Project 6. Oxygen Dissolution via Pump-Turbine –Application to Wastewater Treatment
Large-eddy simulations of a ventilated pump-turbine were conducted using the LES-mixture model. Peripheral and central aeration were applied to study the effectiveness of oxygen dissolution and the pump-turbine performance. The peripheral draft tube aeration was performed by air injection through (1) a continuous orifice and (2) a series of discrete orifices along the surface of the draft tube. The simulations were performed when the system is operating in the turbine mode. Peripheral aeration was more effective for the achieved dissolved oxygen level and the oxygen dissolution efficiency. The mean dissolved oxygen concentration and the dissolution efficiency inside the draft tube were predicted to be 1.8 mg/l and 80% using the continuous or discrete peripheral aeration and 1.4 mg/l and 25% using the central aeration. A mean value ranging from 1.0 mg/l to 2.0 mg/l was considered to be sufficient for aerobic bacteria to treat the wastewater. Aeration resulted in a minor penalty on the power generation, while it provided a significant reduction in the flow-induced vibration. The central aeration was more effective in reducing the amplitude of pressure fluctuations and yielded more stable turbine operating conditions.
Published papers for details:
Daskiran, C., Attiya, B., Altimemy, M., Liu, I.H. and Oztekin, A., 2019. Oxygen dissolution via pump-turbine–Application to wastewater treatment. International Journal of Heat and Mass Transfer, 131, pp.1052-1063.
Daskiran, C., Riglin, J., Schleicher, W.C. and Oztekin, A., 2018. Computational study of aeration for wastewater treatment via ventilated pump-turbine. International Journal of Heat and Fluid Flow, 69, pp.43-54.
Figure 1. (a) The computational domain with all components. The location and direction of air injection for different aeration methods: (b) the central aeration, (c) the continuous surface draft tube (DT) aeration, (d) the discrete surface DT aeration.
Figure 2. The mesh (a) near the draft tube surface, (b) along the single-blade passage with magnification and (c) on the runner hub. The zoom-in image shows the mesh near the blade trailing edge.
Figure 3. The instantaneous (a) velocity and (b) vorticity contours on a central, vertical plane passing through the draft tube after 14 revolutions. The left column and the right column of each subplot depict the single-phase and the multiphase results with the inlet bubble size of 0.1 mm, respectively.
Figure 4. Central injection: The instantaneous contours of (a) bubble size, (b) air volume fraction, (c) interfacial area concentration, (d) dissolved oxygen concentration and (e) oxygen mass faction in the air on a central, vertical plane passing through the draft tube after 14 revolutions for the inlet bubble size of 0.1 mm.
Figure 5. The instantaneous isosurface of Q-criterion in (a) single-phase and (b) multiphase simulation, and the isosurface of (c) the air volume fraction and (d) the DO concentration after 14 revolutions for the inlet bubble size of 0.1 mm.
Figure 6. Dissolution efficiency (top) and mean dissolved oxygen level (bottom) inside the draft tube for the continuous and discrete surface draft tube aeration. The results for the central aeration are shown as an insert on each graph.