Detachment Diameter Research Articles

Dynamic behaviour of the CO2 bubble in a bubble column bioreactor for microalgal cultivation

Carbon dioxide (CO2) gas is a major carbon source for microalgal cultivation. It is usually sparged into photobioreactors in the form of bubbles. The behaviour of the bubbles significantly affects mass transfer, distribution and microalgae growth. In this study, the dynamic behaviour of the CO2 bubbles was compared between a microalgal suspension and pure water. These investigations were carried out via visual methods. The movement and distribution of microalgae at the gas–liquid interface were observed. The effects of gas flow velocity, CO2 concentration and capillary orifice size were analysed. The results indicated that much of the microalgal cells adsorbed onto the surface of the CO2 bubbles in the microalgal suspension, when compared with that in pure water. This resulted in an easier detachment of the bubbles in the microalgal suspension. The growth status of the bubbles were divided into two states according to changes in the Eotvos number and the behaviour of the CO2 bubbles as influenced by gas flow velocity: steady and unsteady state. The critical gas velocity between the two states was achieved. The CO2 bubble rising trajectory can be divided into three main phases: the vertical acceleration phase, the transition phase, and the oscillatory rising phase. During the oscillatory rising phase, the amplitude of the bubble rising trajectory was approximately two times greater than the bubble diameter. In addition, the wavelength of the bubble rising trajectory was approximately 16–18 times the bubble diameter in the microalgal suspension. A smaller capillary orifice size and larger CO2 concentration led to a decrease in the bubble detachment diameter, an increase in velocity and an enlargement in the zone of bubble influence in the horizontal direction. These are advantageous for CO2 transportation. These findings are beneficial for optimizing the design and operation of microalgal photobioreactors.

Impact of Microalgae on CO2 Bubble Growth and Detachment in Bubble Column Bioreactor

The biological capture of carbon dioxide (CO2) using microalgae is a promising technology and has gained a huge momentum due to its environment-friendly, low cost and high CO2 fixation efficiency. In the applications CO2 gas usually was pumped into photobioreactors in bubble form. The bubbles behavior significantly affects the mass transfer and distribution of the microalgae. Also the adhesion of microalgae and absorption of CO2 at the gas-liquid interface will certainly have an important effect on the dynamics of CO2 bubbles in bubble column bioreactor. In this work, visual experiments were conducted for studying the impacts of microalgae on CO2 bubble growth and detachment in bubble column bioreactor. It was found that many microalgae cells adsorbed on the surface of the bubble resulting in easier detachment of bubble in microalgae suspension against with pure water. Besides, it was also found that smaller capillary radius and larger CO2 concentration gave rise to decreasing bubble growth rate and detachment diameter which was conducive to CO2 transportation. Moreover, a critical gas flow velocity of 1.7 m/s in capillary was achieved, which divided the bubble growth status into steady and unsteady state, and different bubble growth and detachment behaviors were observed. These findings will be beneficial for the design of aeration system of photobioreactors for microalgae cultivation to enhance the mass transport at the interface.

Dynamic behaviors of CO2 bubbles coalescing at two parallel capillary orifices in microalgae suspension

In the microalgae cultivation process, CO2 in the bubble form is supplied into photobioreactors as carbon source for microalgae growth. The coalescence of CO2 bubbles greatly affects the mass transport and thus the biomass production rate. In this work, therefore, the dynamic behaviors of CO2 bubbles coalescing at two parallel capillary orifices in microalgae suspension were visually studied and compared with pure water. The movement and distribution of microalgae at gas–liquid interface were also observed. The effects of the center distance of capillary orifices and gas flow rate were also analyzed. It was found that many microalgae and microalgae aggregation adsorbed on the surface of the bubble and tended to distribute between the two bubbles when two bubbles coalesced. Besides, it was also found that in microalgae suspension more time was needed for the coalescence of bubbles, and the coalesced bubbles were more easily detached as a result of low surface tension but with lower terminal velocity after detachment than that in pure water. When the center distance of capillary orifices was 1.5mm (S=3d0), the coalesced bubbles had less growth time and smaller detachment diameter. A critical gas flow rate of 15mL/min was obtained, at which the minimum detachment diameter and terminal velocity of coalesced bubble was achieved. These findings will be beneficial for the design of aeration system of photobioreactors for microalgae cultivation to enhance the mass transport at the interface.

