Bubble Diameter Research Articles

Experiments and CFD modelling of bubble formation at a lateral hole in a top-submerged nozzle

Top-submerged nozzles with lateral holes find applications as passive mixing devices in radiochemical/hazardous industries. We report the mechanism of bubble formation and detachment at a lateral hole in such a nozzle for the first time. Three distinct phases – a growth phase, an elongation phase and finally a rise/detachment phase – could be observed during bubble formation. Effect of gas flow rate, diameter of lateral hole (1, 2 and 3 mm) and physical properties of liquid-liquid system on bubble detachment time and bubble equivalent diameter are reported. Three different liquid-liquid systems (DM water, 2 N and 4 N nitric acid) have been considered. Bubble diameter first reduced, reached a minimum and then started to increase in an oscillatory manner with increase in air flow rate. Before the onset of oscillatory behaviour an increase in hole diameter was seen to reduce bubble diameter. Such a behaviour is typical to bubble formation at lateral hole and is not seen for top-submerged nozzles with central opening. A detailed analysis of the results was carried out and it was observed that inertial forces played a major role. The work also reports a fundamental criterion which must be achieved for onset of oscillations in bubble diameter with flow rate. We also report a 2D CFD model that can simulate bubble formation dynamics at a lateral hole. The model was validated qualitatively as well as quantitatively against the experimental data.

Kolmogorov Turbulence Coexists with Pseudo-Turbulence in Buoyancy-Driven Bubbly Flows.

We investigate the spectral properties of buoyancy-driven bubbly flows. Using high-resolution numerical simulations and phenomenology of homogeneous turbulence, we identify the relevant energy transfer mechanisms. We find (a)at a high enough Galilei number (ratio of the buoyancy to viscous forces) the velocity power spectrum shows the Kolmogorov scaling with a power-law exponent -5/3 for the range of scales between the bubble diameter and the dissipation scale (η). (b)For scales smaller than η, the physics of pseudo-turbulence is recovered.

Bubble manipulates the release of viral aerosols in aeration

Bubble bursting is a common phenomenon in many industrial and natural processes, plays an important role in mediating mass transfer across the water-air interface. But the interplay between bubbles and pathogens remains unclear and the mechanisms of virus aerosolization by the bubble properties have not been well studied. The main objective of this study was to evaluate the water-to-air transfer of viruses by bubbles of different sizes. Unlike the dominant view of smaller bubbles less bioaerosols, it was found that the smaller bubbles could generate significantly more viral aerosols regardless of the virus species (Phi6, MS2, PhiX174, and T7), when the Sauter mean bubble diameters were between 0.56 and 1.65 mm under constant aeration flow rate. The mechanism studies denied the possibilities of more aerosols or better dispersion of viruses in the aerosols generated by the smaller bubbles. However, deeper bubbling could transfer more viruses to the air for MS2, PhiX174, and T7. Their concentrations in aerosols were linearly related to the bubbling depth for these non-enveloped viruses, which demonstrates the bubble-scavenging effect as a main mechanism except for the enveloped virus Phi6. Whereas, unlike these three non-enveloped viruses, Phi6 could survive relatively better in the aerosols generated from the smaller bubbles, though the enhancement of aerosolization by the smaller bubbles was much larger than the improvement of survival. Other mechanisms still remain unknown for this enveloped virus. This study suggests that the attempt of decreasing the bubble size in aeration tank of the wastewater treatment plant might significantly increase the solubility of oxygen as well as the risk of viral aerosols.

