Large eddy simulation of microbubble dispersion and flow field modulation in vertical channel flows

Turbulent liquid–gas vertical channel flows laden with microbubbles are investigated using large eddy simulation (LES) two-way coupled to a Lagrangian bubble tracking technique. Upward and downward flows at shear Reynolds numbers of Re τ = 150 and 590 are analyzed for three different microbubble diameters of 110, 220, and 330 μm. Predicted results are compared with published direct numerical simulation results although, with respect to comparable studies available in the literature, the range of bubble diameters and shear Reynolds numbers considered herein is extended to larger values. Microbubble concentration profiles are analyzed, with the microbubbles segregating at the wall in upflow conditions and moving toward the channel centre in downflow. The various forces acting on the bubbles, and the effect of the flow turbulence on the bubble concentration, are considered and quantified. Overall, the results suggest that the level of detail achievable with LES is sufficient to predict the fluid structures impacting bubble behavior. Therefore, LES coupled with Lagrangian bubble tracking shows promise for enabling the reliable prediction of bubble-laden flows that are of industrial relevance

Dynamics of a two-dimensional upflowing mixing layer seeded with bubbles : bubble dispersion and effect of two-way coupling

The evolution and structure of a spatially evolving two-dimensional mixing layer seeded with small bubbles are numerically investigated. The one-way coupling approach is first employed to show that characteristics of bubble dispersion are dominated by the possibility for sufficiently small bubbles to be captured in the core of the vortices. A stability analysis of the ODE system governing bubble trajectories reveals that this entrapment process is governed by the presence of stable fixed points advected by the mean flow. Two-way coupling simulations are then carried out to study how the global features of a two-dimensional flow are affected by bubble-induced disturbances. The local interaction mechanism between the two phases is first analyzed using detailed simulations of a single bubbly vortex. The stability of the corresponding fixed point is found to be altered by the collective motion of bubbles. For trapped bubbles, the interphase momentum transfer yields periodic sequences of entrapment, local reduction of velocity gradients, and eventually escape of bubbles. Similar mechanisms are found to take place in the spatially-evolving mixing layer. The presence of bubbles is also found to enhance the destabilization of the inlet velocity profile and to shorten the time required for the roll-up phenomenon to occur. The most spectacular effects of small bubbles on the large-scale flow are a global tilting of the mixing layer centerline towards the low-velocity side and a strong increase of its spreading rate. In contrast, no significant modification of the flow is observed when the bubbles are not captured in the large-scale vortices, which occurs when the bubble characteristics are such that the drift parameter defined in the text exceeds a critical value. These two contrasted behaviors agree with available experimental results

Metallic superhydrophobic surfaces via thermal sensitization

Superhydrophobic surfaces (i.e., surfaces extremely repellent to water) allow water droplets to bead up and easily roll off from the surface. While a few methods have been developed to fabricate metallic superhydrophobic surfaces, these methods typically involve expensive equipment, environmental hazards, or multi-step processes. In this work, we developed a universal, scalable, solvent-free, one-step methodology based on thermal sensitization to create appropriate surface texture and fabricate metallic superhydrophobic surfaces. To demonstrate the feasibility of our methodology and elucidate the underlying mechanism, we fabricated superhydrophobic surfaces using ferritic (430) and austenitic (316) stainless steels (representative alloys) with roll off angles as low as 4° and 7°, respectively. We envision that our approach will enable the fabrication of superhydrophobic metal alloys for a wide range of civilian and military applications

On Contact Line Region Heat Transfer, Bubble Dynamics and Substrate Effects during Boiling

Rapid advancement of electronics used in domestic, commercial and military applications has necessitated the development of thermal management solutions capable of dissipating large amounts of heat in a reliable and efficient manner. Traditional methods of cooling, including air and liquid cooling, require large fluid flow rates and temperature differences to remove high heat fluxes and are therefore unsuited for many advanced applications. Phase change heat transfer, specifically boiling, is capable of dissipating large heat fluxes with low temperature gradients and hence is an attractive technique for cooling high heat flux applications. However, due to the complex interactions between the fluid dynamics, heat transfer, and surface chemistry, the fundamental physics associated with boiling is not completely understood. The focus of this work is to get a better understanding of the role played by a nucleating bubble in removing the heat from the substrate. The interfacial forces acting on a bubble, contact line motion, and the thermal interaction with the heater surfaces are some of the important considerations which have not been well understood in literature. The work reported in this dissertation is divided into three parts. In the first part, an analytical study of the effect of evaporation momentum force on bubble growth rate and bubble trajectory was conducted. It was shown that the trajectory of a bubble can be controlled by creating an asymmetric temperature field. This understanding was used to develop a bubble diverter that increased the Critical Heat Flux (CHF) over a horizontal tubular surface by 60% and improved the heat transfer coefficient by 75%. In the second part of the work, additional contact line regions were generated using microgrooves. This enhancement technique increased the CHF with water by 46% over a plain copper surface to 187 W/cm2. Finally, the effect of the heater properties and surface fouling during boiling was evaluated. This included a study on the effect of thermophysical properties of the heater surface on CHF and an investigation of fouling over a heater surface during boiling of seawater