Investigation of Bubble Growth during Nucleate Boiling Using CFD

Boiling process is characterized by the rapid formation of vapour bubbles at the solid–liquid interface (nucleate boiling) with pre-existing vapour or gas pockets. Computational fluid dynamics (CFD) is an important tool to study bubble dynamics. In the present study, CFD simulation has been carried out to determine the bubble detachment diameter and its terminal velocity. Volume of fluid method is used to model the bubble and the surrounding by solving single set of momentum equations and tracking the volume fraction of each of the fluids throughout the domain. In the simulation, bubble is generated by allowing water-vapour to enter a cylinder filled with liquid water through an inlet at the bottom. After the bubble is fully formed, the bubble detaches from the surface and rises up during which the bubble accelerates due to the net balance between buoyancy force and viscous drag. Finally when these forces exactly balance each other, it attains a constant terminal velocity. The bubble detachment diameter and the terminal velocity of the bubble are captured by the monitor function provided in FLUENT. The detachment diameter and the terminal velocity obtained is compared with the established results based on the shape of the bubble. A good agreement is obtained between the results obtained from simulation and the equations in comparison with the established results.

Numerical investigation of quasi-static bubble growth and detachment from submerged orifices in isothermal liquid pools: The effect of varying fluid properties and gravity levels

The present investigation, identifies the exact quantitative effects of fundamental parameters, on the detachment characteristics of isolated bubbles, emanating quasi-statically from submerged orifices into isothermal liquid pools. For this purpose, a Volume of Fluid (VOF) based interface capturing approach is further improved, for the conduction of axisymmetric and 3D numerical experiments on adiabatic bubble growth dynamics. The predictions of the model, are quantitatively validated against literature available experimental data, showing excellent agreement. Two series of numerical experiments are performed, quantitatively exploring the parametric effects of the liquid phase properties in five different gravity levels, and the effect of the gravity vector direction inclination angle, respectively. It is found that the bubble detachment characteristics, are more sensitive in the variation of the surface tension, liquid phase density and gravity, while the effect of liquid phase dynamic viscosity is generally minimal. From dimensionless analysis, two correlations are derived, which for the examined range of Eötvos numbers, are able to predict the equivalent bubble detachment diameter and the bubble detachment time, respectively. It is also found that the bubble detachment characteristics, reduce significantly as the gravity vector direction gradually deviates from being parallel to the bubble injection orifice, following a non-linear decrease.

Numerical investigation of the mechanism of two-phase flow instability in parallel narrow channels

In this paper, the mechanism of two-phase flow instability in parallel narrow channels is studied theoretically, and the characteristic of the flow instability is discussed in detail. Due to the significant confining effect of the narrow channel on the vapor–liquid interface, the two-phase flow resistance in the narrow channel is probably different from that in conventional channel. Therefore, the vapor confined number (Nconf), defined by the size of narrow channel and bubble detachment diameter, is considered in the “Chisholm B model” to investigate the two-phase flow pressure drop. The flow instability boundaries are plotted in parameter plane with phase-change-number (Npch) and subcooling-number (Nsub) under different working conditions. It is found that the predicted result agrees well with the experimental result. According to the predicted result, the oscillation behaviors near the flow instability boundary indicate that the Supercritical Hopf bifurcation appears in high sub-cooled region and the Subcritical Hopf bifurcation appears in low sub-cooled region. Also, a detailed analysis about the effects of key parameters on the characteristic of two-phase flow instability and the flow instability boundary is proposed, including the effects of inlet subcooling, heating power, void distribution parameter and drift velocity.

The impact of intravitreal aflibercept on pigment epithelial detachement morphology in eyes with persistent subretinal fluid despite a minimum of 14 previous treatments with ranibizumab in patients with neovascular age‐related macular degeneration

AbstractPurpose To assess the efficacy of intravitreal aflibercept (2.0 mg) in patients with treatment‐resistant neovascular age‐related macular degeneration.Methods Retrospective analysis of eyes treated with aflibercept with persistent subretinal and/or intraretinal fluid despite a minimum of 14 previous treatments with intravitreal ranibizumab (0.5 mg). All patients were switched to intravitreal aflibercept (2.0 mg) and analyzed after 3 consecutive injections and at month 4 of treatment. Main outcome measures included change in visual acuity, central foveal thickness, and the height and diameter of the largest pigment epithelial detachment on spectral domain optical coherence tomography.Results Fifteen eyes of fifteen patients were analysed. The median number of previous injections of ranibizumab was 19 (IQR 15‐22). At month four, the mean visual acuity and central retinal thickness (CRT) did not change significantly from baseline: logMAR 0.64 (95%CI‐ 0.45‐0.83) and 0.65 (0.44‐0.89; p=0.5); CRT 282 μm (233‐333 μm) and 247 μm (201‐293 μm; p=0.06). However, the maximum pigment epithelial detachment height improved significantly from 279 μm (95%CI 124‐434 μm) to 195 μm (95%CI 65‐326 μm) (p=0.03) and pigment epithelial detachment diameter decreased significantly from 2,515 μm (95%CI 1,705‐3,3324μm) to 1,833 μm (95%CI 256‐1,106 μm) (p = 0.002).Conclusion Intravitreal injections of aflibercept resulted in a significant improvement in the maximum height and width of the PED in eyes with a minimum of 14 previous injections of ranibizumab.

