Numerical Simulation of Bubble Coalescence and Break-Up in Multinozzle Jet Ejector

1Centre for Industrial Mathematics and Department of Applied Mathematics, Faculty of Technology and Engineering, The M. S. University of Baroda, Vadodara, Gujarat 390001, India 2Centre of Computational Engineering and Integrated Design (CEID), Lappeenranta University of Technology, P.O. Box 20, 53851 Lappeenranta, Finland 3Department of Chemistry, Lappeenranta University of Technology, P.O. Box 20, 53851 Lappeenranta, Finland 4Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20, 53851 Lappeenranta, Finland 5Department of Chemical Engineering, Faculty of Technology and Engineering, The M. S. University of Baroda, Vadodara, Gujarat 390001, India


Introduction
There are number of industrial processes in which twophase flows, that is, gas-liquid mixture, in a jet ejector are encountered.Hence, reactions occurring in gas-liquid systems are of great importance in the chemical as well as in the process industry.Mass transfers in dispersions are directly related to the mass transfer coefficients as well as the interfacial area.The jet ejector is one kind of a venturi scrubber and it is widely used for conducting gasliquid reactions in practical applications in industry such as pollution control and waste water treatment.Due to their simple construction, low operating cost, high energy efficiency, and good mass transfer characteristics, the jet ejectors have many advantages when used as the gas-liquid contactors.The experimental observations show that dispersed bubbles towards the bottom of the jet ejector cause highly nonuniform volume distribution in the jet ejector.The gas volume fraction, the interfacial area, and the Sauter mean bubble diameter are the three important parameters that characterize the internal flow structure of gas-liquid flows in the jet ejector [1].The interfacial transport of mass and 2 Journal of Applied Mathematics momentum are proportional to the interfacial area and the driving forces.This is an important parameter required for a two-fluid model formulation [1].The mean bubble diameter serves as a link between the gas volume fraction and the interfacial area concentration [1].An accurate knowledge of local distributions of these three parameters is of great importance to eventual understanding and modeling of the interfacial transfer processes [1,2].Depending on the gas flow rate, two main flow regimes are observed in the jet ejector, namely, the homogeneous bubbly flow regime and the heterogeneous (churn-turbulent flow) regime [3].The homogeneous regime is encountered at relatively low gas velocities and characterized by a narrow bubble size distribution and radially uniform gas holdup and it is the most desirable one for practical applications, because it offers a large contact area [2][3][4].
The bubble size distribution and gas holdup in gas-liquid dispersions depend extensively on the jet ejector geometry, operating conditions, and the physicochemical properties of the two phases.The design of the jet ejector has primarily been carried out by means of empirical or semiempirical correlations based mainly on experimental data.The scale-up of the jet ejector is still poorly understood due to the complexity of flow patterns and their unknown behavior under different sets of design parameters such as area ratio, projection ratio, nozzle diameter, length of free jet, throat, diffuser, convergence angle, divergence angle, and physical properties of the liquid.As a whole, the phenomenon depends strongly on the jet ejector geometry and fluid dynamics involved.It is important to note that the similar kind of study has been developed for bubble column reactor by [3].
