· web viewliquid. once the bubbles have exited the nozzle, the pressure drop causes the bubbles...

Michael EfejukuFaculty Member: Dr. Thomas Shepard

The Effect of Bubble Size on Effervescent Atomization and Spray Stability

INTRODUCTION

Effervescent atomization is the conjunction between effervescent, meaning, gassy, or bubbly, and atomization which means, to reduce to fine spray. Arthur Lefebvre developed the effervescent atomization method in the 1980s (Sovani et al. 2001). Effervescent atomization has the potential to be used in the pharmaceutical industry as it assists with the granulation, coating, and drying process of pharmaceutical applications. Additionally, effervescent atomization can be found in the form of emulsion effervescent atomization, which is the atomization of a mixture of oil, gas bubbles and water. This form of effervescent atomization, emulsion atomization, has the potential to be used in metalworking fluids to provide cooling, surface cleaning, and corrosion protection of the metal (Ochowiak, 2011). Furthermore, effervescent atomization has potential to be used in gas turbines combustors, agriculture, and spray painting (Jagannathan et al. 2010). Combustion engines benefit from small droplets of fuel. A smaller droplet size reduces pollution emitted from combustion engines and improves its performance (NASA Memorandum, 1993); effervescent atomization can achieve this smaller droplet size while using less energy than conventional atomizers (Chou, 2001). Effervescent atomizers are becoming increasingly commonplace in engineering applications where there is a need for liquid to be fragmented into drops (Jedelsky et al. 2009). Due to atomization’s ubiquity in industry, understanding the parameters that affect the performance of the spray can be beneficial (Ochowiak, 2011).

LITERATURE REVIEW

The history of effervescent atomization begins with processes known as flash atomization and dissolved gas atomization. Flash atomization is the breaking up of a liquid through quickly evaporating parts of the liquid. Dissolved gas atomization utilizes a gas dissolved in a liquid to create bubbles that break the liquid up into droplets (Sovani et al. 2001). Both processes have limitations due to being constrained to liquids that had specific characteristics. (Sovani et al. 2001). Dissolved gas atomization is dependent on how much dissolved gas the liquid being atomized could hold. If the liquid could only hold a small amount of gas, atomization could not occur consistently. Comparably, flash atomization is only useful in liquids that evaporates at lower temperature which further limits the selection of liquids in which this process can be used. The need to have liquids with specific properties to conduct flash atomization and dissolved gas atomization increases the price for these processes and consequently, curtails the practicality of industries using these processes. Thus, Lefebvre developed the effervescent atomization process in response to the limitations of flash atomization and dissolved gas atomization (Sovani et al. 2001).

Effervescent atomization utilizes a two-phase flow to produce a spray that exits an atomizer. The two fluids used to produce the spray in most research are air and water. However, Meng et al. (2011) have used liquid water mixed with glycerol in conjunction with pressurized air in the atomizers to a degree of success. Additionally, Woziwodzki et al. (2010) used aqueous solutions of polyethylene oxide to study the effect of the molecular weight of a liquid polymer on effervescent atomization. For more literature on the different experiments that have been run, refer to, (Sovani et al. 2001).

To achieve effervescent atomization, a liquid is mixed with a pressurized gas that is injected into the atomizer. As pressurized gas enters the atomizer, the gas forms bubbles in the

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liquid. Once the bubbles have exited the nozzle, the pressure drop causes the bubbles to expand rapidly and turn the surrounding liquid into ligaments which subsequently turn into droplets as illustrated in Fig. 1.

FIG. 1: Effervescent atomization process (Sovani et al, 2001)

The droplets that form depend on key flow properties in the conduit (Meng et al. 2011; Rahman et al. 2012; Sovani et al. 2001). These key flow properties include injection pressure drop, atomizing gas/liquid mass ratio (GLR), Newtonian/non-Newtonian fluids, viscosity, surface tension, atomizer design and liquid density (Sovani et al. 2001). As GLR increases, the flow regime in the atomizer can change as well (Kim et al. 2009). There are three main flow regimes: bubbly flow, slug flow, and annular flow as shown in Fig 2.

a. Bubbly Flow b. Slug Flow c. Annular Flow

FIG. 2: Bubbly flow, a slug flow, and an annular flow (Shepard, 2011)

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An annular flow is characterized by liquid on the edge of the channel while gas occupies the center. A bubbly flow is characterized by bubbles in the conduit. Lastly, a slug flow which is characterized by large pockets of bubbles followed by portions of liquids (Sovani et al. 2001). Since a bubbly flow is achievable at lower GLRs, lower gas injection rates, and lower energy levels, it is more favorable than the other flow regimes under certain conditions (Sovani et al. 2001).

