REGULAR PAPER

Debjyoti Sen • Miguel A. Balzan • David S. Nobes •

Brian A. Fleck

Bubble formation and flow instabilityin an effervescent atomizer

Received: 30 May 2013 / Revised: 17 July 2013 / Accepted: 22 September 2013� The Visualization Society of Japan 2014

Abstract Bubble formation from a gas jet in a liquid cross flow inside the mixing conduit of an effervescentatomizer is studied using flow visualization. Air injection rates were varied from 0.99 9 10-5 to4.94 9 10-5 kg/s, while maintaining a constant liquid cross-flow rate of 0.298 kg/s. Though gas–liquid ratio(GLR) and flow velocities dominate bubble formation, intermittent choking at the exit nozzle is shown tohave a morphological impact on the bubbly flow. This impact is visible as a pressure wave that propagatesupstream from the nozzle exit. This pulse is found to have a strong influence on bubble formation at the gasinjection location leading to bubble distortion and breakup. In most cases, a liquid jet is observed topropagate through the centreline of larger bubbles as the pressure pulse passes.

Keywords Bubble generation in cross flow � Flow visualization � Shadowgraph imaging � Bubble instability

1 Introduction

Effervescent atomization involves the injection of a gas and a liquid into a mixing chamber and allowing thegas–liquid mixture to escape through a small exit orifice or nozzle (Roesler and Lefebvre 1988). The largepressure drop at the exit leads to a sudden expansion of the gas which shatters the liquid around it into smalldroplets (Lefebvre 1989). Effervescent atomizers have gained popularity due to several advantages overconventional atomizers (Sovani 2001). These atomizers can produce a small droplet size distribution atlower operating pressures; a large nozzle diameter can be used which prevents clogging of the nozzle; andconsistent spray performance in combustors can lead to increased combustion efficiency with reducedpollution (Sovani 2001; Konstantinov et al. 2010).

The two-phase flow phenomena inside the mixing chamber play a major role in controlling the propertiesof the external spray (Lin et al. 2001). The size of the bubbles approaching the exit orifice depends on theupstream conditions which include the formation of bubbles at the gas inlet orifice and bubble transportalong the mixing chamber (Konstantinov et al. 2010). Sovani (2001) observed that gas injection rates to theliquid cross flow have an effect on bubble formation at the gas inlet orifice. With low gas injection rates orlow gas–liquid ratio (GLR), single bubbles are formed at the gas inlet orifice and they have sufficient time toget detached and be convected with the liquid. However, with increasing GLR, the bubbles do not have

Electronic supplementary material The online version of this article (doi:10.1007/s12650-014-0196-3) contains supple-mentary material, which is available to authorized users.

D. Sen � M. A. Balzan � D. S. Nobes (&) � B. A. FleckDepartment of Mechanical Engineering, University of Alberta, Edmonton, CanadaE-mail: david.nobes@ualberta.ca

B. A. FleckE-mail: bfleck@ualberta.ca

J VisDOI 10.1007/s12650-014-0196-3

sufficient time to detach, and a jet of gas is formed at the inlet orifice. The breakup of gas jet into bubbles isa complex mechanism. Badam et al. (2007) characterized jet breakup into the three broad categories of ‘jet-varicose break up’, ‘jet-sinuous break up’ and ‘jet atomization’. Once formed, these bubbles are convecteddownstream through the mixing chamber by the liquid current.

