Research On Junction Mixing For Water Quality In Water Distribution Systems Based On Large Eddy Simulation

The water distribution systems contain many junctions.The solute mixing at the junction has a great impact on the accuracy of the water quality model of the water distribution systems.Traditional water quality models assume that the solute mixing at the junction is instantaneous and complete.But solute mixing at the junction in real systems is not instantaneous and complete.The assumption of complete mixing at the junction is an important cause of errors in the water quality model of the water distribution systems.In this paper,the internal mixing mechanism of the cross junction is studied.The influence of different factors on the solute mixing of the N-type junction,U-type junction,and the cross junction is analyzed.Finally,the solute concentration at the junction outlet is simulated.In this paper,large eddy simulation is used to study the internal mixing phenomenon and mechanism of the cross junction.The solute mixing interface has different mixing mechanism for different Reynolds numbers.When Re<1500,junction mixing is mainly controlled by solute transfer and variation within the shear layer.When Re>1500,the key factor controlling the solute mixing changes to the fluctuation of the mixing interface.The increase of Re destabilizes the mixing interface,and the solute is more fully mixed within the junction.Moreover,the fluid flow state in the outflow pipe section of the cross junction is complex.The outflow pipe section contains a low-velocity backflow region.There are large velocity differences and complex eddy currents at different positions of the outflow pipe section.The closer the distance to the outlet,the smaller the velocity difference on the section,and the smoother the outflow fluid.Through large eddy simulation,the solute distribution,velocity field and flow field results of N-type junction,U-type junction and cross junction after water quality mixing are obtained.Study the effects of inlet Reynolds number ratio,outlet Reynolds number ratio,branch spacing and pipe diameter on solute mixing at junctions.With the increase of the Reynolds number ratio at the junction inlet,the solute inflow flow and the proportion of solute fluid inside the junction increased.The solute concentration at the junction outlet increased.With the increase of Reynolds number ratio at the junction outlet,the flow rate of outlet N at the junction increases,and the flow rate of outlet E decreases.However,the variation law of the outlet concentration of different structural junctions is different.The increase of the L/D and pipe diameter of the junction branch pipe increases the space and contact time of the fluid mixing between the U-type and N-type junctions and promotes the more complete mixing of the junctions.Due to the larger diameter difference of the cross junction,the two inflowing fluids are more fully mixed inside the junctions.At the end of this paper,the simulated and experimental values of the junction outlet concentration of different structures under different working conditions are compared.The results show that using the large eddy simulation to calculate the junction outlet concentration,this method does not need to manually adjust the Schmidt number with reference to the experimental value,and still has a high simulation accuracy;Among them,the overall error of the simulation calculation of the N-type junction and the U-type junction is within 10%;The error between the simulation calculation value of the cross junction outlet concentration and the experimental value is within 5%,and the simulation calculation accuracy of the cross junction is better than that of the N-type and U-type junction.