Research On The Gas-Liquid Two-Phase Flow Mass Characteristics And Structure Optimization Of Microalgae Column Photobioreactors

As the most promising third-generation renewable biomass energy source,microalgae have significant advantages in the field of carbon sequestration and biofuel production,contributing to the achievement of carbon peaking and carbon neutrality targets.The photobioreactor acts as a vessel for the primary cultivation of microalgae,creating an environment suitable for the growth of microalgae,such as light,p H and temperature.Research into photobioreactors can reduce the cost of cultivating microalgae and produce more of them at the same time.The aeration device in the photobioreactor can be used to introduce carbon dioxide gas into the reactor.By improving the performance of the reactor aeration device,the efficiency of microalgae cultivation can be effectively improved,and the aeration structure of the column photobioreactor is optimized using numerical simulation.The Eulerian model was used for the simulations and the k-εmodel was chosen for the turbulence model.The variation of several parameters such as gas content,liquid velocity,turbulent kinetic energy and the flow field in the photobioreactor under different aeration device conditions were studied and analyzed as follows.The effect of aeration on the mass transfer and flow characteristics of the photobioreactor was investigated and it was found that when the aeration structure has the same aperture diameter,the gas content,liquid phase velocity and turbulent kinetic energy all increase with increasing aeration.The increase in aeration velocity also leads to an increase in bubble diameter,which is not conducive to gas-liquid mass transfer.The results show that among the five aeration rates of 400,700,1000,1400 and 1800 m L·min^-1,1400 m L·min^-1is the optimum aeration rate for the column-type photobioreactor.The effect of the height of the photobioreactor on the mass transfer and flow characteristics of the photobioreactor was analyzed,and it was found that the gas content rate increased gradually with the increase of the reactor height,and the turbulent kinetic energy tended to increase and then decrease with the increase of the reactor height.The results show that among the four heights of 20,30,40 and 50 cm,30 to 40 cm is the optimum height range for the bioreactor.The analysis of the effect of the structure of the aeration unit on the gas-liquid transfer and flow characteristics of the photobioreactor showed that among the four structures,spherical,spherical crown,lower hemisphere and flat plate,the average liquid velocity value of the spherical structure increased by about 52%compared to the flat plate structure,and the turbulent kinetic energy value of the spherical structure increased by about 78%compared to the lower hemisphere structure.Optimizing the number of orifices in the aeration unit,the results show that as the number of orifices increases,the orifice flow velocity is gradually decreasing and therefore the turbulent energy in the orifice and the local area nearby is gradually decreasing.Of the six orifice number gradients of 30,40,45,50,70 and 80,an orifice number of 50 is the optimum aeration orifice number.The effect of the aeration device pore size on the column photobioreactor was investigated and the results showed that 30μm was chosen as the optimum aeration pore size out of six gradients of 10,30,50,80,100 and 120μm.In addition,comparing the aeration performance of the dual aeration head with that of the single aeration head,the gas aeration from the dual aeration head is more widely distributed in the photobioreactor with improved turbulent kinetic energy and mass transfer coefficient compared to the single aeration head,which has better aeration effect.On the basis of a more in-depth optimization of the aeration structure,a simulation study was carried out on the aspect of installing baffles in the reactor.A comprehensive analysis of the different cases with spacers has been carried out in relation to the reactor flow field and other parameters（turbulent kinetic energy,gas content,liquid phase velocity）,and the relevant phenomena have been analyzed and discussed.The effect of carbon dioxide concentration on the reactor flow field is first investigated.As the concentration of incoming carbon dioxide increases,the number of carbon dioxide bubbles becomes larger and the gas becomes more widely distributed in the reactor.The rate of CO₂uptake and fixation has a maximum value when the incoming CO₂concentration is 0.3.The turbulent kinetic energy in the middle of the reactor is 0.0048 m²/s²,which is about 23%higher than the turbulent kinetic energy in the upper part of the reactor.On the basis of a more in-depth optimization of the aeration structure,a simulation study was carried out on the installation of baffles in the reactor.Based on the analysis of the flow field in the reactor,the best mass transfer between gas and liquid in the reactor is obtained when the deflector is in the middle position.The effect of the circular and inverted circular deflector on the flow field of the reactor was analyzed.The results showed that due to the inclination of the surface of the deflector,the circular deflector is conducive to enhancing the turbulence intensity and increasing the retention time of the gas in the reactor.In addition,the results show that the gas inclusion rate of the internal partition is only less than that of the circular sill type,which is 0.391,compared with that of the inverted circular sill type,which is about 18.1%higher.The effect of different aeration rates on the flow field of the reactor at the same angle was investigated and the gas content,liquid phase velocity and turbulent kinetic energy of the reactor increased with increasing aeration rate.To investigate the effect of different clamping angles on the reactor flow field at the same aeration rate,it was found that the turbulent kinetic energy decreases when the angle is less than 60 degrees.This may be due to the reduced collision effect of the baffle on the gas caused by the reduced angle,resulting in less vortex in the gas phase.In this thesis,Fluent software is used to optimize the structure of the photobioreactor and its aeration unit.It also provides theoretical support for other studies on the optimization of the structure of photobioreactors,as well as a systematic study and analysis of the aeration structure,which has been little studied by previous researchers.