Bubble Forces Research Articles

Through-Space Electrochemiluminescence Reveals Bubble Forces at Remote Phase Boundaries.

Several groups have reported on the curious chemistry and reaction acceleration in confined volumes. These complex multiphase systems most closely resemble natural processes, and new measurement tools are necessary to probe chemistry in such environments. Generally, electrochemiluminescence (ECL) reports on processes immediately near (within a few micrometers) the electrode surface. Here, we introduce through-space ECL, reporting on dynamics of processes far away (100s of μm) from the electrode surface. We achieved this by collecting reflected ECL light. During the heterogeneous oxidation of C2O42- in an aqueous phase adjacent to a 1,2-dichlorethane droplet, CO2 accumulates in the 1,2-dichloroethane droplet. Upon buildup, we demonstrate that a CO2 bubble forms in the nonaqueous phase and is surprisingly trapped at the water|1,2-dichloroethane interface and continues to grow. The co-oxidation of tris(bipyridine)ruthenium(II) in the aqueous phase lights up the electrode surface and reflects off the edges of the bubble, revealing the bubble growth over time even when the bubble is fractions of a millimeter from the surface. We extend our results to quantifying bubble forces at the water-oil interface at remote distances from the electrode surface.

Euler-Euler Model of Bubbly Flow Using Particle-Center-Averaging Method

The Euler-Euler model is widely used in bubbly flow simulations up to industrial dimensions. The standard Euler-Euler model is based on the phase-averaging method. After averaging, the bubble forces in the field equations are functions of the local void fraction. In simulations, when the bubble diameter is larger than the computational cell spacing, the forces can transport the gas belonging to the same bubble in different directions. By contrast, a closure model for the bubble force is typically developed based on the assumption that the force is a resultant force that acts on the bubble’s center of mass. This inconsistency can lead to a nonphysical gas concentration in the center of a channel or near the channel wall if the bubble diameter is larger than the cell spacing. The purpose of the present contribution is to develop an Euler-Euler model where the bubble force consistency is recovered for two-phase flow simulations where the diameter of the disperse phase can be larger than the cell spacing. Such an Euler-Euler model is developed by combining an existing particle-center-averaged Euler-Euler framework with a Gaussian convolution method. To validate this Euler-Euler approach, a comparison is made with experimental data for the bubbly flows in two different vertical pipes. The results show that the proposed Euler-Euler model recovers the bubble force consistency and alleviates the overprediction of the void fraction peak near the wall, while its simulation results in the axial gas and liquid velocity and the liquid turbulence kinetic energy are similar to the results of the standard Euler-Euler model.

A particle-center-averaged Euler-Euler model for monodisperse bubbly flows

The standard Euler-Euler model is based on the phase-averaging method and each bubble force is a function of the local gas volume fraction. As a result, the coherent motion of each bubble as a whole is not enforced when the bubble diameter is larger than the size of a computational cell. However, the bubble force models are typically developed by tracking the bubbles’ centers of mass and assuming that the forces act on these locations. In simulations, this inconsistency can lead to a nonphysical gas concentration in the center or near the wall of a channel when the bubble diameter is larger than the cell size. Besides, a mesh-independent solution may not exist in such cases.In the present contribution, a particle-center-averaging method is used to average the solution variables for the disperse phase, which allows to represent the bubble forces as forces that act on the bubbles’ centers of mass. An Euler-Euler approach for bubbly flow simulation is formed by combining this averaging method with a Gaussian convolution method to represent the spatial extent of the bubbles. The remediation of the inconsistency in the standard Euler-Euler model by the particle-center-averaging method is demonstrated using a simplified two-dimensional test case. The test results illustrate that the particle-center-averaging method can recover the bubble force consistency and provide mesh independent solutions. Furthermore, a comparison of the standard Euler-Euler model and the particle-center-averaged Euler-Euler model is shown for several bubbly pipe flow cases where experimental data are available. The results show that the particle-center-averaging method can alleviate the over-prediction of the gas volume fraction peaks for wall-peaking and finely dispersed flow cases. The gas velocities simulated by both approaches are similar.