The interaction of the bubble stream with the exit orifice has been studied by several research groups(Konstantinov et al. 2010). Catlin and Swithenbank (2001) used flow visualization to study the mechanismof bubble–orifice interaction. Shock wave-like structures were observed in the external flow when a bubbleexplodes in the low pressure region after passing through the small exit orifice. Kim and Lee (2001) andHuang et al. (2008) studied internal and external flow at the exit orifice location. They both identified threedifferent flow regimes characterized as bubbly, intermittent and annular flow (Konstantinov et al. 2010; Kimand Lee 2001; Huang et al. 2008). With an increase in GLR, the flow regime is reported to change frombubbly, to intermittent, to annular. Annular flow conditions resulted in a spray with the smallest droplet sizeswhile bubbly and intermittent flow conditions resulted in a spray with larger droplet size. The size of thedroplet depends on the thickness of the liquid film which is formed when a bubble squeezes through the exitorifice. A larger bubble size produces a thinner liquid film and hence produces smaller droplets when thebubble explodes. However, larger bubbles passing through the exit orifice can cause instability in the spray(Jedelsky and Jicha 2008; Konstantinov et al. 2010).

Spray instability is a major problem in effervescent atomizers (Konstantinov et al. 2010). An unstablespray can result in an uncontrolled and non-uniform spray pattern which could cause intermittent com-bustion, noise and pollution in combustors (Lefebvre 1988; Konstantinov et al. 2010). Ghaemi et al. (2010)characterized spray steadiness using an optical method. They used two separate gas injection methods: oneinvolving a multi-hole injection system and the other was a porous media injection system. The multi-holeinjection system generated bubbles with a larger diameter compared to the porous media injection system.The resultant spray was measured to have a more consistent spray pattern with the porous media-basedinjection system compared to multi-hole injection system. Maldonado et al. (2008) used flow visualizationand wall pressure fluctuation data to characterize the stability of their spray. They concluded that the flowpattern entering an exit orifice plays a major role in spray stability. A dispersed bubble pattern resulted in amore stable spray pattern and an intermittent flow of bubbles resulted in an unstable spray.

In the present study, a horizontal channel with a square cross section and transparent windows is used toreplicate the internal bubble generation cavity of an effervescent atomizer. The square channel has a slot-shaped exit orifice and gas is injected into the liquid cross flow at a location upstream from the exit orifice.The present study aims at better understanding of the formation of gas bubbles and gas jet breakup. High-speed shadowgraph imaging was used to obtain time-lapsed images of bubble formation. The effect of GLRon bubble formation and gas jet breakup in the region of the gas injection orifice is studied. The interactionof bubbles of different sizes with the exit orifice (downstream location) is also studied. The present studyprovides insight on the internal flow structures and its influence on stability of sprays in effervescentatomizers.

2 Experimental setup

The experimental apparatus consists of a horizontal channel which was designed and constructed in theMechanical Engineering Department at University of Alberta. A schematic of the channel is shown inFig. 1. The body of the channel is made of aluminum. The channel has two flat transparent polycarbonatewindows which allow viewing and illumination of the mixing chamber from either side. The interior of thechannel has a square cross section with a hydraulic diameter, DH of 12.7 mm. The total length of the channelLc = 1.12 m. The exit slot orifice is a replaceable rapid prototyped plastic component fitted inside thechannel as shown in Fig. 1. The width of the exit slot, Deo is 0.8 mm. The two ends of the channel have acylindrical inlet and outlet for liquid flow. The top and bottom of the channel body have small portsdistributed along the length of the channel that serve as the point of air injection and for measurement ofpressure with transducers. There is provision for injection of gas through 12 injection ports along the lengthof the channel; however, the present study required the use of only one gas injection port. In the presentstudy, the effective length of the mixing conduit is 0.54 m. The diameter of the air injection port used in theexperiment is 0.521 mm. The dimensions of the mixing conduit and the ratio Lc/DH = 44, resembles that ofcommercial feed conduits (Maldonado et al. 2008; Gomez et al. 2011). Four pressure transducers (PX 613,Omega) are strategically placed to measure the mean pressure drop along the length of the channel. As

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shown in Fig. 1 the pressure transducers are denoted by P1–P4. The mean pressure values obtained bytransducer P1, P2, P3 and P4 are, respectively, 182.21, 181.91, 181.13 and 88.85 kPa.The pressure dropacross the exit orifice is 99.28 kPa.

