| In order to reduce the slope of the drainage pipeline and eliminate the energy of the water flow in the pipeline,the drop structure of dropshaft is often set up in the drainage system.The water flow in the dropshaft falls in the form of free jet,and the unfavorable hydraulic condition of water tongue blockage often occurs near the intake pipe.Under large water flow rate,the air flow passage in the water inlet pipe area of the dropshaft is seriously narrowed,which is easy to cause large pressure fluctuation and negative pressure growth.At the same time,under the action of the dragging of the discharged water flow and the pressure gradient in the well,the air entrained and by the dropshaft begins to transfer to the downstream drainage pipeline and accumulate continuously,so that the downstream drainage pipeline is often in a state of severe pressure above the water body.Excessive pressure will cause the odor(mainly hydrogen sulfide)in the sewage pipeline to escape into the external environment,causing serious odor problems.In addition,under extreme rainstorm weather,the drainage pipeline is filled quickly,which may lead to blowout events in the drainage system.Excessive negative pressure and excessive entrainment in the dropshaft are the main causes of the above problems.Therefore,the numerical simulation method is used to study the water-gas two-phase flow characteristics in the original shaft model under different inflow conditions.Then,the influence of the inflow tongue on the pressure distribution in the dropshaft is analyzed,and the mechanism of the sudden increase of negative pressure in this area is clarified.Aiming at the rapid growth of negative pressure near the intake pipe,two different structural optimization design schemes were proposed based on the original dropshaft model,and the flow field and pressure field distribution in the optimized shaft model were studied by numerical simulation,then the performance of the optimized structure is evaluated.Finally,the theoretical analysis method was used to calculate the critical position of the horizontal pipe when the air reflux occurs in the dropshaft circulating air flow system.The main results of this paper are as follows:Firstly,the mechanism of sudden increase of negative pressure near the water inlet pipe of the dropshaft was clarified,and the area ratio coefficient of water-gas twophase which can characterize the water passing capacity or ventilation capacity in the dropshaft was defined.The main reason for the excessive negative air pressure near the water intake pipe was that the water tongue blocks the flow channel of the air flow in this area.A air recirculation zone is formed above the water tongue,while the air supply is insufficient below the water tongue,so the negative pressure will suddenly increase.Secondly,a swirling flow-through structure was proposed near the water inlet,which can directly eliminate the water tongue.The water flow in the intake pipe can flow around the two sides of the partition,which avoids the water tongue hitting the dropshaft wall to form a water barrier area,and the water-free cavity area formed in the middle of the partition could be used to ventilate and balance the air pressure of the dropshaft.At the same time,because the water flow rotated around the well wall,the kinetic energy carried by the discharged water flow could be eliminated.Thirdly,an optimization scheme of ventilation pipeline was given,which can connect the recirculation zone above the water tongue and supply air below the water tongue.A water tongue impact model was established to predict the position of the impact point at the end of the water tongue,which provides a theoretical basis for the installation elevation of the ventilation pipeline.The influence of ventilation pipelines at different positions on the pressure distribution in the optimization model was also discussed.Finally,the theoretical calculation model of the critical air pressure and critical position of the air backflow in the dropshaft circulating airflow system was constructed,which aims to provide a theoretical basis for the installation of horizontal pipelines in the dropshaft circulating airflow system.The results of numerical calculation shown that when the horizontal pipeline was installed above the critical position,the air in the circulating air flow system of the dropshaft can be refluxed,and the larger the diameter of the airshaft,the closer the critical position of the horizontal pipeline is to the bottom of the dropshaft. |