A spray’s instability refers to the transient liquid breakup of the spray. Spray instability can be looked at and measured qualitatively by examining how a spray changes with respect to time (Ghaemi et al. 2010). A stable spray should look similar as time increases. Figure 3 shows images selected from a few seconds of operation of an effervescent atomizer with an unstable spray. The images represent the range of spray quality seen over those few seconds. The non-uniform breakup characterizes this unstable spray. This non-uniform breakup can be seen when comparing each picture in the figure to one another. Examining each picture in Fig. 3 reveals the spray is unstable due to the left-most image lacking similarity in appearance compared to the right-most spray image. The left-most spray image is breaking up significantly whereas the right-most spray is not breaking up at all. This lack of break-up is like the stable spray in Fig. 4. The liquid is breaking up slightly. This similarity in spray characteristics such as liquid breakup can be seen in Fig. 4.

FIG. 3: An unstable spray (Shepard, 2011)

FIG. 4: Stable spray (Shepard, 2011)

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Effervescent atomization can create sprays that frequently fluctuate from good to poor (Kourmatzis et al. 2014). This spray’s potential instability can cause problems in effervescent atomizers. An unstable spray can result in a non-uniform spray pattern which can cause problems in combustors and other applications that rely on a steady droplet distribution (Sen et al. 2014). A paper by Ghaemi et al. (2010) suggests that one cause of the instabilities is the pulsations created by the rapid expansion of the bubbles as they exit the nozzle. Kourmatzis et al. (2014) suggests that another cause of this instability is bubble coalescence. Liu et al. (2011) found that spray unsteadiness correlates with GLR: as GLR increases, the spray becomes more unsteady. Sun et al. (2015) and Ramamurthi et al. (2009) suggest that flow regime has a significant impact on spray steadiness and stability. It is suggested that a semi-annular flow and a bubbly flow produce stable sprays while slug flows produce unsteady sprays (Sun et al., 2015).

Though many of the parameters that affect effervescent atomization are known, one parameter continues to be understudied: bubble size. Gomez, 2010; Shepard, 2011; Rahman et al. 2011 and 2013; Sen et al. 2014 have studied how bubble size and characteristics of the bubble in the atomizer have affected the characteristics of the spray. Though each of these researchers differed in terms of the operational conditions, similar conclusions were drawn: bubble size has an influence on effervescent atomization performance. The research shows that the size of the bubbles in the atomizer have a nonlinear correlation with the size of the droplets out of the nozzle orifice (Gomez, 2010; Shepard 2011). As the bubbles in the atomizer decrease in size, the droplets in the spray can become smaller. Though this is not always the case as Shepard, (2011) showed some smaller bubble sizes produce larger droplet sizes.

The current research examines the extent to which bubble size affects effervescent atomization and jet stability. It is hypothesized that bubble size affects effervescent atomization and jet stability to a large enough extent that when industries use the process for applications such as pharmaceutical coatings, combustion, corrosion protection, cleaning, agriculture, or surface cleaning, controlling the size of the bubbles during the process can provide better results than not controlling the size of the bubbles in the process. This research looked at more operating conditions as well as maintain good control on the variance of the bubble size by the methodology employed. Unlike much of the current literature which looks at two bubble sizes this research looked at three different bubble sizes for a given operating condition. Having this many data points allowed a trend to be created to determine if there is an optimal bubble size for a given operating condition. This study adds to the dearth of literature that looks at how bubble size affects effervescent atomization and spray stability. The spray’s stability was qualitatively measured by analyzing pictures of the same spray at different points in time.