Enhanced Pool Boiling Heat Transfer by Flow Modulation and Contact Line Augmentation over Cylindrical Tubes

The miniaturization trend in electronics has spurred the development of efficient thermal management solutions. Single phase techniques are reliable but are limited by large fluid temperature differences and pressure drop. Two phase cooling has very little pressure drop with large heat absorption capacity. Boiling stands out as one of the most effective methods of heat dissipation which utilizes phase change. However, the design of two-phase systems is limited by the critical heat flux condition where a vapor layer prevents the liquid from contacting the heater surface. The current research study is directed towards increasing the CHF and maintaining low wall superheats to design efficient heat removal systems. In this study, different surface modification techniques are studied with an aim to identify various mechanisms that affect the heat transfer. Different surface enhancements in the form of Circumferential rectangular microchannels(CRM) and fin are used over cylindrical surface. Cylindrical tube with outer diameter of 15 mm was used for testing with water as working fluid. Tubular surface with fin attached performed the best yielding the CHF of 115 W/cm2 at wall superheat of 18oC which translated to an enhancement of 76%. The best performance of 110 W/cm2 at 9 oC without reaching CHF was obtained amongst CRM. Different mechanisms were identified by analyzing the results from pool boiling experiments. Area enhancement and contact line substantially affected the heat transfer performance in CRM. Area enhancement increased performance by providing additional area for heat transfer. Contact line region has higher heat flux. Single bubble growing over multiple grooves has increased contact line density which increases heat transfer 4 performance. Increment in CHF was obtained by employing any one of these surface enhancements. High speed imaging enabled to analyze the behavior of bubble after nucleation on the fin surface thus deciphering the flow modulation over the cylindrical surface. Presence of bubble diverter at the bottom surface ensured higher evaporative momentum force towards the cylindrical surface. This displaced nucleating bubble at the bottom away from the fin, enabling liquid to rewet the surface. This allowed the formation of separate liquid vapor pathways which resulted in increased performance

Editing smoke animation using a deforming grid

Abstract We present a new method for editing smoke animations by directly deforming the grid used for simulation. We present a modification to the widely used semi-Lagrangian advection operator and use it to transfer the deformation from the grid to the smoke body. Our modified operator bends the smoke particle streamlines according to the deformation gradient. We demonstrate that the controlled smoke animation preserves the fine-grained vortical velocity components and incompressibility constraints, while conforming to the deformed grid. Moreover, our approach enables interactive 3D smoke animation editing by using a reduced-dimensional subspace. Overall, our method makes it possible to use current mesh editing tools to control the smoke body

Simulation of Microbubble Dynamics in Turbulent Channel Flows

This work investigates microbubble dynamics in four-way (with coalescence) coupled microbubble-laden turbulent channel flows. Upward and downward flows of water at a shear Reynolds number of Reτ = 150 are predicted using direct numerical simulation (DNS). Microbubbles, assumed to be non-deformable and spherical, are injected into the water flow and tracked using a Lagrangian approach. One-way and two-way coupled predictions were successfully compared against other available DNS-based results and used to demonstrate different trends in bubble preferential motion, with bubbles pushed by the lift force towards the wall in upflow and towards the centre of the channel in downflow. Four-way coupled simulations with bubble coalescence clearly demonstrate that the presence of the bubbles, and collisions between them, have a non-negligible effect on the fluid phase. Analysis of bubble collision behaviour highlights that binary collisions most frequently occur at very small approach angles and with low relative approach velocities. Once a collision is detected, the occurrence of bubble coalescence is evaluated, with special attention given to the performance of different bubble coalescence models. The film drainage model returns a 100% coalescence efficiency, while on the other hand the energy model returns a 0% coalescence efficiency, with this large discrepancy requiring further investigation and model development. The knowledge gained from the present results on the mechanisms that underpin bubble collisions is of value to the further development of more advanced coalescence closure models

Multiscale Mechanistic Approach to Enhance Pool Boiling Performance for High Heat Flux Applications

