Bubble Formation, Detachment, Rising & Collapse; VOF Method

Influence of fluid properties on bubble formation, detachment, rising and collapse; Investigation using volume of fluid methodIn various applications of multi-phase flows, a fundamental understanding of the physics for the case of a bubble rising and deforming in a quiescent viscous liquid is essential. Herein, the bubble shapeshave a tendency to vary greatly, depending on where the bubbleslie within the different flow regimes. The bubble rising behaviorscan usually be correlated against four non-dimensional parameterssuch as the Morton number which is defined as [24]:l2gρ DBo -------------σ(12)l 4g( μ )M ------------l 3ρσ(13)The Froude number defined as [29]:2UFr -------ogdo(14)and the Reynolds number defined as:lρ UDRe -----------lμ(15)where, D and do represent the diameter of the bubble and the orificediameter, respectively. The Bond number represents the contributionof the effects of surface tension and buoyancy, whereas the Mortonnumber, which is sometimes referred to as the property group, measures the relative importance of viscous and surface tension forces.Following similar definition, the Reynolds number signifies the contribution between the inertia and viscous effects. Note that mostFig. 1. The solution area of rising single bubble and initial releasedbubble made with 9600 meshes (mesh size 0.25 mm).1351experimental results on bubble rising in liquid are presented usingthe Reynolds number based on the measured bubble terminal risingvelocity (U ), which is given by Reexp ρ lDU /μ l.3. Solution Method, Mesh Dependency and Model GeometryTo find the dependency of the simulation results on the mesh size,a cylindrical bubble column with 20 mm width and 30 mm heightwas simulated in Cartesian coordinate system as shown in Fig. 1.Initially, a static bubble with 4 mm diameter was released and allowed to rise in the stagnant liquid. The model geometry was builtand meshed using GAMBIT and then imported into FLUENT forflow calculations. For the purpose of finding how many computational cells on the solution domain were required to solve the continuity, momentum and volume of fluid function equations, five differentmesh sizes were considered. The grid sizes were 0.2 mm 0.2 mm,0.25 mm 0.25 mm, 0.3 mm 0.3 mm, 0.35 mm 0.35 mm, 0.4 mm 0.4 mm, and the corresponding numbers of grid points were 3825,4902, 6700, 9600, 15150, respectively. The time step was set as0.0001 s.The finite volume method implicit iteration was used to solvethe continuity, momentum and volume of fluid function equations.The first-order upwind scheme was applied to the discretization ofthe flow equations. This scheme is more stable than the secondorder upwind scheme and is used most often with reasonable accuracy [12]. The pressure-velocity coupling was carried out usingthe pressure-implicit with splitting of operators (PISO) method pressure discretization method was pressure staggering option (PRESTO).The numerical simulations were executed on the software platform of Fluent 6.3 running on a high performance 24-core supercomputer. The solution area made with 9600 cells (mesh size 0.25mm) and the initial position of the released bubble are shown inFig. 1. The simulation results for the rising of the single bubble atthree different time steps are shown in Fig. 2. In Fig. 2 blue and redrepresent the liquid and gas phases, respectively. The profiles ofthe single bubble rising with different grid sizes at time of 0.05 sare shown in Fig. 3 (the color coding is similar to that of Fig. 2).These simulation results at time 0.05 s were considered and evalu-Fig. 2. Simulation results of single bubble with 4 mm diameter rising at three different time steps with 9600 meshes.Fig. 3. The profiles of single bubble rising with different grid sizes at time of 0.05 s.Korean J. Chem. Eng.(Vol. 31, No. 8)

1352P. Zahedi et al.Fig. 4. Contour and the plot of vertical velocities at y direction on the center line (x 0, 0.005 mm y 0.015 mm) with various meshes att 0.05 s.Fig. 5. Simulations of bubble formation from an orifice when do 1 mm and Vg 0.2 m/s with 0.25 mm mesh size.ated; therefore, the contour and the plot of vertical velocities at ydirection on the center line (x 0, 0.005 mm y 0.015 mm) withall different meshes was plotted and shown in Fig. 4.The effect of mesh size on simulation results was also examinedby simulating the formation and rising of a bubble with differentmesh sizes for the cases in which the orifice diameter was 0.2 and1 mm. Fig. 5 shows the formation and detachment with 0.25 mmmesh size. The bubble generation includes two stages: expansionand detachment. During the first stage the gas bubble expands andgrows larger while still in contact with the mouth of the orifice (Fig.5 until t 0.1 s). Surface tension force plays a key role in the bubbleexpansion. As the bubble grows larger, the bubble keeps rising, andforms a slender neck which connects the body of the bubble withAugust, 2014the orifice (t 0.1 s). When the buoyancy is greater than the liquiddrag force on the bubble, the bubble detaches from the orifice andmoves up (Fig. 5 at t 0.125 s), during which the bubble keeps deforming. After that another bubble is generated, following by expansion, growth, and detachment.Fig. 6 shows the profiles of formed bubbles with different mesheswhen the gas velocity was 0.2 m/s at time of 0.25 s. Vertical velocities of the first formed bubble with different computing meshes areplotted in Fig. 7 and contour and velocity vectors of vertical velocities at t 0.25 s with 0.25 mesh size are shown in Fig. 8(a) and (b),respectively. It is noticeable that there is a peak indicated in Fig. 7at 0.1-0.15 s time bracket and it is related to the detachment timewhich is associated with an initial increase in velocity which is fol-