Experimental investigation of the thermal interactions of nucleation sites in flow boiling

An experimental setup was designed to perform nucleate boiling experiments in upward saturated flow conditions, in order to investigate the influence of vertically aligned vapor bubble nucleation sites on one another. Experiments were performed by activating two bubble generators, of which the inter-site distance can be varied by steps of 2 mm. Depending on mass flow rate, flow direction and heat fluxes to both bubble generators, nucleation sites have been shown to interact at any nucleation site distance. The results have shown two major trends. The first trend is caused by additional convection (not by bubbles) from the upstream bubble generator, BG2, to the downstream bubble generator, BG1, increasing its nucleation frequency and bubble detachment diameter. The influence of additional convection on bubble frequency and diameter diminishes with increasing inter-site distance, S , and initial bubble nucleation frequency at BG1. The influence of added convective heat is enhanced by increasing the liquid bulk flow rate. The second trend is seen when BG2 initiates its own bubble nucleation. Vapor bubbles that nucleate at BG2 and pass by BG1 in close proximity have an inhibitive effect on bubble nucleation at BG1. The effect of this inhibitive trend diminishes with increasing nucleation frequency at BG1. Because of the significance of the effect of hydrodynamic interaction on the number of active nucleation sites and bubble size at detachment, mechanistic modeling of nucleate flow boiling is expected to benefit from the quantifications presented in this study.

Bubble departure diameter in nucleate pool boiling at saturation: Pure liquids and binary mixtures

Bubble dynamics is the most important sub-phenomenon, which basically affects the nucleate pool boiling heat transfer coefficient. Previous investigations state that the effect of physical properties of liquid and vapor phases on bubble departure diameter are often conflicting. In this article, extensive new experimental data at atmospheric pressure are presented for bubble departure diameters during saturated pool boiling for pure water, ethanol and various binary mixtures, including ethanol/water, NaCl/water and Na2SO4/water over a wide range of concentrations. For all test fluids, experimental results show that bubble diameter increases with increasing heat flux. Also, experimental results show that bubble detachment diameter increases with increasing electrolyte concentration and decreases with increasing ethanol mass fraction. A new model for the prediction of vapor bubble departure diameter, based on Buckingham theory, in nucleate boiling is proposed, which predicts the experimental data with a satisfactory accuracy.

Ex vivo bubble production from ovine large blood vessels: Size on detachment and evidence of “active spots”

Nanobubbles formed on the hydrophobic silicon wafer were shown to be the source of gas micronuclei from which bubbles evolved during decompression. Bubbles were also formed after decompression on the luminal surface of ovine blood vessels. Four ovine blood vessels: aorta, pulmonary vein, pulmonary artery, and superior vena cava, were compressed to 1013kPa for 21h. They were then decompressed, photographed at 1-s intervals, and bubble size was measured on detachment. There were certain spots at which bubbles appeared, either singly or in a cluster. Mean detachment diameter was between 0.7 and 1.0mm. The finding of active spots at which bubbles nucleate is a new, hitherto unreported observation. It is possible that these are the hydrophobic spots at which bubbles nucleate, stabilise, and later transform into the gas micronuclei that grow into bubbles. The possible neurological effects of these large arterial bubbles should be further explored.

The influence of flow rate on inter-nucleation site heat transport

The main topic of this paper is the influence of vertically aligned nucleation sites on each other in upward flow boiling. A setup was constructed to facilitate vertical up-flow of deminiralized water under saturation conditions. The main test section is a glass channel with a set of vertically aligned bubble generators. Each bubble generator is operated independently, where power and wall temperature are registered and the vapour bubbles are visualized by a high-speed camera. During the experiments, the downstream bubble generator (BG1) power is kept constant, while the power fed to the upstream bubble generator (BG2) is incrementally increased. Two main trends have been identified. The first trend is dominated by added convection from one site to the other. Both bubble frequency and detachment diameter on BG1 increase with increased power fed to upstream BG2. This effect decreases with increasing inter-site distance and becomes more significant with increasing liquid flow rate. When vapor bubbles start nucleating from BG2, these vapor bubbles inhibit bubble nucleation BG1 and can even lead to deactivation of this nucleation site. This second trend is only weakly dependent on inter-site distance, since the inhibition originates from bubbles flowing past BG1 in close proximity.

