Near-wall Bubble Research Articles

Study of surface tension effects on near-wall cavitation bubble collapse with a pseudopotential lattice Boltzmann model

A lattice Boltzmann pseudopotential cavitation model with tunable surface tension and large density and viscosity coefficient ratios was used to simulate near-wall cavitation bubble collapse. The influences of the surface tension, bubble–wall distance, and initial pressure difference on the flow field distribution were analyzed, and the relationships between the surface tension and maximum micro-jet and collapse pressure were investigated. The results indicated that a lower surface tension intensifies the deformation of the gas–liquid interface, resulting in a more concentrated micro-jet. In addition, more surface energy is accumulated during cavitation bubble collapse for higher surface tension, strengthening the collapse intensity and increasing the maximum micro-jet velocity and collapse pressure. The time interval between the first and second pressure peaks increases with increasing wall distance. Because of the non-linear attenuation during pressure propagation, the value of the second peak decreases with increasing wall distance. Increasing the initial pressure difference leads to slower growth in the micro-jet velocity and faster growth in the collapse pressure with increasing bubble–wall distance. In addition, increasing the initial pressure difference for the same bubble–wall distance also slows the growth in the micro-jet velocity and increases the growth in the collapse pressure caused by increasing surface tension.

A Phenomenological Investigation on Near-Wall Bubble Behavior Close to Chf in Flow Boiling

A Phenomenological Investigation on Near-Wall Bubble Behavior Close to Chf in Flow Boiling

Flow aeration by an offset aerator: Air entrainment, bubbly flow turbulence, and adaptive-window cross-correlation processing

Chute aerators are used to induce flow aeration to prevent cavitation damages to flood discharge structures. The aerated chute flows are traditionally studied in terms of the air concentration distributions and air entrainment rate, whereas further detailed characterization of the bubbly flow associated with the turbulence modification has been missing. Herein the air–water flow turbulence was studied experimentally for a simple offset aerator model in a sloping chute. Advanced interfacial turbulence analyses were applied to the air–water flow measurement results using a dual-tip phase-detection probe. The new adaptive-window cross-correlation approach was first applied to the turbulence intensity estimate in both regions upstream and downstream the impact point. The overall spatial distributions of basic air–water flow properties agreed favorably with literature, and highlighted different patterns of air–water flow development in different sections of the flow. The turbulence level of the bubbly flow, and the bubble-turbulence interactions, were quantitatively described with the interfacial turbulence intensity, correlation turbulent length scale, and some measures of the bubble clustering behaviors. The interfacial turbulence intensity was obtained in the same order of magnitude as the common water-phase turbulence intensity observations, except for the impact zone. The results revealed the shear flow development and turbulence dissipation in the bottom layer of the aerated chute flow, which affected the local average bubble counts, the relative motions of neighboring bubbles, the diffusive transport of bubbly eddy structures, and consequently the near-wall bubble distributions critical for the protection of chute surface from cavitation erosion.

An equivalent method of jet impact loading from collapsing near-wall acoustic bubbles: A preliminary study

Cavitation damage is a micro, high-speed, multi-phase complex phenomenon caused by the near-wall bubble group collapse. The current numerical simulation method of cavitation mainly focuses on the collapse impact of a single cavitation bubble. The large-scale simulation of the cavitation bubble group collapse is difficult to perform and has not been studied, to the best of our knowledge. In this study, the equivalent model of impact loading of acoustic bubble collapse micro-jets is proposed to study the cavitation erosion damage of materials. Based on the theory of the micro-jet and the water hammer effect of the liquid–solid impact, an equivalent model of impact loading of a single acoustic bubble collapse micro-jet is established under the principle of deformation equivalence. Since the acoustic bubbles can be considered uniformly distributed in a small enough area, an equivalent model of impact loading of multiple acoustic bubble collapse micro-jets in a micro-segment can be derived based on the equivalent results of impact loading of a single acoustic bubble collapse micro-jet. In fact, the equivalent methods of cavitation damage loading for single and multiple near-wall acoustic bubble collapse micro-jets are formed. The verification results show the law of cavitation deformation of concrete using equivalent loading is consistent with that of a micro-jet simulation, and the average relative errors and the mean square errors are insignificant. The equivalent method of impact loading proposed in this paper has high accuracy and can greatly improve the calculation efficiency, which provides technical support for numerical simulation of concrete cavitation.

