Bubble Generation Time Research Articles

Research on the influence of aeration manners on the mass transfer characteristics of bubbles in multi‐column airlift photobioreactor

AbstractMass transfer of bubbles is important in microalgae cultivation. In this study, aiming to improve the microalgae culturing efficiency, computational fluid dynamics (CFD) was adopted to study the influence of the aeration manners on mass transfer of bubbles within ascending columns in multi‐column airlift photobioreactor (PBR) and the hydrodynamic conditions within the PBR under different aeration manners. In addition, the bubble generation time, the detachment diameter, and the average volumetric mass transfer coefficient of bubbles in ascending columns were analyzed. Furthermore, the experimental results were compared with the simulation results obtained from microalgae cultivation. The results showed that the whole aeration manner yields the lowest mixing strength and mass transfer efficiency of bubbles at an aeration rate of 0.2 vvm. Conversely, both the mixing strength of the liquid and mass transfer coefficient of bubbles were enhanced under the half and alternate aeration manners. However, the results demonstrated that the distribution of the flow field was not uniform under the half aeration manner and there were obvious high‐speed and low‐speed zones. In contrast, the flow field distribution in the PBR was more uniform under the alternate aeration manner, which was suitable for microalgae cultivation at high concentration. This study effectively enhanced the mixing strength and CO2 transfer rate in the PBR.

Effect of gas injection rate on bubble generation characteristics and coal flotation

Using a high-speed motion acquisition system, the effect of gas injection rate on dynamic bubble characteristics in the presence of flotation reagents was investigated, and the force analysis during the bubble generation was performed. The corresponding coal flotation tests were conducted to discuss the relationship of the bubble characteristics and the coal recovery. According to a partition criterion of contact angle as 90° between the bubble and the capillary, the bubble generation process was divided into formation, expansion and contraction stages, and the generation time was majorly allocated to the expansion stage. As the gas injection rate increased, the bubble aspect ratio decreased, the bubble size increased and the bubble generation time decreased. The surface tension was analyzed as the main force for adhering the bubble on the capillary, and the buoyancy was deemed as the main force detaching the bubble from the capillary. Furthermore, the increase in net force was analyzed as the essential factor for the increase in bubble generation rate during the expansion stage. The combustible matter recovery increased as the gas injection rate increased, and the flotation rate constant was analyzed closely related to the bubble deformation. The results can provide valuable insights into the development of technology for mineral flotation.

2つのレーザ誘起キャビテーション気泡により生成するマイクロジェットに関する研究

Two cavitation bubbles generated by laser irradiation in water generate microjets due to their mutual interference. The characteristics of microjets vary depending on the size and distance of the bubbles, as well as on the time difference between bubble generation and microjet generation. Therefore, the purpose of this study is to clarify the effect of the bubble generation time difference, T*D in dimensionless form, on the bubble motion and microjet by numerical calculations and experiments. Numerical calculations are performed using the boundary element method and the VOF method, both of which have conflicting characteristics, and two numerical methods are used for efficient analysis. In this study, cavitation bubbles are generated at the top and bottom, and the lower bubble that is generated first is bubble A, and the upper bubble that is generated after a delay is bubble B. When the distance between the bubbles and the bubble size ratio are kept constant and the bubble generation time difference is varied, it is found that there are three types of microjets generated in bubble A due to the mutual interference between the two cavitation bubbles. Upward microjets are generated in bubble A and penetrate upward when the bubble generation time difference is small. This occurs when the expansion of bubble A is slightly disturbed, causing the underside of bubble A to become concave. A downward microjet is generated in bubble A and penetrates downward when the bubble generation time difference is large. This is caused by the upper side of bubble A being pushed in due to the strong influence of the expansion of bubble B. When the bubble generation time difference is in the range of 0.16 ≤ T*D ≤ 0.20, bubble A is separated into two parts, resulting in the creation of upward and downward microjets. This is caused by the predominant horizontal nipping of bubble A.