Experimental investigation about the lift force of a single bubble in the water at a linear shear flow

The rising motion of a single bubble in water at a linear shear flow was experimentally studied. A curved screen was used to generate a stable low-turbulence linear shear flow field. PIV was used to measure the flow field. Bubbles were generated through different sizes of needles by a syringe pump. A high-speed camera obtained two-dimensional photographs of the rising bubble to get the bubble's instantaneous positions, velocities, and forces through image processing. The lift coefficient CL was obtained by quasi-steady-state analysis. This paper analyzed the lift force characteristics of a single bubble under the present experimental conditions and got the critical size for the bubble's lateral migration direction changing. A lift force empirical correlation was proposed based on the difference of bubbles’ lift force characteristics between high-viscosity fluid and the low-viscosity fluid. The predicted results were in good agreement with the experimental results.

The flow and mass transfer characteristics of concentric gas-liquid flow in an advanced static mixer

The fluid dynamic and mass transfer characteristics of concentric upward gas-liquid flow were studied in an industrial static mixer with four equally spaced helical inserts (FKSM). The numerical simulations of the gas volume fraction in a Kenics mixer was in good agreement with the numerical and experimental results provided by Rabha et al. The characteristics of radial gas void fraction and local mass transfer coefficients in the FKSM were evaluated under different operating conditions. The velocity profiles of the concentric air phase accelerated by the bubble forces first became sharp and narrow until z/l = = -3.27 and then slowly decreased and stabilized at z/l = -1.5 before entering the first mixing element. Some extra unimodal profile of radial gas holdup gradually generated near the rectangle cross-sections of the mixing elements. The ?G gradually enlarged from r/R = 0.2 to r/R = 0.55 and then weakened from r/R = 0.65 to r/R = 0.874. The air void fractions in the bulk flow region decreased with the increasing initial uniform bubble diameter. The inlet effect of the first leading edge enhanced the air phase dispersion and local mass transfer coefficients sharply increased from 2.04 to 3.69 times of that in the inlet. The local mass transfer coefficients in each mixing group had unimodal profiles.

Biocompatible Direct Deposition of Functionalized Nanoparticles Using Shrinking Surface Plasmonic Bubble

AbstractFunctionalized nanoparticles (NPs) are the foundation of diverse applications. Especially, in many biosensing applications, concentrating suspended NPs onto a surface without deteriorating their biofunction is usually an inevitable step to improve detection limit, which remains to be a great challenge. In this work, biocompatible deposition of functionalized NPs to optically transparent surfaces is demonstrated using shrinking bubbles. Leveraging the shrinking phase of bubble mitigates the biomolecule degradation problems encountered in traditional photothermal deposition techniques. The deposited NPs are closely packed, and the functional molecules are able to survive the process as verified by their strong fluorescence signals. Using high‐speed videography, it is revealed that the contracting contact line of the shrinking bubble forces the NPs captured by the contact line to a highly concentrated island. Such shrinking surface bubble deposition (SSBD) is low temperature in nature as no heat is added during the process. Using a hairpin DNA‐functionalized gold NP suspension as a model system, SSBD is shown to enable much stronger fluorescence signal compared to the optical‐pressure deposition and the conventional thermal bubble contact line deposition. The demonstrated SSBD technique capable of directly depositing functionalized NPs may significantly simplify biosensor fabrication and thus benefit a wide range of relevant applications.

Influence of wetting conditions on bubble formation from a submerged orifice

The formation of gas bubbles by submerged orifices is a fundamental process encountered in various industrial applications. The dynamics of the contact line and the contact angle may have a significant influence on the detached bubble size depending on the wettability of the system. In this study, the influence of wetting conditions on the dynamics of bubble formation from a submerged orifice is investigated experimentally and numerically. The experiments are performed using a hydrophobic orifice plate and a series of ethanol–water solutions to vary the wettability where the key characteristics of the bubbles are measured using a high-speed, high-resolution camera. An extensive analysis on the influence of wetting conditions on the bubble size, bubble growth mechanism and the behavior of the contact line is given. Bubble growth stages, termed (1) hemispherical spreading, (2) cylindrical spreading, (3) critical growth and (4) necking, are identified based on key geometrical parameters of the bubble and relevant forces acting on the bubble during the growth. The experimental results show that the apparent contact angle varies in a complicated manner as the bubble grows due to the surface roughness and heterogeneity. The experimental findings are finally used to validate the local front reconstruction method with a contact angle model to account for the contact angle hysteresis observed in the experiments.Graphic abstract

