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The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with je...
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The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with jetting and protrusion, are captured using a high-speed camera. Our experimental observations reveal the behaviour of the bubble in terms of the corner angle and the dimensionless standoff distances to the near and far walls in terms of the maximum bubble radius. The bubble remains approximately spherical during expansion except for its surface becoming flattened when in close proximity to a wall. When a bubble is initiated at the bisector of the two walls, the bubble becomes oblate along the bisector during the late stages of collapse. A jet forms towards the end of collapse, pointing to the corner. The closer the bubble to the two walls, the more oblate along the bisector the bubble becomes, and the wider the jet. A bubble initiated near one of the two walls is mainly influenced by the nearer wall. The jet formed is pointing to the near wall but inclined towards the corner. After the jet penetrates through the bubble surface, the bubble becomes a bubble ring, and a bubble protrusion forms following the jet. The bubble ring collapses and subsequently disappears, while the protrusion firstly expands, and then collapses and migrates to the corner.
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We report an experimental study of bubble dynamics in a non-Newtonian fluid subjected to a pressure decrease. The fluid is a hydrogel, composed ofwater and a synthetic clay, prepared and sandwiched between two glass plates in a He...
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We report an experimental study of bubble dynamics in a non-Newtonian fluid subjected to a pressure decrease. The fluid is a hydrogel, composed ofwater and a synthetic clay, prepared and sandwiched between two glass plates in a Hele-Shawgeometry. The rheological properties of the material can be tuned by the clay concentration. As the imposed pressure decreases, the gas initially dissolved in the hydrogel triggers bubble formation. Different stages of the process are observed: bubble nucleation, growth, interaction, and creation of domains by bubble contact or coalescence. Initially bubble behave independently. They are trapped and advected by the mean deformation of the hydrogel, and the bubble growth is mainly driven by the diffusion of the dissolved gas through the hydrogel and its outgassing at the reactive-advected hydrogel-bubble interface. In this regime, the rheology of the fluid does not play a significant role on the bubble growth. A model is proposed and gives a simple scaling that relates the bubble growth rate and the imposed pressure. Carbon dioxide is shown to be the gas at play, and the hydrogel is degassing at the millimeter scale as a water solution does at a smaller scale. Later, bubbles are not independent anymore. The growth rate decreases, and the morphology becomes more anisotropic as bubbles interact because they are separated by a distance smaller than the individual stress field extension. Our measurements show that the interaction distance scales with the bubbles' size.
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This paper is concerned with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water. The behaviour of the bubble was found to be sensitive...
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This paper is concerned with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water. The behaviour of the bubble was found to be sensitive to two geometrical parameters: the dimensionless tube radius alpha = R-T/R-Bmax and the dimensionless eccentricity epsilon = E/R-Bmax, where R-T is the inner radius of the tube, E is the distance from the bubble center at inception to the axis of symmetry of the tube, and R-Bmax is the maximum equivalent bubble radius (similar to 10 mm). The expansion, collapse and rebound of the bubble were captured using a high-speed camera both for the case where alpha > 1 and alpha < 1, with and without eccentricity, respectively. Some new features of the bubble dynamics were observed. In particular, a bubble initiated with eccentricity in a tube for which alpha < 1 was seen to migrate to the distal part of the tube at the end of collapse with formation of a jet also in that direction. This is distinct from the case of a bubble collapsing near a flat surface. A similar phenomenon has been observed previously in the case of a microbubble collapsing in a blood vessel under ultrasound excitation, but was attributed to the elasticity of the vessel wall. The present study suggests that it may in fact be due simply to the geometry of the system. A cloud of microbubbles was observed shortly after the start of rebound of the bubble. Our analysis shows that the microbubbles should be generated from nuclei in tap water with radii in the range of 10(-8) -10(-6) m. (C) 2019 Elsevier Ltd. All rights reserved.
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The synchronization between the air pressure fluctuations and the depth of liquid penetration into the nozzle during bubble departures was investigated using joint recurrence quantification analysis. In the experiment, the bubbles...