The method to gain more knowledge and detailed physical understanding of the hydrodynamics in the jet ejector is Computational Fluid Dynamics (CFD).CFD can be regarded as an effective tool to clarify the importance of physical effects (e.g., gravity, surface tension) on flow by adding or removing them.An increasing number of papers deal with CFD applications of bubble columns [3,[5][6][7].To analyze the flow pattern of the jet ejector both in steady and transient state conditions employing CFD, the majority of researchers use either two-or three-dimensional models, when numerical simulations are usually compared to experimental data.As most of the early CFD studies consider monodispersed bubble size distributions ignoring breakup and coalescence mechanisms, their validity is limited [3].
During the last two decades, significant developments have been done in the modeling of two-phase flow processes because of the introduction of the two-fluid models [1].In the two-fluid models, the interfacial transfer terms are related to the interfacial area concentration and the degree of turbulence near the interfaces [1].Since the interfacial area concentration represents the key parameter that links the interaction of the phases, significant attention has been paid towards developing a better understanding of the coalescence and breakage effects due to interactions among bubbles and between bubbles and turbulent eddies for gas-liquid bubbly flows [1,2,[7][8][9][10][11].The population balance method is a wellknown method for tracking the size distribution of the dispersed phase and accounting for the breakage and coalescence dynamics in bubbly flows [1,[12][13][14][15][16][17][18][19][20][21][22].The population balance method was also used by Hämäläinen et al. [23] and Hämäläinen [24] in paper industry for papermaking suspension flow.
In gas-liquid two-phase systems, bubble break-up and coalescence can greatly influence their overall performance by altering the interfacial area available for the mass transfer between the phases [3].Therefore, in order to develop reliable predictive tools for designing the jet ejectors, it is essential to obtain some insight into the prevailing phenomena through simulation models based on the bubble formation and distraction mechanisms, that is, bubble coalescence and break-up [3,12,15,[25][26][27][28], incorporated into CFD simulation making it possible to calculate hydrodynamic variables such as liquid velocity, gas holdup, and bubble size distributions.
In this work, an attempt has been made to demonstrate the possibility of combining the population balance models with Computational Fluid Dynamics (CFD) for the case of a gas-liquid bubbly flow in the jet ejector.In all these processes gas holdup,   , and bubble size distribution are important design parameters, since they define the gas-liquid interfacial area available for interfacial area mass transfer (), which is given by where  32 is the mean Sauter diameter of the bubble size distribution [3].The MUSIG model implemented in ANSYS Fluent 14.0, which accounts for the nonuniform bubble size distribution in a gas-liquid flow [1,2,6,16,29], is used in this paper.Gas volume fraction, bubble size distribution, number density of bubbles, gas and liquid pressure variation, interfacial area concentration, and gas and liquid velocity variation in jet ejector are predicated.The flow pattern development has been studied at the free jet end, and at the throat end, and at the end of the ejector in detail.In addition, numerical predictions are compared with experiments and the predicated gas interfacial area is in a good agreement with experimental results.

