Bubble Energy Research Articles

Direct numerical simulation of turbulent flow separation induced by a forward-facing step

Direct numerical simulation is used to investigate turbulent flow separations generated by a forward-facing step exposed to an incoming fully-developed turbulent plane channel flow at Reτ=180. The step height is 25% of the inlet channel height. The results are analyzed in terms of the topology of the Reynolds stresses, transport of turbulence kinetic energy (TKE) and unsteadiness of separation bubbles using a broad range of post-processing techniques, including the Lumley triangle, quadrant analysis, auto-correlation, joint probability density function, proper orthogonal decomposition and linear stochastic estimation. The incoming turbulence intensity is elevated as the step is approached and peaks along the mean separating streamline over the step. The topology of the Reynolds stresses switches from two-component to axisymmetric along the mean separating streamline over the step. The production term of TKE peaks in the separated shear layer, while the turbulent diffusion term tends to transport TKE away from the shear layer. An expanded separation bubble upstream of the step is typically associated with a contracted separation bubble over the step. This pattern is captured by the first proper orthogonal decomposition (POD) mode. In the first POD mode, the streamwise and vertical fluctuating velocities switch signs abruptly near the mean separating streamline over the step. This is characterized by two pairs of counter-rotating vortices: one pair of counter-rotating vortices leaning over the step and another pair of opposite-signed counter-rotating vortices near the top surface of the step.

Electromotive force induced by the moving non-magnetic phase in ferrofluids

This paper presents a mechanical-to-electrical energy conversion concept by using the upward movement of air bubbles in ferrofluids. As ferrofluids are magnetized by a magnetic field, the air bubbles passing through the induction coils will change magnetic flux in the coils and produce electromotive force. Therefore, the kinetic energy of air bubbles converts into electricity. The idea possesses a promising prospect to be applied in the non-contact sensor field, which can monitor the flow states of boiling two-phase flow in a heat exchanger and warn the deterioration of heat transfer. A millivolt-level induced voltage with a waveform like a sine wave can be generated. The induced voltage's peak value is approximately proportional to the air bubble's volume and velocity and tends to saturate as the magnetic field strength increases.

Structure and properties of amorphous uranium dioxide

Amorphous uranium dioxide (UO2) has been modelled on the atomic scale using a combination of quantum mechanical (density functional theory) and classical forcefield methods (molecular dynamics and reverse Monte-Carlo). The atomic scale structure of the amorphous state has been predicted and is presented in the form of simulated X-ray diffraction patterns, pair correlation functions and bond angle distributions. These are shown to be consistent with the experimental patterns previously reported. To enable accurate calculation of the energies and magnetic properties, using quantum mechanics, reverse Monte-Carlo was used to generate reduced cells from larger molecular dynamic melt-quenches. This allowed density functional theory energy minimisation for structures consistent with the amorphous state. Building on this, the material's propensity to deviate from stoichiometry, the magnetic structure and amorphous UO2 surface energy were computed. Non-stoichiometry is accommodated more readily in the amorphous system than in crystalline UO2. This indicates that deviations from stoichiometry in fuel (for example as a result of operation) will be accommodated at amorphous phases, if present, leaving a more stoichiometric crystalline phase – impacting processes including fission gas mobility, melting points and a number of other safety relevant properties. The magnetic structure of bulk amorphous UO2 is predicted to be a spin-glass, unlike crystalline UO2 which has anti-ferromagnetic ordering at 0 K. The surface energy of amorphous UO2 is computed to be 0.79 Jm−2, which is similar to the experimentally observed surface energy of grain boundary bubbles in UO2 and comparable to the surface energies reported for crystalline UO2. Results are pertinent to ongoing efforts to understand the nature of grain boundaries in nuclear fuels, to model fission gas release from them, radiation induced amorphization and the impact of dopants and impurities on nuclear fuel's manufacture and in-reactor behaviour.

