Gas-Liquid Flow Mechanism And Optimization Design Research For Inverted Umbrella Aerator

The study is carried out under the financial support from National Natural Science Foundation of China(51879122).As an oxygenation device,the inverted umbrella aerator is an important energy-consuming device in sewage treatment.The internal flow field of the inverted umbrella aerator involves complex flow problems such as rotating free liquid surface,self-admixing gas,water hopping,entrainment,bubble aggregation and crushing.The internal gas-liquid mixed flow state of the inverted umbrella aerator directly determines the efficiency and stability of the entire sewage treatment system.Therefore,the gas-liquid two-phase flow experiment under different working conditions of the inverted umbrella aerator is carried out.A gas-liquid two-phase calculation model suitable for the internal flow characteristics of the inverted umbrella aerator is constructed.The internal gas-liquid flow two-phase flow law is revealed,and the optimal design method for improving the aeration efficiency is mastered.The research results can not only provide theoretical basis and technical support for the study of gas-liquid two-phase of inverted umbrella aerator,but also provide important reference for related research of other aeration equipment.In this paper,the theoretical analysis,numerical calculation and experiment are combined to systematically study the inverted umbrella aerator.The study focuses on internal complex flow problems,internal flow characteristics,bubble characteristics,and gas-liquid two-phase flow calculation models.The optimization design method of the aeration efficiency of the inverted umbrella aerator is also explored.The main work and research results are as follows:1.The dissolved oxygen concentration and PIV test of the inverted umbrella aerator were studied,the oxygen aeration performance and internal flow law under different operating conditions were mastered.The experiment results show that the standard oxygen total transfer coefficient increases linearly with the increase of the rotational speed when the immersion depth is constant.When the rotation speed is constant,the total oxygen transfer coefficient of the standard oxygen increases first and then decreases with the decrease of the immersion depth.The standard oxygen total transfer coefficient is the largest when the immersion depth is-5 mm.The circulating vortex helps the shallow layer to fully mix with the bottom water body,which can accelerate the oxygen mass transfer process and increase the aeration efficiency.2.Bubble characteristic parameters such as the size and distribution had an important influence on gas holdup and gas-liquid two-phase flow.However,there was phenomena such as overlapping bubbles and reflections in the inverted umbrella aerator,and it was impossible to accurately extract the bubble characteristic parameters directly by the high-speed photography.This paper proposed an image processing method including median filtering,watershed segmentation processing,binarization and feature extraction,which realizes the effective extraction of bubble characteristic parameters.Firstly,the median filtering was used to denoise the original image,which can effectively filter the bright and dark spots caused by the impulse noise.Secondly,the watershed segmentation process is used to identify the gray scale,and the bubble accumulation area was divided into small bubbles that are easy to recognize.Finally,a reasonable binarization threshold is determined to achieve efficient segmentation of bubbles and background.The results show that the method can effectively extract the size and distribution of bubbles.The gas enters the water body under the action of water leaching and entrainment,and bubbles of different sizes are generated,and the bubbles are triangularly distributed in the vicinity of the inverted umbrella aerator.When the immersion depth is 0mm,the bubble size is between 0~1.59mm,among which0~0.53mm bubbles account for 40~67%,0.53~0.88mm bubbles account for 21~31%,and 0.88~1.23mm bubbles account for about 8%.1.23~1.59mm bubbles account for about 5%.With the increase of the rotational speed,the number of bubbles and the gas holdup rate both increased.As the immersion depth decreased,both of them increased first and then decreased.3.The internal gas-liquid two-phase flow simulation of the inverted umbrella aerator with a single bubble size neglected the interaction between the bubble groups and the effect of the bubble gathering and breaking phenomenon on the internal flow.It was impossible to accurately reveal the true flow of the inverted umbrella aerator.This paper combined multiphase flow model,population balance model,bubble collapse and coalescence model and k_La mass transfer equation to construct a CFD-PBM model that can accurately simulate the internal flow and aeration performance of inverted umbrella aerator.The results show that the deviation between the simulated value of the interface area and the experimental value is 12%,and the deviation of the total oxygen transfer coefficient is less than 3%,which is consistent with the experimental value.The bubble trapping behavior causes speed loss,while the water jump and entrainment cause the circulation vortex to form.With the influence of the circulation vortex,the entrained gas gradually moves to the bottom,and the gas-liquid mixing is completed due to the stirring action of the inverted umbrella aerator.4.An optimized design method for oxygenation performance of inverted umbrella aerator based on experimental design and response surface was established.The number of blades,the inclination angle of the web,and the gap opening of the web were design variables.A multivariate regression model of major structural parameters and objectives was constructed with the goal of maximizing standard power efficiency.Analyze the interaction of response parameter parameters and determine the optimal combination of parameters.The optimization results were compared and verified by numerical simulation and experiment.The results show that the optimized standard oxygen transfer coefficient is increased by 1.59%and the standard power efficiency is increased by 4.09%.