Jet Ejector
A large choice of gas-liquid contactors, for example, the falling film column, spray column, packed column, plate column, bubble column, mechanically agitated contactors, spray towers, and venturi scrubbers, are available for understanding the mass transfer process.Among these, the venturi scrubber is a wet type design for gas-liquid contactors.In venturi type of scrubber (i) liquid is a medium to absorb objectionable gases and particulates from industrial gaseous waste streams; (ii) a high velocity section of fluid jet is utilized to bring the liquid and gas into intimate contact with each other.
In the first category, it uses mechanical blower to draw a high velocity gas stream through the system.The liquid was originally at rest but once the gas accelerates, it splits into droplets.Particulates and gases are then confined into the comparatively slower moving droplets.This type is called a "high energy venturi scrubber" (HEVS).The scrubbing liquid can be introduced in two ways: (i) If the liquid is introduced through nozzles which is usually at the throat it is known as Pearce-Anthony venturi scrubber.
(ii) In this case liquid is introduced as a film which is usually known as the wetted approach type.
Secondly, a mechanical pump or compressor is used to generate a high velocity to the liquid/fluid jet.This liquid/fluid jet creates suction and gas is entrained into it by transfer of momentum.This type of the venturi scrubber is called an ejector venturi scrubber or "jet ejector." The jet ejectors have some advantages over other types of contactors which are mentioned as follows [31,32]: (ii) Simple construction and being compact.
(iii) Easy installation and operation.
(iv) No moving parts, so little chances of mechanical failure and hence highly reliable.
(v) Being able to deal with wet, hot, and corrosive gases and thick, aggressive, and inflammable particles.
(vi) Ability to separate gaseous pollutant and fine particulate matter simultaneously.
(vii) Being able to handle large gas flow rates.
(viii) High heat and mass transfer rates and interfacial area.
However, it is not energy efficient equipment as a fluid moving device.But it has been reported that it has high efficiency as a gas-liquid contacting device [33][34][35][36].
The jet ejectors are especially effective in chemical and biochemical industries for gas purification and for collaborating gas-liquid reactions like chlorination, oxidation, hydrogenation, and hydroformulation processes.Jet ejectors use high kinetic energy of the operating fluid jet to promote break-up and distribution of the suction fluid into small droplets/bubbles and to pull the gas through the system and push through the connected outlet.
A typical gas-liquid jet ejector is displayed in Figure 1.It consists of a converging section, a throat section, and a diffuser/divergent section.The gas is accelerated to atomize the scrubbing liquid in the convergent section to reach a higher velocity in the throat.Throat is used for interaction of liquid and gases.In the diffuser/divergent section the gas is slowing down allowing some recovery of pressure [37,38].Jet ejectors have favorable mixing and mass transfer characteristics so they are more used among other gas-liquid contactors.A jet ejector is a device in which suction, mixing, and dispersal of secondary fluid take place and are based on the principle of utilizing the kinetic energy of a high velocity motive (primary) fluid jet to entrain the secondary phase to create fine dispersion of two phases (Utomo et al. [39]).The secondary fluid may be dispersed by the shearing action of the high velocity motive fluid or motive fluid may get dispersed when it is captured by a secondary fluid [40].
Figure 1 shows the typical ejector system in which the jet of primary fluid, typically liquid, is pumped into the system through high velocity through a nozzle flowing out of a nozzle which creates a low pressure region in the suction chamber, into which secondary fluid, typically gas, moves according to Bernoulli's principle.The driving force for entrainment of the secondary fluid is developed due to the pressure difference between the entry point of the secondary fluid and the nozzle tip.There is a mixing of gas and liquid phases and a gas-liquid dispersion takes place in the mixing tube.In the diffuser the pressure is recovered.Coaxial-flow and froth-flow are the two principal flow regimes in jet ejectors.In the annular region formed between the jet of primary fluid and ejector wall, a central core of primary fluid with secondary fluid flowing was observed.This regime forms coaxial-flow.Frothflow consists of liquid in which gas phase is completely dispersed in the form of bubble [41].The phenomenon of change from coaxial-flow to froth-flow is termed as mixing shock [42].The small bubbles are generated due to mixing shock which turns into creation of large interfacial area (∼2000 m 2 /m 3 ).Therefore greater rates of reaction and superior gas-liquid mass transfer rates are obtained in ejectors, in comparison to other common gas-liquid contactors [31].
Depending on application, there may be different objectives for design of an ejector which are as follows [43]: Jet ejector may be used as a vacuum producing device as well as jet pump.With the rapid growth of the chemical process industry, their use as entraining and pumping corrosive liquids, slurries, fumes, and dust-laden gases has increased.Their use as mass transfer equipment for liquid-liquid extraction, gas absorption, gas stripping, and slurry reaction, like hydrogenation, oxidation, chlorination, fermentation, and so forth, has increased [31,[44][45][46][47][48][49][50][51][52][53].
Figure 2 shows the major parts like primary fluid inlet, secondary fluid inlet, suction chamber, converging section, throat/mixing zone, and diverging section/diffuser of an ejector.The symbols which we have used for describing different components are as follows: (i) Length of throat (  ), length of diffuser (  ), and distance between nozzle and commencement of throat (  ).
(iv) Angle of converging sections ( convergent ) and angle of diverging sections ( divergent ).
Performance of the ejectors has been studied in terms of (a) area of throat/area of nozzle, that is, area ratio (  =   /  ), (b) length of throat/diameter of throat, that is, throat aspect ratio (  /  ), (c) distance between nozzle tip and the commencement of throat/diameter of throat, that is, projection ratio

