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An experimental study was conducted to study the internal and external two-phase flow in an effervescent atomizer using high-speed imaging. The impact of the aerator tube configuration and mixing zone length on the size of bubbles inside the mixing zone and the spray droplet characteristics at different gas-to-liquid flow rates ratios were studied. An aerator tube with the conical end base was manufactured and tested. It is observed that the separation bubble at the trailing edge is suppressed by this configuration, which resulted in more uniform and smaller bubbles compared to the standard aerator tube with a flat base. The length of the mixing zone was found to have an impact on the bubble size distribution inside the mixing zone; more uniform and smaller bubbles are generated in the shorter mixing zone. The mixing zone length, however, does not show a distinct impact on the droplet velocity and size. It is concluded that a conical base aerator tube and a shorter mixing zone could significantly improve the spray steadiness and the atomization process.

In this research, a new bubble generator to produce fine bubbles was designed and manufactured through the interaction between a strong swirl and coanda flow at the edge of an exit nozzle, a larger bubble could be breakdown into fin microbubbles. A significant vacuum pressure was created at the throat of the exit nozzle to induce the atmospheric air required for bubbles generation process replacing high cost compressed air systems. This study covered coanda radius (0 and 20 mm), water flow rates 21.4 l/min and air flow rates 0.1 to 0.9 l/min. The results indicate, fine average diameter range between 64.3 to 87.6 µm depending on air flow rates and coanda radius. Bubble diameter decreases with increasing coanda radius and reducing the induced air flow rate.

An experimental study is reported that investigated the bubble formation from a novel nozzle design in a liquid cross-flow using high speed imaging. Different configurations and orientations of the novel nozzle design were considered over a range of gas-to-liquid flow rate ratios (GLR) from 0.00031 to 0.00204. The results show that for all cases, the novel nozzle generated smaller bubbles at higher detachment frequency compared to the standard nozzle. At low liquid velocities, the novel nozzle generated bubbles that were 30% smaller in size at a detachment frequency 2–3 times higher than that for the standard nozzle. It was also found that the bubble diameter and the detachment frequency are almost independent of the liquid velocity. The underlying physical process of bubble formation and detachment in the novel nozzle under liquid cross-flow was also investigated. It was observed that the process comprised of three phases: expansion, collapse and pinch off. It was also found that the rebound force of the bubble from a side-hole under the influence of liquid drag force and hydrostatic pressure plays a key role in the early bubble detachment. The results demonstrated that the novel nozzle design performs better than the standard nozzle in the liquid cross-flow, especially at high GLRs.

Reducing the pressurized atomizing gas in the effervescent atomizers with keeping fine spray is a desirable aim. This experimental study focuses on improving the characteristics of the effervescent spray (i.e. drop size and spray cone angle); while an "in-out" gas injection configuration atomizer operates under low gas-to-liquid ratios (GLR). These important characteristics are mainly affected by the internal two-phase flow pattern. Wherefore, shadowgraphy is utilized for visualizing and investigating the internal two-phase flow pattern directly upstream the exit orifice. The atomizer is operated at air gauge pressure of 0.6 Mpa and GLRs of (0.71% – 5.74%). For controlling the internal flow at the operating GLRs, two novel bubble breakers (BB) are designed and tested. The bubble breakers are fixed in the mixing chamber to affect the bubble size. The shadowgraph technique is utilized for visualizing and quantifying bubbles inside the atomizer, spray characteristics and the near nozzle spray. From the visualization and results, the internal two-phase flow patterns inside the mixing chamber vary from bubbly flow at the lowest three GLRs to a bubble-annular flow at the largest GLR, passing through an unsteady transition stage between these two patterns. Also, it is observed that the bubble breakers efficiently disintegrate large bubbles; the largest reduction percentage in the bubble size is 75.63% for the case of flat end BB at GLR of 2.21%. Also, the results showed that the drop size and spray cone angle are strongly affected by the bubble size (i.e. the internal flow structure) subsequently the presence of the bubble breakers. The largest reduction percentage in the droplet size is 68.88% for the case of the cone end BB at GLR of 0.77% and the largest increment percentage in the spray cone angle is 69.97% for the cone end BB at GLR of 0.77 %, all relative to the case of without BB.

Effervescent atomization technique is used in many industrial applications such as gas turbines, internal combustion engines, furnaces and burners, and pharmaceutical sprays. The present study is focused on investigating the influence of a new type of bubble breaker on the two-phase flows inside and outside an effervescent atomizer. Different configurations (size and number of holes) of the new bubble breaker were considered over a range of gas to liquid flow rates ratios (GLRs) from 0.001 to 0.022. High-speed imaging was used to visualize and quantify bubble and spray characteristics. The results showed that the two-phase flow regime and bubbles' size inside the effervescent atomizer, and the spray droplets' size and velocity outside the atomizer are strongly influenced by the bubble breaker. The results demonstrated that the bubble breaker effectively fragments large bubbles into smaller ones. Furthermore, the results illustrate that the diameter and numbers of holes of the bubble breaker affect the bubble size inside the mixing zone as well as the droplet size and velocity.