Experimental study on the effect of the sawtooth nozzle structure on bubble behaviors

The initial formation of bubbles has a significant impact on bubble dynamics and acoustics. Meanwhile, the bubble initial shape is affected by the nozzle boundary. In this study, the processes of bubble generated from sawtooth nozzles have been investigated by means of controlled experiments. A three-dimensional visualization imaging system is used to study bubble behaviors. The morphology and the time-frequency spectrum of the sound pressure signal are analyzed synchronously. Moreover, the effects on bubble trajectory are also studied. Results show that sawtooth nozzle is not conducive to bubble detachment, resulting in larger bubble diameters than the circular nozzle. With the gas flow rate increasing, bubble diameter increases. When the gas flow rate reaches a certain critical value, the surface tension is not sufficient to support the continued development of bubble size. Due to effect of the sawtooth structure, bubbles tend to break in the necking stage, accompanying by small bubbles. The generation of accompanying bubbles lead to the irregular generation of nearby bubbles. This study aims to reveal the effect of sawtooth nozzle structure on the initial formation of bubbles, so as to predict the flow patterns and sound signal characteristics of the bubbles.

NEW PIDHID UP TO THE MOVEMENT OF FIRE SAFETY WATER SYSTEMS

When obtaining estimates that characterize the level of fire safety of the hydrogen storage and supply system, there is an error due to the subjective nature of its occurrence. It is noted that it is possible to weaken the influence of the subjective nature on these estimates by using the probabilistic characteristics of failure of the main elements of the hydrogen storage and supply system. Such basic elements include a gas generator. It is shown that the diffusion processes between the hydroreactive sample and the liquid - water are accompanied by the appearance of gas bubbles located on the reacting surface. This process - chemical boiling is characterized by internal characteristics. Internal characteristics were obtained experimentally for hydroreactive samples based on sodium aluminum hydride, which are approximated by polynomials of the fourth order and represent the dependence of the diameter of gas bubbles and the generation frequency on the diameter of the reacting surface. It is noted that the characteristics of the gas generator depend on the orientation of the reacting surfaces of the hydroreactive sample. The growth rate of basic bubbles practically does not depend on the diameter of the reacting surface, and the size of this surface for the vertical arrangement of the reacting surface is several percent larger than for its horizontal arrangement. The most unfavorable mode of gas generation is the case when the reactive surface of the hydro-reactive sample is oriented downwards. It is shown that increasing the reliability of the gas generator of the hydrogen storage and supply system is possible due to the implementation of a hydroreactive sample with a polylike one - a passive method and due to the formation of hydrodynamic forces acting on gas bubbles - an active method. These recommendations are embodied on the example of a hydrogen storage and supply system with vibrational movement of a hydroreactive sample, which is made in the form of a set of identical long cylinders assembled in a mesh cassette. Keywords: water saving and supply system, gas generator, fire safety, reliability.

Nozzle diameter and expansion ratio of compressed air foam system

Variations of nozzles are studied for Compressed Air Foam Systems (CAFS). The CAFS is a self-contained fire suppression system with the capability of injecting compressed air into the foam solution to create a dense mist. Even though this type of foam has a tighter, denser bubble structure that allows it to adhere to vertical and horizontal surfaces and penetrate the fire more deeply before the bubbles burst, thereby making it more effective, the nozzle of CAFS is weak. The experimental study has been done in order to find the expansion ratio of the nozzle. Four sizes of diameter nozzles are 5 mm, 10 mm, and 20 mm, and three holes are 20 mm. Nozzle diameter size has a correlation with bubble size. The highest expansion ratio is found in the nozzle with three holes 20 mm in diameter. It is followed by a nozzle with a 20 mm diameter, then a 10 mm diameter. The last finding is a nozzle with five diameters. The other founding is the linear correlation between nozzle diameter size and bubble size. But it is on the other way than the correlation between nozzle diameter and bubble size. The shorter the nozzle diameter, the faster the fire extinguishing time.

