پديد آورندگان :
حسنيان شيروان، ساناز دانشگاه سيستان و بلوچستان , محمدعلي نژاديان، دامون دانشگاه شيراز , كاويانپور، محمدرضا دانشگاه صنعتي خواجه نصيرالدين طوسي تهران
كليدواژه :
پديده كاويتاسيون , دريچه تخليه , ضريب هوا دهي , ميدان جريان , دريچه اضطراري
چكيده لاتين :
Introduction
Drain valves are usually constructed to control and drain flood, regulate flow, drain tank in critical cases, discharge sediment, and transfer current. Therefore, the study of their hydraulic conditions during design and operation should be considered by researchers and designers. As the height of the dam increases, the flow velocity in the semi-open valves of the dam also increases and as a result, the local pressure decreases, which consequently causes the cavitation phenomenon The presence of air near the rigid boundaries of the flow greatly reduces the destructive effect of cavitation and therefore the method of aeration and its effects and the percentage of air bubbles in the vicinity of these boundaries to prevent cavitation is one of the points to know the different types of aeration mechanism and bubble placement, and the type of valve according to the flow conditions. As mentioned, one of the phenomena that can endanger the safety of valves is cavitation. In these valves, the two-phase flow of air is transmitted at high speed. Due to the separation of the flow lines, a sharp drop in the downstream values of the valve occurs.
Methodology
Siazakh rock dam is located in a place called Siazakh and at the junction of two tributaries of Ghezelozen river named Kaqli and Sheikh Haidar, 7 km from Divandere. The level of the dam on the riverbed is 1756 meters above sea level. The purpose of constructing this dam is to supply agricultural water, control and control river floods. In the middle of the duct, the control system is located, consists of an emergency sliding valve and a sliding service valve. The physical model of the valve includes a repair valve, a metal cover with a rectangular cross section, a duct inlet, a valve groove, a middle duct, an emergency valve, an emergency valve chamber, its grooves, a service valve, a vent between two valves and the entire downstream duct. In order to provide the required water height and required discharge, an open metal tank has been used. This tank is in the form of a cylinder with a diameter of 5 meters and a height of 6 meters.In order to measure the pressures on the valve, 8 piezometers are installed on the valve and all these piezometers are connected to the tightly connected hoses. The experiments were performed for four different heads. Two pumps and an outlet adjustment valve were used to adjust the head, so that only one pump was switched on at the lower heads and the output valve was bypassed to adjust the head. This time was chosen according to the turbulence of the air flow and minimizing its error by trial and error.
Results and discussion
After adjusting the head, the service valve was placed in the pre-planned openings and the emergency valve was displaced so much that the most critical situation occurred. The criterion for detecting this critical state is the velocity of air suction from the aeration pipe between the two valves into the duct, which was measured by a hot wire. To measure the air velocity, the hot wire is placed inside the aeration tube in the center of the tube for one minute. After the desired time, the average inlet air velocity is recorded by the hot wire device. The results show that the most critical situation occurs when the jet passing under the emergency valve hits exactly the lower edge of the service valve. In this case, a severe disturbance occurs between the empty space of the two valves, which causes severe suction of air into the aeration pipe. According to observational experience, this condition is usually achieved when the percentage of emergency valve opening is up to about 5% less than the service valve opening.
Conclusion
The results of this study showed that when the emergency valve is broken in a certain opening and consequently the emergency valve enters the circuit, the most critical situation is when the amount of emergency valve opening is equal to the service valve. By measuring the amount of incoming air from the aeration tube in 24 different laboratory modes, and comparing them with different parameters, a relation was provided to determine the aeration coefficient which is a function of the landing number and includes a range between the lower and upper limits of the data. Also, by examining the amount of inlet air flow from the aeration tube for 24 different experiments, it was observed that this amount of air has a relative maximum at two points, the first maximum being related to low openings.
