Study On The Aeration Process And Internal Gas-liquid Two-phase Flow Characteristics Of The Jet Mixer

Gas-liquid two-phase flow is widely used in chemical engineering,biological engineering and environmental engineering,such as liquid phase hydrogenation,biological fermentation,and wastewater treatment.Aerator is the core equipment of wastewater treatment.Its performance directly determines the effect and cost of wastewater treatment.Jet mixer is a traditional jet aeration device with a simple structure,no moving parts,and a strong turbulent mixing state in the gas-liquid contact part.It can break the gas into a large number of small bubbles or even microbubbles,resulting in excellent mass transfer efficiency.Strengthening the aeration process of jet mixer is an important research direction to further improve aeration efficiency,regulate gas-liquid mass transfer and reaction rate.It is also one of the research hotspots in the field of chemical engineering.This paper focuses on a self-aspirating jet mixer aeration device.The aeration process and the gas-liquid two-phase flow characteristics inside the jet mixer by combining experimental research and numerical simulation,aiming to reveal the strengthening law of gas-liquid two-phase flow in the jet mixer from the mechanism,and guide the development and industrial application of efficient aeration devices.Firstly,the aeration process of the jet mixer with swirling design is studied by high speed camera method,and the aeration performance of the mixer under two operation modes of self-aspiration and blowout is investigated.For the blower aeration process,the effects of gas volume flow rate,liquid volume flow rate,immersion height at the outlet of the mixing tube,and liquid to gas volume flow rate ratio(abbreviated as liquid to gas ratio)on the penetration depth and morphology of the jet flow are investigated.It is found that when the immersion height and gas volume flow rate are constant,the penetration depth and axial diffusion range of the jet flow gradually increase with the increase of liquid volume flow rate.Compared to liquid to gas ratio and immersion height,the volume flow rate ratio of liquid to gas is the key factor affecting the maximum penetration depth.As ratio of liquid to gas increases,the maximum penetration depth increases approximately linearly.For the self-priming aeration process,the effects of immersion height and liquid volume flow rate on penetration depth,jet flow morphology and suction capacity are investigated.It is found that when the immersion height is constant,the penetration depth and inspiratory capacity increase gradually with the increase of liquid volume flow rate.When the liquid flow rate is fixed,the inspiratory capacity gradually decreases with the increase of immersion height.Therefore,to achieve greater inspiratory capacity and penetration depth,a smaller immersion height and a larger liquid volume flow rate should be selected.Secondly,the structure of the jet mixer is optimized,and two types of jet mixer aeration devices with swirling addition and non-swirling addition are designed.The effects of different structural parameters on the gas-liquid mass transfer performance in the self-inspiratory aeration process are investigated.The gas-liquid dispersion characteristics in aeration tank with different liquid flow forms are compared.When the configuration is fixed,increasing liquid flow rate results in an obvious increase in the gas suction rate.Swirling flow is the main factor affecting the inspiratory efficiency.When the diameter of the mixing tube is constant,the inspiratory capacity of the jet mixer with swirling addition is much larger than that without swirling addition.Immersion height is another key factor affecting the efficiency of inspiratory efficiency.Higher immersion depth leads to an increase in internal pressure,which reduces the pressure difference between the gas inlet and the inside of the jet mixer and results in a reduction in the driving force of gas suction.Therefore,the gas suction rate gradually decreases.When the liquid flow rate is constant,the penetration depth obtained under the swirling condition is lower than that in non-swirling condition.The bubble diameter in the aeration tank with swirling design are greater than that without swirling design.Thirdly,the Eulerian-Eulerian two-phase flow model with RNG k-ε turbulence model is used to simulate the aeration process of the jet mixer with swirling addition.The interfacial forces,including the drag force,lift force,turbulent dispersion force,and wall lubrication force are considered in the simulation.The gas volume fraction,static pressure,and velocity streamlines are discussed in the self-inspiratory aeration process.When the liquid enters the mixing tube in the form of swirling flow,a negative pressure zone is generated in the upper part of the mixing tube due to the obvious tangential velocity component and strong centrifugal force.The gas is continuously self-sucked into the mixing tube.Under the effect of centrifugal force and liquid swirling flow,the gas continuously converges to the center of the mixing tube in the form of spiral flow.Increasing the liquid flow rate leads to an obvious increase in the pressure drop,which causes more gas to be sucked in.With the increase of liquid flow,the kinetic energy carrying gas flowing downwards increases,resulting in a deeper penetration depth.Then,the established numerical simulation method is used to simulate the gasliquid two-phase flow process inside the optimized jet mixer.The gas volume fraction,static pressure,and velocity streamlines are discussed in the jet mixer with or without swirling design.The results show that the pressure drop between the gas inlet and the outlet with swirling addition is significantly larger than that without swirling.There is an apparent positive linear correlation between the gas suction rate and the pressure drop.The swirling flow can generate a centrifugal force that pushes the gas towards the center of the pipe and the liquid to the vicinity of the wall due to the density difference between air and water.This opposite movement is not conducive to the efficient dispersion of gas in the liquid phase.Therefore,the liquid flow rate and the swirling intensity should be well designed to balance the gas inhalation capacity and the final gas-liquid dispersion state.At the same time,it is necessary to take further strengthening measures to improve the efficient dispersion of gas-liquid two phases in the mixing pipe under swirling design.Finally,the jet mixer is strengthened by adding internal components to generate smaller bubbles and enhance the gas-liquid mass transfer efficiency.A metal screen is added to the jet mixer to balance the gas suction efficiency and the final gas-liquid dispersion state.The effects of mesh size,layer number,and spacing on the bubble flow pattern,inspiratory capacity,and bubble size are investigated.The research shows that the addition of metal screen can generate smaller bubbles,and distribution of gas-liquid two phase is more uniform.When the mesh number is large,the diameter of the sieve hole is small,which increases the resistance of fluids passing through the sieve and reduces the pressure difference between the gas inlet and the interior of the jet mixer.Therefore,the gas suction rate significantly reduces.Similarly,as the number of layers of the sieve increases,the internal resistance of the mixing tube becomes larger,and the suction capacity gradually decreases.In the case of swirling flow,the increase in number of screen layers has small influence in the bubble diameter.While in the case of non-swirling flow,the bubble size gradually decreases with the increase of the number of screen layers.The size of screen spacing has little effect on gas-liquid twophase flow pattern and bubble diameter.In addition,the bubble diameter distribution and gas-liquid two-phase flow pattern in the mixing tube are analyzed by adding glass spheres to the jet mixer.The results indicate that adding glass spheres can produce smaller and more uniform bubbles in the mixing tube compared with no glass spheres addition.With the increase of liquid flow rate,the average bubbles diameter becomes smaller under both swirling and non-swirling flow conditions,and the bubbles are evenly distributed in the mixing tube.Whether there is a swirling design has little influence on the bubble size.A larger filling height is beneficial for generating bubbles with smaller diameters.