Schemes of hydroelectric stations with pressure conduits and surge tanks are used rather widely. The upstream surge tanks are usually constructed in the case of a large length of diversion pressure conduits, when their constant of inertia exceeds 4-6 see. However, there can be cases, for example, in the presence of a sudden change in the longitudinal profile of the pressure conduit, when the danger of discontinuity of the flow arises, and a surge tank is needed when the length of the diversion conduit is comparatively small. The use of layouts with downstream pressure conduits, as is known, is permissible only provided there is no discontinuity of the flow behind the turbine runner during regulation of the discharge [i, 2]. The maximum permissible length of downstream pressure conduits mainly depends on the depth of the turbines below the lower pool level, which, as a rule, is taken miminum from the condition of predcluding cavldation. This length, usually does not exceed 100-150 m, and additional deepening of the turbine is required for increasing it, which in most cases is not advisable. Under these conditions a surge tank is constructed even in the case of a relatively small length of the pressure conduitS, for example, 200-300 m.The principles of calculation and the main calculated relations when determining fluctuations in systems with surge tanks have remained practically unchanged for many years. However, a number of factors require additional refinement.In particular, the problem of taking into account losses and the velocity head at the junction of the surge tank with a pressure tunnel has recently attracted attention [3,6].The essence of the problem reduces to the following. where Ld, Fd, Qd are the length, cross-sectional area, and discharge of the diversion conduit; k d is a coefficient taking into account losses over the length and local losses in the diversion conduit; k s is a coefficient taking into account losses at the junction of the transition, in which case Qs = Qd -Qt; kv = i/2gFd = is the velocity head coefficient (Fig. la).The right-side of Eq. (i) is set up so that it corresponds to the initial and final conditions of the transient, since during a steady regimeHowever, during an unsteady process, where the level in the surge tank changes, such a form of consideration of losses and velocity head introduces a certain error. Actually, the results of investigations of forks [2,5,6] show that the head losses in a branch can include a variable component of the velocity head; the structure of the losses in the transition changes as a function of the direction and relation of the water discharges in the branches.As regards these principles, they can be illustrated by two extreme regimes of separation of the flows in the usual branch of a conduit at a right angle; flow regime when passing through, when the discharge into the branch is equal to zero (Fig. Ib), and the regime when the entire discharge enters the branch (Fig. ic).In the first case (dz/dt = 0) the branch can be represented as a piezom...
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