| The Circular-ring sediment desilting basin(Hereinafter referred to as Circular-ring) is a new secondary sediment treatment facilities. It has initially been applied in the head of Hutubi Aweitan canal in Xinjiang province, with sediment deposition in diversion channel solved and provision for agricultural water guaranteed in Aweitan irrigated area. Since 2006, the Circular-ring has performed well. Taking Circular rings as a prototype, the author applies the methods of physical model experiment, numerical simulation and theoretical analysis to analyze the distribution rules of the flow field and sediment concentration, and the characteristics of desilting sediment. Meanwhile the mechanism of separation between water and sediment and the optimized structure are explored. The main conclusions are as follows:(1)The distribution of radial velocity is done based on the model experiment in Circular-ring under different conditions. The results show that: the radial velocity and its gradient present uneven distribution under the condition that the water-outflowing central ring does not be heightened in the basin. The value of radial velocity and its gradient goes to peak interval arranging from 135° to 225°, especially when its cross-section is towards 180° radial direction, their value goes to peak; the value of radial velocity and its gradient present a negative correlation with Circular-ring’s radius along the radial direction, and also the value of radial velocity presents a negative correlation with depth of water along the vertical direction; the value of radial velocity, its gradient and turbulence intensity of water flow is higher in the circle within 1/2 radius from the center, while the value is low in the rest range of Circular ring; there is recirculation zone in radial cross section. When the height of the overflow weir is raised, the radial velocity in Circular-ring decreased. It is beneficial for desilting efficiency. However, the state of uneven distribution of radial velocity cannot be changed in the basin. When the height of the water-outflowing central ring is raised to 9cm, the distribution of radial velocity and its gradient is uniform, which shows more beneficial for desilting efficiency.(2) The results based on RNG k-? model and VOF method which can accurately calculate the air-water two phase flow field, are relatively reliable under two working conditions which one is to keep the height of water-outflowing central ring in Circular-ring same, and the other is to increase the its height to 9cm. Hence, the model experiment can be replaced by the numerical simulation effectively to some extent in similar projects, which improves the research efficiency and also covers the shortage of physical model experiments. The results after numerical simulation show that: the air-water two phase flow field is three-dimension flow field made up by radial velocity, tangential velocity and axial velocity. The value of radial velocity is higher than that of tangential velocity and axial velocity, while the value of axial velocity is the lowest. There is the same distribution law between turbulent flow energy of different radial sections and radial velocity along the vertical direction, which indicates that the radial velocity plays a key role in the Circular-ring. The recirculation zone which count against the setting of fine sediment, is yielded along the radial direction in Circular-ring. The distribution of Flowing velocity and turbulent flow energy in Circular-ring presents uneven state, especially in the range between 135° and 225°. When the height of water-outflowing central ring is raised to 9cm, however, their distribution presents even state with the significantly narrowing of upwards flowing area, which is more beneficial for setting of silt particles.(3) The model experiment is used to test the characteristics of sediment concentration distribution and desilting of Circular-ring under different conditions. The results show that the distribution of sediment concentration, desilting range and diameter are non-uniform in the working condition which the height of water-outflowing central ring in Circular-ring stays the same, in particular, the value of the three indexes are higher in the range between 135°and 225°than the rest. However, the sediment concentration and diameter are inversely proportional to the radius of Circular-ring along the radial direction, and the sediment concentration varies inversely with water depth along the vertical direction within 1/2 radius from the center, the vertical distribution is more non-uniform while the sediment diameter is wider than in other value. When the height of water-outflowing central ring is raised to 9cm, the distribution of sediment concentration, desilting range and diameter are more uniform than in other conditions. The phenomenon of sorting sediment particle physically exists in the Circular-ring and the separation of water and sediment relies on gravity sedimentation. Under the designed flows and sediment concentration conditions, the desilting rate is 90.9%, the water consumption is only 6.6 % and the rang of sediment diameter is between 1mm < D ≤ 84 mm in the working condition which the height of water-outflowing central ring in Circular-ring stays the same. When the height of water-outflowing central ring is raised to 9cm, the desilting rate is 94.2 %, the water consumption is only 4.37 %, and the sediment diameter is between 1mm<D≤84mm, while the capacity of treating suspended load is increased. The regression model of desilting time and influent flow is t=1545.8Q-0.997 in the working condition which the height of water-outflowing central ring in Circular-ring stays the same, while model comes to t=2152.55Q-1 when the height raised to 9cm. The Circular-ring is a kind of sediment processing facility discharging sediment which works by means of the kinetic energy of inpouring water, steep bottom slope of inverted cone, the self-weight sediment as well as high-speed circulation; it has an advantage when working in the rivers whose sediment mainly are bed load.(4) Considering the study results of model experiment, numerical simulation and theoretical analysis, the water-sediment separation mechanism of Circular-ring is detected from the perspectives of its structure, current velocity, sediment concentration and the character of how the sediment diameter distributing. The length of overflow weir is the key point influencing the water-sediment separation result of the Circular-ring, the longer, the better. Under the conditions of designed flows, the regression model between Circular-ring radius and the maximum value of overflow sediment diameter is d=-2.43 + 52.04/R, when the ratio between the diameter of central outflow ring and the diameter of overflow weir, that is D0/D=1/15, for the sediment whose diameters wider than 1mm and totally submerged in the Circular-ring, the desilting rate of these sediment is high up to 100%. With the increasing of the Circular-ring diameter, the value of D0/D diminishes, the maximum value of overflow sediment diameter decreases fast, and the sediment diameter whose desilting rate is 100% decreases as well. Under the same condition of inpouring water, with the increasing of Circular-ring diameter, the length of overflow weir grows fast, and the desilting rate also increases while the water consumption reduces. When the diameter value of Circular-ring keeps still, once the value of flow and sediment concentration is lowered than the designed ones, the desilting rate grows while the water consumption reduces. |
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