METHODOLOGY

Figure 5 shows an example of the experimental setup. Tap water or salt water was filled into a 60-liter tank that is pressurized with compressed air. The water flow rate is maintained at 2.3 liters-per-minute. This flow rate was measured by an Omega flow meter (PX409-10WDWU5V) with a differential pressure sensor. The flow of gas into the atomizer is measured by the (Alicat scientific M250SLPM) gas mass flow meter. Prior to use, each device was calibrated to ensure data collection was accurate. The fluids met in a channel of height 16.2cm, a channel width of 1.2cm, channel depth of 0.787cm and left the channel at an exit diameter of 2mm. An example of this can be seen in Fig. 6.

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FIG. 5: Setup of the experiment

To change the size of the bubbles, porous plates that have media grades measured in micrometers received air through a hole of with a diameter of 11mm. To change the size of the bubbles further, inserts that create a localized acceleration point in the liquid flow were used. This insert speeds up the water thus reducing the bubble sizes. A smaller insert creates a larger area for the liquid to flow through whereas a larger insert creates a smaller area for the liquid to flow through. An example of this insert can be seen in Fig. 6.

FIG. 6: CAD of the channel with flow dimensions and acrylic window with an insert

Once the atomizer is creating a spray, digital images of the spray and the bubbly flow was captured using a Photron Fastcam SA1.1 camera and a Navitar 12x zoom lens with 0.25x

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adapters lens and a 2x F-Mount. For processing, 2000 spray images at speeds of 24000 frames-per-second and a 9.87 microsecond shutter speed were gathered to analyze spray stability using a custom code at a resolution of 320 x 640. 100 different bubble images at a frame rate of 5400 frames per second and a shutter speed of 141 microseconds were gathered for bubble analysis at a resolution of 1024 x 1024.

To measure the bubbles once they were imported to MATLAB, a total of 500 bubbles were analyzed. These bubble images were chosen at random to ensure an accurate representation of the average bubble diameter for each given operating condition. Once the images of the bubbles were selected, a custom MATLAB code analyzed each bubble image and returned a bubble size and a standard deviation. These numbers were converted to inches using dimensional analysis. The resulting bubble sizes standard deviations can be found in Table 1. An example of the bubbles that were measured in the atomizer can be seen in Fig. 7.

Additionally, an average image from 2000 spray images was made for each operating condition. From this average image, dark core length, the average length from the darkest point at jet’s exit to the end of the jet, and average jet angle were calculated using dimensional analysis and trigonometry. To calculate the dark core length, the darkest point on the jet’s length was found using MATLAB functions. The y – coordinate corresponding to the darkest point on the jet was added to the y – coordinate at the lowest point of the jet. This sum was divided by 2 and the resulting quotient was the dark core length. To calculate the average jet angle, a vertical line was drawn at the 2mm exit extending to the end of jet. The diagonal was determined by looking at the picture’s most intense point and moving left or right until that intensity began to decrease at the spray’s edges. The vertical line and the diagonal were then connected by a horizontal line, and the angles were calculated as shown in Fig. 7 using trigonometry. This method was utilized on the right and left side of the jet.

FIG. 7: Dark core length, jet angle calculations and bubbles (bottom: Db = 0.595 mm; top: Db = 1.60 mm; both at 0.0012 GLR)

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Liquid Flow Rate QL (LPM)

Gas-to-liquid ratio

Average Bubble Diameter (mm)

Standard Deviation(mm)

Channel Height

Porous Plate (microns)

Salt Content or Fresh Water

2.3 0.0006 0.619 0.21 Largest 7.13mm

40 1.50%

2.3 0.0006 0.91 0.27 Largest 40 Fresh2.3 0.0006 1.3 0.35 Smallest 40 Fresh2.3 0.0012 0.595 0.25 Largest 40 3%2.3 0.0012 0.904 0.20 Largest 40 Fresh2.3 0.0012 1.6 0.55 Smallest

4.06mm40 Fresh

TABLE 1: Operating conditions

RESULTS

In Figs 8 - 13, one can see that as the range of spray quality for a given condition moves from a good spray to a bad spray, the spray is breaking up significantly less. This phenomenon can be seen in each of the figures starting from Fig. 8 and ending at Fig. 13. At a smaller bubble size, the jet does not expand as far radially as it does at a larger bubble size. As the bubble size increases, the liquid jet is broken into sections more often as evidenced by comparing the best spray in Fig. 8 to the best spray in Fig 13. In Fig. 8 and Fig. 9, one can see the circular clumps of liquid forming from the bubbles exiting the atomizer’s orifice. In these two figures, it appears that the liquid took the shape of circular clumps.

