Chemical Engineering Science ] (]]]]) ]]]–]]]

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Science

0009-25

http://d

n Corr

E-m

Pleasresis

journal homepage: www.elsevier.com/locate/ces

Comprehensive study of regime transitions throughout a bubblecolumn using resistivity probe

Mohsen Shiea, Navid Mostoufi n, Rahmat Sotudeh-Gharebagh

Multiphase Systems Research Lab., Oil and Gas Centre of Excellence, School of Chemical Engineering, College of Engineering, University of Tehran,

P.O. Box 11155/4563, Tehran, Iran

H I G H L I G H T S

c Flow regime transitions were determined in a bubble column by employing a double-needle resistivity probe.c The resistivity probe method is applicable to detect transition between discrete bubble and coalesced bubble flows.c It is necessary to place the probe far from the distributer and column wall to be able to determine the onset of the slug flow.c A flow regime map was presented based on the measured data.

a r t i c l e i n f o

Article history:

Received 7 August 2012

Received in revised form

8 November 2012

Accepted 17 January 2013

Keywords:

Bubble columns

Bubble

Hydrodynamics

Multiphase flow

Regime transition

Resistivity probe

09/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.ces.2013.01.047

esponding author. Tel.: þ98 21 6696 7797; f

ail address: mostoufi@ut.ac.ir (N. Mostoufi).

e cite this article as: Shiea, M.,tivity probe. Chem. Eng. Sci. (2013),

a b s t r a c t

Flow regime transitions were experimentally characterized using several local bubble properties

measured by a double-needle resistivity probe. Experiments were carried out in a bubble column of

0.09 m diameter and 1.8 m height. The superficial gas velocity was changed from 0.005 to 0.070 m/s

and the superficial liquid velocity from 0.003 to 0.060 m/s. Average bubble chord length, bubble

frequency and sauter mean bubble chord length were obtained at various locations of the column by

placing the probe at three axial and six radial positions. Transitions between dispersed bubble flow,

discrete bubble flow, coalesced bubble flow and slug flow were detected at these locations.

A comparison between transition points at different locations verified that regime transition occurs

throughout the bubble column at nearly the same superficial gas and liquid velocities. A flow regime

map was also obtained based on the local measurements of the bubble properties at different locations.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Bubble columns are extensively used as gas–liquid contactorsin many industrial applications due to their various advantagessuch as simple construction, low operation cost, high heat/masstransfer rate and easy temperature control. Performance ofbubbles columns depends on the prevailing operating flowregime. Modeling and design of bubble columns require a properunderstanding of different flow regimes because hydrodynamiccharacteristics of various flow regimes can be quite different.Discrete bubble, dispersed bubble, coalesced bubble, slug, churn-turbulent, bridging and annular flow are common regimesreported in literature. Many investigations have been carriedout to characterize these flow regimes and determine the transi-tion points (Letzel et al., 1997; Monahan et al., 2005; Narinder,1982; Nedeltchev et al., 2011; Shaikh and Al-Dahhan, 2005; Taitel

ll rights reserved.

ax: þ98 21 6696 7781.

et al., Comprehensive studhttp://dx.doi.org/10.1016/j

et al., 1980; Zhang et al., 1997) and several methods have beenused to study flow regimes including visual observation, gas hold-up measurement, wall pressure fluctuations and various probemeasurements.

Visual observation is one of the earliest and simplest methodsemployed to characterize flow patterns and transition points (Taitelet al., 1980; Tzeng et al., 1993). However, there are no quantitativecriteria for detecting regime transitions. In addition, applying visualobservation is impossible or limited in high pressure columns.Measuring the variation of gas hold-up with superficial gas velocityhas been used to determine the transition points (Deckwer andField, 1992). This method is only capable of detecting homogenousto heterogeneous regime transition. Besides, the criterion used fordetecting transition point is inapplicable for some cases especiallywhen the gas is mal-distributed. Different methods of analysis ofvarious signals measured in bubble column were used to identifyregime transitions. Measurement of the wall pressure fluctuations isthe most attractive technique employed to study hydrodynamics ofbubble columns (Letzel et al., 1997; Matsui, 1984; Narinder, 1982;Vial et al., 2001). However, interpretation of pressure signals is

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M. Shiea et al. / Chemical Engineering Science ] (]]]]) ]]]–]]]2

difficult because the relationship between pressure and flow struc-ture is not straightforward.

