Effect Of Bubble Size Research Articles

Enhancement of SPR effect and sensing characteristics of no-core fiber with bubble structure

In this work, we propose the SPR fiber sensor based on a bubble structure. Conventional SPR optical fiber sensors have an anomalous broadening of the resonance dip in the near-infrared band. Therefore, we propose to utilize a bubble structure to improve the FWHM of the spectrum. The sensor utilizes no-core fiber as the sensing platform and multimode fiber as the transmission fiber, and fabricates a bubble at the fusion of the two fibers. The effect of bubble size on the SPR effect is investigated experimentally, and the results show that bubbles can improve the sensitivity and reduce the FWHM of SPR sensor. The refractive index sensing test was implemented on the proposed SPR bubble sensor. The experimental results showed that the bubble structure enhanced the maximum sensitivity of the SPR sensor by 927.7 nm/RIU. Finally, the effect of the bubble on the response time of the sensor is discussed. The method of using a bubble structure to improve sensor performance is simple to operate and easy to implement.

Investigation of the bubble deflection using graphics processing unit accelerated multiple relaxation time lattice Boltzmann method: The effect of bubble size and liquid viscosity

Bubble deflection is important for bubble motion and bubble mass transfer in multiphase reactors. However, the mechanism triggering the instability is controversial. In this study, we first perform a graphics processing unit-accelerated multiple relaxation time lattice Boltzmann method (MRT-LBM) to simulate the motion process of single bubbles (1.8–8.0 mm) freely rising in stationary liquids with different viscosities. Then, we further analyze the influence of the bubble size and liquid viscosity on the characteristics of the time-dependent bubble motion, including the bubble shape, velocity, and deflection, based on the high-precision numerical simulation results. The results show that the formation and shedding of bubble wake vortices are responsible for the oscillation of the bubble shape. The interaction of bubble wake vortices and bubble shape is the primary cause of bubble deflection. Finally, an empirical model, taking into account the bubble acceleration and deflection angle, is developed to characterize the effect of bubble size and liquid viscosity on the drag coefficient of a rising bubble. This study provides insight into the bubble deflection and the bubble dynamics.

The Effect of Bubble Size on Lock-Exchange Density Currents through Bubble Screens

The Effect of Bubble Size on Lock-Exchange Density Currents through Bubble Screens

Impact force of an air-in-liquid compound drop

We investigate numerically the transient forces produced by an air-in-liquid compound drop impacting onto a solid surface. We demonstrate that the presence of a bubble in the drop decreases the impact force. This decrease can be mainly explained by a reduced area over which the impact pressure is applied. We systematically vary the bubble size to understand its effect on the maximum normal force. A simple scaling model for the maximum impact force is developed to accommodate the effect of bubble size.

Near-wall bubble migration and wake structure in viscous liquids

The migration and wake structure of a single bubble in a viscous medium near a vertical wall have been experimentally investigated using a two-camera orthogonal shadowing method combined with a particle image velocimetry system. The effects of bubble size, liquid properties, and migration modes on the motion of a near-wall bubble and its wake structure are discussed. It is shown that, unlike migration in pure water, collisions between bubbles and walls are rare in viscous liquids. As viscosity increases, the mobility of the bubble near the wall changes from 3D to 2D and finally exhibits 1D motion with only one degree of freedom. While 1D motion is dominated by viscous forces, 2D migration is influenced by both wall effects and viscous forces, while 3D migration is primarily affected by bubble size. After analyzing the wakes produced by different factors, it was found that the wakes generated by mobile bubbles in three dimensions exhibit similar shedding patterns to Karman vortex streets, while two-dimensional wakes show stronger continuity. The average topological flow field structure of the bubble wake is obtained through periodic phase averaging, and the effect of different factors on the evolution of near-wall bubble wakes is analyzed.

Bubble-particle detachment behavior during bubble coalescence: Role of bubble size

Studies have reported that bubble coalescence causes particle–bubble detachment in the flotation pulp and foam phases. Most coalescence experiments have been conducted using bubble pairs with the same diameter; however, this is not the case in real flotation. In this study, particle–bubble detachment behavior during the coalescence of two bubbles with different sizes was investigated using a high–speed camera, and the variation of the coalescing bubble area was fitted to a damped oscillatory equation and analyzed. To explore the intrinsic detachment mechanism of the bubble size effect, the flow field variation in the bubble periphery was simulated using COMSOL 6.1. Results show that the particle–bubble detachment probability decreased with increasing bubble size difference. The released energy is greatest when the two equal bubbles coalesced, thus leading to the strongest oscillation and highest detachment probability. The simulation results show that turbulent kinetic energy affects particle-bubble stability and is an important reason for inducing particle-bubble detachment. Notably, particle–bubble detachment did not occur at the maximum turbulent energy, because there was a delay due to the time required for the sliding contraction of the three–phase contact line. The research finding provides new insights into the mechanism of particle–bubble detachment.

