Undulations on the surface of elongated bubbles in confined gas-liquid flows

© 2017 American Physical Society. A systematic analysis is presented of the undulations appearing on the surface of long bubbles in confined gas-liquid flows. CFD simulations of the flow are performed with a self-improved version of the open-source solver ESI OpenFOAM (release 2.3.1), for Ca=0.002-0.1 and Re=0.1-1000, where Ca=μU/σ and Re=2ρUR/μ, with μ and ρ being, respectively, the viscosity and density of the liquid, σ the surface tension, U the bubble velocity, and R the tube radius. A model, based on an extension of the classical axisymmetric Bretherton theory, accounting for inertia and for the curvature of the tube's wall, is adopted to better understand the CFD results. The thickness of the liquid film, and the wavelength and decay rate of the undulations extracted from the CFD simulations, agree well with those obtained with the theoretical model. Inertial effects appear when the Weber number of the flow We=CaRe=O(10-1) and are manifest by a larger number of undulation crests that become evident on the surface of the rear meniscus of the bubble. This study demonstrates that the necessary bubble length for a flat liquid film region to exist between the rear and front menisci rapidly increases above 10R when Ca>0.01 and the value of the Reynolds number approaches 1000

Pore-scale analysis of the minimum liquid film thickness around elongated bubbles in confined gas-liquid flows

© 2017 Elsevier Ltd The fluid mechanics of elongated bubbles in confined gas-liquid flows in micro-geometries is important in pore-scale flow processes for enhanced oil recovery and mobilization of colloids in unsaturated soil. The efficiency of such processes is traditionally related to the thickness of the liquid film trapped between the elongated bubble and the pore's wall, which is assumed constant. However, the surface of long bubbles presents undulations in the vicinity of the rear meniscus, which may significantly decrease the local thickness of the liquid film, thus impacting the process of interest. This study presents a systematic analysis of these undulations and the minimum film thickness induced in the range Ca=0.001-0.5 and Re=0.1-2000. Pore-scale Computational Fluid Dynamics (CFD) simulations are performed with a self-improved version of the opensource solver ESI OpenFOAM which is based on a Volume of Fluid method to track the gas-liquid interface. A lubrication model based on the extension of the classical axisymmetric Bretherton theory is utilized to better understand the CFD results. The profiles of the rear meniscus of the bubble obtained with the lubrication model agree fairly well with those extracted from the CFD simulations. This study shows that the Weber number of the flow, We=CaRe, is the parameter that best describes the dynamics of the interfacial waves. When We 0.1, a larger number of wave crests becomes evident on the surface of the rear meniscus of the bubble. The liquid film thickness at the crests of the undulations thins considerably as the Reynolds number is increased, down to less than 60% of the value measured in the flat film region. This may significantly influence important environmental processes, such as the detachment and mobilization of micron-sized pollutants and pathogenic micro-organisms adhering at the pore's wall in unsaturated soil

Dynamics of long gas bubbles rising in a vertical tube in a cocurrent liquid flow

© 2019 American Physical Society. When a confined long gas bubble rises in a vertical tube in a cocurrent liquid flow, its translational velocity is the result of both buoyancy and mean motion of the liquid. A thin film of liquid is formed on the tube wall and its thickness is determined by the interplay of viscous, inertial, capillary and buoyancy effects, as defined by the values of the Bond number (Bo≡ρgR2/σ with ρ being the liquid density, g the gravitational acceleration, R the tube radius, and σ the surface tension), capillary number (Cab≡μUb/σ with Ub being the bubble velocity and μ the liquid dynamic viscosity), and Reynolds number (Reb≡2ρUbR/μ). We perform experiments and numerical simulations to investigate systematically the effect of buoyancy (Bo=0-5) on the shape and velocity of the bubble and on the thickness of the liquid film for Cab=10-3-10-1 and Reb=10-2-103. A theoretical model, based on an extension of Bretherton's lubrication theory, is developed and utilized for parametric analyses; its predictions compare well with the experimental and numerical data. This study shows that buoyancy effects on bubbles rising in a cocurrent liquid flow make the liquid film thicker and the bubble rise faster, when compared to the negligible gravity case. In particular, gravitational forces impact considerably the bubble dynamics already when B

Coupled atomistic–continuum simulations of nucleate boiling

Boiling is a striking example of a multiscale process, where the dynamics of bubbles is governed by the interplay between the molecular interactions responsible for nucleation, and the macroscale hydrodynamic and thermal boundary layers. A complete description of this phenomenon requires coupling molecular- and continuum-scale fluid mechanics into a single modelling framework. This article presents a hybrid atomistic–continuum computational model for coupled simulations of nucleate boiling. A domain decomposition coupling method is utilised, where the near-wall region is solved by a Molecular Dynamics description, which handles nucleation and the moving contact lines, while the bulk flow region is solved by a continuum-scale description based on the Navier–Stokes equations. The latter employs a Volume Of Fluid method to track the evolution of the liquid–vapour interface and the interphase mass transfer is computed via the Hertz–Knudsen–Schrage relationship. Boiling of a Lennard-Jones fluid over a heated wall is simulated and the hybrid solution is validated against a fully molecular solution. The results obtained with the coupled framework in terms of time-dependent bubble volume, phase-change rates, bubble dynamics and evolution of the temperature field agree quantitatively with those achieved by a MD-only simulation. The coupled framework reproduces the bubble growth rate over time from nucleation until a bubble diameter of about 70 nm, demonstrating the accuracy and robustness of the coupling architecture. This also demonstrates that the fluid dynamics description based on the Navier–Stokes equations is capable of correctly capturing the main heat and mass transfer mechanisms responsible for bubble growth at the nanoscale. The proposed modelling framework paves the way towards multiscale simulations of boiling, where the necessary molecular-level physics is retained in a computational fluid dynamics solver

How service design cues help in service failures

Terres, M. D. S., Herter, M. M., Pinto, D. C., & Mazzon, J. A. (2020). The power of sophistication: How service design cues help in service failures. Journal of Consumer Behaviour, 19(3), 277-290. https://doi.org/10.1002/cb.1816By analyzing three experimental studies, this research tests how and when sophisticated service environment designs (compared to modest service designs) can minimize consumers' negative emotions and increase repurchase intentions after a failure. Drawing on part-list cueing literature, this research proposes that when a service failure occurs in a sophisticated (vs. modest) environment, consumers will rely on the sophisticated style of design as cues for service quality. We argue that sophisticated (vs. modest) service designs work as strong cues for quality that restrict the retrieval of negative information by consumers and can minimize the negative impacts of service failure, reducing consumers' negative emotions and increasing repurchase intentions. We further advance our theorizing by showing how choice failure consequences (i.e., the risk or consequence related to the service choice) moderate the effects via associative pathways of retrieval. The findings contribute to theory and practice by revealing how service designs can serve as cues to mitigate adverse consequences of service failure.authorsversionpublishe