Modelling and Measurement of Bubble Formation and Growth in Electroflotation Processes

Abstract Electroflotation is used in the water treatment industry for the recovery of suspended particles. In this study the bubble formation and release of hydrogen bubbles generated electrolytically from a platinum cathode was investigated. Previously, it was found that both the growth rate and detachment diameter increased with increasing wire diameter. Conversely, current density had little effect on the released bubble size. It was also found that the detached bubbles rapidly increased in volume as they rose through the liquid as a result of decreasing hydrostatic pressure and high levels of dissolved hydrogen gas in the surrounding liquid. The experimental system was computationally modelled using a Lagrangian-Eulerian Discrete Particle approach. It was revealed that desorption of gaseous solutes from the electrolyte solution, other than hydrogen, may have a significant impact on the diameter variation of the formed bubbles. The simulation confirmed that liquid circulation, either forced or induced by the rising bubble plume, influences both the hydrogen supersaturation (concentration) in the neighbourhood of the electrode and the size of the resulting bubbles.

Effect of the PTFE content in the gas diffusion layer on water transport in polymer electrolyte fuel cells (PEFCs)

The dynamic behavior of a liquid water droplet emerging and detaching from the surface of the gas diffusion layer (GDL) is investigated. The droplet growth and detachment are studied for different polytetrafluoroethylene (PTFE) contents within the GDL and for different superficial gas velocities flowing in the gas channel. To simulate the droplet behavior in the cathode and anode of an operating polymer electrolyte fuel cell, separate experiments are conducted with air and hydrogen being supplied in the gas channel, respectively. Both the superficial gas velocity and the PTFE content within the GDL are found to impact the droplet detachment diameter. Increasing the superficial gas velocity increases the drag force applied on the droplet sitting on the GDL surface. It is observed that the droplet detaches at a smaller diameter for higher superficial gas velocities. The droplets also detach at smaller diameters from GDLs with a higher amount of PTFE. Such observation is justified according to two different points of view: (1) heterogeneous through-plane PTFE distribution through the GDL and (2) reduced GDL surface roughness caused by PTFE loading.

Numerical analysis of the bubble detachment diameter in nucleate boiling

The present paper presents a tri-dimensional CFD (Computational Fluid Dynamics) model to investigate the fluid flow around bubbles attached to heated walls. Transient solutions of the governing field equations in a domain containing the bubbles and the surrounding liquid have been obtained. The nucleation, growing and detachment processes have been analyzed. Concerning the software, the open source OpenFOAM has been used. Special attention has been given to the bubble detachment diameter. Two mechanisms have been considered as physically related to the detachment: surface tension and buoyancy. As expected, it has been verified that the bubble detachment diameter depends on the contact angle, operating pressure and properties of the fluid. Several fluids have been considered (water, R134a, ammonia and R123), as well as several operating pressures (between 0.1 and 10 bar) and contact angles (between 10 and 80°). It has been concluded that the detachment diameter depends strongly on the contact angle and fluid properties and slightly on the pressure. A correlation for the bubble detachment diameter has been developed based on the obtained numerical results. Data from this expression compare reasonably well with those from other correlations from the literature.

A Model for the Prediction of Bubble Detachment Diameters in Vapor Compression Distillation Assembly

The vapor compression distillation assembly is a phase-change water recovery technology which will reclaim water from urine, whose evaporator is a rotating container involving boiling heat transfer. The production rate of the apparatus depend on the bubble detachment diameter. In the article, based on the force equilibrium of bubble, the bubble detachment diameter formula is obtained, in which, the flow velocity along axis is neglected because it is very slow. The formula is very significant to the design of the apparatus and other relative apparatus.

Prediction of bubble detachment diameter in flow boiling based on force analysis

Abstract Bubble detachment diameter is one of the key parameters in the study of bubble dynamics and boiling heat transfer, and it is hard to be measured in a boiling system. In order to predict the bubble detachment diameter, a theoretical model is proposed based on forces analysis in this paper. All the forces acting on a bubble are taken into account to establish a model for different flow boiling configurations, including narrow and conventional channels, upward, downward and horizontal flows. A correlation of bubble contact circle diameter is adopted in this study, and it is found that the bubble contact circle diameter has significant effect on bubble detachment. A new correlation taking the bubble contact circle diameter into account for the evaluation of bubble growth force is proposed in this study, and it is found that the bubble growth force and surface tension force are more significant in narrow channel when comparing with that in conventional channel. A visual experiment was carried out in order to verify present model; and the experimental data from published literature are used also. A good agreement between predicted and measured results is achieved.