The channel operates as a closed loop. For all experiments, water was used as the working liquid and wassupplied into the channel from a reservoir using a centrifugal pump (Model A-97568304, Grundfos). Afterthe water passed through the channel, the air/water mix was collected in a separate reservoir to allowdegassing. The reservoirs were connected to each other allowing passage due to gravity, which maintains asteady circulation of water in the system. A water bypass connection into the reservoir is placed just beforethe inlet of the channel and valves were used to control flow of water into the channel. Water mass flowmeters (FLR8340D, Omega) are installed in the bypass line and in the main inlet line and the difference ofwater flow rates between the inlet and the bypass line indicates the amount of water flowing through thechannel. An air compressor (GX4FF, Atlas Copco) was used to provide a steady source of air at constantpressure up to 551.58 kPa. A gas mass flow controller (MC-20SLPM, Alicat Scientific) was used to regulateand monitor the flow of air into the channel.

Experiments were conducted with a fixed water flow rate, QL of 0.299 L/s resulting in mean liquidvelocity of 1.86 m/s and the calculated Reynolds number, ReL = 27,060. The gas mass flow rate, (mG), atinjection was varied from 0.99 9 10-5 to 4.94 9 10-5 (kg/s) resulting in mean gas injection velocity, Uinj

of 21.55–108.53 m/s. Using the above flow rates for liquid, resulted in GLR values ranging from3.30 9 10-5 to 16.54 9 10-5. The experimental parameters are listed in Table 1.

High-speed imaging of bubble evolution was carried out using a 1,280 9 1,024 pixel 8bit CCD camera(Lightning RDT Motion Pro, Redlake Inc). A sub-region (1,280 9 168 pixels) of the CCD was scanned sothe camera could operate at 3,000 frames per second. The channel was back-illuminated so that bubbleimages were collected as a shadowgraph. The camera and the light sources were mounted on a three axiscontrol traverse allowing accurate positioning and repositioning of the camera. A 35-mm lens (AF-S35 mm, Nikkor) was used to obtain a field of view of 68 9 9.7 mm. An exposure time of 200 ls wasselected to freeze the bubble and fluid motion. The high-speed images were taken at the injection location(upstream) and near the exit orifice (downstream). The regions of interest (ROI) are denoted in Fig. 1 as ROIA and ROI B. A calibration target is placed on the outer window of the channel at ROI A and ROI B toconvert from pixel to real space with a determined calibration factor of 0.0577 mm/pixel.

3 Results and discussion

3.1 Gas injection location, ROI A

Example images of the breakup of the gas jet into bubbles at different GLR conditions (Case ID: G1-G5) areshown in Fig. 2. The images have been selected from a time series to show the gas jet after it breaks up intobubbles. The liquid cross flow is in the positive x direction and the gas is injected into the liquid cross flowfrom below. The liquid momentum bends the gas jet and drags it to the positive x direction while buoyancy

Fig. 1 Schematic of the experimental rig. All dimensions are in mm

Bubble formation and flow instability in an effervescent atomizer

and gas injection momentum act on the jet in positive y direction. At low GLR conditions, the jet breaks upinto small, roughly spherical bubbles as shown in Fig. 2a, b. At these GLR conditions, the hydrodynamicdrag of the liquid cross flow acts strongly to drag the bubbles downstream while buoyancy has a smaller butnoticeable influence and continues to raise the bubble in the positive y direction. With an increase in the gasflow rate and GLR, the jet length increases and the jet breaks up into larger elongated bubbles as shown inFig. 2c through Fig. 2e. With increasing GLR, there is an increase in the size of the largest bubblegenerated.