CFD-DEM investigation into flow characteristics in mixed pulsed fluidized bed under electrostatic effects

Static electricity has an important effect on gas–solid fluidized bed reactor fluidization performance. In the process of fluidization, electrostatic interaction between particles will obviously accelerate particle agglomerate formation, which consequently reduces the fluidization performance. Pulsed gas flow injection is an efficient method to enhance particle mixing, thereby weakening the occurrence of particle agglomerate. In this study, the two-dimensional hybrid pulsed fluidized bed is established. The flow characteristics are studied by using the coupled CFD-DEM numerical simulation model considering electrostatic effects. Influences of different pulsed frequencies and gas flow ratios on fluidized bed fluidization performance are investigated to obtain the optimal pulsed gas flow condition. Results show that in the presence of static electricity, the bubble generation position is lower, which is conducive to the particle flow. Pulsed gas flow can increase the particle velocity and improve the diffusion ability. The bubble generation time is different at different frequencies, and the frequency of 2.5 Hz has the most obvious effect on the flow characteristics. Different gas flow ratios have significant impacts on the particle movement amplitude. When the pulse gas flow accounts for a large ratio, the particle agglomerate tends to be larger. Therefore, in order to improve the fluidization effect, the ratio of pulsed gas flow to stable gas flow should be appropriately reduced to 0.5 or less.

Developing microporous fibrous-diaphragm aerator to decrease bubble generation diameter for improving microalgal growth with CO2 fixation in a raceway pond

A novel microporous fibrous-diaphragm aerator (FDA) was proposed to generate pressurized gas with check valve to penetrate through millions of micropores (6–126 μm) of expanded fibrous diaphragm on internal support, thus decreasing bubble generation diameter and increasing bubble residence time for improving microalgal growth with CO2 fixation in a raceway pond. When installation angle of FDA internal support increased from 0° to 45°, bubble generation time and diameter first decreased (to valley bottoms of 4 ms and 0.45 mm at 22°) and then increased. Compared to traditional strip aerator, bubble generation time and diameter decreased by 50% and 60% through FDA with support installation angle of 22° and average pore diameter of 28 µm, while gas-solution mixing time decreased by 22% and mass transfer coefficient increased by 40%, leading to increased actual photochemical efficiency (by 80%) and increased biomass yield (by 38.5%) of Arthrospira cells with pure CO2 aeration through FDA.

Reduced generation time and size of carbon dioxide bubbles in a volute aerator for improving Spirulina sp. growth

A novel volute aerator was proposed to generate shear force and break gas flow through water centrifugation to promote mass transfer for CO2 fixation with microalgae. The bubble evolution and gas-liquid mixing processes in volute aerator were numerically simulated. The bubble generation time and diameter were measured on a high-speed camera and assessed with level-set method. By optimizing gas inletpipe angle and water/gas inlet velocities of volute aerator, the bubble generation time decreased by 60.1% to 3.3 ms and outlet bubble diameter decreased by 50.7% to 1.8 mm, compared with traditional strip aerator. The corresponding gas-liquid mixing time reduced by 15.6% and mass transfer coefficient increased by 42.2%. The volute aerator was used as an alternative to traditional strip aerator to culture microalgae under high-purity CO2 condition, which promoted average growth rate and biomass production by 26.6% and 50.7%, respectively.

Alternatively permutated conic baffles generate vortex flow field to improve microalgal productivity in a raceway pond

Alternatively permutated conic (APC) baffles were proposed to generate vertical and horizontal vortex flow to intensify mixing and mass transfer in a raceway pond. Both clockwise vortexes were generated before and after conic baffles in the main stream to increase perpendicular velocity by 40.3% and vorticity magnitude by 1.7 times on vertical cross section. Self-rotary flow around conic baffles and vortex flow among conic baffles were generated to increase perpendicular velocity by 80.4% and vorticity magnitude by 4.2 times on horizontal cross section. The bubble generation time and diameter decreased by 25.5% and 38.7%, respectively, while bubble residence time increased by 84.3%. The solution mixing time decreased by 48.1% and mass transfer coefficient increased by 34.0% with optimized relative spacing (ε) and height (ω) of conic baffles. The biomass productivity of Spirulina increased by 39.6% under pure CO2 with APC baffles in a raceway pond.