Experimental study on turbulent characteristics of dispersed bubbly flow in a narrow rectangular channel by two-phase PIV

Narrow rectangular channels are widely used in high heat flux applications, due to the high heat transfer efficiency and compact structure. However, a rare understanding was known about the two-phase turbulent characteristics. In this study, an experimental investigation of bubble and turbulent characteristics of dispersed bubbly flow in the narrow rectangular channel was carried out by using the high-speed camera and two-phase particle image velocimetry. The bubble size effect was studied independently using the continuous spectrum bubble generator. According to the results, the bubble size distribution conforms to a Weibull distribution, and the wall-peak void fraction distributions were found. Both turbulent enhancement and suppression were observed in current conditions. Large bubbles enhance turbulence intensity, while small bubbles suppress it. The eddy viscosity ratio ϕ characterizes the modification of turbulence intensity by bubbles, with a critical value of ϕ=1.

Combined Effect of Bubble Size and Gas Volume Fraction on Natural Convective Heat Transfer Enhancement in Homogeneous Bubbly Flow: Eulerian–Eulerian Numerical Simulations

Abstract Natural convective heat transfer can be enhanced through either fins or riblets, wall roughness elements, or the injection of bubbles in the flow. Bubble injections in a quiescent (pseudo-turbulent) liquid phase or an already turbulent liquid phase had been shown to enhance the natural convective heat transfer from literature. However, study of the combined effect of bubble size and gas volume fraction rather than individual effect on natural convective heat transfer enhancement for homogeneous bubbly flow is lacking. The present work intends to fill in that data gap through conducting numerical simulations to study the combined effect of bubble size and gas volume fraction on natural convective heat transfer enhancement. The present numerical work employs the validated interphase force models and the Eulerian–Eulerian model. ansysfluent is used to simulate a bubbly flow in a three-dimensional rectangular channel with a natural convective heat transfer. Bubbles ranging from microto millimeter diameter with inlet gas volume fraction varied in the range of 0.351–3.725% are injected upward to a quiescent liquid phase in a rectangular channel with a heated left wall and a cooled right wall. The flow regime is homogeneous without bubble coalescence and breakup effect. Validated computational models are employed to study the combined effect of bubble size and gas volume fraction on heat transfer enhancement. A relation between Nusselt number, bubble Reynolds number, Rayleigh number, nondimensional bubble size, and inlet gas volume fraction is constructed using the power regression method.

Isothermal and non-isothermal growth of a vapor bubble in pool boiling – Effect of translational velocity of bubble

In the present work, we have numerically simulated the effect of bubble translation on the growth of a vapor bubble in a pool of superheated liquid based on the isothermal and non-isothermal model. The isothermal growth uses the ODE solver and the non-isothermal growth the PDE-ODE solver. The overall model at present considers the forces acting on the bubble to compute the bubble motion and the energy balance at the bubble surface to include the effect of bubble size on its motion. Thus, it computes equivalent bubble radius and bubble velocity simultaneously. The numerical simulation shows a remarkable improvement by our method over the previous numerical model while validating with the experimental and numerical results on bubble growth with the phase motion. The improved model has significantly reduced the deviation from the experiments reported in literature on the growth of a moving bubble. The aspect ratio of the bubble is computed to see the shape change of the bubble and is used to model the drag force accurately. It has been shown that the model of the isothermal growth can better explain the bubble growth at the initial stage, and the model of the non-isothermal growth at the final stage. The non-isothermal growth of bubble is accompanied by the slowdown of the bubble translation in the superheated liquid.

A model for foam fractionation with spatially varying bubble size

A model is developed for a foam fractionation process, supposing that bubble size increases moving up the foam column. Predictions are obtained for liquid flux through the foam column, liquid fraction at the top and enrichment of surface actives within the foamate. Compared to previous models with fixed bubble size, fractionation performance is shown to be improved. A synergy is revealed between the effect of the foam becoming drier (and hence richer in surface actives) as the foam column becomes taller, and the effect of bubble size increasing moving upwards (which makes the foam even drier and hence richer still). A dimensionless parameter m⁎ quantifies the relative variation in bubble size between top and bottom for a foam column of typical height. Even small m⁎ values lead to behaviour qualitatively different from a system with no bubble size variation whatsoever, whilst increasing m⁎ leads to even better fractionation performance.

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.