On bubble forces in turbulent channel flows from direct numerical simulations

The prediction of void fraction, which relies on interfacial force models, is a major issue in the context of boiling. The two-fluid model requires the modelling of the momentum transfer between phases. When bubbles are small (particle hypothesis), the momentum transfer is related to interfacial forces acting on bubbles. However, the splitting of these forces into drag, lift, added mass, etc., is not straightforward from the local point of view, where only the total interfacial force is defined as an integral of the constraint over the interface. For large-size bubbles, the particle hypothesis can be questioned. The momentum transfer can then be connected to the forces acting on a fluid element of the vapour phase. Based on the local and averaged formulations of the Navier–Stokes equations, a new balance equation for forces enables us to define lift, drag, added-mass and dispersion forces acting on a fluid element of the vapour phase. This equation gives a local definition for all the forces responsible for spatial distribution of bubbles and reflects the meaning usually assigned to the interfacial forces in the particle approach. Through this means, the link between the local formulation and physical phenomena is established and a new way of modelling the lift force is proposed. Furthermore, a new laminar dispersion force which relies on surface tension and pressure effects is introduced. The analysis of the budget equation on our direct numerical simulation database brings into light the large influence of this laminar dispersion force in the migration process. Different well-known physical behaviours can be modelled via this new force: the horizontal clustering of spherical bubbles in laminar flows and the oscillating trajectories of deformable bubbles.

Investigation of single air bubble dynamics and the effect of nanoparticles in rectangular minichannels

Bubble dynamics and understanding related mechanisms based on force analysis are necessary for better understanding two-phase flow phenomena in small channels. To address this subject, experiments were conducted with injected single air bubbles into rectangular minichannels containing flows of pure water, pure ethanol and TiO2-nanoparticle-based nanofluids, which had a nanoparticle mass fraction of 0.005 wt% for both water and ethanol base fluids. For a range of fluid flow rates, bubble movement and temperature profiles were captured along the channel using high-speed and infrared (IR) cameras, respectively. Upon heating, when using nanofluids, deposition of TiO2 nanoparticles occurred. The results in the channels with cross sectional dimensions of 2 mm × 4 mm and heated length of 7 cm were compared with their counterparts on plain surfaces. Heat fluxes were applied by means of a tantalum film heater on the outer surface of the channel. Bubble dynamics and forces acting on the bubbles were quantitatively analyzed in relation to the fluid type, heat flux, flow rate and deposition. This study highlights the effects of TiO2-nanoparticles (dispersed in two different base fluids) on single-bubble dynamics in minichannels. The nanoparticle deposition was found to have a retarding effect on the bubble movement and led to a more elliptical shape rather than a spherical bubble shape. The bubble behavior is comprehensively assessed in the light of the visualization data and acting forces.

Validation of a closure model framework for turbulent bubbly two-phase flow in different flow situations

Abstract In the present paper a set of closure relations for interphase momentum exchange and for bubble-induced turbulence within the Euler-Euler framework is presented and validated against a set of tests performed at the HZDR-facility MT-Loop. The facility was equipped with wire-mesh sensors that allow cross sectional distributions of gas fraction, gas velocity, and bubble sizes to be measured at different distances from the gas injection. The here applied models were derived for the simulation of fully developed turbulent vertical upward bubbly flow in a pipe. The radial gas fraction profile for this kind of flow is the result of the ratio of the radial force components of the so-called non-drag forces and can be used for model validation. However, only the ratio, not the absolute value of the bubble forces can be tested in this way. In the present paper more detailed information from the experiment is exploited by consideration of the evolution of gas fraction distribution particularly after the gas injection region. The change of cross sectional gas volume fraction distribution is the result of the action of non-drag forces. In addition to vertical pipe flow tests further insight is obtained from the investigation of the effect of a slight tube inclination which shifts the gas distribution. Here the disturbance of the cylindrical symmetry of a vertical pipe gives hints on the absolute value of the non-drag force components. The results show that the presented model framework at least is able to describe the phenomena qualitatively. Possible reasons for quantitative deviations are discussed and require further investigations.

Euler–Euler multiphase CFD-simulation with full Reynolds stress model and anisotropic bubble-induced turbulence

In the present work, Euler–Euler modeling of bubbly flows is combined with a full Reynolds stress model for the turbulence in the liquid carrier phase. Reynolds stress models have only rarely been explored in this context, although effects requiring this level of description are frequently encountered in industrial applications towards which the Euler–Euler approach is geared. In particular, source terms describing the additional bubble-induced contribution to the liquid phase turbulence with proper account for its anisotropy have not firmly been established yet. A formulation based on the direction of bubble motion relative to the liquid is given here. Two well-known variants of Reynolds stress models due to Launder, Reece and Rodi and Speziale, Sarkar and Gatski are compared. Closure relations for the bubble forces are applied that have been shown previously to work well over a range of conditions. The model is validated by comparison with a set of pipe flow data that contains variations of liquid and gas flow rates as well as different pipe diameters. An important criterion for the selection of the data was to provide measurements of individual components of the Reynolds stress tensor.