Application of the exact regularized point particle method (ERPP) to bubble laden turbulent shear flows in the two-way coupling regime

Direct numerical simulations of a bubbly laden homogeneous shear flow have been carried out using the exact regularized point particle method as the inter-phase momentum coupling approach. The aim of this study consists in addressing the modulation of shear turbulence and the bubble clustering geometry in the presence of different inter-phase momentum coupling conditions. Suspensions with different combinations of the void fraction and Kolmogorov-based Stokes number, in the dilute regime, have been addressed. Bubbles suppress the turbulent kinetic energy and turbulent dissipation as well. Turbulent modulation occurs via the direct change of the Reynolds shear stress. In fact, the bubble energy source is proved to be negligible in the scale-by-scale turbulent energy budget. The bubble clustering, in agreement with the literature, occurs in the form of thin elongated structures. The clusters are aligned with the principal strain direction of the mean flow, as usual in shear flows. The bubble clustering and turbulent modification are strictly related: both increase with the Stokes number and are independent of the void fraction. The data show that the turbulent modification is disadvantaged when the bubble distribution is homogeneous (i.e., small Stokes number). Finally, the small-scale bubble clustering is slightly reduced by two-way coupling effects even though the clustering anisotropy still persists at small scales as it occurs for inertial particles.

Molecular dynamics simulation of cavitation in a Lennard-Jones liquid at negative pressures

Spontaneous cavitation in a stretched Lennard-Jones liquid at temperatures above and below the triple point has been studied by the direct forward flux sampling and the seeding methods. A change in the nucleation rate of 230 orders of magnitude has been traced. The data obtained have been compared with calculations according to classical nucleation theory in a capillarity approximation. A discrepancy reaching 16 orders of magnitude in nucleation rate between the results of theory and simulation has been found. It has been shown that the reason for this discrepancy consists in the size dependence of the surface free energy of critical bubbles. The first-order and second-order corrections to the surface free energy of these bubbles with respect to the curvature of the interface have been determined.

Effect of the Aluminum Particle Size, Solid Content, and Aluminum/Oxygen Ratio on the Underwater Explosion Performance of Aluminum-Based Explosives

Several aluminum-based explosives are prepared and their underwater explosion performances, including the specific bubble energy and specific shock wave energy, are measured in underwater explosion experiments. The formulation characteristics of the explosives involve the aluminum/oxygen (Al/O) ratio, aluminum particle size, and the total content of all solid components, including ammonium perchlorate (AP), aluminum (Al), and cyclotrimethylenetrinitramine (RDX). The results indicate that the Al/O ratio of the explosives produces a great effect on the specific shock wave energy and specific bubble energy of the explosives. The total specific energy of the explosive reaches the maximum value at Al/O = 0.44. It is notable that an increase in the total solid content (AP, Al, and RDX) in the explosive formulation can effectively increase the specific total energy of the explosives. When the total solid content is increased by 2% (by weight) in the explosive formulation, the total energy of the explosive can be increased by about 0.1 times the TNT equivalent. Moreover, the particle size of the aluminum powder can also significantly affect the energy of the explosives. The smaller particle size of the aluminum powder is beneficial to the energy release of aluminum and can increase the total explosion energy.

Effect of baffles on bubble behavior in a bubbling fluidized bed for chemical looping processes

A bubbling fluidized bed is generally used as the fuel reactor in chemical looping processes. However, insufficient gas–solid contact remains a steadfast problem. As such, we propose the use of internal baffles to split bubbles at a high gas velocity and to restrict the movement of large-scale solids in the fluidized bed. We investigated the effect of baffle arrangement on the bubble size and energy, pressure gradient, particle distribution, and particle velocity in a fluidized bed using a computational particle fluid dynamics simulation based on our experimental results. We discuss the main influencing factors, which include presence of caps, number of baffles, and the baffle opening ratio. The baffle structure with caps efficiently broke the large bubbles without creating jet flow. Three baffles were deemed suitable in the bed owing to the small bubble size and uniform gas–solid distribution in each compartment. We selected a baffle opening ratio of 20.5%, which improved the flow pattern without causing a significant increase in the pressure gradient and particle velocity through the baffles. We also discussed the potential for scaling up this baffled fluidized bed for industrial applications.

Study of cavitation bubble collapse near a rigid boundary with a multi-relaxation-time pseudo-potential lattice Boltzmann method

Due to the complex phase change and heat transfer processes, the mechanisms of cavitation bubble collapse near a rigid boundary are well recognized to be complicated. Based on a modified large-density ratio multi-relaxation-time pseudo-potential lattice Boltzmann model, a single and a dual cavitation bubble collapse process near a rigid boundary with large-density and various viscosity ratios are simulated in the present study. Effects of density ratio, viscosity ratio, initial pressure difference, and distance between the cavitation bubble and wall on the cavitation process are studied. Furthermore, the evolution of maximum pressure, micro-jet velocity, lifetime, deformation index, and the first introduced total kinetic energy of cavitation bubbles are analyzed in the development of cavitation. Simulations show that the interaction mode of the bubbles and the distance between the rigid boundary and the lower bubble are key factors in determining the effect of aeration reduction. The study also shows that the proposed lattice Boltzmann pseudo-potential model is a robust and effective tool for studying the collapse of near-wall cavitation bubbles and has potential to predict the interaction of cavitation bubbles in the presence of complex boundaries.