Bubble breakage and aggregation characteristics in a vortex pump under bubble inflow

The behaviors of bubbles in a vortex pump play a crucial role in its performance when handling gas–liquid flows. However, not much research has been done on the distribution of the gas–liquid phases and the characteristics of bubble breakage and aggregation in vortex pumps. This lack of understanding hinders the improvement of pump performance in gas–liquid flow transport. This study aims to investigate the bubble characteristics in a vortex pump using the population balance model, focusing on the variation of bubble size and the influence of the inlet gas volume fraction (IGVF) on bubble breakage and aggregation. The results show that as the IGVF increases, the gas volume fraction in the impeller becomes larger than that in the bladeless chamber. The majority of bubbles in the impeller are concentrated near the hub, while they also remain in the circulating-flow zone of the bladeless chamber. Under low IGVF conditions, the average diameter of bubbles decreases from the pump inlet to the outlet. The bladeless chamber has a larger average bubble diameter and a higher percentage of large bubbles compared to other parts of the pump. Moreover, the bubble number density at the pump outlet increases with the IGVF, indicating the production of more bubbles, while the increase in IGVF also results in an increase in the percentage of large bubbles. The study also discusses the mechanism of bubble breakage and aggregation in vortex pumps. It suggests that the effective breakage frequency and effective aggregation frequency are responsible for bubble breakage and aggregation in the vortex pump. The gas volume fraction and turbulent dissipation rate are identified as important parameters affecting the effective breakage and aggregation frequency. These findings provide new insights into understanding the characteristics of bubble breakage and aggregation in vortex pumps.

Superior performance of a membrane bioreactor through innovative in-situ aeration and structural optimization using computational fluid dynamics.

The optimization of membrane bioreactors (MBRs) involves a critical challenge in structural design for mitigation of membrane fouling. To address this issue, a three-dimensional computational fluid dynamics (CFD) model was utilized in this study to simulate the hydrodynamic characteristics of a flat sheet (FS) MBR. The optimization of the membrane module configuration and operating conditions was performed by investigating key parameters that altered the shear stress and liquid velocity. The mixed liquor suspended solids (MLSS) concentration was found to increase the shear stress, leading to a more uniform distribution of shear stress. By optimizing the appropriate bubble diameter to 5mm, the shear stress on the membrane surface was optimized with relatively uniform distribution. Additionally, extending the side baffle length dramatically improved the uniformity of the shear stress distribution on each membrane. A novel in-situ aeration method was also discovered to promote turbulent kinetic energy by 200 times compared with traditional aeration modes, leading to a more uniform bubble streamline. As a result, the novel in-situ aeration method demonstrated superior membrane antifouling potential in the MBR. This work provides a new approach for the structural design and optimization of MBRs. The innovative combination of the CFD model, optimization techniques, and novel in-situ aeration method has provided a substantial contribution to the advancement of membrane separation technology in wastewater treatment.

Effects of surfactants on bubble-induced turbulence

We use experiments to explore the effect of surfactants on bubble-induced turbulence (BIT) at different scales, considering how the bubbles affect the flow kinetic energy, anisotropy and extreme events. To this end, high-resolution particle shadow velocimetry measurements are carried out in a bubble column in which the flow is generated by bubble swarms rising in water for two different bubble diameters (3 and 4 mm) and moderate gas volume fractions (0.5 %–1.3 %). We use tap water as the base liquid and add 1-Pentanol as an additional surfactant with varying bulk concentration, leading to different bubble shapes and surface boundary conditions. The results reveal that with increasing surfactant concentration, the BIT generated increases in strength, even though bubbles of a given size rise more slowly with surfactants. We also find that the level of anisotropy in the flow is enhanced with increasing surfactant concentration for bubbles of the same size, and that for the same surfactant concentration, smaller bubbles generate stronger anisotropy in the flow. Concerning the intermittency quantified by the normalized probability density functions of the fluid velocity increments, our results indicate that extreme values in the velocity increments become more probable with decreasing surfactant concentration for cases with smaller bubbles and low gas void fraction, while the effect of the surfactant is much weaker for cases with larger bubble and higher void fractions.

