Investigations into the turbulent bubbly wake of a ventilated hydrofoil: Moving toward improved turbine aeration techniques

Abstract

The use of aerating hydroturbines to mitigate the problem of low dissolved oxygen in the discharge of hydroelectric power plants has recently attracted a lot of attention. The design of a ventilated hydroturbine requires a precise understanding of the dependence of the operating conditions (viz. liquid velocity, air ventilation rate, hydrofoil configuration, etc.) on the bubble size distribution generated in the bubbly wake and the consequent rise in dissolved oxygen in the downstream water. In the current research, experiments are conducted in the wake of a ventilated NACA0015 hydrofoil by systematic variation of hydrodynamic conditions allowing for quantitative analysis of aeration statistics and capabilities for turbine blade hydrofoil designs. The data concerning bubble velocity distributions, bubble locations and size distribution, void fraction, etc. are reported for a chosen reference case. In addition, trends in the variation of bubbly wake are explored particularly in the light of wake physics. It is found out that an increase in Reynolds number (Re) led to greater breakup, while an increase in normalized air ventilation rate (CQ) favored greater coalescence events in the wake. Further, the PDF(d̃) of the normalized bubble size d̃=d/d32, where d32 represents Sauter mean size distribution, is found to have a universally similar shape independent of either Re, CQ or hydrofoil angle of attack. Finally, a numerical formulation is proposed for the bubble sizes in the hydrofoil wake. This rich dataset will also contribute to the development of a numerical turbulence model, to investigate turbulence effects on bubble size distribution and predict the rate of air entrainment and the oxygen transfer occurring in the wake at different hydrodynamic conditions.