Influence of fluid properties on bubble formation, detachment, rising and collapse; Investigation using volume of fluid method1353Fig. 6. Bubble profiles with different computing meshes when vg 0.2 m/s at time of 0.25 s.E, of the bubble was calculated according to the expression:dE -----vdhFig. 7. The first bubble Vertical velocity versus time with different meshes when vg 0.2 m/s.lowed by a decrease in bubble velocity.To more deeply analyze the mesh dependency the aspect ratio,(16)where, dv and dh are the vertical and horizontal diameters, respectively; this ratio for the first formed bubble with different meshes isplotted in Fig. 9. From Figs. 6-8, the values of simulated size aspectratio with 0.25 mm, 0.2 mm grid sizes are very close. Similar trendswere found in the profiles of bubbles at different grid sizes. Thebubble profiles during the bubble growth with 0.25 mm and 0.2mm mesh sizes provided in Fig. 6 were approximately the same.These results indicate that the simulations are grid independent forthe mesh sizes studied here and that reasonable accuracy can bereached with a mesh size of 0.25 mm in this 2D simulation. Basedon this consideration and the computational time a the grid size of0.25 mm was chosen in the 2D simulations. Of course, the meshsize can be reduced further but the computational time will increasedramatically.4. Simulation VerificationThe experimental set up used to investigate the bubble formation under constant flow conditions is shown in Fig. 10. A glasscolumn with a square cross section (with each side of 10 cm) andFig. 8. Contours (a) and vector (b) of vertical velocities at t 0.25 s with 0.25 mesh size.Korean J. Chem. Eng.(Vol. 31, No. 8)

1354P. Zahedi et al.Fig. 9. Aspect ratio (E) with different meshes plotted against time.Fig. 10. Schematic diagram of the experimental set-up.Fig. 11. Comparisons of simulations and experiments of bubble formation and rising motion from an orifice of 4.5 mm diameter withconstant flow rate of 2.5e-7 m3/s (a) simulations and (b) experiments.August, 2014

Influence of fluid properties on bubble formation, detachment, rising and collapse; Investigation using volume of fluid method1355Table 1. Comparisons of calculated and measured bubble velocities in numerical and experimental methodBubble velocity (m/s)Experimental methodNumerical simulationRelative error (%)0.2240.2459.000with a height of 50 cm was used. In experiments, water was usedas the liquid phase, with the static liquid level maintained at 10 cmfrom the bottom of the column. Air was introduced in the columnthrough the orifice with 4.5 mm diameter as shown in Fig. 10. Theair flow rate was constant at 2.5e-7 m3/s using an air pump withflow controller. A high-speed digital camera (speed of 60 frames/s)recorded the bubble formation process. The bubble formation periods were characterized by analyzing the recorded movie files withsnapshots taken every 0.016 s.Comparisons between simulations and experiments of bubbleformation and rising motion from orifice with air flow rate of 2.5e7 m3/s and stagnant liquid are shown in Fig. 11. In addition, riseFig. 12. Example of modeled region with axisymmetric boundarycondition.Fig. 13. Comparisons of experiments and simulations of bubble formation from an orifice of 1.0 mm diameter and rising motion whenvg 3 m/s (a) 2D simulation without axisymmetric boundary condition, (b) 2D axisymmetric simulation, (c) Experiments [30] and(d) 3D simulations [30].Korean J. Chem. Eng.(Vol. 31, No. 8)