Trajectory Analysis of Rising Spheroidal Bubbles in a Rolling Column

The rise of spheroidal bubbles is emulated using a hexapod with periodic angular perturbations to mimic bubble paths in columns onboard seagoing vessels. Bubble trajectories are determined using high-speed camera imaging/ultrasound Doppler velocimetry in vertical, inclined, and rolling columns. The performances of the measurement techniques are compared based on the bubble rise velocity time series and frequency spectra. A hierarchy of reference frames is developed to relate the bubble velocity/acceleration components in the (non-)Galilean frames and to infer from trajectography the lift and drag forces applying to the bubble in the moving-bubble frame. Wall disruption during bubble flights in the rolling column is scrutinized using various geometric/dynamic metrics to underline the dissimilarities between near-wall and centerline bubble path oscillations. Although Coriolis, centrifugal, and Euler accelerations play roles neither on the bubble streamwise rise nor the bubble accelerations, the time-varying character of the buoyancy force brings in dynamic features to the bubble rise in a rolling column that are distinct from those observed in static inclined columns.

Effect of stand-off distance and spatial resolution on the pressure impact of near-wall vapor bubble collapses

We consider the collapse behavior of cavitation bubbles near walls under high ambient pressure conditions. Generic configurations with different stand-off distances are investigated by numerical simulation using a fully compressible two-phase flow solver including phase change. The results show that the stand-off distance has significant effects on collapse dynamics, micro-jet formation, rebound, and maximum wall pressure. A relation between cavitation induced material damage and corresponding collapse mechanisms is obtained from pressure-impact data at the wall. We analyze the resolution dependence of collapse and rebound and the observed maximum pressure distributions. The comparison of the results on six different grid resolutions shows that main collapse features are already captured on the coarsest resolution, while the peak pressures are strongly resolution dependent.

Investigations of near-wall bubble behavior in wire heaters pool boiling

Current work establishes a pool boiling critical heat flux prediction method based on percolation theory. For the first time, we observe the experimental bubble footprint?s power-law distributions with almost the same exponent in wire heaters? water pool boiling crisis, which is borne out strongly that boiling crisis is a typical continuum percolative scale-free behavior, and its characteristics seems not to be influenced by the critical heat flux value. The proposed 1-D Monte-Carlo method successfully simulates the phase transition of interactive near-wall bubbles. This research enriches and extends applications of continuum percolation theory in boiling phenomena, and could be an instruction for the followed critical heat flux enhancement studies.

Numerical assessment of cavitation-induced erosion using a multi-scale Euler–Lagrange method

A multi-scale Euler–Lagrange method was developed and applied to numerically assess cavitation-induced erosion based on the collapse dynamics of Lagrangian bubbles. This approach linked macroscopic and microscopic scales and captured large vapour volumes on an Eulerian frame, while small vapour volumes were treated as spherical Lagrangian bubbles. Interactions between vapour bubbles and the liquid phase were considered via a two-way coupling scheme. A verification and sensitivity study of the developed procedure to transform vapour volumes between Eulerian and Lagrangian frames was performed. First, the developed method was validated for bubble dynamics, using analytical and experimental data. Second, the cavitating flow through an axisymmetric nozzle was simulated using a measurement-based distribution of cavitation nuclei. Details of single bubble collapses were used to assess cavitation erosion. Based on well-recognised fundamental experiments and theoretical considerations from the literature, model assumptions were derived to account for the effects of a bubble’s stand-off distance on the bubble’s motion and its radiated pressure during an asymmetric near-wall bubble collapse. Computed maximum collapse radii of bubbles correlated well with diameters of measured erosion pits. Considering a nonlinear dependence of erosion on impact pressure, calculated erosion potentials compared well to measured erosion depths.