Various non-intrusive and intrusive probes can also be used tomeasure local properties of multiphase flow in bubble columns,including electrical resistivity (or conductivity) probe (Herringe andDavis, 1974), fiber optic probe (Lee et al., 1990), electrical conduc-tance probe (Barnea et al., 1980), X-ray void measurement (Jones andZuber, 1975), laser Doppler anemometry (Olmos et al., 2003) andcomputed tomography (Shaikh and Al-Dahhan, 2005). These mea-surements can be utilized to identify regime characteristics andtransitions. Resistivity probe is one of the intrusive methodsemployed for studying flow regimes and their transitions using themeasured properties of the gas bubbles. Das and Pattanayak (1993)used the probability distribution function (PDF) of void fractionsignals measured by conductivity probe in order to detect bubble toslug, slug to churn and churn to annular transitions. Zhang et al.(1997) introduced consistent criteria for detection of regime transi-tions based on bubble/void characteristics measured by conductivityprobe in two- and three-phase systems. They presented a flow regimemap for the two-phase flow in bubble columns using those criteria.However, their measurements were limited to only one axial positionand the center of the column. They stated that the obtained map isonly applicable at the axial location of the probe measurements,although visual observations suggested that regime transitions occurthroughout the height of bubble column at approximately the samevalues of superficial gas and liquid velocities.

Fig. 1. Schematic of the

Please cite this article as: Shiea, M., et al., Comprehensive studresistivity probe. Chem. Eng. Sci. (2013), http://dx.doi.org/10.1016/j

No studies are reported in literature to investigate whether theprobe measurement location influences the detection of regimetransition or not. In this experimental work, flow regime mapswere obtained by placing resistivity probe at different axial andradial locations and the potential of different measurementlocations for detection of regime transitions was assessed. Itwas also clarified whether or not the regime transition pointschange along the height of the bubble column.

2. Experimental setup and measurement technique

The experiments were carried out in a lab-scale cylindricalPlexiglas bubble column of 0.09 m diameter and 1.8 m height shownin Fig. 1. A perforated plate with 100 circular holes of 0.001 mdiameter was placed at the bottom of the column as the distributor.Gas and liquid phases were introduced to an engagement sectionbelow the distributor. The height of the engagement section was0.1 m and was filled with glass beads (0.01 m in diameter) for bettermixing of two phases. Air was used as the gas phase and its flow ratewas set by a mass flow controller. Tap water, as the liquid phase,was pumped to the engagement section from a storage tank. Threeparallel rotameters of different sizes were used to measure theliquid flow rate. The superficial gas velocities ranged from 0.005 to0.07 m/s and the superficial liquid velocities ranged from 0.003 to0.06 m/s in the experiments.

experimental setup.

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Fig. 2. Schematic design of the double-needle resistivity probe.

Fig. 3. (a) Output sinusoidal signal of the probe and (b) Amplitude signal.

Fig. 4. Amplitude signal (a) before de-noising and (b) after de-noising.

M. Shiea et al. / Chemical Engineering Science ] (]]]]) ]]]–]]] 3

A double-needle resistivity probe was used to measure proper-ties of gas bubbles. Schematic design of the probe is shown inFig. 2. Sensing component of the probe consisted of two beveledtip surgical stainless steel needles with 0.2 mm in diameter. Theneedles were placed in a 1.3 mm outer diameter tube and coatedwith polyester resin except for the tips. The axial distancebetween the needle tips was approximately 2.2 mm. Anotherlarger tube with outer diameter of 2.1 mm was used as the holderof the arrangement. The holder tube was also served as thecommon ground electrode of the probe. A circuit was designedto supply an alternating-current (AC) voltage at a frequency of5 kHz to be applied between the needles and the commonelectrode. The passage of different phases at the probe tip led toa shift in the amplitude of the output signal of the circuit whichwas sampled by Advantech A/D converter (PCI 1712-L) at afrequency of 120 kHz. Period of the measurement was set to180 s in order to detect sufficient number of bubbles for statisticalanalyses. The measurements were carried out at three axiallocations above the distributor (30 cm, 100 cm, and 165 cm)while the probe was at the center of the column. In addition,measurements at axial location of 165 cm above the distributorwere performed at six radial positions (r/R¼0, 0.2, 0.4, 0.6, 0.8 and0.9). Shiea (2012) showed that axial symmetry exists in theprofile of bubble properties in a similar bubble column. Thus,radial measurements were only carried out along one side of thecolumn diameter.