Numerical investigation of central breakup of large bubble induced by liquid jet

A large spherical bubble rising in quiescent liquid generally leads to the formation of a toroidal bubble (central breakup). In this paper, we investigate the bubble dynamics during the central breakup process using the three dimensional Volume of Fluid method implemented in OpenFOAM. The potential energy of the large bubble is converted into the kinetic energy of the liquid jet, resulting in the formation of the toroidal bubble. Before the central breakup of the bubble, a high pressure zone is formed on the top of the bubble due to the collision of the liquid jet with the top of the bubble. We report for the first time that a protrusion is formed on the top of the toroidal bubble for a large spherical bubble rising in quiescent liquid. The velocity of the gas inside the toroidal bubble around the liquid jet is much faster than that in other places after the central breakup, which leads to the formation of the protrusion against the restriction of the surface tension force. We find that the bubble size, liquid viscosity, and density can influence the formation of the toroidal bubble, while the influence of surface tension is negligible. We summarize the above influencing factors into two dimensionless numbers: Galilei (Ga) number and Eötvös number (Eo). In the end, we discover a simple linear relation between the jet Reynolds number and the Ga number by analyzing all numerical experiments.

Propulsion effects after laser ablation in water, confined by different geometries

A Nd:YAG laser with 7-ns pulses and pulse energies up to 10 mJ is used to induce an optical breakdown in the front surface of an aluminum rod, covered by a water layer. The rod is part of a ballistic pendulum. In this way, we study the propulsion effects by means of coupling coefficient and energy-conversion efficiency with respect to different confining geometries, volumes of water applied to the front surface of the rod, and the distance of this surface from the laser-beam focus. Holes with different dimensions are drilled on the target surface and filled with different volumes of water to examine the influence of the confinement by the liquid (a free boundary) and a solid-surface geometry on laser ablation effects. The rod movement and the water ejection after laser ablation are acquired by a high-speed camera with 10k frames per second. The results show that the confinement by cavity substantially increases the propulsion effects by shaping the ejected flow of the liquid; while the cavitation bubble, induced inside the water layer, plays a significant role in propulsion efficiency. From the presented results, it follows that the laser-propelled rod carries below 0.5% of the total mechanical energy after propulsion, while the rest of this energy represents the kinetic energy of the ablated water. As expected, moving the target surface away from the focal position decreases the ablative-propulsion efficiency. When the focus is moved inside the solid target, the decrease occurs due to lower conversion of the pulse energy into the energy of the cavitation bubble. If the focus is moved from the surface outward, the bubble moves towards the liquid–gas interface and it is not able to efficiently eject all the liquid from the target.

A Simple Method for Reliable Estimation of the Bubble Energy in the Underwater Explosion

The bubble energy is the main part of the chemical energy of the explosive that is formed upon the propagation of a shock wave through the water for the underwater explosion. A simple method is introduced for reliable prediction of the bubble energy of composite explosives containing aluminum (Al) and/or ammonium perchlorate (AP). It is shown that the bubble energy of composite Al/AP explosives depends on the number of moles of chlorine, carbon, and Al atoms. Experimental data of 56 composite Al/AP explosives are used to construct and test the novel model of the bubble energy. Statistical parameters of the new model, in external validation containing 35 composite Al/AP explosives as the training set, have the values 0.43 and 1.15 MJ·kg–1 for the root mean squared error (RMSE) and maximum of errors (Max Error) of the new model, respectively. The values of RMSE and Max Error for 21 composite Al/AP explosives as test set are also 0.60 and 2.22 MJ·kg–1, respectively. Cross validations of the new method corresponding to the coefficients of determination for leave‐one‐out (Q2LOO) and the fivefold cross validation (Q25CV) are 0.8573 and 0.8403, respectively, which confirms goodness‐of‐fit, goodness‐of‐prediction, accuracy and precision of the novel model. It is shown that the novel correlation can be applied for pure and composite explosives, which do not contain Al/AP.