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.

Comparison of Shock/Cooling-Film Interaction for Cooled and Isoenergetic Injection

High-speed particle-image velocimetry measurements and temperature measurements are performed to investigate the interaction of shock waves with film cooling flows at varying injection temperature. A laminar jet is tangentially injected beneath a turbulent boundary layer at a freestream Mach number . Two cooling film injection Mach numbers are considered, at a total temperature ratio and at a total temperature ratio . For the low injection Mach number, the shock is generated by a flow deflection of ; for the high injection Mach number, the flow deflection angle is . For both injection Mach numbers, two shock impingement positions are investigated. For comparison, literature data with comparable shock parameters and isoenergetic injection are considered. The current results show that the flow structure of the shock/cooling-film interaction is highly sensitive to the injection temperature. The increased blowing rate at a lower injection temperature, that is, increased streamwise mass flux in the cooling film, drastically stabilizes the flow. This prevents mean separation and leads to reduced turbulent mixing downstream of the shock interaction region. In consequence, the turbulent heat transport toward the wall decreases, which implies a higher cooling effectiveness when the injection temperature decreases. The findings also show that flow cases with different injection temperatures but similar near-wall mass flux yield separation bubbles with similar streamwise extent.

Numerical modeling of the acoustically driven growth and collapse of a cavitation bubble near a wall

This paper describes the first high-order accurate, fully compressible, multiphase model to simulate the expansion and collapse of a near-wall cavitation bubble in a low-frequency ultrasound field. The model captures the compressibility of the fluids, subsequent shocks, and a physically correct representation of the acoustic input through an immersed moving boundary that represents the active face of the ultrasound transducer face. The model’s predictions of bubble dynamics are compared to existing models that are able to capture the collapse of a near wall bubble, (1) the Rayleigh growth and collapse model and (2) the Rayleigh-Plesset growth initialized collapse model, highlighting the limitations of the previously developed models.

An improved DNB-type critical heat flux prediction model for flow boiling at subcritical pressures in vertical rifled tube

Based on the near-wall bubble coalescence model and the structural characteristics of the rifled tube, improvements and optimization of the model for rifled tube were conducted. DNB-type CHF phenomenon in the rifled tube under subcritical and near-critical pressures was investigated by numerical calculation. A new CHF prediction model of flow boiling in rifled tube with low qualities was established and compiled by FORTRAN language. The model used the Staub model which considering the influence of the bubble contact angle to calculate the bubble detachment diameter. The surface friction coefficient of the rifled tube was calculated by the classical Kohler and Kastner formula. And a new critical void fraction of the wall bubbly layer αb correlation was proposed based on a large number of CHF experimental data. The present model was verified by the CHF look-up values and CHF experimental values. Based on the CHF experimental data of the rifled tube, three classic CHF prediction models and the present CHF model were evaluated, and it was found that the present modified CHF model has the highest prediction accuracy for the rifled tube, which is the best agreement with the experimental values, indicating that the present model is appropriate for DNB-type CHF prediction of the rifled tube. In this paper, the effects of pressure, mass flux and inlet fluid subcooled enthalpy on CHF were studied. It was found that, the effect of pressure and mass flux on CHF is obvious in the low quality region, with the increase of vapor quality, the influence of pressure and mass flux on CHF weakens.

Comparative scaling analysis of two different sized pilot-scale fluidized bed reactors operating with biomass substrates

