The design of a spillway, called a countereddy spillway, was,proposed and investigated at the V. V. Kuibyshev Moscow Civil Engineering Institute (MISI).The spillway (Fig. i) consists of an inlet pressure conduit 1 from which branch six conduits 2 of smaller diameter conveying water to a cylindrical chamber 7 separated by a deflector 6. The chamber with a length of 2-3 diameters passes into the outlet conduit, which can be both pressure and freeflow. The water is delivered to the chamber by conduits 2 tangentially, whereupon three conduits (section A-A) swirl the flow in one direction and the other three (section B-B) in the opposite direction.Both swirled flows, by-passing the separating deflector 6, enter the chamber, where their interaction and intense dissipation of the energy of the swirl (kinetic and pressure energy) occur. Also provided for is the entry of an axial flow into the dissipation chamber through conduit 3, which should promote stabilization of the flow and reduction of cavitation phenomena.Conduits 2 and 3 are equipped with gates 4, which can be in two positions:closed or completely open. There is also an air duct with a gate 5 through which air can be fed into the central part of the flow. The discharge is regulated by bringing into operation a various number of tangential conduits 2.An important advantage of such a spillway compared to the scheme of separating and joining two swirled flows [i] is that the zone of intense interaction of the two flows is located within the common flow and does not close up on the walls of the dissipation chamber, which eliminates the danger of occurrence of dynamic actions on the spillway structure.The swirled flows increase the pressure on the chamber walls within the limits of the deflector and tangential inlets, which promotes a decrease of velocity and prevents the occurrence of cavitation.The energy is dissipated on a short length of chamber 7.
The dynamic loads absorbed by pressure conduits of hydroelectric stations (HESs) and pumped-storage stations (PSSs) are composed of water:hammer, occurring during transient processes and caused by a change in the operating regime of the unit, and of fluctuations occurring both during steady and transient processes. Whereas the phenomenon of water hammer has been studied rather thoroughly and many scientific investigations and works have been devoted to this problem, the problem of fluctuations is still in the initial stage of study. Yet fluctuations can have a substantial effect on the reliability and life of pressure conduits and equipment and in connection with this their investigation and development of methods of taking them into account during design and operation is an urgent problem.Various types of pulsation loads are possible in pressure conduits. Sometimes selfexcited pressure fluctuations occur, which are created in a hydroelastic system having a device acting on the flow rate and characterized by the fact that with increase of head (pressure) the discharge being passed decreases. This system is hydraulically unstable and strong and even dangerous fluctuations with a pronounced resonance frequency can occur in it. Such fluctuations induced by vibrations of a spherical valve were noted, for example, at the Bersimis HES [6]. Stretches of unstable flow, for example, in forks, which occur in the presence of an unbalanced relationship of the discharges of the branches, when fluctuating stagnant zones are created, can be exciters of pressure fluctuations. But these are all special cases. The most common are forced pressure oscillations induced by disturbances created by the operation of the units. It is necessary to note that the nature of these disturbances has still notbeen completely revealed and on this account various hypotheses are expressed. It is considered that the cause can be vane disturbances (impacts) [I, 2], rotational separation of the flow in which the axial symmetry of flow past the circular rows of the blades of the unit is disturbed [3], and vortex filaments behind the runner and their movement (precession) in the draft tube [2, 4, 8, 9, i0]. Obviously, all the factors mentioned can act to a greater or less degree depending on the geometry and operating regime of the units.For calculations of pressure fluctuations in pressure cond@its the resultant action should be expressed in the change in the discharge being passed by the unit at a given head or in the change in the head being developed at a given discharge. So long as there is insufficient data on the actual characteristics of the disturbances it is ncessary to base oneself on some kind of working hypothesis. In the majority of works it is assumed that the disturbance can be represented in the form of the fluctuation of the discharge capacity or reduced discharge AQ'_.I for a given opening of the unit. Then the change in the discharge in dimensionless parameters Aq will be represented by the equa~lon Aq = Aq~ ~l + ~ + +~,where Aq = AQo...
The availability of a spillway system capable of conveying and regulating large flows within a wide range of heads up to 200-300 m and over would permit reducing subsrantially the volume of work and the construction costs for hydraulic developments; thus, the problem of creating such spillway systems becomes extremely important. However, this calls for the solution of many complex problems involved in the construction of high-head gates and the protection against cavitation and dissipation of hydraulic energy.A11 known gates in hydraulic developments operate on the principle of flow contraction, that is, they regulate the flow by varying the area of the hydraulic passage (Fig. la), which leads to the development of high fiowvelocities. The velocity at the contracted section is determined from the equationin which H is the head and ~ is the velocity coefficient (q = 1).Thus, downstream from the gate practically ali the excess energy is transformed into kinetic energy, which quickiy increases as the head increases and which is the cause of the difficulties.The cavitation coefficient or parameter for any element around which the water flows can be expressed by the equation (P/'Oabs-(p/y) wp ~ca~-
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