Effects of Sparger Holes on Gas‐Liquid Hydrodynamics in Bubble Columns

AbstractA multiphase second‐order moment turbulent model was developed including an improved closure Reynolds stress transport to describe effectively the anisotropic characteristics of bubble‐liquid turbulent flow. The two‐phase hydrodynamics in a bubble column were numerically simulated by an in‐house code. The effect of the amount of jetting holes on bubble‐liquid hydrodynamics in relationship with a cell culture was investigated. Firstly, the bubble energy production correlation was proposed to verify the higher cell damage rate near gas inlet regions. The larger amounts of jetting holes resulted in an increase of bubble stress, bubble kinetic energy, and energy dissipation, particularly bubble energy production at the near gas inlet, which boosted cell damage.

The Influence of Atmospheric Pressure on Air Content and Pore Structure of Air-entrained Concrete

To study the effect of atmospheric pressure on the properties of fresh and hardened air-entrained concrete, three kinds of air entraining agents were used for preparing air-entrained concrete in the plateaus (Lhasa, 61 kPa) and the plains (Beijing, 101 kPa). Air content, slump, compressive strength and pore structure of the three air-entrained concretes were tested in these two places. It is found that the air content of concrete under low atmospheric pressure (LAP) is 4%-36% lower than that of concrete under normal atmospheric pressure (NAP), which explaines the decrease of slump for air-entrained concrete under LAP. Pore number of hardened concrete under LAP is reduced by 48%-69%. While, the proportion of big pores (pore diameter >1 200 μm) and air void spacing factor are increased by 1.5%-7.3% and 51%-92%, respectively. The deterioration of pore structure results in a 3%-9% reduction in the compressive strength of concrete. From the results we have obtained, it can be concluded that the increase of critical nucleation energy of air bubbles and the decrease of volumetric compressibility coefficient of air in the concrete are responsible for the variation of air content and pore structure of concrete under LAP.

Energy characteristics of a radiating thin layer of bubbles in a liquid

The radiation of a thin bubble layer in an infinite liquid excited by a single wave of large amplitude is studied. It is shown that, after excitation, the layer emits a strong nonlinear wave in both directions. The radiation of the layer becomes close to sinusoidal with one line in the spectrum at large times. It is shown that in the nonlinear phase of the sound wave radiation, the time variation of the bubble energy in the layer is characterized by a stepwise dependence.

Numerical study on dual-frequency ultrasonic enhancing cavitation effect based on bubble dynamic evolution.

Ultrasonic cavitation is a physical dynamic phenomenon of bubbles inflation, compression, and collapse in liquid. A dual-frequency ultrasonic cavitation dynamics model is established in this paper to investigate dynamic evolution of bubble under single and dual frequency ultrasonic modes. The variation of bubble radius, pressure, energy, temperature, and number of water vapor molecules inside the bubble in single and dual frequency ultrasonic modes are analyzed, respectively. The results show the oscillation of cavitation bubbles is more unstable and easier to collapse in dual-frequency ultrasound field than those in single-frequency ultrasound field. With the increase of the ultrasonic frequency, cavitation effect is weakened due to the shortage of oscillation period. Under the same ultrasonic power, the maximums of bubble radius, pressure, and water vapor molecules number inside the bubble in the dual-frequency mode are larger than those in the single-frequency mode. Under the ultrasonic excited by 50 kHz + 70 kHz, the maximum bubble radius and pressure can reach 36.061 μm and 2285.9 MPa, respectively, which are much larger than 18.183 μm, 730.61 MPa at 50 kHz and 14.576 μm, 332.25 MPa at 70 kHz. The calculation results of three different frequency combinations (30 kHz + 50 kHz, 40 kHz + 60 kHz and 50 kHz + 70 kHz) indicate dual-frequency ultrasound can significantly enhance the cavitation effect.