Towards a unified approach for modelling uniform and non‐uniform bubbly flows

AbstractAn important ingredient of closure relations for the Euler‐Euler two‐fluid model is the description of turbulent fluctuations. Models proposed in the literature disagree concerning the treatment of such on all scales. The large‐scale fluctuation structures as well as the bubble‐induced turbulence might be neglected, resolved, and/or modelled respectively in different ways. Each treatment has been demonstrated to work for a certain application but a unified perspective is lacking so far.To this end a set of closure relations for the fluid dynamics of bubbly flow has been collected that represents the best available knowledge and may serve as a baseline for further improvements and extensions. This model comprises a set of bubble forces as well as a turbulence model including turbulence modification due to the bubbles and has been successfully validated for bubbly flows in pipes and bubble columns. Here it is applied to two sets of data representing non‐uniform and uniform flows in bubble columns which are dominated by large‐scale fluctuations and bubble‐induced turbulence respectively.

Transient force analysis and bubble dynamics during flow boiling in silicon nanowire microchannels

A study on bubble growth mechanisms and underlying physical phenomena of flow boiling in silicon nanowire (SiNW) microchannels has been performed and compared these results with plainwall microchannels. A new approach in studying bubble dynamics and forces acting on liquid–vapor (L–V) interface of growing bubble has been proposed based on theoretical, experimental and visual studies. Bubble size, liquid film thickness, interfacial properties are measured and L–V interfaces are detected from high speed visualization data and results are analyzed by vision-based approach. Force analysis during instantaneous bubble growth from bubble nucleation to formation of annular flow regime has been performed for both the SiNW and plainwall microchannel configurations. Results from force analysis show the dominance of surface tension at L–V interface of growing bubble which resulted higher heat transfer contact area, lower thermal resistance and higher thin film evaporation. Whereas, inertia force is dominant at L–V interface of fully grown bubble and it helps in bubble removal process and rewetting before flow reversal. Significant differences between SiNW and plainwall microchannels have been observed in bubble growth mechanism, heat transfer mode and forces acting on L–V interface.

Evaluation of the physical forces exerted on a spherical bubble inside the nozzle in a cavitating flow with an Eulerian/Lagrangian approach

An Eulerian/Lagrangian approach is used to calculate the physical forces acting on a spherical bubble. Reynolds average Navier–Stokes (RANS) equations for the Eulerian approach are solved with a finite volume scheme. The SIMPLE algorithm is utilized for pressure and velocity linkage. To model convective fluxes, an upwind scheme is used. The Reynolds stress transport model (RSTM) is used to calculate the turbulent parameters. In the Lagrangian approach, a modified form of the Reyleigh–Plesset (RP) and Maxey equations are solved with MATLAB programming software for evaluation of bubble motion and bubble dynamics. The carrying fluid in this study is diesel fuel. Continuous filter white noise (CFWN) is solved parallel to the Maxey and RP equations to calculate fluctuating terms of velocity in x and y directions. Six forces exerted on the bubble during its motion are investigated inside the cavitating flow regime. The cavitating regime can be extremely effective on bubble force and increase bubble forces up to several thousand times. Added mass force in the y direction has the highest value among all forces exerted on the bubble during its motion inside the nozzle.

Baseline Model for the Simulation of Bubbly Flows

AbstractThe bubble size is a key parameter appearing in closure relations for the Euler‐Euler two‐fluid model. The bubble size distribution is established as a result of bubble coalescence and breakup processes. These processes are very complex, and therefore, a two‐step procedure is adopted for model validation where, in a first step, measured values are substituted for the bubble size distribution. In this way, the uncertainties of the less developed modeling for bubble coalescence and breakup are bypassed and a validation of the other parts of the overall model becomes possible. In a second step, the previously qualified models for bubble forces and bubble‐induced turbulence are used without any change and the validity of the models for bubble coalescence and breakup can be assessed.