The advent of cloud computing and the complex packaging architecture of next generation electronic devices drives methods for advanced thermal management solutions. Convection based single-phase cooling systems are inefficient due to their large pressure drops, fluid temperature differences and costs, and are incapable of meeting the cooling requirements in the high power density components and systems. Alternatively, phase-change cooling techniques are attractive due to their ability to remove large amounts of heat while maintaining uniform fluid temperatures. Pool boiling heat transfer mechanism centers on the nucleation, growth and departure of a bubble from the heat transfer surface in a stagnant pool of liquid. The pool boiling performance is quantified by the Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC) which dictate the operating ranges and efficiency of the heat transfer process. In this work, three novel geometries are introduced to modify the nucleation characteristics, liquid pathways and contact line motion on the prime heater surface for a simultaneous increase in CHF and HTC. First, sintered microchannels and nucleating region with feeder channels (NRFC) were developed through the mechanistic concept of separate liquid-vapor pathways and enhanced macroconvection heat transfer. A maximum CHF of 420 W/cm2 at a wall superheat of 1.7 °C with a HTC of 2900 MW/m2°C was achieved with the sintered-channels configuration, while the NRFC reached a CHF of 394 W/cm2 with a HTC of 713 kW/m2°C. Second, the scale effect of liquid wettability, roughness and microlayer evaporation was exploited to facilitate capillary wicking in graphene through interlaced porous copper particles. A CHF of 220 W/cm2 with a HTC of 155 kW/m2°C was achieved using an electrodeposition coating technique. Third, the chemical heterogeneity on nanoscale coatings was shown to increase the contribution from transient conduction mechanisms. A maximum CHF of 226 W/cm2 with a HTC of 107 kW/m2°C was achieved. The enhancement techniques developed here provide a mechanistic tool at the microscale and nanoscale to increase the boiling CHF and HTC

Numerical Simulation of Gas-Liquid Bubbly Flows

Gas-liquid bubbly flows exist in many engineering processes. However, limitations in understanding prevent the optimal design and operation of multiphase equipment. The bubble size distribution is a key parameter in such flows as it governs the interfacial area and the rate of exchange of mass, momentum and energy between the phases. Evolution of the bubble population is to a large extent driven by the coalescence and breakup of bubbles. Due to the lack of experimental studies of these phenomena, accurate predictions from numerical models are of value in improving understanding, and for use in developing engineering models. The work described furthers our insight of and ability to predict bubbly flows by combining large eddy simulation and Lagrangian bubble tracking. Horizontal and vertical channel flows of water over a range of shear Reynolds numbers and air bubble diameters are considered. Coalescence and breakup are favoured in upflows, with high turbulence levels impacting bubble interaction. Coalescence is dominant at low turbulence levels, and increases with decreasing bubble size, whereas breakup is favoured at high turbulence levels. The breakup of air bubbles, under the flow conditions studied, is almost negligible. The simulations are therefore extended to bubbles of refrigerant R134a, with a considerably lower surface tension than air bubbles, with significant levels of breakup detected at high Reynolds numbers. The investigation is a novel contribution to the literature and provides a comprehensive study of next generation predictive techniques. The model developed can predict microbubble behaviour in turbulent flows up to the level of four-way coupling, where inter-bubble collisions, coalescence and breakup are accounted for. Its application extends existing knowledge of these flows, including the effect of bubbles on the carrier fluid. Overall, the tool developed and the understanding generated are of value to industry in allowing the design of more efficient flow processes

Direct numerical simulations of bubbly flows

The work presented considers direct numerical simulation (DNS) based studies of bubbly flows in turbulent channel and pipe flows. A variety of bubble sizes are simulated by means of Lagrangian tracking (LPT) and the volume of fluid (VOF) approaches to gain an in-depth knowledge of the physical phenomena involved in, and provide a predictive capability for, turbulent bubbly flows. DNS coupled with LPT is used to investigate microbubble dynamics in four-way coupled channel and pipe flows. Microbubbles, assumed to be non-deformable and spherical, in channel flows show that one- and two-way coupled predictions demonstrate different trends, with microbubbles pushed by the lift force towards the channel wall in upflow and towards its centre in downflow. Analysis of bubble collision behaviour highlights that binary collisions most frequently occur at very small approach angles and with low relative approach velocities. This trend is confirmed in pipe flows, with bubble coalescence in both geometries predicted using film drainage and energy-based models. DNS of turbulent channel flow with large deformable bubbles is studied using the VOF method. The motion of single bubbles is considered, with 8 mm bubbles travelling in roughly rectilinear paths in upflow owing to their higher deformability than smaller bubbles which move towards the walls. A method for estimating the drag coefficient is proposed, with good agreement found with experimental data. Time-averaged liquid turbulence statistics are evaluated to quantify bubble-induced turbulence, and bubble clustering is analysed by simulating bubble swarms with a horizontal alignment of colliding bubbles found. The work reported contributes new understanding of the phenomena involved in bubbly flows down to the smallest scales, with the VOF method, in particular, allowing highly accurate results to be generated that improve understanding. The DNS-LPT technique, for the foreseeable future, will remain the main predictive tool for modelling practically relevant bubbly flows