Air Entrainment And Energy Dissipation In Drop Structures In Drainage System

Drop structures like dropshafts and drop manholes are usually inserted in the pipelines to transfer wastewater (or storm water) from a higher elevation to a lower one. Two of the most important concerns of them are air entrainment and energy dissipation. Large amount of air entrained by the drop structures, especially by the dropshafts, can cause the odor issue of drainage system, which annoys the neighbor residents much. High speed water flow caused by insufficient energy dissipation in drop structures can lead to poor hydraulic conditions and even pipe erosion in the drainage systems. Therefore, a good understanding of air entrainment and energy dissipation in drop structures are of practical importance.In this thesis, the interaction of air and water was studied first. Experiments were conducted to study the effects of air core generated by the vortex on the shape and discharge of the outflow in a cylindrical water tank with a bottom well-designed nozzle. With the air core in the tank, outflow shape can vary from a divergent shape to a smooth spindle shape with air core, and to a string of small spindles. The existence of the penetrated air core can dramatically reduce the outflow discharge, with the discharge coefficient decreasing with the increasing of nozzle diameter.An experimental study on the breakup of a turbulent round water jet in still air was also included. The visual structure of the falling water jet was recorded by a high-speed camera. As the jet fell, it was gradually bent into a helical shape and the amplitude of the surface disturbance nearly grew in an exponential form. The jet disintegrated in the bag breakup regime, which happened when the dynamic air pressure imposed on the jet surface was larger than the restraining surface tension pressure. The onset of jet breakup was found about 100 times of the nozzle diameter away from the nozzle. A theoretical model was developed for predicting the onset of jet breakup and the prediction matched the experimental results well.A large-scale model was used to investigate the mechanisms of air entrainment in a tall plunging dropshaft. The water flow in the dropshaft was observed to break up into small drops near the bottom, where was called the "rain-like zone". The drops increased the drag force imposed on the air by water significantly and thus, large amount of air could be entrained into the dropshaft. The air pressure increased from the top to the bottom of the dropshaft and the largest air pressure gradient was found in the "rain-like zone". An analytical model was developed to predict the air pressure gradient in the "rain-like zone" based on the momentum transfer from the water drops to air. The prediction compared well with the measurements at small flow rates but overestimated the air pressure gradient at large flow rates due to the restriction of boundary conditions. Different downstream condition was also tested by installing a weir in the outlet pipe. The weir would cause a significant blockage in air passage, resulting in larger pressure, even positive pressure in the dropshaft and reduced air entrainment.An experiment on the retrofitted dropshaft was conducted to study the performance of the air circulation system on reducing the amount of entrained air. It suggested that the effect of the air circulation system was significant at large water flow rates. It helped increase the air pressure inside the dropshaft, reduce the pressure gradient and the amount of entrained air. But it did not work well under small water flow rates. Besides, based on the experimental results and the data from filed monitoring, prediction of air movement in the prototype of dropshaft retrofitted with the air circulation system was conducted.A summative work was conducted in aspects of the energy dissipation in drop manholes or dropshafts. Four regimes of flow behaviors in the drop manhole were divided:free overfall flow, orifice flow, full pipe flow and fully submerged flow. Theoretical predictions of energy dissipation in the drop manhole under different flow regimes were conducted and the prediction matched the experimental data well. A possible design criteria for the drop manhole was suggested in this paper, which aimed to make the drop manhole a subcritical downstream flow, together with a design example.