| Tunnel plays an important role as one of the main buildings when dam discharging. However, influenced by terrain and geological factors, many non-pressure tunnels not only have big bottom slopes but also have long tunnels. The big head difference between the entrance and exit of tunnel makes the velocity of flow growing more than16m/s immediately, and this kind of tunnel belongs to high flow rate tunnel, so energy dissipater is essential to control the energy. Traditional dissipation tools can only effect before the entrance or after the exit of tunnel, which cannot protect the tunnel body. Furthermore, because of limited terrain, the downstream of some projects does not have enough space to build bottom-flow dissipation and does not meet the condition of ski-jump energy dissipation. Therefore, if step dissipater can be set in tunnel to dissipate energy, huge benefits will be produced.This dissertation formulate the flow condition, flow pattern, flow profile, time averaged pressure and fluctuating pressure of horizontal and vertical plane, step aeration characteristic long the sectional and frictional direction, absolute energy dissipation effect by model test and numerical modeling. The main contents of the studies and achievements are as follows:1. Study on flow condition of step dissipater in hydraulic tunnel. This dissertation proposes that flow with step dissipater in tunnel can be divided into three flow conditions: nappe flow, transition flow and skimming flow. Froude number in hydraulic tunnel ranges larger than in spillway. Under the same unit discharge, different Froude numbers in the top of discharge trough will bring out different flow condition along steps. It shows that flow condition is closely related to Froude number in the top of discharge trough and has put forward empirical formula of boundary between nappe flow and transition flow.2. Study on the influence of little wedge step, which is set at the top of step dissipater in tunnel on flow. With a little wedge step set at top, if the height of first three steps is0.5m, height of3th-10th steps is0.5-1.0m, height of step after10th is1.0-2.0and the top steps are set as wedge-shape step incline downwards and then become horizontal steps, the unit discharge occurs skimming flow will become smaller, and the angle between nappe and the horizontal plane will be smaller, and the impact force between nappe and bottom will be smaller consequently, which will make water become smoother and will improve unfavorable flow regime like jets. 3. Study on a continuous bucket followed by arc or ogee bottom. The gradient of curve bottom becomes sleeper and can solve problems like ultra-low Fr, and ponding in aerator with gentle bottom. Flow with ultra-low Fr is easily having ponding problem. This study shows that jet should fall on the middle or tail of arc or ogee bottom slope, otherwise, the curve bottom slope cannot restrain from going back. With the same horizontal length, the bigger the dispersion of arc or ogee bottom slope is, water can be removed more. The gradient of the top of ogee bottom slope is gentler, and the middle is steeper, it can restrain water from going back better, and can clear away water in cavity completely, and increasing the aeration efficiency.4. Study on the relationship between throughput of aerator qs, unit discharge q and Froude number of flow Fr. By systematic researching, author has thrown out empirical formula. In addition, in a practical engineering, the throughput of aerator can be calculated depending on the biggest flow and the biggest Froude number.5. Study on the position of aerification equal point and aeration properties of step dissipater. The trend of aeration along depth direction of each step is:the air concentration increases slowly at0.1-0.3times depth of water and it decreases substantially at0.4-0.6times depth of water, and increases gradually above0.8times depth of water. At0.4-0.6times depth of water. With the increase of unit discharge, air concentration along depth direction becomes smaller; in addition, lower air concentration changes bigger than the upper part. With the increase of Froude number, gradient of bottom slope and height of steps, the entirely air concentration becomes bigger. Moreover, the change of Froude number has a greater effect on the air concentration of middle of upstream steps, and gradient of bottom slope and height of steps have little influence on air concentration at the same section. However, gradient of bottom slope has great influence on the air concentration of top or middle of flow.6. Study on the change rule of Froude number of water on steps. The bigger unit discharge is, the earlier Froude number along steps tends to stability, and when it is stable, the bigger relative Froude is. With the increase of Froude number at the beginning of discharge chute, the relative Froude number along steps decreases, the Froude number at downstream step section tends to stability earlier. In addition, with the increase of gradient of bottom slope and height of steps, Froude number along steps increases gradually, and is volatile, and more downstream steps are needed to achieve a stable Froude number. In fact, the relative Froude number of downstream achieves stability around the10th step.7. Study on formula of step energy dissipater along steps in tunnel. Started with the formula of absolute energy dissipater of step dissipater and with the condition of fixed flow condition at the top of discharge chute, function between absolute energy dissipater along steps and average Froude number of flow section of downstream step Fr2can be pushed out. Moreover, this dissertation systemically studied influences on flow Froude number along steps Fr2, such as unit discharge q, flow Froude number at the top of discharge chute Fr1, gradient of bottom slope i and height of step h, and pushed out formula of absolute energy dissipater, which agrees well with measurement data, and point out that the energy dissipater procedure is the same as decreasing flow Froude number on-way in essence.8. Study on affecting factors of energy dissipater. With the increase of number of steps, energy dissipater on-way increases nonlinearly, and the rate of growth becomes slower. In addition, at the same part of step, with the increase of rate of flow, energy dissipater decreased gradually; and with the increase of Froude number at top of discharge chute, energy dissipater increases; and with the increase of gradient of discharge chute bottom, energy dissipater decreases linearly. Among them, the change of Froude number mainly affects the energy dissipater. When Froude number of flow along steps reaches the steady value, energy dissipater at same part is unrelated to height of steps. Moreover, under the same condition of same gradient of bottom slope, same unit discharge and same Froude number of flow at top of discharge chute, and when Froude number along steps reaches the steady value, energy dissipater at the same section is unrelated to height of steps, in other words, it is irrelevant to the number of step.9. Study on the reasonable step style and length of step. When step reaches a certain length, it is not economically to promote energy dissipater by increasing the length of steps.which can be implied from increase-decrease characteristics of functions that energy dissipater n is a monotone increasing concave function that depends on relative section dispersion (?)H1-2/y1. If step is high, the volatility of flow along steps will be big, and it will need more steps for Froude number along steps to reach stability, and when it reaches stability, the height of steps has no influence on energy dissipater, so wedge steps can be used to control flow regime. When the bottom is slope and the unit discharge is big, about50-130steps are need to reach higher energy dissipater. But in practical engineering, the volume of it is huge, so such project is not suitable to use step dissipater.10. Study on potential cavitation, erode and vibration of step dissipater in tunnel. This dissertation first analyzed the influences of unit discharge, flow Froude number, gradient of bottom slope and height of steps on time average pressure of horizontal and vertical plane of steps, intensity of fluctuating pressure, maximum and minimum fluctuating pressure and energy spectrum of pulse, and then evaluate safety problems as cavitation, scour and vibration in tunnel with step dissipater. The study shows that, the maximum average pressure p/h=3.5, the maximum fluctuating pressure p/h=9.5. In addition, the first step’s maximum negative pressure p/h=-0.8, the maximum negative pressure along steps p/h=-0.6and the minimum fluctuation pressure p/h=-2.5m. Under the condition of adequate aeration, the maximum positive pressure on horizontal plane of step and maximum negative pressure on vertical plane step are under the control of concrete. Flow on step dissipater has a great difference with it, so it will not induce vibration on step dissipater.When designing step dissipater in practical hydraulic tunnel, the type and length of steps should take flow regime, aeration property, energy dissipation into consideration. If there is enough space in tunnel, trying to make flow become skimming flow, and make sure the middle and downstream flow is aerified sufficiently, and should also control the flow velocity of the exit of tunnel to make sure erosion damage of downstream riverbed and bank slope. If the space of tunnel is limited, energy dissipation and flow condition should be considered first, and then the uniformity aeration of flow. |
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