Helium bubble size effects on the surface morphological response of plasma-facing tungsten

Abstract We report a simulation study on the effects of helium (He) bubble size on the morphological evolution and pattern formation on the surface of tungsten used as a plasma-facing component (PFC) in nuclear fusion devices. We have carried out a systematic investigation based on self-consistent dynamical simulations of surface morphological evolution according to an atomistically-informed, 3D continuum-scale model that captures well the relevant length and time scales of surface nanostructure formation in PFC tungsten. The model accounts for PFC surface diffusion, driven by the biaxial compressive stress originating from the over-pressurized He bubbles in the near-surface region of PFC tungsten as a result of He plasma exposure, combined with the formation of self-interstitial atoms in tungsten that diffuse toward the PFC surface and the flux of surface adatoms generated as a result of surface vacancy-adatom pair formation upon He implantation; this transport of surface adatoms contributes to the anisotropic growth of surface nanostructural features due to the different rates of adatom diffusion along and across step edges of islands on the tungsten surface. Our detailed analysis reveals that varying the average He bubble size plays an important role in the PFC surface growth kinetics as well as the resulting surface topography. Specifically, we find that an increase in the He bubble size leads to a deceleration in the growth rate of the tungsten nanotendrils that emanate from the PFC surface. We also find that the separation distance between the resulting surface features increases with increasing He bubble size, as well as over time. This coarsening effect is a thermally activated process resulting in an accurate description of the temperature dependence of the average surface feature separation by an Arrhenius relation.

The effect of bubble size in descaling process of bubble flow pipes based on leaky ultrasonic guided waves

Crystal scaling in gas-liquid two phase flow pipes is a common issue in many fields, such as chemical engineering, water industries, and energy industries, posing a great threat to the production process and the environment. The characters of air bubbles in the bubble flow have a serious impact on the ultrasonic guided wave descaling inside a pipe. The interaction mechanism between the air bubbles with various sizes and the cavitation bubbles are studied in this paper by combined the finite element method and the bubble dynamics equation, qualitatively analyzing the impact of the air bubble size and the acoustic pressure in the fluid field on the leaky ultrasonic guided waves descaling. The numerical and experiment results show that the maximum expansion radius ratio of cavitation bubble is suppressed from 2.4 to 1.3 when the radius of air bubble grows from 0.5 mm to 4.5 mm. The Ca content removal ratio at 35.5 cm was 81 %, 66 % and 28 %, while the Ca content removal ratio at 60 cm was 76 %, 56 % and 16 % when the gas outlet orifice is 100 μm, 200 μm and 300 μm respectively in the experiment. The results indicate that large bubbles in two phase flow inhibit the cavitation effect of cavitation bubbles and reduce the effectiveness of descaling.

Simulating vibro-acoustic damping of bubbles in fluids

Bubbles in fluids are common occurrence due to impurities, thermal or flow variations. They are sometimes considered detrimental to the flow and more importantly surrounding structural integrity. On the other hand, bubbles were used positively towards damping vibrations in fluid filled structures [1] and as barriers or curtains in the removal of plastic waste from water ways [2] and also in noise attenuation from offshore windfarms [3]. In the latter two cases, engineers hypothesized these curtains help marine life from sound pollution from surrounding machinery and ships. In this paper the authors discuss two cases to study the effect of bubble size and bubble density. In the first experiment a curtain of air bubbles was introduced in a water body and sound transmission loss (STL) through this curtain was studied. In a second experiment, a stainless-steel tank filled with liquid mercury is infused with helium bubbles and the vibration levels were monitored.

Effect of bubble size and spatial distribution on the flow and mixing of steel

ABSTRACT The size and spatial distribution of the argon bubbles have a significant impact on the refining effect when refining steel by blowing argon gas. Based on the VOF model, this paper simulated the rising behaviour of incipient bubbles in steel during ladle argon blowing. The effects of the bubble rising trajectory, velocity, offset coefficient, and active zone coefficient for steel on the effect of steel mixing were analysed. It was found that when the distance between the two incipient bubbles in the horizontal direction is greater than four bubble diameters, it will produce the best stirring effect. Moreover, when the same amount of argon gas generates several small bubbles, it provides more stirring power to the steel than when a single large bubble of equal volume is generated, and the stirring effect is superior.

Bubble Loading in a Fluidized Column: Effects of Bubble Size, Particle Size, Contact Angle and Particle Density

ABSTRACT The size of bubbles and particle properties (including particle size, density and contact angle) play an important role on the degree of particle loading on bubbles and this has been studied extensively. However, experimental results on the influence of these properties on bubble loading under fluidized-bed flotation conditions (where the probability of detachment is expected to be negligible) are scarce. In this work, the loading fraction (L) of individual bubbles was studied using a high-speed video camera (Phantom Miro LC111). Particles of silica (−75 + 53 µm to −1018 + 850 µm) and magnetite (−425 + 300 µm) were fluidized in a specially designed perspex column. Specific sizes of air bubbles were generated using a combination of a syringe pump and nozzles with different internal diameters. The generated bubbles rose through a bed of fluidized particles, which resulted in bubble loading. The results indicate that fractional bubble loading decreased with an increase in both bubble and particle size. With a 2 mm bubble, the loading fraction of silica declined from circa 0.9 to circa 0.1. However, with coarse particles (−850 + 600 µm and −1018 + 850 µm), bubble loading increased with an increase in bubble size, until an optimum bubble size of 2 mm and then started to decrease as bubble size increased. An increase in particle contact angle showed a corresponding increase in loading fraction, while the density of the material seemed to play no role in terms of the loading fraction. An increase in loading fraction had a negative effect on bubble rise velocity.

