Numerical study on air movement in steep sewer pipes

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Yu Qian, David Zhu, Nallamuthu Rajaratnam

Tuesday 30 june 2015

17:33 - 17:36h at Europe 1 & 2 (level 0)

Themes: (T) Water engineering, (ST) Computational methods, Poster pitches

Parallel session: Poster pitches: 7E. Engineering - Computional

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In modern drainage systems, sewer pipes with steep slopes may be constructed and hydraulic jumps may exists if downstream flow is subcritical. Odour issues may occur if air moves in an undesired way. Therefore, to provide guidance for sewer system designing in hilly region, it is important to know how air interacts with supercritical sewage flow with different downstream condition. In present study, a numerical model was built to predict the air requirement in a circular pipe system with changing slopes. A Volume of Fluid (VOF) method was adapted and CFX-14.0 was used for the modelling. A corresponding physical experiment was conducted to calibrate the numerical model and verify the accuracy of the numerical model. Water flow rate, air flow rate, water depth upstream and downstream of the jump and air velocity profile in headspace were measured and plotted with numerical modelling outcomes. It is found that the measured air flow rate is generally larger than published value. If hydraulic jump exists and downstream flows in open channel condition, the ratio of air and water flow rate (_=Q_a/Q_w ) ranges from around 0.8 to 1.2. By comparison, the published data is maximally 0.2 when Froude number is 13. For conduit jump case, the ratio (_) gets close to published value but is still 50% larger than previous researches. No air entrainment is observed if the tailgate is closed from top and downstream pipe flows in full. When the tailgate is fully opened, supercritical flow occurs and the Froude number stays at 4. The value of _ ranges from 0.8 to 1.6 and is found to be independent with Froude number. Pressure gradient and the existence of water surface drag may act together on affecting the bulk air flow rate. The numerical model suggests that the bulk air flow rate can be modeled. The simulated flow structures for selected case have good agreement with measurements. And the model is believed to be able to model more complicated cases in the future. Keywords: Numerical method, Hydraulic jump, Air entrainment, Multi-phase flow