Study On The Interaction Of Water And Air In Drop Structures Under Different Ventilation Conditions

Drop structures that consist of inflow pipe,dropshaft and outflow pipe are widely used in urban drainage systems including building drainage systems connecting two pipes with different elevations.However,earlier studies on drop structures are mostly under fully open ventilation condition while the actual operation of urban drainage system is mostly water-air two-phase flow or multi-phase flow with limited ventilation condition.Therefore,it is of great practical significance to study the the interaction of water and air and related influence factors in the drop structures under different ventilation conditions.A physical model study was conducted to investigate the flow regimes in a plunging type dropshaft under the conditions of no air supply and limited air supply.When the top of the dropshaft is sealed,three flow regimes are observed with the increase of the water flow rate:(1)Free-flow Regime,where the falling water partially occupies the cross-section and air can move freely with pressure atmospheric inside the dropshaft;(2)Water-plug Regime,where a water plug is formed at the bottom of the dropshaft intercepting the air flow and causing sub-atmospheric pressure;(3)Choked Regime,where the water plug rises and submerges the inflow pipe,and the dropshaft operates under a choking condition with full pipe flow.The flow regime changes from Free-flow Regime to Water-plug Regime at the dimensionless water flow rate Q*= Qw/(gDs5)1/2=0.24 ± 0.06,where Qw is the water flow rate,Ds is the diameter of the dropshaft,and g is gravitational acceleration.The flow changes to Choked Regime when Q*= 0.48 ± 0.04.A large drop height leads to an early formation of Water-plug Regime while the diameter of the inflow pipe has little influence on the formation of Water-plug Regime.The energy dissipations in in different flow regimes especially in Choked Regime is discussed.When the top of the dropshaft is partially open to the outside atmosphere by a valve,Water-plug Regime appears at Q*=0.30 ± 0.06,slightly later than the cases with the dropshaft top sealed.Choked Regime is not observed under the tested condition,as the magnitude of the negative pressure inside the dropshaft is reduced due to the air supply.The entrained air flow rate reaches a maximum value of about 0.5 time of the water flow rate with the limited air supply.In this thesis,a physical model study was conducted to observe and investigate the water-air interaction under both top-sealed condition and top-open condition.Under top-sealed condition,the air could be supplied only through the downstream outflow pipe.Three air supply stages were defined with the increase of the water flow rate:(1)air column stage,(2)transition stage and(3)air bubble stage.Under top-open condition,the air was mainly supplied through the air tube at the dropshaft top.As the magnitude of the negative pressure inside the dropshaft was reduced for the reason of air supply at the same water flow rate,only two air supply stages were observed at low water level:(1)air column stage and(2)transition stage.The air flow rate at air bubble stage was estimated.The relationship between air flow rate Qa and water flow rate Qw under top-open condition was studied.The influences of different top ventilation conditions on the splash height after water flow impinging on the wall was also studied.A numerical modeling method is adopted in the present study to investigate the effect of water flow rate on the flow regimes in a dropshaft under top sealed condition based on commercial computational fluid dynamics(CFD)software ANSYS CFX.k-ε turbulence model is adopted to analysis three typical flow regimes at Qw = 4 L/s,9 L/s and 15 L/s based on the results of physical model study with inflow pipe diameter Di = 100 mm,drop height H = 1.5 m and the dropshaft diameter Ds = 150 mm.The effect of extreme water flow rate(Qw = 30 L/s and 50 L/s)on the flow regime in the dropshaft which is hard to carry out subject to the test condition is also predicted by volume of fluid(VOF)model.Modeling results show that water in the dropshaft can be choked up and the water level rises to submerge the inlet under top sealed condition.A full pipe flow is formed finally in the dropshaft but the water level does not rise much after that.The pressure variation in a horizontal pipe of single-stack drainage system was investigaeted based on a physical model experiment.Four parameters affecting the pressure variation,i.e.water flow rate(Qw),inlet height(H),ventilation condition and outlet condition were studied.Five pressure transducers were installed along the horizontal pipe to record the pressure variation.When the top of the vertical drainage stack and the outlet were fully-open to the outside atmosphere,the flow regime in the horizontal pipe changed from free surface flow to free-surface-pressurized flow with Qw increasing.The mean values and amplitudes of pressure fluctuation increased significantly at different measuring points with Qw increasing but decreased along the outflow direction.The relative air flow rate was also estimated.Different inlet heights upstream had similar influence on the pressure variation in the horizontal pipe at a certain water flow rate.Three ventilation conditions,i.e.,top fully-open condition,half-open condition and sealed condition were tested by controlling the cross-section of the vertical drainage stack with a plate.A choking flow was formed in the vertical drainage stack and the pressure in the horizontal pipe decreased about 51 Pa under the top sealed condition.Three outlet conditions,i.e.,fully-open condition,half-submerged condition and submerged condition were tested by controlling the water level at the outlet of the horizontal pipe.The pressure increased about 652 Pa under the submerged condition.