Study On Flow Regimes, Mixing Mechanism And Enhancement In Planar Opposed Jets Reactors

Impinging jets is one of important technologies for enhancing mixing performance, which has been applied successfully for increasing industrial processes, e.g. gasification and nanoparticle synthesis. Understanding the flow regimes and mixing mechanism of opposed jets is crucial for the development, design and operation of impinging jets reactors. Present work is devoted to studying the deflecting oscillation of planar opposed jets, as well as the oscillation behavious, mixing mechanism and enhancement in planar opposed jets reactors based on combination of the experimental study and numerical simulaton. The main contents and results are summarized as follows:1. The deflecting oscillation of planar opposed jets are experimentally studied by a flow visualization technique, combining with a particle imaging velocimetry (PIV) and a high-speed camera and numerically simulated by Large-eddy simulation (LES). Effects of the jet Reynolds number, the nozzle separation, the confinement of boundary and the width-to-height ratio of jet on the deflecting oscillation of planar opposed jets are investigated. And the causes of the deflecting oscillation are revealed. Results show that the deflecting oscillation of planar opposed jets is originally caused by the nature instability of the planar jets and the impinging instability in the impingement region, while is self-sustained by the variation and transition of velocity and pressure fields. The period of deflecting oscillation is mainly determined by the jet velocity and the nozzle separation, and also affected by width-to-height ratio of jet and boundary confinement degree. Compared with Reynolds-averaged Navier-Stokes turbulence models, the LES results agree best with experiment results.2. The flow behaviors in planar opposed jets reactors are experimentally studied for various jet Reynolds numbers and geometry parameters, with focus on a half deflecting oscillation that has not been reported in literature. Effects of the jet Reynolds number, the jet separation, the width-to-height ratio of jet, the depth and the headspace of the chamber on the half deflecting oscillation are investigated. And the causes of the half deflecting oscillation in planar opposed jets reactors are revealed. Results show that there are different flow regimes, i.e., the segregated flow, the engulfment flow, the flapping oscillation of the impingement plane and the half deflecting oscillation in planar opposed jets reactors. The half deflecting oscillation occurs below the injectors plane and its dimensionless period is about half of that of the unconstrained opposed jets. The critical Reynolds number decreases with the jet separation and the width-to-height ratio of jets. The half deflecting oscillation is originally caused by the nature instability of the planar jets, while is self-sustained by the variation and transition of velocity and pressure fields. The positive pressure in the headspace of the chamber inhibits the jets deflecting upto the headspace of the chamber.3. Based on the instability of planar opposed jets, two novel devices of excited oscillation of planar opposed jets are design. Effects of the Reynolds number, the excitation frequency, the excitation amplitude and the geometry parameters on the oscillation behaviors are investigated. Oscillation characteristics of planar opposed jets and reactors with excitation are obtained. Results show that the acoustic excitation results in the horizontal periodic oscillation, whose frequency is equal to the excitation frequency. The acoustic excitation of oscillation amplitude less than 10% has negligible influence on the deflecting oscillation. For synchronous or asynchronous excitation with higher amplitude, the transition from the deflecting oscillation to a centered oscillation or a horizontal oscillation occurs. The transition of the oscillation regimes results from the change of vortex structures of planar jets under the acoustic excitation.The excited flapping oscillation of impingement plane in planar opposed jets reactors is caused by pulsed inflow and the interaction between the impingement plane and the confined wall of reactor, whose frequency is determined by the excitation frequency. The amplitude of excited flapping oscillation is proportional to excitation amplitude and the inlet jet velocity, but is inversely proportional to excitation frequency, when the oscillation amplitude is less than half of jet separation.4. The liquid mixing and enhancement with excitation in planar opposed jets reactors is studied by planar laser induced fluorescence (PLIF) technique. Effects of the fluid viscosity, the Reynolds number, the excitation frequency and amplitude and geometry parameters of reactors on the flow regimes and mixing performance are investigated. Results show that the mixing performance of fluids is poor in the segregated flow regime, and significantly increases when the segregated flow translates to the self-sustained oscillation regime. The deflecting oscillation at low Reynolds number results in good mixing performance in planar opposed jets reactor with lager jet separation. The mixing performance is enhanced with excitation in the segregated flow regime.