Developing a water-circulating column photobioreactor for microalgal growth with low energy consumption

A water-circulating column photobioreactor (WCC–PBR) was developed to decrease bubble generation time and mixing time for growing microalgal biomass at low energy consumption. Bubble generation time was decreased by 60.4% and mixing time was decreased by 41.5% owing to an enhanced solution velocity with a water pump. Bubble residence time was decreased by 31.1% and mass transfer coefficient was decreased by 0.4% owing to a reduced distance between air aerator and solution surface. Microalgal growth rate was decreased by 12.7% from 128.9mg/Lday in an air-lifting column photobioreactor (ALC-PBR) to 112.6mg/Lday in a WCC–PBR because of the decrease in residence time of bubbles and an additional shear of cells in a water pump. However, total energy consumption of a WCC–PBR with an air compressor and a water pump was lower by 21.1% than that of an ALC–PBR with only an air compressor.

Improving microalgal growth with small bubbles in a raceway pond with swing gas aerators

A novel swing gas aerator was developed to generate small bubbles for improving the mass transfer coefficient and microalgal growth rate in a raceway pond. A high-speed photography system (HSP) was used to measure the bubble diameter and generation time, and online precise dissolved oxygen probes and pH probes were used to measure the mass transfer coefficient and mixing time. Bubble generation time and diameter decreased by 21% and 9%, respectively, when rubber gas aerators were swung in the microalgae solution. When water pump power and gas aeration rate increased in a raceway pond with swing gas aerators and oscillating baffles (SGAOB), bubble generation time and diameter decreased but solution velocity and mass transfer coefficient increased. The mass transfer coefficient increased by 25% and the solution velocity increased by 11% when SGAOB was used, and the microalgal biomass yield increased by 18%.

Improving CO2 fixation with microalgae by bubble breakage in raceway ponds with up–down chute baffles

The aeration gas was broken into smaller bubbles with enhanced local solution velocity to improve CO2 fixation with microalgae in raceway ponds with up–down chute baffles. A high-speed photography system and online precise pH probes were used to measure bubble generation and residence times, which were affected by paddlewheel speed, aerator orifice diameter, gas flow rate, and solution depth. Bubble generation time (from gas reaching aerator orifice surface to completely escaping from the aerator) decreased because of the enhanced local solution velocity, whereas bubble residence time increased because of the vortex flow field produced by up–down chute baffles. Bubble generation time decreased by 27% and bubble residence time increased by 27% when paddlewheel speed was 10r/min with an aeration gas rate of 0.03vvm. The decreased generation time and increased residence time of aeration bubbles promoted microalgae biomass yield by 29% in optimized flow fields in raceway ponds.

Computational study on microscale behavior of bubble generated by aeration in a plug-flow aeration tank

The microscale hydrodynamics of bubbles generated by aeration is directly related to the oxygen transfer efficiency and the overall performance of the activated sludge wastewater treatment process. To gain a deeper insight on the microscale phenomena of dispersed bubble occurring in this process, a three-dimensional direct simulation method is developed to study the effects of the liquid cross-flow on microscale behavior of bubble generation in a plug-flow aeration tank. The numerical simulations are performed using the level set method coupling with the governing equations of a single fluid with variable properties. The governing equations are solved using the finite-volume technique. The simulation results are validated through comparison with experimental observations. The study indicates that the liquid cross-flow has a strong impact on the bubble generation. Compared to that generated under quiescent liquid conditions, the bubble under liquid cross-flow conditions grows downstream along the tilted axis. The bubble generation time tends to decrease noticeably and the bubble at detachment has significantly smaller size. The bubble size and generation time also increase with the increase of gas velocity. The relation of such results to the oxygen transfer efficiency of the wastewater treatment process is also discussed.