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The synchronization between the air pressure fluctuations and the depth of liquid penetration into the nozzle during bubble departures was investigated using joint recurrence quantification analysis. In the experiment, the bubbles were generated from a glass nozzle into distilled water. During the analysis, the recurrent rate coefficients were calculated for the depth of liquid penetration into the glass nozzle and pressure changes in the gas supply system. The study was conducted by two air volume flow rates, i.e. 0.023 l/min and 0.026 l/min. The air volume flow rates were selected so that the appearance and disappearance of period bubble departures were clearly visible. It has been shown that the synchronization of the pressure changes and the depth of liquid penetration appears when periodic changes in the depth of liquid penetration occur in a relatively long period of time. The process of changing the distance between the extremes of liquid penetration into the nozzle and pressure changes in the gas supply system was observed. It has been found that the decrease in the distance between these extremes is responsible for the appearance of periodic bubble departures. This behaviour has not been reported in previous papers. This process was modelled by numerical simulations.
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Directional solidification experiments were performed using succinonitrile saturated with nitrogen gas to examine the effects of in situ processing pressure changes on the formation, growth, and evolution of an isolated, cylindric...
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Directional solidification experiments were performed using succinonitrile saturated with nitrogen gas to examine the effects of in situ processing pressure changes on the formation, growth, and evolution of an isolated, cylindrical gaseous pore. A novel solidification facility capable of processing small cylindrical samples (I.D.≤1.0mm), under controlled pressure conditions, was developed for the experiments. A new experimental method for growing the isolated pore from a seed bubble is introduced. The experimental results indicate that a step-wise processing pressure change will result in either a transient change in pore diameter or a complete termination of pore growth, demonstrating that pressure changes can be used as a control parameter to influence bubble growth. During steady-state growth, however, pore size shows no dependence on processing pressure. A simple analytical model has been introduced to explain the experimental observations.
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The effective removal of carbon dioxide (CO_2) in the anode microchannels is of great importance to the performance of a micro direct methanol fuel cell (uDMFC). The gathered bubbles block part of the meth-anol mass transfer area ...
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The effective removal of carbon dioxide (CO_2) in the anode microchannels is of great importance to the performance of a micro direct methanol fuel cell (uDMFC). The gathered bubbles block part of the meth-anol mass transfer area from anode flow field to catalyst layer, causing a reverse effect on cell performance. Unlike the flow in the straight channel, the gas emission at the corner is more critical because of the corner effect. The work investigates the two-phase transport between CO_2 bubble dynamics and micro-fluid at the corner from a Mesoscopic perspective. A two-dimensional, nine-velocity Lattice-Boltzmann model coupled with surface tension, buoyancy force and the fluid-solid interaction force is adopted in this work to simulate the dynamics of micro flow. Simulation results indicate that the emission speed of CO_2 gas patterned with the circular corner is faster than that with orthogonal corner. To verify the simulation results, a visual study of the CO_2 bubble behavior in the micro channels of a transparent uDMFC is conducted, and the cells patterned with different structures are tested. Compared with the cell with orthogonal corner, the cell patterned with circular corner exhibits a substantial increase of 21% in peak power density.
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This paper is concerned with the bursting of a large bubble at a free surface. The numerical modelling is based on the boundary integral method. To validate the numerical model, experiments are carried out for the bursting of a sp...
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This paper is concerned with the bursting of a large bubble at a free surface. The numerical modelling is based on the boundary integral method. To validate the numerical model, experiments are carried out for the bursting of a spark generated bubble at a free surface in a low pressure tank, captured by using a high speed camera. Our numerical results agree qualitatively with the experiments. We further carry out numerical analysis for the bursting of an underwater explosion bubble at a free surface. We have considered the bursting of singly connected bubbles as well as toroidal bubbles. When a bubble is initiated very close to a free surface, the bursting occurs during the expansion phase, thus, resulting in a cone shaped spike at the free surface. However, when the bubble is initiated away from the free surface, it expands and collapses below the free surface and rises to the free surface due to buoyancy. An upward liquid jet forms during the later stage of collapse, which subsequently penetrates through the bubble. The toroidal bubble rises and bursts at the free surface, which results in a much higher water column due to the high speed bubble jet. (C) 2015 Elsevier Ltd. All rights reserved.