Fig. 5. Converting the de-noised signal to square wave.

3. Data analysis

3.1. Signal processing

Output digital data of the resistivity probe was processed infour steps. The output signal was a sinusoidal wave with varyingamplitude. The change in the amplitude corresponds to thechange in the detected phase. Thus, the first step of the signalprocessing was focused on detecting changes in the amplitude ofthe output sinusoidal signal of the probe. The amplitude signalshows phase change as well as the original signal with a muchfewer data points. Fig. 3 demonstrates both the output signal ofthe probe and its amplitude. The inherent noise in the amplitudesignal was then reduced by applying an interval-dependenthard thresholding de-noising, using wavelet decomposition ofthe signal at level 9 with Sym5 wavelet (Mallat, 2009). Fig. 4

Please cite this article as: Shiea, M., et al., Comprehensive studresistivity probe. Chem. Eng. Sci. (2013), http://dx.doi.org/10.1016/j

demonstrates the amplitude signal before and after de-noising.The de-noised signal shows a near exponential rise in the voltagewhen a bubble hits the sensor, but falls rapidly as the bubbleleaves the tip of the sensor. Therefore, it is necessary to convertthe signal into a square wave. This was done by adjusting a

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threshold level of 8% of the maximum rise of the voltage in thesignal. This conversion is shown in Fig. 5. The square wave wascalled gas-phase density function which takes a value of unitywhen the needle tip is in the gas phase and a value of zero whenthe needle tip is in the liquid phase:

jg ¼1 ðfor gas phaseÞ

0 ðfor liquid phaseÞ

(ð1Þ

Each pulse in the gas-phase density function corresponds to abubble which has passed the probe. The duration of the pulse isequal to the contact time (t) of the bubble with the probe. Thecontact time t depends on the bubble size, bubble velocity, bubbleshape, the point at which the bubble hits the sensor and the anglebetween the direction of bubble motion and probe orientation.

The last step was to calculate the bubble and void propertiesusing the gas-phase density functions of both needles. Gas hold-up and bubble frequency were calculated from the gas-phasedensity function of the front needle. Bubble frequency wasdefined as the number of bubbles hit the probe per unit of time:

f ¼Nb

Tð2Þ

Time-averaged local gas hold-up was calculated by

eg ¼

Piti

Tð3Þ

The bubble velocity and bubble chord length were obtained byanalyzing gas-phase density functions of both needles:

Vbi¼

L

Dtið4Þ

lbi ¼ 103tiVbi ð5Þ

A program was utilized to include only bubbles travelingupward. The conditions used in this program are explained byRevankar and Ishii (1992).

Fig. 6. Frequent flow regim

Please cite this article as: Shiea, M., et al., Comprehensive studresistivity probe. Chem. Eng. Sci. (2013), http://dx.doi.org/10.1016/j

3.2. Determination of regime transition points

The flow regimes studied in the present work were dispersedbubble flow, discrete bubble flow, coalesced bubble flow and slugflow. These flow regimes are shown in Fig. 6. The transitions betweenmentioned flow regimes are explained in following sections.

3.2.1. Transition between discrete bubble flow and coalesced bubble

flow

When both gas and liquid flow rates are low, the gas phaseexists in the form of small bubbles in the column. As the gas hold-up and the number of bubbles are low, the distance betweenbubbles is large compared to their size. Therefore, the bubbles donot have sufficient collisions to coalesce. This flow pattern iscalled discrete bubble flow. As the gas flow rate increases, largerbubbles are formed and the number of bubbles increases. As aresult, the distance between bubbles decreases and the coales-cence of bubbles occurs as the bubbles ascend in the column. Thisregime is referred to as coalesced bubble flow. Zhang et al. (1997)used the plot of bubble frequency versus superficial gas velocityto obtain the transition between discrete bubble flow andcoalesced bubble flow. They stated that the bubble frequencyshows a linear relationship in the discrete bubble flow regime;therefore, the point at which the plot deviates from a linearrelationship corresponds to the transition to the coalesced bubbleflow. The same criterion was used in this work to find regimetransition between discrete bubble flow and coalesced bubble flow.