By examining the figures, one can confirm that the worst spray is also the most stable spray. The worst spray picture in Fig. 8 is uniform throughout the jet, however, it is not breaking up which contributes to its uniformity. This phenomenon is seen throughout each bubble size’s spray image. The worst spray is consistent throughout Figs. 8 – 13, they look the same. On the contrary, the best spray is not uniform throughout its column which makes it unstable. As the bubble size increases, the third picture, which is a poorer spray compared to the best spray, becomes better as one moves from Figs. 8 – 13. From this analysis, one can assert that as the bubble size increases, the rate at which the liquid column breaks-up increases as well as evidenced by the best spray Fig. 10; one can see the individual separation of liquids in the column. In Fig. 8, one can see that the best spray is found in small controlled chunks of liquid which is not present in the higher bubble sizes. In higher bubble sizes, the spreading appears to occur not only more randomly, but also more often. Overall, at lower bubble sizes, the best sprays have clumps that do not spread out much. This can be seen in Figs. 8 – 9. However, in Figs. 10 – 13, the clumps spread out more radially and the break up is better.

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FIG. 8: Spray Breakup: 0.0012 GLR 0.595mm Db




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From Fig. 14, it is evident that Db, the average bubble diameter, does not affect the dark core length. This is evidenced by the 2.4% difference between the dark core length of the 1.6mm Db measurement and the dark core length of the 0.595mm Db measurement, the 4.4% difference between the 0.619mm Db and the 1.6mm Db, the 3.8% difference between the 0.904mm, Db and the 1.6mm Db, the 1.6% difference between the 0.91mm Db measurement and the 1.6mm Db, and lastly, the 4.7% difference between the 1.30mm Db and the 1.60mm Db. This data suggests that as Db increases, DCL remains relatively constant for the conditions studied. Figure 15 shows the jet angle in degrees vs the average bubble diameter. Looking at average jet angle at a GLR of 0.0006, Fig. 15 suggests that as bubble size increases, the jet spreads less. This suggest that, at the given operating conditions, a smaller bubble size causes the jet to break up more and thus spread more increasing the average jet angle. On the other hand, as the GLR changes from 0.0006 to 0.0012, the characteristics of the jet do not change significantly and, consequently, one cannot see a difference between the spray images as GLR increases.

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0.4 0.6 0.8 1 1.2 1.4 1.6 1.802468

101214

Dark Core Length Vs Db Avg

0.0006 GLR 0.0012 GLR

Average Bubble Diameter (mm)

Dark

Cor

e Le

ngth

(mm

)

FIG. 14: Dark Core Length Vs Average Bubble Diameter

0.4 0.6 0.8 1 1.2 1.4 1.6 1.80123456

Average Jet Angle vs Db Avg

Average Jet Angle 0.0006 GLRLinear (Average Jet Angle 0.0006 GLR)Average Jet Angle 0.0012 GLRLinear (Average Jet Angle 0.0012 GLR)

Db Avg (mm)

Angl

e, D

egre

es

FIG. 15: Jet angle (degrees) Vs Average Bubble Diameter

CONCLUSION

In conclusion, there is a relationship between spray characteristics and bubble size in an effervescent atomization. As bubble size increases the spray becomes more unstable and the breakup becomes non-uniform. Additionally, as the average bubble size increases, the angle at which the spray develops tends to decrease at a lower GLR. Furthermore, the dark core length does not increase as the average bubble size increases for either GLR.

For the future researcher, one could look towards higher gas-to-liquid ratios and study the effects of bubble size at those operating conditions. At those operating conditions, one might see a change in the average jet angle as Db increases or see a similar trend when observing dark core length vs bubble size. Additionally, one can also look at smaller bubble sizes. Unfortunately, due to research limitations, such as leaking around the porous plate, this research was only able to

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look at low GLRs thus reducing the scope of potential applications. Research down the road could also look toward finding a solution to the leaks thus enabling the atomization process to reach different operating conditions. Furthermore, one could find ways to further affect the bubble sizes by changing liquid properties.