Photoelectrochemical Water Splitting at Elevated Pressure: A Model-Based Evaluation

Photoelectrochemical (PEC) water splitting is one of the few truly renewable pathways towards “green” H2. Although PEC water splitting demonstration devices have been mainly operated at atmospheric pressure, most applications and processes that use H2 require it to be supplied at elevated pressure. Although PEC-generated H2 can be pressurized afterwards with e.g. mechanical compression, operating the PEC water-splitting device itself at elevated pressure offers an intriguing alternative and several possible advantages. For example, bubble formation is strongly reduced at elevated pressure, which lowers bubble-induced electrode deactivation and bubble-induced product crossover rate.[1,2] In addition, the optical reflection and diffraction losses induced by bubbles can be minimized.[3] Finally, the electrochemical production of hydrogen at higher pressure requires only a small (though not insignificant) increase of the thermodynamic cell voltage (29 mV for 10-fold increase in pressure).[4] The main disadvantage of a higher pressure is the increasing mechanical demands on the cell design, and the challenge is to find a pressure range at which optimum conditions will be achieved.To evaluate these pressure-induced effects, we have developed a two-dimensional multiphysics model that considers semi-empirical pressure-dependent bubble characteristics (e.g., bubble formation efficiency, bubble diameter) and their impacts on the multi-phase flow, electrochemistry, and mass transport in a membrane-free PEC water-splitting device. Based on the simulation results, a quantitative comparison between two technical approaches to generate pressurized H2 from a PEC device, namely operating PEC water-splitting at atmospheric vs. elevated pressure, has been conducted, as shown in the schematic diagram in Fig. 1a. Various performance loss factors in a PEC water-splitting device were considered, including bubble-induced product crossover, optical loss, and pH gradient-induced concentration overpotential. These losses (translated to kWh unit) were then compared with the specific work required to mechanically compress H2 from the collection pressure (p 1 in Fig. 1a) to 700 bar (p 2 in Fig. 1a). Our results show that bubble-induced optical loss is the predominant factor among all the losses considered. The overall loss diminishes as pressure increases, as shown in Fig. 1b. By comparing the benefit of conducting PEC water-splitting at elevated pressure (defined as the amount of losses avoided vs. operation at 1 bar) with the thermodynamic cell voltage penalty (see Fig. 1c), we found that no significant benefit can be gained by operating beyond 6 – 8 bar. Therefore, this pressure range represents a desirable range for the operation of a membrane-free PEC water-splitting device. Keywords : (photo)electrochemistry, water splitting, elevated pressure, multiphysics modeling, quantitative evaluation.

Multi-Scale Simulations of Bubble Transport in an Alkaline Water Electrolyzer

The existence of bubbles can lead to the loss of performance by decreasing the electrochemically active surface area and disrupting the transport of hydroxide ions. These effects caused by the bubbles have yet to be sufficiently clarified. A comprehensive understanding of the mechanisms of dynamics inside an electrolyzer from the molecular scale up to the whole electrolyzer scale is needed. Furthermore, establishing relations between these dynamics at a wide range of scales is important. In this study, numerical simulations of bubble transport coupling fluid dynamics, mass transport and electrochemical reactions were conducted on micro and macro scales, respectively. The micro scale bubble simulation (Fig. 1(a)) incorporates the concentration field of the chemical species into the numerical model and deals with how bubbles grow on and detach from an electrode. In this model, the chemical species generated by the electrochemical reactions dissolve into the liquid phase, and their transfer into the gas phase leads to bubble growth. Since the spatial size distribution of the bubbles can be one of the factors that affect electrolysis performance, in the macro scale bubble simulation (Fig. 1(b)), we study how different sizes of bubbles are transported and distributed inside the electrolyzer. To simulate this phenomenon, a four-phase flow analysis is performed in which small, medium, and large diameters are selected as representatives of bubble diameters, and each is treated as a different phase.[Acknowledgements]This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (P14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).Fig. 1 Simulation systems of bubble transport at (a) micro scale and (b) macro scale. Figure 1

Experimental investigation on the characteristics of XG/GG/HPAM gel foam and prevention of coal spontaneous combustion

