Numerical and laboratory investigation of the cavitation phenomenon in the lower discharge ducts

Academic Thesis for Master's Degree in Civil Engineering

Trend of Hydraulic Structures

Abstract:

Obviously, water resources are essential for the life of living beings and especially human life. Dischargers are a set of structures that are used to transfer water from the dam lake to the discharge point downstream. Therefore, due to the importance of this part of the dam, the analysis of the discharge function, including the conduit, valves, and its outlet, is of particular sensitivity. The presence of pressurized flow upstream of the discharge valve, the presence of flow energy loss due to various factors, as well as very low values ??of the ratio of valve opening to water head on the discharge valve, causes the use of relationships and results obtained from theoretical methods to cause significant errors in determining various parameters related to dischargers, including the pressure drop values ??of the valves and its water-passing capacity. This thesis, with the aim of investigating the pressure distribution in different points of the discharge duct, determining the water-passing capacity of the valve and calculating its water-passing coefficient, investigating the possibility of cavitation, comparing the modes presented for aeration after the service valve and after the emergency valve in case of the combined operation of the two valves, as well as determining the coefficients of the main drops in the duct including; Friction loss, conversion loss and valve loss have been done using the data obtained from the physical model of the lower drain of Narmashir dam. Based on this, the physical model of the conduit and valves (service and emergency) built in the laboratory of the Soil Protection and Watershed Research Center was used and the necessary tests were performed. The values ??of the pressure head at different points and the flow rate of the valve at the maximum, normal and minimum levels of the measurement tank and its results were presented in the relevant tables and graphs. Also, using the Flow 3D software, the numerical model of the discharger in this head and at three openings of 60, 80 and 100% was simulated in order to make a comparison between the experimental and numerical results and to compare the previous results in this thesis. In the following, we will see that in both the case of single valve operation and in the case of combined operation, the cavitation index in critical areas such as the groove of the valves and between the valves is in the permissible range in the case of single valve and practically eliminates the risk of cavitation.

1-1-Introduction

For many years, water consumption by humans has started in various cases, agriculture, liquor, industry and electricity production. In fact, water is considered one of the important factors in the development of different societies in the world, and its transfer is decisive both technically and economically.

Although many researchers always research about different water transfer systems and get precious results from their local, laboratory and numerical research every day, but it seems that in some cases, unexpected issues cause different views of each function to appear. A letter with a comprehensive and complete definition of the lower dischargers by examining the flow field and analyzing the cavitation mechanism in these structures to address the parameters affecting these events and the need for research in this field. Lower drains are used. The common parts of drains are: intake channel [1] or water intake building, water channel, waterway or tunnel [2], downstream valve room, shot or slide [3], energy absorber and outlet channel. A drain may include all or some of the mentioned components (Abrishmi, Hosseini 2017). Drains are also often used to divert water during construction and may also be used to help drain design floods if they are highly reliable. These structures are relatively smaller in arch or weight dams, but large in earthen dams (Johnson, 1988).. To reduce the length of the pressure part, a large discharger is usually divided into two pressure parts (which are controlled by a valve with a high head) and the outlet tunnel (which transfers the subcritical flow to the atmosphere) (Vischer and Hager, 1998).

Generally, the dischargers are classified as follows:

- based on whether the discharge channels are open or closed; Unloader arm unloader under pressure

- based on the type of hydraulic exploitation of it; Valved and non-valved discharger

- based on the position of the discharger; bottom discharger and surface discharger - based on where the discharger leads, channel discharger (dischargers that flow into the canal) and fluvial discharger (dischargers that flow into the river) (Smith, 1990).  

- In order to adjust the amount of water passing through the draining channel

In this way, the lower outlets are made in line with the following goals:

- Filling the tank

- Lowering the water level in the tank and washing the sediments

- Flood deflection and residual flow

Considering that the speed at the outlet of the lower drain is high, that is, from the Torrisli formula V = (2gH) follows, therefore, vacuum generation [4], wear [5], and air and water flow [6] are special hydraulic problems in these structures. In the Torisli relationship, H is the load on the outlet and g is the acceleration of gravity. Usually, two valves are provided for the discharger (Levin, 1995): - Service valve [7] or regulator valve [8] with Variable opening - safety valve [9] or emergency valve [10] is usually either completely open or fully closed.

Classification of types of valves:

a) Wedge valve [11]: This valve moves vertically and is completely closed only when it is completely sealed.

b) Sliding valve [12]: This type of valve is used as a normal lower outlet valve. For those with a high water load, roller or wheel sliding valves [13] are used, and it is known that these valves have less ability to vibrate.

C) Radial valves [14]: These valves do not have grooves and their sealing is simple. The forces entering the valve are concentrated on its arms or levers and the supports are subjected to high pressure. This type of valves is actually one of the most suitable valves. The types of outlet valves are lower, and they are generally used in any type of operation where the flow rate is high.

d) Hollow jet valve [15]: They are similar to the ring valve, but it has a kind of aeration device to prevent the condensation of the outlet jet. Its hydraulics are excellent. On the other hand, it can be considered as a kind of extension, which is able to move axially. In this way, a ring jet with low drops due to maintaining the flow is developed. The mentioned flap valve can be used in the form of a regulatory device or a safety device for high pressures without causing any problems in terms of leakage. It disperses and the problems of vacuum generation are eliminated due to proper ventilation. Due to the fact that all the parts of this valve are located at the outlet of the lower discharger, so their maintenance is simple. In such a situation, there will be no need for a relaxation pond, but the outlet must be covered to prevent splashing. It is the dams that require sufficient assurance of the water intake capacity and the proper functioning of the conduit and related hydraulic facilities, including valves and hydromechanical facilities. Based on this, in this thesis, in order to check the hydraulic performance of the lower discharge channel of the dams, the Normashir dam discharge model was built and tested in the soil and watershed protection research center laboratory, and all the effective parameters in the flow field, including the flow coefficient, landing number, water speed, discharge, air speed, etc. was investigated.