This paper presents a comparative scaling analysis of two different sized pilot-scale fluidized bed reactors operating with biomass substrates. A multiphase Eulerian-Eulerian 2-D mathematical model was implemented, coupled with in-house user-defined functions (UDF) built to enhance hydrodynamics and heat transfer phenomena. The model validation was attained by comparison to experimental data gathered from both reactors. A grid refinement study was carried out for both geometries to achieve an appropriate computational domain. Hydrodynamics was deeply studied for both reactors concerning the scale-up effect. Mixing and segregation phenomena, solid particle distribution and biomass velocity were matters of great concern. Results showed that UDF implementation successfully minimized deviations and increased the model’s predictability. The largest deviations measured between experimental and numerical results for syngas composition were of about 20%. Solids mixing and segregation was found to be directly affected by the particles size, density, and superficial gas velocity, with the larger reactor revealing improved mixing ability. Improved mixing occurred for smaller particles size ratio (dbiomass = 3 mm), smaller particles density ratio (ρbiomass = 950 kg/m3), and higher dimensionless superficial gas velocities (U0/Umf=3.5). The larger unit showed an increase in near-wall velocity, lateral dispersion, and bubble size. As for the smaller reactor, higher velocities were obtained at the center region due to a more pronounced wall boundary layer. Similarities were found between the two reactors regarding the bubble distribution, dimensionless average bed pressure drop and biomass velocity vector profiles when dimensionless parameters were employed.

Analysis of the effect of impact of near-wall acoustic bubble collapse micro-jet on Al 1060

The bubble collapse near a wall will generate strong micro-jet in a liquid environment under ultrasonic field. To explore the effect of the impact of near-wall acoustic bubble collapse micro-jet on an aluminum 1060 sheet, the cavitation threshold formula and micro-jet velocity formula were first proposed. Then the Johnson-Cook rate correlation material constitutive model was considered, and a three-dimensional fluid-solid coupling model of micro-jet impact on a wall was established and analyzed. Finally, to validate the model, ultrasonic cavitation test and inversion analysis based on the theory of spherical indentation test were conducted. The results show that cavitation occurs significantly in the liquid under ultrasonic field, as the applied ultrasonic pressure amplitude is much larger than liquid cavitation threshold. Micro pits appear on the material surface under the impact of micro-jet. Pit depth is determined by both micro-jet velocity and micro-jet diameter, and increases with their increase. Pit diameter is mainly related to the micro-jet diameter and dp/dj≈0.95–1.2, while pit’s diameter-to-depth ratio is mainly negatively correlated with the micro-jet velocity. Wall pressure distribution is mostly symmetric and its maximum appears on the edge of micro-jet impingement. Obviously, the greater the micro-jet velocity is, the greater the wall pressure is. Micro pits formed after the impact of micro-jet on aluminum 1060 surface were assessed by ultrasonic cavitation test. Inversion analysis results indicate that equivalent stress, equivalent strain of the pit and impact strength, and velocity of the micro-jet are closely related with pit’s diameter-to-depth ratio. For the pit’s diameter-to-depth ratio of 16–68, the corresponding micro-jet velocity calculated is 310–370m/s.

Critical heat flux prediction model for low quality flow boiling of water in vertical circular tube

Based on the viewpoint of bubble crowding in near-wall bubble layer, a critical heat flux (CHF) prediction model was developed for low quality flow boiling of water in uniformly heated vertical circular tube under high pressure and low flow rate conditions. In this model, a CHF formula was derived from the conservation equations of mass, momentum and energy, where the transverse mass transport between the near-wall bubble layer and core is assumed to be limited. Taking account of the convective shear effect caused by the frictional drag on the wall-attached bubbles, the limiting transverse interchange of mass flux crossing the bubble layer–core interface was determined from the momentum balance equations. A new formula of bubble departure diameter considering the effect of buoyancy was put forward and used to solve the model with numerous other empirical correlations (e.g. bubble departure point, turbulence velocity profile, void fraction and so on). The model shows good agreements with the experimental data of uniformly heated circular tube. Based on this, the effects of flow variables such as pressure, mass flux and inlet subcooling on CHF prediction results were also discussed.