Some unique features are also observed in Fig. 2b–e at the time step immediately after the gas jet hasfirst divided into bubbles. The largest bubbles, generated at what would have been the end of the gas jet,show the appearance of an inward and concave-shaped deformation in the surface of the leading face of thebubble. At the center of this deformation, more distinctly seen in Fig. 2c–e, a counterflow water jet forms,passing through the air bubble in the negative x direction. This phenomenon is more clearly seen in themovies of the jet growth and breakup for flow conditions denoted by Case ID: G1 and Case ID: G4 providedin the Online Resource 1 (Ref. Electronic Supplementary Material). From this observation, it seems that asudden pressure pulse exerted on the bubble, traveling in the negative x direction might be responsible forsuch structures. With an increase in GLR (Case ID: G2 through Case ID: G5) the pressure pulse appears togrow in strength, which is evident from the nature of deformation of the gas bubble. The penetration of thewater jet inside the air bubble increases with increase of GLR, which also suggests an increase in thestrength of the pressure pulse. The penetration of the water jet increases from 7.1 to 13.1 mm for the datapresented in Fig 2. The effect of the pressure pulse is not observed for the flow condition with the lowestGLR condition (Case ID: G1).

A detailed time-lapsed series of images for flow condition denoted by Case ID: G4 in Table 1 is used todescribe the breakup mechanism of the gas jet in Fig. 3. Figure 3a shows the image of gas jet just before thearrival of the pressure pulse. The jet has a blob shaped head which starts to distort with the onset of arrival ofthe pressure pulse (Fig. 3b). The distortion occurs through the center of the blob head of the gas jet.Immediately after the arrival of the pressure pulse, the jet begins to break up as shown in Fig. 3c. In Fig. 3cthe pressure pulse also results in the formation of the water jet which passes through the center of the newlyformed bubble. Three jet pinch-off locations (JPL) are identified in the Figure. The jet thickness is at aminimum at JPLs and the jet breaks up from these locations. As the pressure pulse travels through the liquid,regions denoted by JPL become thinner until the gas jet collapses. Figure 3(f) shows that the jet breaks upinto three oblong bubbles. Continued observation would show that the oblong bubbles would advectdownstream and stabilize into roughly spherical bubbles. The small gas jet shown in Fig. 3f would continueto grow until the next bubble breakup event.

More detailed images of the breakup of the gas jet are shown in Fig. 4. At time t = 0, the jet is fullygrown and the deformation of the jet begins at t = 0.67 ms. The initial thickness of the gas jet at location JPL1 and JPL 2 is, respectively, 1.1 and 1.27 mm. The jet collapses and pinches off at locations JPL 1 and JPL 2after 2 and 2.67 ms, respectively. While this is occurring, the water jet is traveling inside the gas bubble andgrows in length to become 6 mm long in 3.33 ms. This is shown in more detail on the right side of the timeseries in Fig. 4. These images also show the smooth surface of the bulbous head of the gas jet. The firstbubble to break-off is deformed as a surface waves move over the liquid/gas interface. Figures 2 and 3describe one instance of a pressure pulse deforming the jet and assisting in jet breakup, several suchinstances can be observed by watching the videos provided as Supplementary Electronic Materials.

3.2 Exit orifice location: ROI B

Time series images were also taken at the downstream location near the nozzle exit orifice with the sameflow conditions as described in Table 1. The images of bubbles approaching the exit orifice at two time steps

Table 1 The gas flow rates are varied while maintaining a constant liquid flow rate

Case ID mG (kg/s) 9 10-5 Uinj (m/s) Reinj GLR 9 10-5

G1 0.99 21.55 1,300 3.30G2 1.97 43.21 2,600 6.60G3 2.99 66.32 3,950 10.02G4 3.95 86.80 5,220 13.23G5 4.94 108.53 6,520 16.54

QL = 0.299 (L/s), mL =0.298 (kg/s), ReL = 27060. The values of pressure in the channel measured by transducers P1, P2, P3 andP4 are, respectively, 182.21, 181.91, 181.13 and 88.85 kPa

