Research On Measurement Of The Viod Fraction And The Instability Pattern Of The Bubble Plume

Bubble plume is a complex two-phase fluid generated in aerating process, the flow of gas-liquid two-phase is stochastic and the flow pattern is diverse. Through the calculation parameter of void fraction and the research of its oscillation mechanism to further reveal the instability law of gas-liquid two-phase flow and improve the efficiency of gas-liquid two-phase flow.In this study, image processing and numerical calculation were used to obtain void fraction of the parameters of the bubble plume based on particle image velocimetry techniques. And the influences of void fraction distribution to the movement of bubble plume and aeration efficiency are researched in the different conditions. On the basis of the experimental study, frequency spectra of bubble plume oscillation at different water depths and frequency spectra of bubble fluctuation velocities were obtained by image processing and Fast Fourier Transform, and further study the instability character of the bubble plume and the characteristics movement of bubbles in a detailed spatial and temporal.Some conclusions can be obtained as follows:(1) The local averaged brightness method and the real local void fraction is suitable to measure the void fraction of gas-liquid two-phase flow based on image processing, this method can simply and directly obtain the void fraction distribution even in high void fraction.(2) The void fraction distribution is mainly affected by aspect ratio and gas volume. The movement pattern of the bubble plume is determined by Aspect ratios, gas volume is the main factor affecting the void fraction distribution of bubble plume. With the increasing pressure and gas flow rate, the oscillation of bubble plume intensified and the number of freedom movement bubbles are increasing under the water, which have brought about changes in the void fraction distribution of bubble plumes. When the aspect ratio is 1.0, the void fraction distribution is not affect the structure of the bubble plume and the structure is stability, bubbles can be stayed a longer time in the water and the mass transfer efficiency of oxygen can be improved.(3) The results can be found by Frequency spectra of bubble plume oscillation and frequency spectra of bubble fluctuation velocities in the different conditions, when the pressure is increasing, the frequency and the peak of bubble plume oscillation is detected in the low, fluctuations peak of bubble is also detected in the low, but the bubble fluctuation velocities is found in the high frequency.(4) Change the pressure and void fraction, The influence of bubble plume movement structure in the three aspect ratios are as follows:The whole structure of the bubble plume is stability when the aspect ratio of 1.0; The pattern and structure in the middle part of bubble plume is instability when the aspect ratio of 1.5; The pattern and structure in the upper part of bubble plume is instability when the aspect ratio of 2.0.