Two-phase Flow Research And Numerical Simulation Of Air-water Pulse Cleaning Process For Water Distribution Pipe

Water supply network is the basic infrastructures in city and the component of water supply systems. The irregular growth rings gradually formed along the pipeline inwall because of the corrosion of longtime running. The growth ring is the mixture of the sediment, corrosion, contamination and biofilm, which not only causes the secondary pollution, but also reduces the water-crossing section and increases the resistance coefficient, directly affecting the water supply capacity and pressure. How to remove the growth ring has became the trouble of water suppliers. Air-water pulse washing method is an effective for removing growth ring, being simple to operate, highly adaptable, no environmental pollution. However, the application of barometric Pulse is still in the period of experience, the mechanisms and regulations of washing need to be studied thoroughly. In this study, combined experiments and numerical simulation was used to reveal the flow characteristics of two-phase flow in air-water pulse tube, simultaneously, various elements affecting wall shear stress was analyzed.The pressure drop in the process of washing was also analyzed by detecting the pressures, and the regulation of pressure drop in the process of air-water pulse tube was determined by combined the experimental results and homogeneous flow model of gas-liquid two phase flow. Compared with no air, the specific pressure drop in the pipes increased dozens of times when high pressure air was filled into flow, and the same changes of pressure drops was observed in every wash cycle. The effects of air pressure and water flow on pressure drops in the pipes was studied by setting different experimental conditions. When water flow and impulse frequency were constant, there was significant linear correction between the pressure drops and air pressure. When the air pressure and impulse frequency were constant, the relationship between pressure drops and flow can be expressed through a binary cubic polynomial. The conversion of flow pattern between air and water in the process of washing was observed by picture processing technology, and the slug flow was determined to be the main flow pattern playing the significant role in washing.Based on the study on the mechanisms about air-water two phase flow in the horizontal pipes, mathematical model in the washing process of water-air pulse flushing was established, CFD numerical simulation was conducted by VOF method, and the reasonability of model was verified by experimental results. The flow characteristics such as flow patterns, dynamic pressures, turbulence intensity and wall shear stress in the straight and bended pipes were respectively analyzed, which provided the theoretical basis for removing growth ring in air-water pulse method. In the formation of slug flow of air-water flow, when l>100D or l>15D in the horizontal or bended pipes, the elastic cells tended towards stability. After high pressure air was filled into flow, the dynamic pressure in the pipes was significantly increased, and continuously increased wit the extent of inflation time. It is observed that the turbulence intensity accelerated, the turbulent fluctuation aggravated, and the wall shear increased, simultaneously, the change trends of wall shear along the length of pipes was in accordance with the change of the air-water two phase flow regimes.The flow and wall shear stresses at different pipe diameters, inclinations, corrosion situation, inlet nozzle size and position were calculated by numerical simulation, which provided the guidance for determining the parameters in practical application. Other conditions being equal, when the pipe diameter was less than50mm, the effective of pipe diameter on flow and was small; when the pipe diameter was bigger than50mm, the wall shear stress was changed significantly and increased first and then decrease with the increase of pipe diameter. The wall shear stress in the downward upward inclined pipe and horizontal pipe was higher than that in the upward inclined pipe, and the wall shear stress in roughened tubes was bigger than that in the smooth pipe. When the roughness was lesser, the wall shear stress increased with the increase of roughness. With the roughness increased to a certain degree, the wall shear stress did not changed obviously. At the same position of nozzle of air supply, the flow and wall shear stress was smaller when the nozzle of air supply was less than0.5D (D is pipe diameter), while the wall shear stress was not affected when the nozzle of air supply was more than0.5D. With the extent of inlet time, the wall shear stress at different inlet position changed largely. In most of the section of pipe, the flow and wall shear stress values was the lowest.Research result can improve the cleaning effect of air-water pulse pipeline cleaning engineering, is advantageous to the application of air-water pulse pipe cleaning technology, solve the problem of network secondary pollution, reduce energy consumption of water supply, provide strong support of ensure people’s health, reduce energy consumption and environmental pollution.