3.2.2. Transition between coalesced bubble flow and slug flow

Further increase of the gas flow rate leads to slug flow regimein the column. Slug flow regime is characterized by largeelongated bullet-shape bubbles called Taylor bubbles which havediameters almost equal to the diameter of the column. Taylorbubbles are followed by liquid slugs containing small gas bubbles.Since the conductivity probe detects both the Taylor bubbles andthe trailing small bubbles, the arithmetic mean of measuredbubble chord lengths does not change significantly as the regimetransition occurs. Zhang et al. (1997) used the sauter mean bubblechord length to detect the transition point. The advantage of using

es in bubble columns.

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sauter mean is that it gives more weight to large bubbles than tosmall ones. The point at which the sauter mean bubble chordlength reaches the column diameter indicates that Taylor bubblesare formed and slug flow regime exists in the column. The samecriterion was used in this study to detect transition from coa-lesced bubble flow regime to slug flow regime.

3.2.3. Transition between discrete/coalesced bubble flow and

dispersed bubble flow

As mentioned above, discrete or coalesced bubble flow existsin the column when both gas and liquid flow rates are low. In thissituation, increasing the liquid flow rate makes the bubblessmaller and their size distribution narrower. This flow regime iscalled dispersed bubble flow. The bubble behavior in this regimeis almost independent of the type of the gas distributor. It will beshown in the following section that the average bubble chordlength (lb) increases slightly in the region of discrete/coalescedbubble flow as the liquid flow rate is increased, while it started tofall sharply with further increase of the liquid flow rate. In thepresent study, the first point at which the value of lb reachesbelow the values in the region of the discrete/coalesced bubbleflow was assumed to be the transition point between discrete/coalesced bubble flow and the dispersed bubble flow.

Fig. 8. Average bubble chord length against superficial liquid velocity for

Ug¼0.03 m/s: (a) at different axial positions and center of bubble column and

(b) at different radial positions and height of 165 cm above distributer.

4. Results and discussion

4.1. Flow regime transition

4.1.1. Transition between discrete/coalesced bubble flow and

dispersed bubble flow

The plot of the average bubble chord length versus the super-ficial liquid velocity is shown in Fig. 7 at various superficial gasvelocities at 100 cm above the distributor. It can be seen in thisfigure that the value of the average bubble chord length increasesslightly at low liquid velocity (0.0031–0.015 m/s) and then beginsdecreasing below the values related to discrete/coalesced bubbleflow region corresponding to superficial liquid velocities between0.0031 and 0.0295 m/s. At 0.05 m/s superficial liquid velocity, thevalue of lb reaches below the values related to lower superficialliquid velocities. This indicates the occurrence of the regimetransition between discrete/coalesced bubble flow and dispersedbubble flow. For example, consider the curve corresponding toUg¼0.02 m/s in Fig. 7. The value of the average bubble chordlength is about 5 mm at the superficial liquid velocity of0.0031 m/s. This value increases to about 11 mm as the superficialliquid velocity is increased, then it decreases to approximately3 mm at UL¼0.05 m/s which is smaller than the initial value of

Fig. 7. Average bubble chord length against superficial liquid velocity at 100 cm

above the distributor and r/R¼0.

Please cite this article as: Shiea, M., et al., Comprehensive studresistivity probe. Chem. Eng. Sci. (2013), http://dx.doi.org/10.1016/j

the average bubble chord length (corresponding to the coalesced/discrete bubble flow region). Thus, the dispersed bubble flowoccurs at superficial liquid velocity of 0.05 m/s when the super-ficial gas velocity is equal to 0.02 m/s.

Similar plots were obtained at other axial and radial positionsand almost all results were consistent with each other. A samplerelated to the superficial gas velocity of 0.03 m/s is shown inFig. 8. In the dispersed bubble flow regime, the bubble sizedistribution is narrow and the bubbles are small and uniform insize. Therefore, the liquid circulation caused by ascending of largebubbles is suppressed. In addition, high liquid velocity decreases

Fig. 9. Variation of bubble frequency with respect to the superficial gas velocity at

165 cm above the distributor and r/R¼0.