Mathematical Model
The poly dispersed multiphase flow is observed in jet ejector.It means that the dispersed phase covers a wide range of size groups.One of the characteristics of this flow is that the different sizes of the dispersed phase interact with each other through the processes of break-up and coalescence.A population balance equation is framed to deal with this kind of a flow.It is well known that the population balance model is best fitted method to calculate the size distribution of a dispersed phase which includes break-up and coalescence effect [3].Our model is in parallel to the model developed by Mouza et al. [3].The general form of the population balance equation is where   ,   ,   ,   ,   , and ⃗   represent the number density of size group , the birth rate due to break-up, the death rate due to break-up, the birth rate due to coalescence, the death rate due to coalescence, and the gas velocity, respectively.The relation between number density   and the volume fraction   is given by where   and   represent the volume fraction and the corresponding volume of a bubble of group , respectively.The coalescence of two bubbles occurs in three steps [3,19]: (i) First, the bubbles collide trapping a slight amount of liquid between them.
(ii) Secondly, this liquid film drains until it reaches a critical thickness.
(iii) At the end, the film breaks and the bubbles join together.
The process of coalescence depends on the collision rate of the two bubbles and the collision efficiency.Coalescence is a function of   and   , where   is the time required for coalescence and   is the contact time.Collision is as a result of turbulence ( Tu  ), laminar shear ( Ls  ), and buoyancy ( By  ).The total coalescence rate is Collision is also developed due to the difference in rise velocities of bubbles with different sizes.The two conditions observed in jet ejector in parallel to the work of Mouza et al. [3] for bubble column are as follows: (i) Jet ejectors operate at the homogeneous regime.Due to the narrow bubble size distribution in homogenous regime, the relative effect of buoyancy can be neglected which leads to the assumption that bubbles rise with the same velocity regardless of their size.
(ii) Collision occurs as a result of strong circulation pattern in jet ejector.However this happens at gas rates higher than those encountered in the homogeneous regime and hence laminar shear term may be neglected.
Hence, there is only the turbulent contribution in the model which is given by where   ,   , and   are the concentrations of bubbles of radius   , concentrations of bubbles of radius   , and the average turbulent fluctuating velocity, respectively.The collision cross-sectional area (  ) of the bubble, the time required for coalescence, and the contact time are given by ( 6), (7), and (8), respectively: where   ,   , , , ℎ 0 , and ℎ  are the equivalent radius, the density of the liquid phase, the surface tension, the turbulent energy dissipation, the film thickness when collision begins, and the film thickness when ruptures of the film occur, respectively.The birth rate of group  due to coalescence of group  and group  bubbles and the death rate of group  due to coalescence with other bubbles are given by ( 9) and (10), respectively: The break-up of bubbles in turbulent dispersions employs the model developed by Luo and Svendsen [28].The break-up rate of bubbles of size  into bubbles of size  is given by where ,   , ,   , and  represent the turbulent energy dissipation rate, the bubble diameter, the dimensionless size of eddies in the inertial subrange of isotropic turbulence, the critical dimensionless energy for break-up, and the total number of groups, respectively.The birth rate of group  bubbles due to break-up of larger bubbles and the death rate of group  bubbles due to break-up into smaller bubbles are given by ( 12) and ( 13), respectively: The mass conservation equation for the liquid phase, the mass conservation equation for the gas phase, and the momentum conservation equation in the Eulerian framework are given by ( 14), (15), and ( 16), respectively: where   , ⃗   ,   ,   , ,   , and  represent the density, the velocity, the volume fraction, and the viscosity of the th phase, the pressure, the interphase momentum exchange between phase  and phase , and the gravitational force, respectively.
The momentum exchange between gas and liquid phase is given by where   is the interfacial drag coefficient.