Numerical Simulation of Bubble-Liquid Two-Phase Turbulent Flows in Shallow Bioreactor

An improved second-order moment bubble-liquid two-phase turbulent model is developed to predict the hydrodynamic characteristics of the shallow bioreactor using two height-to-diameter ratios of H/D = 1.4 and H/D = 2.9. The two-phase hydrodynamic parameters, the bubble normal and shear stress, the bubble energy dissipation rate, the bubble turbulent kinetic energy, etc. were numerically simulated. These parameters increased along with flow direction and constituted a threat to cells living at far distance away from the gas jetting inlet regions, rather than a finding of higher cell damage at near the jetting inlet region, as reported by Babosa et al. 2003. A new correlation named the turbulent energy production of bubble-liquid two-phase flow was proposed to successfully verify this experimental observation. A smaller H/D ratio makes more contributions to the generation of lower turbulent energy productions, which are in favor of the alleviation of cell damage. The extremely long and narrow shape of the bioreactor is deteriorative for cell living.

Cavitation Induced by Janus-Like Mesoporous Silicon Nanoparticles Enhances Ultrasound Hyperthermia.

The presence of nanoparticles lowers the levels of ultrasound (US) intensity needed to achieve the therapeutic effect and improves the contrast between healthy and pathological tissues. Here, we evaluate the role of two main mechanisms that contribute to the US-induced heating of aqueous suspensions of biodegradable nanoparticles (NPs) of mesoporous silicon prepared by electrochemical etching of heavily boron-doped crystalline silicon wafers in a hydrofluoric acid solution. The first mechanism is associated with an increase of the attenuation of US in the presence of NPs due to additional scattering and viscous dissipation, which was numerically simulated and compared to the experimental data. The second mechanism is caused by acoustic cavitation leading to intense bubble collapse and energy release in the vicinity of NPs. This effect is found to be pronounced for as-called Janus NPs produced via a nano-stopper technique, which allow us to prepare mesoporous NPs with hydrophobic inner pore walls and hydrophilic external surface. Such Janus-like NPs trap air inside the pores when dispersed in water. The precise measurement of the heating dynamics in situ enabled us to detect the excessive heat production by Janus-like NPs over their completely hydrophilic counterparts. The excessive heat is attributed to the high intensity cavitation in the suspension of Janus-like NPs. The present work elicits the potential of specifically designed Janus-like mesoporous silicon NPs in the field of nanotheranostics based on ultrasound radiation.

Effect of Aluminum Powder on Underwater Explosion Performance of CL‐20 Based Explosives

AbstractThe influences of particle size distributions of the nano and micro‐aluminum powders on the underwater explosion were analyzed experimentally. Both shock waves and the second pressure pulses induced by the CL‐20 based explosives were measured. Furthermore, the shock wave energy, the bubble energy, the reacting quantity of aluminum powders in the CL‐20 based explosives during the detonation and secondary reaction processes were calculated. The results indicate that: the nano‐aluminum powders have significant influences on both the detonation reaction and the secondary reaction, leading to the higher peak pressure of the shock wave while the lower peak pressure and narrower pulse width of the secondary pressure pulse; the gradations of both nano and micro‐aluminum powders improve the energy outputs of the explosives; the nano‐aluminum powders enhance the shock wave pressure while the micro‐aluminum powders maintain higher bubble energy.

Bubble evolution mechanism of pulse spark discharge in seawater

Underwater spark discharge could generate high-pressured bubble oscillations, which has broad application prospects in marine acoustic exploration and medical therapy. Much research has been focused on the first oscillation period, while little attention has been paid to the following periods and discharge efficiency. In this work, we studied the bubble evolution characteristics of three oscillations, as well as the multi-factor effects on radius, oscillation periods and energy efficiency. An experimental method is presented to control several influential factors to analyse their influences on bubble evolution and energy transformation. Notably, an image process algorithm is proposed to accurately describe the bubble evolution and the energy transformation efficiency. In addition, we analysed the jet flow formation during the first bubble growth and the non-uniformly distributed bubble dispersion. These results could provide references to the future optimization of bubble dynamic models and design of controllable acoustic sources.

Numerical Study on the Explosive Effect of RDX-based Aluminized Explosives on Stiffened Plates

The finite element software LS-DYNA was used to numerically investigate the damage of stiffened plates due to underwater explosive loads provided by RDX-based aluminized explosives. A finite element analysis model of fluid-solid coupling was established in order to study the elastoplastic dynamic buckling behavior of a stiffened plate structure. The effects of the RDX-based aluminized explosive energy release rate, shockwave energy, bubble energy and shockwave pressure on damage are discussed. A connection is established between aluminum content, the energy output of the underwater explosion and the response of the stiffened plate.