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Fig. 10. Variation of bubble frequency with respect to the superficial gas velocity

for UL¼0.0031 m/s: (a) at different axial positions and center of bubble column

and (b) at different radial positions and height of 165 cm above distributer.

Fig. 11. Plot of sauter mean bubble chord length versus superficial gas velocity at

axial position 165 cm above the distributor and radial position of r/R¼0.2.

M. Shiea et al. / Chemical Engineering Science ] (]]]]) ]]]–]]]6

the bubble velocity fluctuations due to the intensified inertialforce of the water. These effects result in a uniform dispersion ofbubble throughout the bubble column. Thus, the position of themeasurement probe is not important for detection of regimetransition between discrete/coalesced bubble flow and dispersedbubble flow.

4.1.2. Transition between discrete bubble flow and coalesced bubble

flow

Fig. 9 displays the variation of bubble frequency with respectto the superficial gas velocity for different superficial liquidvelocities at 165 cm above the distributor and r/R¼0. The pointsat which the plots deviate from the linear relationship are shownin this figure. These points specify the superficial gas velocities atwhich the transition between discrete bubble flow and coalescedbubble flow occurs at each superficial liquid velocity. For exam-ple, consider the curves corresponding to UL¼0.0031, 0.0064 and0.009 m/s in Fig. 9. The trends are linear in the superficial gasvelocity range of 0.005–0.02 m/s. However, they deviate fromlinearity at higher superficial gas velocities (0.03–0.07 m/s). Thefirst point of deviation from linearity corresponds to regimetransition from discrete bubble flow to coalesced bubble flow.At the two highest liquid velocities (0.05 and 0.0602 m/s), how-ever, the bubble frequency reaches a maximum and then falls.This trend can be explained by the fact that there is highernumber of bubbles in the dispersed bubble flow compared to thediscrete bubble flow. Therefore, the coalescence of bubblescorresponding to regime transition results in a sharper fall inthe bubble frequency. Besides, further analysis of the data showedthat at the first data point following the peak of the bubblefrequency plot, superficial gas velocity is exactly the same as thatobtained by the criterion used for detection of transition to slugflow at liquid velocities of 0.05 and 0.06 m/s. In other words, notransition between dispersed bubble flow and coalesced bubbleflow has occurred at those two superficial liquid velocities.Instead, the dispersed bubble flow directly changes to the slugflow as the superficial gas velocity increases.

Comparison between the plots corresponding to different axialand radial positions, shown in Fig. 10, revealed that it is not possibleto detect the transition between discrete bubble flow and coalescedbubble flow by means of resistivity probe at radial positions greaterthan r/R¼0.4. In fact, large bubbles formed by coalescence tend toascend at the center of the column. In addition, it was found that thecoalesced bubble flow occurs at higher superficial gas velocities atlocation 30 cm above the distributor compared to higher positions. Asan explanation, the bubbles do not have efficient collisions to formlarge bubbles at axial positions close to the distributor.

4.1.3. Transition between coalesced bubble flow and slug flow

Fig. 11 shows calculated sauter mean bubble chord length versussuperficial gas velocity for various superficial liquid velocities at165 cm above the distributor and r/R¼0.2. For each superficial liquidvelocity, the point at which the sauter mean bubble chord lengthpasses the diameter of the column represents the transition betweenthe coalesced/dispersed bubble flow and slug flow. Similar plots wereobtained at other axial and radial positions to examine the potentialof different probe locations for detection of transition between thecoalesced/dispersed bubble flow and the slug flow. Fig. 12 is a sampleplot for superficial liquid velocity 0.0602 m/s. It was found that thesauter mean bubble chord length does not reach the diameter of thecolumn at 30 cm above the distributor and radial positions r/R¼0.8and r/R¼0.9. Gas slugs (Taylor bubbles) are formed by coalescence ofsmall bubbles. However, at distances close to the distributor thebubbles have not reached their maximum size to form gas slugs sincethe bubbles do not have enough time to merge and grow in size in

Please cite this article as: Shiea, M., et al., Comprehensive studresistivity probe. Chem. Eng. Sci. (2013), http://dx.doi.org/10.1016/j

such a distance. In addition, gas slugs rise at the center of the columnwhile the liquid and smaller bubbles exist in the regions close to thewall. Therefore, detection of gas slugs is not possible when theresistivity probe is placed close to either the distributer or the wall.

