Dynamic Models Of Hypolimnetic Aeration In Stratified Reservoirs

Internal pollution is becoming the dominant factor influencing the water quality ofstratified lakes and reservoirs. Hypolimnetic aeration is an effective technology ofcontrolling the internal pollution of stratified reservoirs, but the current dynamic modelsof hypolimnetic aeration can t be used to satisfactorily predict the flow velocity andoxygen concentration, and are not able to guide the optimized design and operation ofhypolimnetic aerators.A one-dimensional hydrodynamic model and an oxygenation model weredeveloped using the combined methods of theoretical analysis and experiments, keymodel parameters were reasonably determined, and the superficial water velocities andoxygen concentrations in hypolimnetic aerators were well predicted under differentoperational conditions. Effects of air flowrate, bubble diameter and water depth abovethe air diffusors on mixing and oxygenation were also investigated using the developedhydrodynamic model and oxygenation model. Optimization methods of designing andoperating hypolimnetic aerators were also proposed.Main research outcomes of this study were listed as follows:(1)Energy input and energy losses acting on the gas-liquid two-phase flow in ahypolimnetic aerator were systemically analyzed, a one-dimensional hydrodynamicmodel of hypolimnetic aeration was developed based on the energy balance equationand gas-liquid drift equation, a real-domain method based on MATLAB was alsodeveloped to solve the hydrodynamic model, the dimensionless energy loss coefficientdue to bubble wakes (KW) and energy loss coefficient due to entrances and exits (KT)were properly determined. Based on the operational data of hypolimnetic aeratorsinstalled in Lake Prince and Lake Western Branch in USA, the developed hydrodynamic model was validated satisfactorily, the prediction errors of superficial water velocityunder different operational conditions were all reduced from precious ranges of±25%to±8%.(2) Hypolimnetic aeration experiments were conducted in a series of laboratoryhypolimnetic aerators under various aerator configurations and operational conditions,the experimental results showed that the superficial water velocities increased with theair flowrate, but decreased with the bubble diameter and the water depth above the airdiffusors, and this was mainly due to the redistribution of different kinds of energy; thedimensionless energy loss coefficient due to bubble wakes (KW), energy loss coefficientdue to entrances and exits (KT) and gas-liquid drift velocity were also properlydetermined based on the experimental data, the superficial water velocities underdifferent operational conditions were well predicted within the errors of±10%usingthe developed hydrodynamic model.(3)Based on the configuration sizes of hypolimnetic aerators installed in LakePrince and Lake Western Branch and key model parameters, dependency of thepredicted superficial water velocities on the operational conditions was similar to thosefor the case of laboratory hypolimnetic aerators.(4)Oxygen transfer process from the bubbles to the water was mainly analyzed inthe rising zone of a hypolimnetic aerator, parameters characterizing the oxygentransfer were properly determined，an oxygenation model of hypolimnetic aeration wasdeveloped based on the discrete bubble equation and the double-layer theory，thesuperficial water velocities and gas volume fractions of the hypolimnetic aeratorsinstalled in Lake Prince and Lake Western Branch were first predicted using thehydrodynamic model, and the oxygenation effectiveness was then well predicted usingthe developed oxygenation model of hypolimnetic aeration.(5)Based on the predicted results from using the oxygenation model ofhypolimnetic aeration, decreasing the bubble diameter and increasing the water depthabove the air diffusors were helpful to improve the oxygenation effectiveness in therising zone of the hypolimnetic aerators, especially when the bubble diameter was ofmicrometer level; if the configuration conditions of hypolimnetic aerators were fixed,when the air flowrate increased to a critical value, the oxygenation effectiveness in the rising zone of the hypolimnetic aerators would become weaker.(6) Taking the oxygen concentration at the top of the aeration chamber as theoptimized objective, the developed hydrodynamic model and oxygenation model can beused to guide the optimization design and operation of hypolimnetic aerators.