The Research Of The Critical Velocity Of Sewage Flowing As Two-Phase Fluid In City Drainpipe

In current design of city drainage system, self-clean velocity is normally controlled as the minimal design value and this value generally is obtained through bringing forward firstly a fixed value based on wastewater pipe, rainwater pipe or combination of them and then treating it as a single and even phase. However, this method only can be received when solid impurity is simple and its concentration is too low. Such and homothetic treatment method have no solid theory to support and have some limitation from the point of view of real operation and recent study. Actually, it is not pure clean water moreover some impurities such as bedload in the drainage system and bedload makes their characteristics different from pure water at the concentration, size, density, viscosity and so on. In this paper, wastewater in drainage pipe system would be looked as two-phase liquid. Based on recent studies of two-phase liquid, especially on the energy study of Professor Fei xiangjun, calculational model of hydraulic gradient about full and non-full flowing, wastewater pipe system has been established from energy conservation and then series of average velocity U and hydraulic gradient J are fitted by least square method. After that, critical velocity (namely the lowest design velocity) in pipe can be deducted through the lowest data point on the two-phase liquid figure of I-U, because corresponding function value of this point is just the critical velocity at which sand could start to settle.Through the established calculational models of hydraulic gradient and critical velocity, these conclusions can be led to:(1)Critical velocity in full flowing pipeline is relative with pipe diameter and characteristics of solid impurities. This velocity will increase with the increments of pipe diameter, size and density.(2)Critical velocity in non-flowing pipeline is closely relative with solid size, density, pipe diameter and so on. This velocity increases to different extents with the increments of diameter, size and density, and flux capacity of pipe increases with the decrease of coarseness rate. When designed value is above the lowest design velocity (Critical velocity), velocity in pipe of low coarseness will be higher than that in pipe of high coarseness, that is to say, sewage elimination capacity of plastic pipe is more favorable than concrete one.(3)Through the I-U figure of non-full flowing pipeline, critical velocity (lowest design velocity) can be obtained and then lowest design gradient also can be found...