Études théoriques et expérimentales de l'évolution de l'énergie dans les ouvrages aval des installations hydroélectriques de basse chute

Theoretical and experimental studies on energy changes in low-head hydroelectric plant tailwater structures

¹ E.D.F., Direction des Etudes et Recherches, 6, quai Watier, 78 - Chatou.
² SOGREAH, Grenoble.

Résumé

There still is quite a considerable amount of kinetic energy in the flow discharging from a low-head axial turbine (equivalent to 11 percent of the head in one particular case mentioned). When it leaves the diffuser after a sudden widening of the flow passage, the water is conveyed to the tail water canal. Varying proportions of its energy can be recovered__ i.e. "saved" for the positive side of the plant's energy balance__ depending on canal shape and cross-sectional area relationships. The "remote downstream head" was defined by taking the tail water canal cross-section at which steady flow is restored as the reference cross-section and trying to establish the relationship between the remote downstream head and the downstream head measured at the draft tube exit according to normal experimental rules. The loss of head in the canal must of cause also be allowed for if assessing the turbine alone. By generalizing the Borda-Carnot formula to apply to free-surface flow with a non-uniform velocity distribution, a relationship was found which gave the remote downstream head from known draft tube exit data (formula 1 to 4). Two series of tests were carried out (one in Grenoble and one in Chatou) to confirm these results. In addition to supplying information on the form of the flow and its spatial diffusion, these tests also showed up the important effect of the shape of the downstream structures on energy loss and the considerable distance the flow has to travel after discharging from the turbine before becoming similar and uniform again.