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Numerical and experimental studies of the dynamics of a cavitating bubble near a resilient metal surface were performed. To augment the experimental flow visualizations of a collapsing bubble, numerical simulations were conducted ...
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Numerical and experimental studies of the dynamics of a cavitating bubble near a resilient metal surface were performed. To augment the experimental flow visualizations of a collapsing bubble, numerical simulations were conducted to more thoroughly identify the collapse dynamics and analyze the flow. A bubble collapse was captured using a high-speed camera and back illumination. The metal sample was made of pure aluminum placed near a collapsing cavitation bubble at various distances from the metal surface. Width, depth, and volume of the induced material deformations were measured using an optical microscope and a three-dimensional profilometer and then compared against existing experimental data from the literature. The cavitating bubble's dynamics and the related flow were simulated numerically using the open source finite volume based flow solver CavitatingFOAM. This code solved the Navier-Stokes equations for compressible two-phase flows using an Euler-Euler approach, including the barotropic equations of state. Bubble shapes, collapse times, and obtained damage parameters were compared to experimental observations. Impact velocities, pressures, shear rates, and various flow phenomena were discussed, providing broad insight into bubble dynamics and the induced damage. (C) 2019 Elsevier Ltd. All rights reserved.
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Three-dimensional dynamics of a transient bubble inside a corner formed by two rigid curved parabolic plates (walls) is studied numerically using boundary integral method (BIM) based on the potential flow theory. The bubble dynami...
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Three-dimensional dynamics of a transient bubble inside a corner formed by two rigid curved parabolic plates (walls) is studied numerically using boundary integral method (BIM) based on the potential flow theory. The bubble dynamics, including the expansion and collapse phases until the jet impact, are investigated for different corner angles associated with different focal lengths k of the parabolas. However, for all the simulations, the dimensionless initial vertical standoff distance of the bubble's center from the corner edge (h*) is fixed at 4. The bubble remains almost spherical during expansion, except for parts of its surface that flattens near the walls. When the bubble is initiated at the bisector plane of the two intersecting walls, it oscillates symmetrically with respect to the bisector plane and becomes oblate during the late stages of the collapse phase. A high-speed liquid jet forms towards the end of bubble collapse, pointing to the corner. If the corner angle decreases, the bubble becomes more oblate along the bisector plane making the ensuing liquid jet wider and slower. In addition, a bubble initiated closer to one of the two walls is mainly influenced by the closer wall, oscillates non-symmetrically with respect to the bisector plane and the liquid jet formed in this case is inclined towards the closer wall due to the greater Bjerknes force of that wall.
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Structured enhanced surfaces are widely used to promote nucleate boiling heat transfer in refrigeration and process industries. Despite the wide usage of the enhanced surfaces, there appears a significant lack of understanding on ...
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Structured enhanced surfaces are widely used to promote nucleate boiling heat transfer in refrigeration and process industries. Despite the wide usage of the enhanced surfaces, there appears a significant lack of understanding on the boiling mechanism or predictive models. In this study, bubble dynamic data -bubble departure diameter, bubble generation frequency, nucleation site density - were obtained for R-134a boiling on horizontal surfaces having circular pores. Nine samples included pore diameters from 0.1 mm to 0.3 mm and pore pitches from 0.75 mm to 3.0 mm. Tests were conducted at 4.4 ℃ and 26.7 ℃ saturation temperatures. Results showed that bubble departure diameter increased as pore diameter increased, or pore pitch and saturation temperature decreased. Furthermore, nucleation site density increased as heat flux increased or pore pitch decreased. Bubble generation frequency generally increased as pore diameter or pore pitch increased. The existing bubble dynamic models did not adequately predict the present bubble dynamic data, and a new model was developed.
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