Polymer gel foam shows excellent development potential and application prospects in the prevention of coal spontaneous combustion. To reveal the relationship between the characteristics of gel foam and the prevention of coal spontaneous combustion, XG/GG/HPAM gel foam consisting of polymer xanthan gum (XG), guar gum (GG), and polyacrylamide (HPAM) is prepared. The characteristics of gel foam are investigated in terms of microstructure, viscosity, thermal stability, and film-forming. The effects of gel foam on gas products, characteristic temperature, combustion characteristics parameters, and functional groups during coal oxidation are studied. The results show that polymer delays the foam coarsening and improves the foam stability. XG/GG/HPAM gel foam exhibits a slow foam coarsening process, concentrated bubble diameter distribution, shear-thinning behavior, excellent thermal stability, and film-forming properties. Moreover, the inhibition rate of XG/GG/HPAM gel foam on coal is 43.63% higher than that of ordinary foam. XG/GG/HPAM gel foam effectively improves the apparent activation energy of coal, the maximum weight loss rate of the coal is decreased by 7.55%, and the combustibility, combustion stability, and comprehensive combustion index are diminished by 8.50%, 7.64%, and 10.17%, respectively. Simultaneously, XG/GG/HPAM gel foam reduces the oxidation activity of aliphatic hydrocarbons and oxygen-containing functional groups in coal, blocks the chain reaction, and thus inhibits coal spontaneous combustion. The results provide support for achieving efficient fire prevention performance of gel foam.

CFD-DEM study of impacts of the porous distributor medium on fluidization characteristics of a 2D-fluidized bed

Gas–solid bubbling fluidized beds are widely used in chemical, energy, construction, and other industrial fields due to their excellent fluidization performance. 2D-fluidized beds with planar rectangular columns of finite thickness are widely used in fluidized bed studies, and it is essential for understanding the hydrodynamics of gas-particle systems. Moreover, the distributor (porous medium) at the bottom of the bed has a crucial influence on the fluidization performance of the 2D-fluidized bed. In this work, the fluidization mechanism and gas–solid dynamic characteristics of the fluidized bed under three porous media are studied by CFD–DEM coupled with the porous medium model, and the accuracy of numerical simulation is verified by a high-speed photography experiment. Results show that with the increase in the flow resistance of the porous medium, the average and standard deviation of the bubble diameter decrease, and the time and position of particles in the dead zone to participate in the core–annular flow advance. At a low fluidization velocity, the dead zone in the bed can be considerably reduced by increasing the flow resistance.

Transport phenomena of bubble growth coupled with chemical reaction during laser-induced photothermal reduction and deposition processes

Laser assisted low-temperature nano/microfabrication of functional materials on a flexible or rigid substrate using water-based liquid precursor have gained significant attention owing to its high process efficiency and low environment impact. Both of which are essential for the green and low-carbon manufacturing technologies needed for the next generation of electronic devices. Laser induced photo-hydrothermal synthesis (LIPHS) utilizes thermally induced reductive reaction in a liquid precursor to synthesize and deposit nano/micro-patterns on temperature sensitive substrates. During LIPHS, microbubble generation is inevitable as a result of byproduct gas formation and liquid-vapor phase change of the precursor. Transport phenomena of the laser-induced reduction reaction coupled with microbubble dynamics on the liquid-vapor-solid triple contact interface significantly affects the quality of fabricated nano/microstructures. Herein, we investigated microbubble formation and growth on the substrate surface during a LIPHS process for silver on a polyimide substrate surface. Framewise growth of microbubble was captured for different laser powers and solute concentrations to reveal the time dependent bubble diameter variation. It was found that the bubble size and growth rate decreased with increasing solute concentration owing to increased boiling point of the precursor. Bubble temperature and reduction reaction rate were obtained by theoretical analyses on the microbubble growth rate. Transient solid-liquid-vapor triple contact point temperature of the microbubble during the LIPHS process was obtained from a numerical simulation to develop a comprehensive model the first time for predicting the chemical reaction rate. The proposed model was validated experimentally with a relative error of 6.2% at the highest reaction rate.