Measurement of bubble detachment diameters from the surface of the boiler heat exchanger in a domestic central heating system

Abstract Wet central heating systems are used throughout the world and the domestic variants are known to account for 16% of the carbon dioxide emissions in the UK. It is an industry wide fact that significant quantities of air leak into the system’s closed loop circuit, mostly due to thermal cycling during routine use. The water in the system results in high dissolved gas saturation levels thus resulting in microbubble nucleation and detachment at the boiler wall. An in depth knowledge into the behaviour of microbubbles in such systems could result in better deaeration systems thus improving the overall system efficiency. In this study, we will report a measurement of the bubble detachment sizes in this system at different operating conditions, these being, the system flow rate, pressure, heating load and saturation ratio. The Winterton models for zero and finite contact angles for the prediction of the bubble detachment diameters in supersaturated solutions were used as a comparison with the test results. The model for the prediction of bubble detachment diameters at finite contact angles in round pipes has yielded good predictions. A new correlation has been proposed so as to include the effects of pressure and heat flux on the bubble characteristics which were not represented through the Winterton model.

Bubble Dynamics for Nucleate Pool Boiling of Electrolyte Solutions

Bubble dynamics is the most important subphenomenon, which basically affects the nucleate pool boiling heat transfer coefficient. Previous investigations state that the effect of physical properties of liquid and vapor phases on bubble departure diameter are often conflicting. In this article, extensive new experimental data are presented for the bubble departure diameter for various electrolyte aqueous solutions over a wide range of heat fluxes and concentrations. Experimental results show that the bubble detachment diameter increase with increasing either boiling heat flux or electrolyte concentration. Experimental results also present a close relation between the dimensionless capillary and bond numbers. A new model for the prediction of vapor bubble departure diameter in nucleate boiling for the electrolyte solutions is proposed, which predicts the experimental data with a satisfactory accuracy.

Convective Boiling Between 2D Plates: Microgravity Influence on Bubble Growth and Detachment

The experiment detailed in this paper presents results obtained on the nucleation, growth and detachment of HFE-7100 confined vapour bubbles. Bubbles are created on an artificial nucleation site between two-dimensional plates under terrestrial and microgravity conditions. The experiments are performed by varying the shear flow by changing the convective mass flow rate, and varying the bubble nucleation rate by changing the heat flux supplied. The experiments are performed under normal (1 g) and reduced gravity (μg). The distance between the plates is equal to 1 mm. The results of these experiments are related to the detachment diameters of bubbles on the single artificial nucleation site and to the associated effects on the heat transfer by the confinement influence. The experimental device allows the observation of the flow using both visible video camera and infrared video camera. Here, we present the results obtained concerning the influence of gravity on the bubble detachment diameter and the images of 2D bubbles obtained in microgravity by means of an infrared camera. The following parameters: nucleation site surface temperature, bubble detachment diameter and bubble nucleation frequency evidence modifications due to microgravity.

Numerical investigation of water droplet dynamics in a low-temperature fuel cell microchannel: Effect of channel geometry

The sensitivity of liquid water to geometry of cathode gas microchannel in low-temperature fuel cells is investigated numerically. The two-phase flow is resolved using 3D CFD simulations with the volume-of-fluid (VOF) method. Simulations for microchannels with different cross-sections, including rectangle with aspect ratios in a range of 0.1–2, trapezoid, upside-down trapezoid, triangle, rectangle with a curved bottom wall, and semicircle are compared. The 0.5 aspect ratio rectangle yields the longest detachment time and the largest detachment diameter, whereas the longest removal time occurs for the 0.25 aspect ratio case. With decreasing aspect ratio for the rectangle the pressure drop increases and the coverage ratio decreases. The 0.1 and 2 aspect ratios rectangles have the largest water saturation. For microchannels with different cross-sections, the detachment time, detachment diameter, and removal time of the water droplet are found to be in this order: triangle < trapezoid < rectangle with a curved bottom wall < rectangle < upside-down trapezoid. The friction coefficient increases by a factor of 2–4 in the presence of water. The upside-down trapezoid yields the maximum coverage ratio and water saturation, while the rectangle with a curved bottom wall results in the minimum values.