Simulation Parameter for Jet Ejector Geometry
Figure 3 shows the details of the geometry of ejector.Agrawal [31,111,112] has done the experiments based on the experimental setup as shown in Figure 4.It is important to note that these experiments were conducted on the industrial stage ejector.Data for the geometry of the ejector is shown in Table 1.

The CFD Simulation
Accurate modeling of the jet ejector requires a Population Balance Modeling (PBM) to be solved simultaneously with a CFD solver because of the presence of bubble-bubble contacts or bubble-liquid contacts.The PBM-CFD model, that is, MUSIG model (ANSYS Fluent 14.0), is implemented in this study which combines the population balance method with the break-up [28] and coalescence [19] models in order to predict the bubble size distribution of the gas phase.This model uses the Eulerian-Eulerian model.The MUSIG model has been extensively used for different systems [1-3, 11, 16, 20, 27, 29].The equations of continuity, momentum, and turbulence for the continuous and dispersed phases give a standard two-phase flow calculation.It can be extended to include number density of bubbles within several size groups using the MUSIG model.
The size range of the bubbles is split into several groups with, for example, groups of equal diameter.Equations are then solved for the number density in each group.These size fractions provide a more accurate measure of the interfacial area density.In this study, the simulation is done in three dimensions and three geometries of the ejector, namely, with one nozzle, three nozzles, and five nozzles, as described in Figure 3.In the present study, bubbles ranging from 1.4472692 × 10 −5 m (bin-20) to 9.7005842 × 10 −4 m (bin-0) in diameter are equally divided into 21 classes (see Table 2) as the experimental observation of the gas volume fraction is 100% and therefore we consider 0.005, 0.010, 0.015, 0.018, 0.020, 0.025, 0.032, 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060, 0.065, 0.070, 0.075, 0.080, 0.085, 0.090, 0.095, and 0.100 which represent the volume fraction of the bubbles of group , ( = 0, 1, 2, 3, 4, 5, . . ., 20).In view of computational time's resources, we consider the subdivision of the bubble sizes into 21 size groups.All computational results are based on the discretization of the bubble sizes into 21 groups.ANSYS Fluent 14.0 gives solution to the coupled sets of governing equations for the balances of mass and momentum of each phase.The conservation equations were discretized using the control volume technique.Similar kind of study   Bin-20-fraction Table 3: Operating conditions.A second-order discretization scheme was used for the convective terms.At the inlet, gas, liquid, and the average volume fraction have been specified.At the outlet, a relative average static pressure of zero was specified.The operating conditions are summarized in Table 3.The fluid data are taken at room temperature (25 ∘ C).

Results and Discussion
In this work, we have made an effort to study Chlorine-Aqueous Sodium Hydroxide system.As stated earlier, the bubble sizes were distributed into twenty groups (bins) with diameters between 1.4472692 × 10 −5 m and 9.7005842 × 10 −4 m.

Gas Volume Fraction and Liquid Volume Fraction. Figures 5 and 6
show the plot of the predicted gas volume fraction and predicted liquid volume fraction for liquid velocities of 4.6 m/s and gas velocity of 0.2866 m/s for nozzles 1, 3, and 5, respectively.It can be observed that most of the gas volume fraction tends to migrate towards the bottom of the ejector.It is also observed that the gas volume increases in the area of the throat as well as near the gas inlet.As the number of nozzles increases (from nozzle 1, to nozzle 3, to nozzle 5), the gas volume fraction decreases with the same trend in all the nozzles.As we proceed from nozzle to end of jet ejector we are not adding any gas or liquid so it should be parallel to axis.There are three clear cut zones (a) from discharge of liquid nozzle to beginning of the throat, (b) from commencement to the end of the throat, and (c) from end of the throat to end of the diffuser.In zone (a) there is a negative pressure so same quantity of gas has more volume at constant temperature which is clear by peak zone.In the throat there is almost constant pressure which is shown as almost parallel line in this zone.The volume fraction of gas should reduce since the pressure is recovered and higher in zone (c).This fact is evident from Figure 5 as the volume fraction of gas is slanting downward.

Liquid Velocity and Gas
Velocity.Figures 7 and 8 represent the plots of the velocity magnitude of the gas and liquid phases, respectively.These results show that the axial liquid velocity profile has a slight degree of asymmetry due to the presence of the gas flow.The liquid velocity in the upper region of the jet ejector is higher than in the lower region.This is due to the increase in cross-sectional area at the bottom of the ejector.size distribution varies with the position because of the nonuniform profiles of the gas volume fraction entrapped by liquid stream and dissipation rate (see [1,3]).The number densities of the bubbles along the vertical direction which is middle line of the jet ejector for bin-0, bin-  (b), the bubble size reduces due to the intimate mixing.It can be seen from these figures that the bubble size distribution function reaches an independent state as determined by the balance between birth and death processes that depend on the local flow conditions.Figures 15-17 represent the discrete number density of the bubbles for nozzles 1, 3, and 5, respectively, for the entire volume.represent the discrete number density of the bubbles for nozzles 1, 3, and 5, respectively, for the plane near to gas inlet.It is evident from the figure that as the diameter increases the number density decreases for entire volume as well as the plane near to throat.