Effect of foam temperature on foam stability of foamed concrete and stabilization mechanisms

In this study, a novel method for increasing foam stability based on adjusting the temperature of surfactant solution used to prepare foam (foam temperature) is presented. The effect of foam temperature on the compressive strength, and stability of the foamed concrete was studied. Meanwhile, laser displacement sensors, X-ray computed tomography, automated surface tensiometer, and electron microscopy were adopted to explore the action mechanisms. Experimental results reveal that the method of adjusting the proper foam temperature can decrease the surface tension of surfactant, reduce the diameters of bubbles, and retard bubble coarsening, thus enhancing the foam stability. The fresh foamed concrete with a foam temperature at 20 °C displays the highest stability level. The foamed concrete with a foam temperature at 20 °C possesses a more uniform pore structure, which explains the reasons for the better properties of foamed concrete.

On the flow of a bubble pair rising in temperature-sensitive gel propellants

The gel propellant is a novel type of propellant created by introducing gelling agents to the liquid propellant. Similar to other types of propellants, the occurrence of air bubbles in the gels is common and unavoidable during the manufacturing and casting process. The presence of bubbles can adversely impact the stable combustion of the engine and may even pose safety hazards when they enter the combustion chamber. In order to gain a better understanding of the bubble interaction within the gel propellant, we have conducted a study on the motion of a pair of bubbles rising in a quiescent water-gel. In this study, the volume of fluid (VOF) method is adopted for modeling the multiphase feature of the flow. The shear and temperature dependent viscosity of the gel has been modeled using the temperature-coupled Carreau–Yasuda model. The effects of various factors, including the initial distance between the bubble pair, bubble diameters, and the temperature, on the motion characteristics of the bubble pair are studied. It is found that an increase in the initial distance leads to a reduction in the bubble interaction; for side-by-side bubble pairs, the bubbles exhibit slower ascent rates and reduced lateral displacement; for inline bubble pairs, the coalescence of the bubble pair becomes slower. Increasing the diameter of the dominant bubble accelerates the coalescence of the bubble pair and the ascent. Elevating the temperature results in a decrease in viscosity, thereby accelerating the coalescence and ascent of inline bubble pairs and producing viscosity reduction around the bubble pair.

Coupled bubble dynamics and interaction mechanisms of adjacently nucleated vapor bubbles under subcooled pool boiling regime

The present work investigates the plausible modes of interaction between two adjacently located vapor bubbles growing on high wettability surface during subcooled nucleate pool boiling. The study becomes important as the phenomenon of bubble interaction has a significant impact on the resulting bubble dynamic parameters as well as on microlayer dynamics of the respective vapor bubbles. As against the symmetric growth of microlayer in the case of isolated pool boiling, microlayers of mutually interacting vapor bubbles no longer remain symmetric and tend to deplete relatively faster, thereby influencing the resulting wall heat transfer rates. Toward this, the microlayers and dynamics of adjacently located interacting vapor bubbles have been mapped in a simultaneous manner using thin-film interferometry and high-speed rainbow schlieren deflectometry as a function of varying subcooling levels. Results of the experiments revealed that parameters such as non-dimensional spacing between the two vapor bubbles, supplied heat flux, and degree of subcooling have a strong impact on the mechanism of bubbles' interaction, which, in turn, influence the bubble departure frequency, bubble diameter, and microlayer dynamics. Three dominant modes of bubble interaction, hydrodynamic interaction (HI), thermal interaction (TI), and coalescence (C), were identified as a function of the level of subcooling. Quantification of the experimental data showed that the equivalent diameter of the vapor bubble increases with an increase in the level of heat flux, while it decreases with an increase in the subcooling level. Additionally, the secondary nucleation affects the liftoff time and movement of the primary bubble and can hydrodynamically deplete the microlayer of the first bubble even when the bubbles are far apart, with the depletion rates showing a strong dependence on the growth rate and location of the secondary nucleation.