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are shown in Fig. 5 for the different GLRs, to view the bubble denoted by B1 as it passes through the exitorifice. The bubble denoted by B2 is further upstream than B1. Note the shape of bubble B2 immediatelybefore and after the passage of B1 through the exit orifice. At low GLR as shown in Fig. 5a the bubblesapproaching the exit orifice are generally smaller in size and small compared to the exit orifice. Thesebubbles squeeze through the exit orifice without blocking it. The entire bubble passes through the exit orificein 0.67 ms. With an increase in GLR, larger bubbles arrive intermittently and are accompanied by smallbubbles (also shown in the Online Resource 2). Figure 5b–e shows the effect of passage of a large bubblethrough the exit orifice on trailing bubbles. Bubbles denoted by B2 are deformed as B1 passes through theexit orifice. This is not the case for the lowest GLR condition where the B2 bubble is not deformed and theB1 bubble is small compared to the exit orifice as shown in Fig. 5a.

The interaction of a large bubble with the exit orifice is shown in a time series of images in Fig. 6. Astrong change in the shape of bubbles marked as B2 and B3 located further upstream can be observed.Figure 6a shows that as bubble B1 is deformed and squeezes through the small orifice, there is contractionof volume and the surrounding liquid accelerates to fill the volume. This results in a sudden increase in the

Fig. 2 Shadowgraph images of gas jet breakup and evidence of a pressure pulse at different GLR (refer Table 1) are shown.The evidence of pressure pulse becomes more prominent with an increase in GLR values a Case ID: G1, b Case ID: G2, c CaseID: G3, d Case ID: G4, e Case ID: G5

Bubble formation and flow instability in an effervescent atomizer

liquid velocity that is also observed in Online Resource 2. Similar observations for flow acceleration werereported by Huang et al. (2008) and Konstantinov et al. (2010). The effect is observed in Fig. 6b wherebubble B3 develops a bulge outwards (convex) in the positive x direction. This deformation is also apparentin bubble B2, which has a convex right bubble wall and concave left bubble wall. Other smaller bubbles in

Fig. 3 Time-lapsed shadowgraph images of breakup of a gas jet in liquid cross flow are shown. The images are for flowconditions denoted by Case ID: G4 (refer Table 1). Images show the breakup of the jet immediately after arrival of a pressurepulse. The cross flow is in positive x direction while the pressure pulse is traveling in opposite direction. a Full grown jet. bArrival of pressure pulse. c Jet breaks up at JPL 1. d Jet breaks up at JPL 3. e JPL 3 becomes thin. f Jet breaks up at JPL 3

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the images show similar characteristics. The shape of the bubbles is due to the increase in liquid pressure inthe positive x direction. As bubble B1 squeezes through the small orifice further, it chokes the orifice. Thisacts like the sudden closing of a valve and a pressure pulse develops which travels in the negativex direction. This could be envisaged as a water hammer effect and corresponding pressure wave that travelsback along the channel. The effect is observed in Fig. 6c, B2 disintegrates and B3 develops a concave rightwall. Another factor which is believed to contribute to the pressure pulse is the sudden expansion of thebubble as it completely passes through the orifice. Figure 6d shows that bubble B1 has completely passedthrough the orifice and it suddenly expands. The sudden expansion of the bubble can cause strong pressurewaves (Catlin and Swithenbank 2001). The effect can be seen on bubble B3. Further deformation of bubbleB3 occurs and the concavity of the right side of the bubble increases (refer Fig. 6).

The pressure pulse observed while studying ROI A is found to originate in the downstream location nearthe exit orifice in ROI B. The pressure pulse originates when large bubbles compared to the size of the exitorifice passes through it. With an increase in size of the bubbles passing through the orifice the effect ofpressure pulse becomes more prominent (refer Fig. 5). The same observation is made at upstream location

Fig. 4 A zoomed in section of gas jet formation is shown here. The deformation of the bulbous head of the gas jet occurs uponarrival of the pressure pulse. The pressure pulse travels in negative x direction. A water jet is found to travel inside the gasbubble. The gradual thinning of jet pinch-off locations denoted by JPL 1 and JPL 2 is shown

Bubble formation and flow instability in an effervescent atomizer

ROI A. The large bubbles which are produced from breaking up of the gas jet (refer Fig. 2) seem to havemore prominent effect. This can be observed by looking at the liquid jet penetrating into the gas bubbles(Fig. 2 through Fig. 4). While for Case ID: G1, small uniformly dispersed bubbles pass through the exitorifice without blocking it seems to have negligible effect for both locations ROI A and ROI B. Moreevidence can be found by studying the videos provided as electronic supplementary material.

