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Currently, large dams in the world, due to high amount of sediments in the reservoir, especially around the intake, have operational problems. One of the solutions for this problem is pressure flushing which is an efficient method for extracting the accumulated sediments behind the dams, where, the valves and turbines are placed over there. In this type of flushing, previously deposited sediments are removed by opening the bottom outlets. Sediment is scoured and a funnel shaped crater is created in the vicinity of the bottom outlet opening. Amount of the flushed sediments depend on many parameters such as water depth on the bottom outlets, discharge released through bottom outlets, size of the outlets, geometry of the reservoir, size and kind of the deposited sediments in the reservoir. But the extent of flushing impact range is limited. On the other hand, since in this method, the waste of water in the reservoir is relatively low; therefore, an appropriate solution for increasing the efficiency of the pressure flushing process, would be able to increase the dam’s lifetime with minimal amount of water waste. In laboratory experiments carried out in this study, the effect of expansion of bottom outlet channel within the reservoir is investigated on the volume and dimensions of the flushing cone. In order to achieve the objective of this study, experiments done by means of a physical model with length 7.1 m, wide 1.4 m and height of 1.5 m. Experiments performed with three bottom outlet channel lengths 10, 20 and 30 cm, three water heights 47.5, 55 and 64.5 cm over center of outlet and three discharge flows equal to 1, 2 and 3 (l/s) for each height. Preliminary results showed that, expansion of bottom outlet channel within the reservoir has positive and tangible effects on the size of the flushing cone. As, with the expansion of bottom outlet channel within the reservoir, new hydraulic conditions are introduced, which affects the mechanism of flushing and by increase in the length of outlet channel, dimension of flushing cone increases. But the rate of increase in dimensions decreases with increment in expansion amount. So the relative amount of bottom outlet channel expansion for 0.5, 1 and 1.5 times height of the sediment in the reservoir, leads to increase in flushing cone volume for average amount of 50, 74 and 96% compared to the case with the no developed bottom outlet channel. according to the experimental data, non-dimensional equations are derived for estimation of the flushing cone size. These equations show high regression coefficients and provide good estimations. Also the results indicate that, In the higher discharges of flow, effect of the expansion size of bottom outlet channel on the amount of the sediment discharged is high.

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Outlet conduits are one of the important parts of dams. Due to the high flow rate and pressure drop, problems such as cavitation can affect these structures. Considering these problems, detailed design is necessary. Laboratory studies are usually carried out which are expensive, thus, numerical models for determining complex flow characteristics have attracted the attention of the designers. In this study, the numerical simulation of Jegin dam outlet conduit in south of Iran with the scale of 1:10 is provided and the results are verified by experimental information taken from physical model built and tested at Water Research Institute. Gate opening in this research is always 70% and the water head is the constant value of 38.6m. The research is focused on the intake gate and not the service one, so the service gate is always fully opened. FLUENT computer code is considered for the numerical model studies. In the numerical simulation the Finite volume mixture two phase flow scheme is used together with k–e turbulence model. The flow discharge and air supply from the air vent downstream of the gate is then computed by 3D numerical model for different channel geometries. Reasonable agreement between the numerical model and experimental results shows reliable performance of the numerical model. This study showed the ability of the numerical model to simulate the complex air water flow in high speed gated tunnels. This study also includes the effect of the height to width ratio of the conduit on flow discharge and aeration downstream of the gate. To do this, different numerical models are simulated among which the difference is the height to width ratio of the conduit. Height and width of the conduit are measured at the gate section and changes are applied in two cases of constant height (depth) and variable width, and constant width and variable height. Results show more aeration and more flow discharge while heightening and widening of the outlet. Flow discharge has also been determined as function of the height to width ratio of the conduit at gate section of the channels. One of the important results is that in comparison with the width changes, height changes of the channel affect hydraulic characteristics of flow more and the diagram rates vary more sharply. Researches show that 1.5 to 2 ranges for height to width ratio is the best range hydraulically and other ranges have effect on reducing aeration and Air demand ratio β (β in this research agrees more with the relation Kalinske and Robertson presented), so the pressure in a conduit may fall considerably below atmospheric pressure which results in cavitation and vibration. To avoid these problems, suggests not to heightening the conduit more than a specific value.

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