Numerical analysis of bubble size effect in a gas-liquid two-phase rotodynamic pump by using a bubble coalescence and collapse model

The inlet bubble sizes and their coalescence and collapse behavior are the key factors affecting the performance and transportability of the multiphase pump. In this paper, a bubble coalescence and collapse model is applied for the simulation of a gas-liquid two-phase rotodynamic pump. The external characteristic, gas-liquid flow, phase interaction, and pressure fluctuation are analyzed using computational fluid dynamics (CFD) methods as the inlet bubble diameter d0 varies from 0.1 mm to 2.2 mm. The CFD results show that the pump’s efficiency and pressure rise decrease sharply at 0.1 mm ≤ d0 ≤ 0.4 mm, slow down at 0.4 mm < d0 ≤ 1.3 mm, and become stable at 1.3 mm < d0 ≤ 2.2 mm. The effects of inlet bubble size on flow characteristics in the impeller, such as gas-liquid separation, flow separation, vorticity change rate, and interphase force, are larger than that in the guide vane. The drag is the most significant interphase force at d0 = 0.1 mm, while the lift force plays a dominant role at d0 = 0.7 mm and 1.9 mm. The effect of the turbulent dispersion force is weakest at d0 = 0.1 mm, 0.7 mm, and 1.9 mm. Rotor-stator interaction is the key driver of the pressure fluctuation in the gas-liquid two-phase pump but is restrained by serious phase separation, flow separation, and interphase interaction at the impeller outlet. The most obvious fluctuation occurs near the guide vane inlet at d0 = 0.1 mm, 0.7 mm, and 1.9 mm. The maximum fluctuation at d0 = 0.7 mm and 1.9 mm are 5.98 times and 7.17 times that at d0 = 0.1 mm.

Effect of bubble size and column depth on diffused aerator efficiency

Effect of bubble size and column depth on diffused aerator efficiency

Evaluation and sensitivity analysis of three-dimensional numerical simulation for aerated deflector-generated jets in spillways

Abstract Aeration is an efficient way to prevent cavitation erosion of a discharge structure and alleviate damage to trajectory nappe on the downstream energy dissipator. Due to the complexity of the aeration jet, it is still a big challenge to understand the aeration mechanism at the water-air interface. This study applied the two-fluid model to simulate three-dimensional entrainment characteristics of jet flow, and also analyzed effects of bubble size, wall roughness, drag force model, turbulence model, and numerical model dimension on clear-water length. Results reveal that the two-fluid model can accurately and effectively simulate clear-water length, thickness variation, and average cross-section air concentration of jet flow with maximum error of 4.1%, 6.5% and 4.2% (x = L1, defining clear-water length with 1% air concentration as L1), respectively. Wall roughness height has a crucial influence on clear-water length. Taking L1 as an example, clear-water length decreases by 13.9% when roughness height increases from 0 to 3 mm. The drag models and bubble particle sizes affect clear-water length only when the profile aeration concentration is greater than 15%. Turbulence model and numerical model dimension have a minor effect on clear-water length.

Destratification of thermally stratified water columns by air diffusers

This study aims at improving the understanding in order to optimise an aeration system for artificial destratification to control cyanobacteria growth in the reservoirs. Previous applications for artificial destratification in reservoirs were based on installations based on computational methods, where neither the effect of air bubble size and configuration nor the effect of air density in the bubble plume could be investigated. This study seeks for an optimized design with the help of experimental and numerical analyses. In order to perform experimental studies, a novel water tank enabling the heating/cooling of the water column as desired and a diffuser system were manufactured. During the experimental studies, effect of bubble size, bubble slip velocity, and other parameters of air diffuser on destratification efficiency were investigated. Based on the nondimensional parameters, a new destratification efficiency formula is obtained by the Genetic Algorithm (GA) approach. Additionaly, the hydrodynamics of the water tank during the mixing process by air diffuser was simulated via 3D numerical model and validated with experimental results. The Eulerian multiphase model with the ‘degassing’ boundary condition and k-ω turbulence model are found to be suitable for the purposes of the study. Based on the error analysis of comparisons of the model and observations, the best configuration of air diffuser is proposed, and the numerical model is found to be successful in simulating the destratification of thermally stratified water columns by air diffuser.