| Urban water supply network is an important part of municipal construction.Tap water is transmitted to each resident of the city through the network system.In the pipe network system,the branch pipe of municipal fire hydrant is directly connected with the water supply pipe network,and there is more water in the pipe,which is easy to form the dead-end of stagnant water in the long period.The water in the branch pipe at the deadend is easy to react with the pipe wall,and the iron element generated will flow into the main water with the main pipe flow,and finally reach the user’s tap,resulting in the "red water" phenomenon.Excessive iron content will seriously affect the water quality,residents’ health and the operation of pipe network.The existing control mode is to drain the fire hydrant water regularly and flush the pollutants at the dead-end branch pipe through water flow.However,a large number of municipal fire hydrant branch pipes lead to heavy cleaning workload,the management is complex,and the cleaning cycle is long,which is easy to cause water waste and bring great challenges to the operation of pipe network and the safety of water quality.The joint between the branch pipe at the dead-end of the municipal fire hydrant and the main pipe can be regarded as a typical cavity structure,which involves shear layer instability,vortex generation and development,material mixing and other related physical processes.Computational fluid dynamics(CFD)solves the governing equations of fluid flow through the numerical method of discretization,so as to study the law of fluid motion.Therefore,in this paper,through geometric modeling,mesh generation,mesh independence verification,numerical model construction,sub grid stress model comparison,the large eddy simulation method was used to build the numerical model of iron diffusion at the dead-end branch pipe of municipal fire hydrant.The experimental platform of pollutant migration and diffusion at the dead-end branch pipe of municipal fire hydrant was designed and built.The pollutant concentration at the outlet of main pipe was recorded.The numerical model of iron diffusion at the dead-end branch pipe of municipal fire hydrant was checked,and the results showed that the accuracy of the model met the requirements.Based on the numerical model of iron diffusion at the dead-end branch pipe of municipal fire hydrant,with different length depth ratio,pollutant concentration and Reynolds number as control parameters,the amount of iron diffusion at the dead-end branch pipe under different working conditions was obtained.The results show that when the length depth ratio is between 1 and 0.1,the amount of pollutant diffusion decreases with the decrease of length depth ratio.When the length depth ratio is reduced to 0.05,the average distribution coefficient of export quality has always remained stable.There is a positive correlation between pollutant concentration and diffusion.At low Reynolds number,the amount of pollutant diffusion after stabilization is large,while at high Reynolds number,the amount of pollutant diffusion after stabilization is small.Based on the cavity flow theory,combined with streamline diagram and concentration distribution nephogram,the flow field characteristics of iron diffusion at the dead-end branch pipe of municipal fire hydrant were analyzed.The results show that the iron diffusion phenomenon at the dead-end branch pipe is closely related to the vortex structure in the flow field.The position,strength and concentration of the vortex structure determine the amount of pollutant diffusion.The passive fluid control method was used to control the flow field of the cavity structure by setting a spoiler at the leading edge of the dead-end branch pipe,so as to indirectly reduce the diffusion of pollutants.The iron diffusion control model of the branch pipe at the dead-end of the municipal fire hydrant was modeled and checked.The results show that the simulated value of the base state numerical model coincides with the measured value,which meets the accuracy requirements of the model.Under the conditions of low Reynolds number and high Reynolds number,the iron diffusion control model of the dead-end branch pipe of the municipal fire hydrant was calculated with fixed inclination angle and the height of the spoiler as the independent variable,and then the effect evaluation,trend analysis and mechanism research of the spoiler were carried out.The results show that under the condition of low Reynolds number,the different height of the spoiler can control the diffusion of pollutants,and the higher the height of the spoiler,the stronger the control effect.Under the condition of high Reynolds number,the0.3 and 0.5 height of the spoiler has obvious effect on promoting the diffusion of pollutants or has no obvious effect on controlling the diffusion of pollutants,while the1.0 and above height of the spoiler has obvious effect on controlling the diffusion of pollutants.The control mechanism of the spoiler was studied.It is considered that the position,strength and pollutant concentration of the vortex structure can be affected by raising the position of the free shear layer impacting the trailing edge of the dead-end branch pipe,so as to control the pollutant diffusion behavior. |