1356P. Zahedi et al.velocities both in numerical simulations and experiments are listedin Table 1. The time of releasing the first bubble from orifice is considered as the starting time of experiment. The comparisons of bubblerise velocity between simulated results and those calculated fromthe experiments indicate that the agreement is good with a relativeerror of approximately 9% (Fig. 11). However, the behavior of 2Dbubbles obtained from the numerical simulation is different to someextent from that of the bubbles measured in the experiment. Moreover the experiment of bubble formation was done under approximate constant gas flow rate while the simulation was carried out atstrict condition of constant gas flow rate.5. Axisymmetric SimulationThe assumption of axisymmetry implies that there are no circumferential gradients in the flow, but that there may be non-zerocircumferential velocities. Examples of axisymmetric flow are inFig. 12.Yujie et al. [30] conducted a research based on two-dimensionalnumerical studies on single bubbling behavior. Three-dimensionalnumerical simulations on bubble formation in bubble columns werealso investigated using the volume of fluid (VOF) model using Fluent6.3, and the results were validated with the experimental observations.In the present research, 2D axisymmetric simulations in Cartesiancoordinate system and without axisymmetric boundary conditionwere carried out and the results were compared with the experimentaland 3D numerical simulation results by Yujie et al. [30].Comparisons between simulations and experiments of bubbleformation and rising motion from an orifice with 1 mm diameterand a high orifice gas velocity of 3.0 m/s in the experimental method[30], 2D axisymmetric simulations and without axisymmetric boundary condition are shown in Fig. 13. The physical characteristics ofgas and liquid phases in the simulations are listed in Table 2. Bubblediameter and rise velocity are also listed in Table 3. It is found fromFig. 13 and Table 3 that axisymmetric simulations have a high relative error in comparison with 3D simulation and simulations without axisymmetric boundary condition. In axisymmetric simulation,bubble velocity and bubble diameter showed 37% and 18% relative error, respectively, compared with experiments. In axisymmetric simulation, bubble motion was incorrectly rising just in centerTable 2. Properties of gas and liquid phases in the 0Table 3. Comparisons between simulations and experiments of characteristic data of bubble behavior form an orifice with 1.0mm diameter and vg 3.0 m/sBubbleBubblediameter (mm) velocity (m/s)Experiments[30]3D simulation [30]2D axisymmetric simulation2D simulation without axisymmetricboundary conditionAugust, 20147.007.125.707.650.2750.2620.1730.253Fig. 14. Regime map of experimentally observed rising bubble shapeaccording to Bhaga and Weber [31]: S-spherical, OE-oblateellipsoid, OED-oblate ellipsoidal (disk-like and wobbling),OEC-oblate ellipsoidal cap, SCC-spherical cap with closed,steady wake, SCO-spherical cap with open, unsteady wake,SKS-skirted with smooth, steady skirt and SKW-skirtedwith wavy, unsteady skirt.line of bubble column; however, in 2D simulation without axisymmetric boundary condition and in Cartesian coordinate system thebubble velocity and bubble diameter had 8% and 9% error, respectively. Indeed there is reasonable agreement between experimentsand simulations of bubble behavior in the process of bubble generation and rising motion without axisymmetric boundary condition.According to Bhaga and Weber [31], the shapes of a single risingbubble under a range of Reynolds and Bond numbers have beenobserved and reported, as shown in Fig. 14. In general, small bubbles that experience low Reynolds or Bond number rise in a steadymanner and maintain spherical shape (Re 1 or Bo 1). At intermediate Reynolds and Bond numbers, the shapes of bubbles willbe significantly affected by the flow conditions (1 Re 100 and1 Bo 100). Bubble shapes such as oblate ellipsoid, disk-like, oblateellipsoidal cap, skirt bubble and spherical-cap are observed. In spiteof the difference in shapes, the bubbles maintain a straight path upwards in the liquid. At high Reynolds number (100 Re 500) thebubbles begin to deform into a toroidal shape in the high bond numberregime (100 Bo 500), spherical-cap shape in the intermediate bondnumber regime (30 Bo 100) and oblate ellipsoid in the low Bondnumber regime (1 Bo 30). As the bubble size increases further,the onset of turbulent wake developing behind the bubbles becomesmore prevalent, which subsequently leads to unsteady bubble motion.The bubbles may rise in a wobbly path, oscillate about a mean shapeand could even coalesce or break up. When the Reynolds and Bond