4.2. Flow regime maps

The flow regime maps were obtained using regime transitioncriteria at different axial and radial positions except axial position30 cm above the distributor and radial positions r/R greater than0.4 since some regime transition criteria were not applicable inthese positions. The map is presented in Fig. 13. It can be seen

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Fig. 12. Plot of sauter mean bubble chord length versus superficial gas velocity at

axial position for UL¼0.0602 m/s: (a) at different axial positions and center of

bubble column and (b) at different radial positions and height of 165 cm above

distributer.

Fig. 13. Flow regime map based on data at two different axial locations.

M. Shiea et al. / Chemical Engineering Science ] (]]]]) ]]]–]]] 7

from this map that the discrete bubble flow occurs at lowsuperficial gas and liquid velocities. As the superficial gas velocityincreases, the coalesced bubble flow appears in the column andexists only for a limited range of superficial gas velocity. Withfurther increase in the superficial gas velocity, slug flow regimeoccurs in the bubble column. Transition to slug flow regime doesnot practically depend on the superficial liquid velocity in therange that measurements were carried out. The dispersed bubbleflow can be observed at high liquid velocities. Transition betweenthe discrete/coalesced bubble flow and the dispersed bubble flowis almost independent of the superficial gas velocity in the range

Please cite this article as: Shiea, M., et al., Comprehensive studresistivity probe. Chem. Eng. Sci. (2013), http://dx.doi.org/10.1016/j

of experiments of this work. Transition trends shown in the flowregime maps are in a good agreement with the one reported byZhang et al. (1997). However, the values of superficial gas andliquid velocities at transition points are a little bit different.

The map obtained at two different axial positions (shown inFig. 13) reveals that the transition points differ slightly from oneposition to another. This confirms that regime transition occursthroughout the bubble column at nearly the same superficial gasand liquid velocities. However, this is not the case for the regionsclose to the distributor (up to 30 cm above the distributor in thiswork). For example, while the slug flow exists at elevated heightsof the column, coalesced bubble flow can be seen in the regionsnear the distributer. It should be noted that aforementioned resultis applicable only for the bubble column studied in this work andthe effect of any changes in the design of bubble column,especially changes in the diameter of the column and the typeof distributor, should be examined before generalizing the results.In addition, the closeness of the regime transition points atvarious axial and radial positions reveals that the resistivity probemeasurements for detection of regime transitions are reliable aslong as the location of the probe is kept close to the center of thecolumn and far from the distributor.

5. Conclusions

Flow regime transition points between dispersed bubble, discretebubble, coalesced bubble and slug flows were determined at differentlocations of the bubble column by employing a double-needleresistivity probe and the criteria proposed by Zhang et al. (1997).However, a different criterion was found to detect the transitionbetween discrete/coalesced bubble flow and dispersed bubble flow. Itwas shown that the resistivity probe method is applicable to detecttransition between discrete bubble and coalesced bubble flows onlywhen the probe is located close to the center of the column.Furthermore, it is necessary to place the probe far from the distributerand column wall to be able to determine the onset of the slug flow. Itwas also shown that regime transition points do not differ signifi-cantly along the height of the bubble column except in regions nearthe distributer. A flow regime map was presented based on the dataobtained at locations in which it was found that the resistivity probemethod is applicable.

Nomenclature

Dcolumn diameter of the column (mm);f bubble frequency (Hz);lb bubble chord length (mm);lb average bubble chord length (mm);L distance between the tips of the needles (m);Nb number of pulses in gas-phase density function;r radial position of the probe (mm);R radius of the column (mm);t bubble contact time (s);T total sampling time (s);Ug superficial gas velocity (m/s);UL superficial liquid velocity (m/s);Vb bubble velocity (m/s).

Greek symbols

Dt time interval between pulses in the gas density func-tions of needles (s);

eg gas hold-up;jg gas-phase density function.

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Subscripts

b bubble;g gas;l liquid.

Acknowledgments

The authors gratefully thank Ms. M. Abbasi for her great helpsand useful hints during this work.

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