Bubbly flows with fixed polydispersity: Validation of a baseline closure model

For practical applications the Euler–Euler two-fluid model relies on suitable closure relations describing interfacial exchange processes. In dispersed gas–liquid multiphase flow, closures are needed for bubble forces, bubble-induced turbulence, as well as bubble-coalescence and -breakup. The quest for models with a broad range of applicability allowing predictive simulations is an ongoing venture. Reasonable success has been achieved so far for flows that are amenable to a monodisperse approximation for the bubble size which limits the latter to no more than a few mm.In the present work we extend the validation to flow in which bubbles with a broad distribution of sizes up to ∼10mm are present, but the shape of the distribution remains unchanged during the flow development. The existence of such conditions, which we term “fixed polydispersity”, is deduced from the experimental data. For this kind of situation the complexity of the closure problem is reduced since a balance between bubble-coalescence and -breakup prevails that allows to neglect these processes and simply impose a fixed bubble size distribution. Conclusions towards best practice guidelines for modeling bubbly flows are drawn and needs for further research identified.

Baseline closure model for dispersed bubbly flow: Bubble coalescence and breakup

A set of closures for two-fluid modelling of adiabatic bubbly flows has been defined as baseline model, which provides a common basis for further improvement and development. It includes closures for bubble forces, bubble-induced turbulence as well as bubble coalescence and breakup. In this work, the baseline model is implemented in the commercial code ANSYS CFX 14.5 and applied to the case of adiabatic upward vertical pipe flows. Special attention is paid to the performance of the newly proposed coalescence and breakup model. The comparison with measurements shows that the baseline model is able to capture the evolution of bubble size distribution, gas volume fraction and velocity profiles along the pipe over a wide range of flow conditions.

Transient simulation for large scale flow in bubble columns

The transient simulation of large scale bubbly flow in bubble columns using the unsteady Reynolds averaged Navier Stokes (URANS) equations is investigated in the present paper. An extensive set of bubble forces is used with different models for the bubble induced turbulence. Criteria are given to assess the independence of the simulation time and the time step length. Using these criteria it is shown that a simulation time, time step length and mesh independent solution can be obtained for complex bubbly flows using URANS equations under certain requirements. With the obtained setup the contribution of the resolved turbulence to the total turbulence and the influence of the bubble induced turbulence modeling on the resolved turbulence is investigated. Further, it is pointed out that the virtual mass force is not negligible. The simulations are compared to data from the literature at two different superficial velocities, which cover monodisperse and polydisperse bubbly flows.

Closure models for turbulent bubbly flows: A CFD study

For practical applications the Euler–Euler two-fluid model relies on suitable closure relations describing interfacial exchange processes. In dispersed gas–liquid multiphase flow, bubble-induced turbulence is one such process for which a satisfactory model is still not available. A common approach to its solution is to add source terms to the single phase two-equation turbulence models. Here we report a comparison of different models of this type some of which have been used previously, some of which are new. To qualify the validity of the different models a set of reference data has been selected from the literature. Together with a suitable model for the bubble forces the most promising variants can be identified. Special attention in this respect is given to the wall force. Guidelines for modeling bubbly turbulence are proposed and needs for further research identified.

CFD simulation of DEBORA boiling experiments

CFD simulation of DEBORA boiling experimentsIn this work we investigate the present capabilities of computational fluid dynamics for wall boiling. The computational model used combines the Euler/Euler two-phase flow description with heat flux partitioning. This kind of modeling was previously applied to boiling water under high pressure conditions relevant to nuclear power systems. Similar conditions in terms of the relevant non-dimensional numbers have been realized in the DEBORA tests using dichlorodifluoromethane (R12) as the working fluid. This facilitated measurements of radial profiles for gas volume fraction, gas velocity, bubble size and liquid temperature as well as axial profiles of wall temperature. After reviewing the theoretical and experimental basis of correlations used in the ANSYS CFX model used for the calculations, we give a careful assessment of the necessary recalibrations to describe the DEBORA tests. The basic CFX model is validated by a detailed comparison to the experimental data for two selected test cases. Simulations with a single set of calibrated parameters are found to give reasonable quantitative agreement with the data for several tests within a certain range of conditions and reproduce the observed tendencies correctly. Several model refinements are then presented each of which is designed to improve one of the remaining deviations between simulation and measurements. Specifically we consider a homogeneous MUSIG model for the bubble size, modified bubble forces, a wall function for turbulent boiling flow and a partial slip boundary condition for the liquid phase. Finally, needs for further model developments are identified and promising directions discussed.