Influence of fluid properties on bubble formation, detachment, rising and collapse; Investigation using volume of fluid methodnumbers are not too high (Re 200 and Bo 200), the rising bubbles generally have axisymmetric shapes.In most of the presented simulated cases in this study, for Bondnumbers between 30 and 100 the predicted shapes agree well withthe experiments as observed in Fig. 13. After detachment, the leading bubble takes the shape of a spherical cap with a strong wake(having high upward velocity at its center) behind it. This high-velocityjet in the center of the vortex behind the leading bubble forces itsrear surface to move faster than the front surface of the bubble. Thisleads to merging of the front and rear surfaces of the leading bubbleand results into the formation of a toroidal bubble. The previouslyformed large spherical cap bubble influences the growth of the second bubble significantly.6. Influence of Liquid Surface Tension on Bubble behaviorThere are many factors influencing the bubble behavior, including physical properties of the fluid such as surface tension, liquiddensity and viscosity. Fig. 15 shows volume fraction contours ofsimulation results of bubbling formation and rising process withvaried liquid surface tension force for three different orifice diameters (0.5, 1 and 1.5 mm),when the constant gas mass flow is 1e-7m3/s. The simulation time for all cases in this simulation was 0.25 s.The effects of surface tension on bubble diameter and detachmenttime are shown in Figs. 16 and 17, respectively. From Fig. 16 thereis an upward trend for the bubble diameter by increasing surfacetension; moreover, detachment time saw a similar trend as shownin Fig. 17. However, bubble formation frequency is decreased. Sur-1357face tension increasing at constant orifice diameter and mass flowleads to reduction in bubble formation frequency, but bubble diameter and detachment time are increased. Detachment time delay makesmore gas get into the growing bubble. Meanwhile, the increase ofsurface tension inhibits the bubble generation, which results in longaverage cycle and low frequency of bubble generation. Therefore,the average bubble diameter is relatively large. According to Eqs.(13) and (14), at constant Froude number and by decrease of Mortonnumber caused by surface tension rise, bubble diameter and detachment time are increased.7. Influence of Liquid Viscosity on Bubble behaviorThe impact of liquid viscosity on bubble behavior was investi-Fig. 16. Effects of surface tension on bubble diameter at constantgas mass flow equals to 1e-7 m3/s and varied orifice diameter.Fig. 15. Simulation results of bubble behavior under different surface tension forces at constant gas mass flow equals to 1e7 m3/s and t 0.25 s, (a, f, k) σ 0.0364 N/m, (b, g, l) σ 0.0546 N/m, (c, h, m) σ 0.0728 N/m, (d, I, n) σ 0.1092N/m, (e, j, o) σ 0.1456 N/m, (a-e) do 0.5 mm, (f-j) do 1mm, (k-o) do 1.5 mm.Fig. 17. Effects of surface tension on detachment time at constantgas mass flow equals to 1e-7 m3/s and varied orifice diameter.Korean J. Chem. Eng.(Vol. 31, No. 8)

1358P. Zahedi et al.Table 4. Property parameters for studying the effect of liquid viscosity on bubble behaviorμl (kg/m·s)ρl (kg/m3)σ (N/m)Morton e-74.07e-61.59e-4Fig. 19. Effects of liquid viscosity on bubble diameter at constantgas mass flow equals to 1e-7 m3/s and varied orifice diameter.Fig. 18. Simulation results of bubble behavior under different liquid viscosity at constant gas mass flow equals to 1e-7 m3/sand t 0.25 s, (a-d) do 0.5 mm, (e-h) do 1 mm, (a, e) μl 1.003e-3 kg/m·s, (b, f) μl 1.0e-2 kg/m·s, (c, g) μl 2.0e-2 kg/m·s, (d, h) μl 5.0e-2 kg/m·s.gated by liquids whose physical parameters are listed in Table 4.Simulation results at t 0.25 s and 1e-7 m3/s are shown in Fig. 18;as it is seen, no obvious change of bubbling behavior is found forvaried liquid viscosity. Bubble diameter approximately stayed constant at varied liquid viscosity as it is shown in Fig. 19. The effectof liquid viscosity on detachment is given in Fig. 20. It is noticeable that bubble detachment time grew negligibly with the increasein liquid viscosity. However, these results indicate that the effect ofliquid viscosity on bubbling behavior is not considerable at presentsimulations. It is worth noting that again, bubble formation frequency is reduced by increasing orifice diameter at constant massflow.8. Influence of Liquid Density on Bubble behaviorDensity variation has a significant impact on bubble behavior,motion and rising. The physical parameters of the different liquidphases that have been used in order to investigate the density effec

are in fact intimately linked. Single bubble behavior plays an impor-tant role in determining the flow, mass and heat transfer character-istics in the bubble columns and fluidized beds, since its generation and rise can stir up the liquid and intensify the interphase distur-bance. This disturbance makes sufficient inter-phase contact and effi-