Analysis Of Three-Dimensional Flow Field Characteristics And Prediction Of Air Demand During Operation Of Free-Flow Flood Discharge Tunnels

During the operation of the high-head free-flow flood discharge tunnel,air is dragged downstream by the water flow,causing a large negative pressure in the tunnel,which may affect the normal operation of the air injection and erosion reduction facilities.When the crosssection design of the ventilation air-supply well is unreasonable,a large wind speed will be generated which will cause a large wind noise at protrusions or other sharp places,affecting the normal work of personnel.This paper starts from the numerical simulation of practical engineering and analyzes the flow characteristics of different parts of the free-flow tunnel under different openings.By analyzing the real flow state in the tunnel,help is provided for the layout of air-supply facilities such as ventilation holes in the tunnel.A numerical simulation of a single air-supply well is carried out to study the influence of air-supply well.Finally,a simplified geometric model is used for numerical simulation to study the influencing factors of air flow velocity distribution in the tunnel and obtain a predictive formula for the required air volume in the ventilation hole of the tunnel.The main work is as follows:(1)Comparing the calculation results of different multiphase flow models and related parameters,the selection and setting of multiphase flow models and other parameters are summarized.The Euler multiphase flow model is more accurate than the VOF model in simulating water self-aeration.The selection of the drag force model has little effect on the flow velocity distribution in the air-supply hole.The velocity distribution trends calculated by the Schiller and Naumann model and the Symmetry model are the same.The Schiller and Naumann model calculates slightly larger than the Symmetry model.The discrete phase radius has a relatively small effect on the volume fraction near the water surface,and the droplet size in the tunnel is in millimeters.The discrete phase radius is determined to be 7×10-3m.(2)A 1:1 model of a free-flow tunnel for a certain water conservancy project was established to reveal the flow characteristics of different parts of the tunnel under different gate opening degrees.The results show that with the increase of gate opening degree,the backflow in the flat section of the tunnel decreases,the negative pressure in the tunnel gradually increases,and the total air demand in the tunnel increases.When the gate opening degree is greater than 0.75,with the increase of opening degree,the influence of the decrease of tunnel area on air demand gradually occupies a dominant position,and the air demand in the tunnel begins to decrease.Due to momentum input and other reasons,the flow state of the air-supply hole and gate chamber is relatively chaotic and consumes a lot of energy.The air injection weir has little effect on the airflow state in the tunnel and can be almost ignored,while the outlet of the tunnel has a greater effect on air injection in the tunnel.(3)A geometric model of a single air-supply hole was established to explore the influence of the size and layout of the air-supply hole and tunnel on the airflow state in the tunnel.The change of the radius of the air-supply hole has a greater influence on the air pressure in front of the air-supply hole.Due to the increase of flow velocity behind the air-supply hole,the air pressure drops sharply,but the overall pressure does not change much with the change of radius.The closer the air-supply hole is to the gate chamber,the smaller the negative pressure in the tunnel.The air-supply hole should be located downstream of the air injection weir to alleviate the impact of negative pressure.The length change of the air injection well has little effect on the negative pressure in the tunnel and can be ensured to be as short as possible according to the terrain.The clearance at the top of the tunnel has little effect on the airflow velocity distribution within a certain range,but has a greater effect on air pressure.The negative pressure in the spillway tunnel increases,with the decrease of clearance at the top of the tunnel.(4)The influence of water velocity and other parameters on the distribution of airflow velocity is systematically analyzed,and fitted the calculation formula for air demand in the tunnel according to the Couette flow theory.The air velocity above the water surface is proportional to the water flow velocity.When there is sufficient clearance at the top of the tunnel,the change of air velocity with height is almost the same.When the width of the tunnel is above 6m,the air velocity in the tunnel is the same in the width direction and only rapidly decays near the wall.The Couette flow theory is used to calculate the pan-Couette flow in the tunnel,and the formula for air distribution above the tunnel is obtained by fitting.The flow velocity error is within 10%compared with the measured results,which can provide help for predicting air demand in the initial design stage.