Vortex ring and bubble interaction: Effects of bubble size on vorticity dynamics and bubble dynamics

Bubbly turbulent flows involve complex interactions between bubbles and vortices, in which their size ratio plays a critical role. The present work investigates an idealization, namely, the interaction of a single air bubble with a (water) vortex ring, with the focus being on the effects of the bubble-to-vortex core size ratio (Db/Dc,o) on the bubble and ring dynamics (Db = bubble diameter and Dc,o = initial vortex core diameter). The interaction is studied for size ratio, Db/Dc,o, of 0.6–1.7, over a large Weber number range from 10 to 500 [We=0.87ρ(Γ/πDc,o)2/(σ/Db), Γ = circulation]. On the bubble dynamics side, in the initial stages of the interaction after the bubble's capture by the ring, the bubble's radial equilibrium position, its azimuthal elongation, and breakup pattern are influenced by both Db/Dc,o and We. However, at longer times, the results show that the We alone decides the broken bubbles to Db ratio and scales as We−0.13, which can be contrasted with the scaling of We−0.6 in isotropic turbulence [R. Shinnar, J. Fluid Mech. 10, 259–275 (1961)]. On the ring dynamics side, increasing Db/Dc,o leads to larger deformation of the vortex ring core at low We, and these effects are significant above a critical Db/Dc,o of about 1.2. Under these conditions, the vortex core can fragment, leading to large reductions in the ring's measured convection speed and axial enstrophy, both of which follow a similar scaling, (Db/Dc,o)2/We; the reduction in enstrophy being reminiscent of bubbly turbulent flows. These results and scalings should help us to better understand and model bubble–turbulence interactions.

Multi-scale characteristics investigation of sheet/cloud cavitation

Sheet and cloud cavitation involve the formation of large-scale cavities and microbubbles across a wide range of length scales. In this study, a two-way coupling Eulerian-Lagrangian algorithm was utilized to simulate the cavitating flow around a NACA66 hydrofoil at an incidence of 8 degrees and a cavitation number of 1.2. The large eddy simulation (LES) and volume of fraction (VOF) methods are used to capture the large scale vapor structures in Eulerian frame. Meanwhile, the discrete bubble model (DBM) and simplified Rayleigh-Plesset equation are implied to solve the dynamic of Lagrangian microbubbles smaller than local grid scale. The results indicate that large-scale cavities are periodically shed downstream, influenced alternatively by the re-entrant jet and shock wave mechanisms, resulting in periodic variations of both the number and Sauter mean diameter of microscale bubbles. During the quasi-periodic evolution process of sheet/cloud cavitation, the microbubbles scatter around the large scale cavities where have strong turbulence intensity and high vorticity, and the probability density function of discrete bubble diameter similarly conforms to Gamma distribution with the dominant bubble size between 30 and 70μm. During the growth of an attached cavity, the number density of bubbles follows a power-law scaling with a value of -5/3 on radius. For the other stages, the bubble size spectrum of smaller microbubbles exhibits a -2/3 power-law scaling when the diameter of the microbubble is less than 200 μm. For larger bubbles with a diameter greater than 300 μm, the bubble density is proportional to the bubble radius to the power of -6.

Gas bubble coalescence in laser directed energy deposition of irregular HDH titanium alloy powder feedstock

In this study, the synchrotron X-ray imaging technique was used to investigate the coalescence of gas bubbles during laser directed energy deposition (L-DED) of irregular hydride-dehydride (HDH) titanium alloy particles onto a titanium alloy substrate. The objective is to better understand the coalescing mechanism of gas bubbles during the L-DED process of unique feedstock materials. The coalescence frequency of gas bubbles is the percentage of bubbles merging and is dependent on collision frequency and coalescence efficiency. Forces in the melt pool flow such as Marangoni forces, buoyancy force, vaporization pressure, and acoustic waves, along with the number of generated bubbles, were the major factors for increasing the collision frequency between bubbles. In addition, the random forest model determined that the diameter of the smaller bubble during collision was the most significant factor impacting the efficiency of coalescence, which was equal to 48% when the smaller diameter was larger than a threshold of 76 μm and 3.5% and when the diameter was smaller than 76 μm. The results also showed that 6.5% of the formed bubbles in the melt pool led to coalescence. Overall, this work can help mitigate pores, verify simulation models, and promote irregular or recycled powder feedstock as effective replacement to atomized feedstock.