The qualitative study presented here could explain some of the phenomena observed earlier byresearchers (Catlin and Swithenbank 2001; Ghaemi et al. 2010; Maldonado et al. 2008). They had observedthat small dispersed bubbles in the mixing chamber of an effervescent atomizer produces a stable spray withless pressure fluctuations, however, a detailed explanation was not provided. The present study showsqualitatively that the pressure fluctuations in the channel and stability of the spray are related to the size ofthe bubbles which are interacting with the exit orifice.

3.3 Electronic supplementary material

High-speed image sequences in the form of videos are provided. The sequence entitled ESM1.avi shows theformation of the gas jet for Case ID: G1 and Case ID: G4 concurrently. The movie shows formation ofsingle dispersed small spherical bubbles at low GLR (Case ID: G1) and gas jet formation and breakup intolarger elliptical bubbles (Case ID: G4). Several occurrences of bubble deformation and jet breakup due topressure pulse are shown for Case ID: G4, whereas for Case ID: G1 no such evidence was observed.

The video entitled ESM2.avi, shows the interaction of bubbles with the exit orifice for Case ID: G1 andCase ID: G4. For Case ID: G1 small uniform bubbles are found to easily squeeze through the small exitorifice without blocking it. For the higher GLR values, Case ID: G4, larger bubbles appear intermittentlywith numerous small bubbles. The smaller bubbles pass through the orifice smoothly whereas large bubblesinteract with the exit to create pressure surges that propagate upstream in the flow channel.

Fig. 5 The interaction of bubble with the exit orifice is shown. Each sub figure shows images of bubbles at two time steps. Theshape of the bubble B2 is observed immediately before and after the passage of bubble B1 through the exit orifice. a CaseID:G1, b Case ID: G2, c Case ID: G3, d Case ID: G4, and e Case ID: G5 (refer: Table 1)

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4 Conclusions

A shadowgraph flow visualization technique has been used to investigate the mechanism for gas jet for-mation and breakup into bubbles in a liquid cross flow. Important flow properties that could affect stabilityof an effervescent spray are identified. Pressure pulses which originate downstream due to the interaction oflarge bubbles with the exit orifice are observed. With increasing GLR, larger bubbles are formed inter-mittently and when these bubbles interact with the exit orifice, a pressure pulse is generated. The pressurepulse is found to affect gas jet breakup and bubble formation at the upstream location. The effect of thepressure pulse increases with an increase in GLR, which is a direct consequence of the bubble size. Thebubble size depends on the jet breakup mechanism which is affected by the pressure pulse. It can be

Fig. 6 Interaction of gas bubble, marked as B1, with the exit orifice is shown. The change in shape of bubble B2 and B3 isobserved. The images are for flow conditions denoted by Case ID: G4 (refer Table 1). a Bubble denoted by B1, contracts andsqueezes through the small orifice. b B1 clogs the nozzle and creates a pressure wave which travels in negative x direction. cSudden expansion of B1 produces additional pressure pulse in negative x direction. d B2 shatters due to pressure pulse and B3is deformed

Bubble formation and flow instability in an effervescent atomizer

concluded here that there is a direct coupling of the large bubble exit to the breakup and generation of largerbubbles at the gas injection location at larger GLRs.

Acknowledgments The authors acknowledge funding support for this research from Alberta Ingenuity Fund, the NaturalSciences and Research Council (NSERC) of Canada, and the Canadian Foundation of Innovation (CFI).

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