Near-wall rising behaviour of a deformable bubble at high Reynolds number

The dynamics of a large deformable bubble ($\mathit{Re}\sim \mathit{O}(10^{3})$) rising near a vertical wall in quiescent water is experimentally investigated. The reference (without the wall) rising path of the considered bubble is a two-dimensional zigzag. For a range of wall configurations (i.e. initial wall distance and boundary condition), using high-speed shadowgraphy, various rising behaviours such as periodic bouncing, sliding, migrating away, and non-periodic oscillation without collisions are measured and analysed. Unlike low-Reynolds- and Weber-number bubbles, the contribution of the surface deformation to the transport between the energy components becomes significant during the bubble’s rise. In particular, across the bubble–wall collision, the excess surface energy compensates the deficit of kinetic energy. This enables a large deformable bubble to maintain a relatively constant bouncing kinematics, despite the obvious wall-induced energy dissipation. The wall effect, predominantly appearing as energy loss, is found to decrease as the initial distance from the bubble centre to the wall increases. Compared to the regular (no-slip) wall, a hydrophobic surface enhances or reduces the wall effect depending on the wall distance, whereas a porous surface reduces the energy loss due to the wall, regardless of the initial distance from the wall. Furthermore, the bubble–wall collision behaviour is assessed in terms of a restitution coefficient and modified impact Stokes number (ratio of the inertia to viscous forces), which shows a good correlation, the trends of which agree well with the variations in the energy components. The dependence of near-wall bubble motion on the wall distance and boundary condition may suggest a way of predicting or controlling the near-wall gas void-fraction distribution in gas–liquid flow systems.

Dynamics of detonation waves in an annular layer of a round pipe

The dynamics of axisymmetric detonation waves in a round pipe containing an annular near-wall bubble layer with a combustible gaseous mixture are studied. The situations in which within the annular layer there are a pure liquid or a bubbly mixture with an inert (unexplosive) gas are considered.

The characteristics of gas–solid flow and wall heat transfer in a fluidized bed reactor

Numerical study using computational fluid dynamics has been carried out to investigate the heat transfer characteristics of a laboratory fluidized bed reactor. The fluidized bed reactor of vTI (Johann Heinrich von Thunen-Institute)-Institute of Wood Technology and Wood Biology is modeled. For the simulation of multiphase flow and thermal fields, an Eulerian–Eulerian approach is applied. The flow and thermal characteristics of the reactor are fully investigated for the wide range of superficial gas velocities and two different particle diameters. In particular, the contributions of the gas bubble and emulsion phase flows on the wall heat transfer are scrutinized. From the predicted results, it is fully elucidated that particular near-wall bubble motions mainly govern the wall heat transfer.

Numerical Simulation of a Near-Wall Bubble Collapse in an Ultrasonic Field

Ultrasonic cleaning device was modeled and the collapse of a single bubble in an ultrasonic pressure field was studied using a finite-volume 2D axisymmetrical model. The pressure field was generated with the bottom of a container oscillating at 33 kHz. Spherical air bubble of resonance size was induced into water near the bottom where it violently collapsed. Compressibility of both phases was taken into account and the interface between air and water was captured using Volume-Of-Fluid approach. Maximal pressures, temperatures and velocities, generated during the collapse, were studied with regard to the initial bubble distance from the bottom. The results were compared for different time step sizes and grid densities where some great differences were found. For validation of the model a separate numerical simulation was run for a bubble collapsing near a rigid wall in a uniform pressure field. The computed bubble shapes were compared to the experimentally observed bubble shapes of Philipp and Lauterborn (1998) and a very good agreement was found. Although the collapse of just one bubble was studied, the results obtained gave good insight into conditions that happen due to cavitation in a near-wall region of ultrasonic cleaning devices.

Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction

To investigate the phenomena of skin-friction drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy's William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct drag-reduction phenomena were investigated; bubble drag reduction (BDR) and air-layer drag reduction (ALDR).The streamwise distribution of skin-friction drag reduction was monitored with six skin-friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0ms−1. Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 <Y< 5mm) bubble populations were estimated with a bubble imaging system. The instrument suite was used to investigate the scaling of BDR and the requirements necessary to achieve ALDR.Results from the BDR experiments indicate that: significant drag reduction (>25%) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the ‘initial zone’ scaling, which provides little information on downstream persistence of BDR.ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3ms−1. These results indicated that there are three distinct regions associated with drag reduction with air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3ms−1; the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5ms−1with the surface fully roughened (though approximately 50% greater volumetric air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small faired step (10mm height in the experiment) that helps to create a fixed separation line.