shaft by a cushion of water, the depth of which is determined by the elbow connecting the shaft to the tailrace. The latter should preferably be of the free-flow type, in order to separate the air out from the emulsified water. The results of this study were applied in designing a relief bypass system for the Oraison power plant, in which the water falls through a shaft 7 metres in diameter from a height of 65 metres ; the peak discharge is 88 cubic metres/sec. 62 per cent of the energy is dissipated by wall friction in the shaft, and the remainder in the bottom elbow, and the flow very nearly reaches the velocity limit. This system has been operating satisfactorily for 1,600 hours.