REFERENCES

Ghaemi, S., Rahimi, P., and Nobes, D., Effect of Bubble Generation Characteristics on Effervescent Atomization at Low Gas-Liquid Ratio Operation, Atomization and Sprays., vol. 20, pp. 211-225, 2010.

Gomez, Johana., (2010). Influence of Bubble Size on an Effervescent Atomization

Jagannathan, T.K., Nagarajan, R., Ramamurthi, K. Effect of Ultrasound on Bubble Breakup Within the Mixing Chamber of an Effervescent Atomizer., Chemical Engineering Processing: Process Intensification., vol. 50, pp. 305 - 315, 2011.

Jedelsky, J., Jicha, M., Slama, J., Otahal., Development of an Effervescent Atomizer for Industrial Burners, Energy Fuels., vol 23, pp. 6121-6130, 2009.

Kim, J., and Lee, S., Dependence of Spraying Performance on the Internal Flow Pattern in Effervescent Atomizers, Atomization and Sprays., vol. 11, 22 pages, 2001.

Kourmatzis, A. Lowe, A. and Masri, A.R., Atomization Instabilities in Bubble Induced Break-up Proc. of 19th Australasian Fluid Mechanics Conference, 2014.

Kourmatzis, A., Lowe, A., Masri, A.R., Combined Effervescent and Airblast Atomization of a Liquid Jet, Experimental Thermal and Fluid Science., vol. 75, pp 66-76, 2016.

Liu, M., Duan, Y., Zhang, T., Xu., Y, E Evaluation of Unsteadiness in Effervescent Sprays by Analysis of Droplet Arrival Statistics – The Influence of Fluids Properties and Atomizer Internal Design., Experimental Thermal and Fluid Science, vol. 35, pp. 190-198, 2011.

NASA Technical Memorandum 105888 Application of Jet-Shear-Layer Mixing and Effervescent Atomization to the Development of a Low-NO Combustor, 1993.

Ochowiak, M., The Effervescent Atomization of Oil-In-Water Emulsions, Chemical Engineering and Processing, vol. 52, pp. 92-101, 2012.

Rahman, A. Amin, A. Hossain, A. and Fleck, B., Numerical Investigation of Two-Phase Nozzle Flow, Procedia Engineering, vol 90, pp 346-350.

Rahman, M.A., Balzan, M., Heidrick, T., Fleck, B.A., Effects of the Gas Phase Molecular Weight and Bubble Size on Effervescent Atomization., International Journal of Multiphase Flow vol. 38, pp. 35-52, 2012.

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Ramamurthi, K. and U. K. Sarkar., Performance Characteristics of Effervescent Atomizer in Different Flow Regimes, Atomization and Sprays., vol. 19, pp. 41-56, 2009.

Sen, D. Balzan, M. Nobes, D. and Fleck, B., Bubble Formation and Flow Instability in an Effervescent Atomizer, Journal of Visualization., vol. 17, no 2, pp 113-122.

Shepard, Thomas G.. (2011). Bubble Size Effect on Effervescent Atomization.. Retrieved from the University of Minnesota Digital Conservancy, http://hdl.handle.net/11299/113573.

Sovani, S., Chou, E., Sojka P.E., Gore, J.P., Eckerle, W.A., Crofts, J.D., High Pressure Effervescent Atomization: Effect of Ambient Pressure on Spray Cone Angle, Fuel, vol. 80, pp. 427-435, 2001 (a).

Sovani, S.D., Sojka, P.E., Lefebvre, A.H., Effervescent Atomization, Progress in Energy and Combustion Science, vol 27, pp. 483 – 521, 2001 (b).

Sun, C. Ning, Z. Lv, M. Yan, K. and Fu, J., Time-Frequency Analysis of Acoustic and Unsteadiness Evaluation in Effervescent Sprays, J. Chemical Engineering Science, vol. 127, pp 115-125.

Woziwodzki, S., Ochowiak, M., Broniarz-Press, L., Atomization of PEO Aqueous Solutions in Effervescent Atomizers, International Journal of Heat and Fluid Flow., vol. 31, pp. 651-658, 2010.

Zaremba, M., Weib, L., Maly, M., Wensing., M., Jedelsky., J., Low Pressure Twin Fluid Atomization: Effect of Mixing Process on Spray Formation, International Journal of Multiphase Flow., vol. 89, pp 277-289, 2017.

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