Interfacial Area.
The present simulation results show the interfacial area variation along the vertical direction which is middle line of the jet ejector as shown in Figure 22.The variation of interfacial area in vertical middle plane is also shown in Figure 21.From Figures 5 and 22, it can be seen that the appearance of interfacial area concentration looks   similar to the gas volume fraction.But the interfacial area is depending not only on the volume fraction of the gas, but also equally on the distribution of the bubble size.Since the measurements of the volume fraction and the interfacial area are independent of each other, the data required for calculating an interfacial area from population balance model provides a valuable test for model prediction [1,3].Hence our results show that the birth and the death processes modeled in the population balance model are appropriate to describe the dynamics of bubbles.It can be also seen that the interfacial area concentration reaches the maximum value of 7000 m 2 /m 3 , 5600 m 2 /m 3 , and 4500 m 2 /m 3 in the mixing zone of the jet ejector for the nozzles 1, 3, and 5, respectively.It is also observed that the interfacial area is decreasing when the number of nozzles is increased.The interfacial area is in good agreement with experimental result [31].
The range of measured values of the interfacial area in the jet ejector is about 3000 to 13000 m 2 /m 3 [31].The experimental values of the interfacial area for nozzle 1, nozzle 3, and nozzle 5 are nearly in the range of 4000-5500 m 2 /m 3 , 3800-4500 m 2 /m 3 , and 2500-3500 m 2 /m 3 , respectively, for different concentrations of gas and liquid [31].The simulated values of the interfacial area concentration are in the same order of magnitude with the experimental results [31].More detail on the experimental results can be found in Agrawal [31].As shown in Figure 23, the correlation shows that diameter is less in zone (a) but increases in zone (b) and is almost constant in zone (c).In zone (c) there is sudden rise of the diameter in the case of nozzle 1, whereas in the cases of nozzles 3 and 5 the diameter does not show appreciable change.The reason is that in nozzle 1 there is only single jet.Its singularity nature is not disturbed while fluids are passing through the jet ejector.Hence there is fast collision of bubbles and it shows sudden rise in the bubble diameter, while, in the cases of nozzles 3 and 5, the water jet is divided into three and five jets of the same velocity, respectively, so these streams maintain their diversity.While fluid stream enters into the diffuser, the bubbles entrapped in streams are not easily disengaged to form higher size of bubbles and for this reason the two nozzles do not show appreciable increase in their average diameter.that larger recirculation region occurs in the mixing as the number of nozzles increases.

Conclusion
Population balance approach, combined with the coalescence and break-up models, is presented to simulate the operation of the jet ejector, using CFD software.The population balance approach was demonstrated by using the equation of bubble number density for gas-liquid flows using ANSYS Fluent 14.0 to explain the temporal and spatial changes of the gas bubble size distribution.The CFD results have been analyzed to determine bubble size distribution, liquid velocity, gas velocity, and volume fraction in the jet ejector with the Cl 2 -NaOH system.Computational results are compared with the experimental results [31].The simulated interfacial area is in good agreement with the experimental values.
In order to see the validity of the CFD model we require more experimental data on liquid-to-gas ratio, geometry of the jet ejector, properties of gas and liquid, reactivity of fluids, and so forth.The coupling of all these properties gives a good platform to predict the best model and as a consequence we may predict the performance of the jet ejector.
(a) To achieve greater entrainment of the secondary fluid.(b) To yield deep mixing between the two fluids.(c) To inject fluids from a region of low pressure to a region of higher pressure.

S3Figure 3 :
Figure 3: Detail of the ejector used in the experimental setup[31,111,112].

Figure 5 :
Figure 5: Plot of the volume fraction of gas.

Figure 6 :Figure 7 :
Figure 6: Plot of the volume fraction of liquid.

6. 5 . 3 )Figure 15 :Figure 16 :
Figure 15: Plot of the number density of bubble in the entire volume for nozzle 1.

Figure 17 :
Figure 17: Plot of the number density of bubble in the entire volume for nozzle 5.

Figure 18 :Figure 19 :Figure 20 :
Figure 18: Plot of the number density of bubble in a plane near to the gas inlet for nozzle 1.

Figure 23 :
Figure 23: Plot of the diameter of bubble.

Table 2 :
Diameter of each bubble class tracked in the simulation.