Bubble column reactors are used in industrial practices due to their intrinsic advantages of good mixing ability, high heat transfer and operational versatility. Generation of small bubbles in bubble column is a crucial step to improve their performance. The present investigation introduces a new approach for bubble breakup in an upward co-flowing liquid using a honeycomb monolith breaker with square cell structure. The experimental measurements were conducted using high speed imaging at different superficial liquid velocities and gas flow rates ranging between 8 to 50 cm/s and 165 to 1000 ml/min, respectively. A comparison between the bubbles generated from the monolith breaker and those generated from the nozzle shows that the monolith breaker reduces the bubble size by approximately 60% over the given range of liquid superficial velocities and gas flow rates. It is observed that at low superficial liquid velocities and low gas flow rates, the bubble size at the breaker exit follows log-normal distribution, which becomes more symmetric at higher superficial liquid velocities and gas flow rates. The main contributor of large bubbles formation at the monolith breaker exit is the bubbles׳ coalescence. Different mechanisms of bubbles coalescence at the breaker outlet are observed and classified into three types; multi- and successive “accumulative” coalescence, multi- and non-successive coalescence, and bubbles coalescence in the vicinity of the breaker outlet. The efficiency of the breaker is quantified in terms of the fractional conversion of bubbles׳ kinetic energy into the surface energy. A strong dependency of the breaker efficiency on the superficial liquid velocity is observed. The results indicate that an optimal liquid velocity exists that corresponds to the minimal bubble coalescence at which the breaker efficiency is maximum.

Aerated-liquid atomization, also called "effervescent atomization", is a technique that has a wide range of applications such as gas turbine combustors, internal combustion engines, furnaces and burners, and pharmaceutical sprays. We report on an experimental study conducted to investigate the two-phase flow in an Effervescent atomizer. A novel aerator tube base was implemented and tested. It is observed that the novel configuration suppresses the separation bubble at the trailing edge and results in more uniform and smaller bubbles compared to the standard flat base aerator. It has been found that the more uniform and smaller bubbles are generated as the mixing chamber length is reduced. It is concluded that by using a conical base aerator and by reducing the mixing chamber length, the spray steadiness and the atomization process can be significantly improved., ASME 2014 4th Joint US-European Fluids Engineering Division Summer Meeting, FEDSM 2014, Collocated with the ASME 2014 12th International Conference on Nanochannels, Microchannels, and Minichannels, 3 August 2014 through 7 August 2014

The bubbling of liquid with gases is required in a number of important industrial processes such as; gas–liquid contactor, gas–liquid separator, bubble absorption, bubble column, fermentation, metallurgy and waste water treatment. The size of generated bubbles has a significant effect on the hydrodynamics and mass transfer performance in these technological processes. In the present experimental study an innovative nozzle design for reducing the size of bubbles generated in stagnant liquid, at constant gas flow conditions, was tested and evaluated for two nozzles diameter as well as different gas flow rates. This technique is based on the creation of one or two side holes in the nozzle near the main nozzle hole so that the diameter of these side holes is lower than the inner nozzle diameter. Results show that the new nozzle design produced smaller bubbles with higher detachment frequency in comparison with those bubbles generated from the standard nozzle with no side holes.Copyright © 2013 by ASME

Injection of bubbly gas flow into a liquid stream has wide application in chemical and biochemical industries. In these applications, generation of smaller bubble at higher frequency is desirable to achieve better reaction efficiency. We report on an experimental study conducted to investigate the bubble formation in liquid cross-flow using a novel nozzle design developed by Gadallah and Siddiqui [1]. This design is based on the creation of two side holes in the standard nozzle near the main nozzle hole. The experiments were performed in an acrylic square flow channel and high speed imaging was used to capture bubble dynamics. An image processing algorithm was used to quantify the size and frequency of detached bubbles and to track each individual bubble at different gas flow rates and liquid velocities. The results show that the new design of nozzle generates more bubbles of smaller size compared to the standard nozzle with no side hole at low liquid flow rates. It was observed that while the liquid velocity has crucial affect on the bubble size and detachment frequency from a standard nozzle, these parameters are weakly dependent on the liquid flow rate in the novel nozzle. The results show that at low liquid flow rates or in the stagnant liquid, the novel nozzle generates bubbles that are more than 20% smaller in size and more than two times faster compared to the standard nozzle. Copyright © 2013 by ASME and Her Majesty the Queen in Right of Canada., ASME 2013 Fluids Engineering Division Summer Meeting, FEDSM 2013, July 7-11, 2013, Incline Village, Nevada, USA, Series: FED

Gas injection into a liquid cross flow from a nozzle causes bubble formations which have potential applications in industry such as chemical plants, waste water treatment and bio- and nuclear-reactors. The purpose of this study is to experimentally investigate the effects of nozzle shape and configuration with respect to the liquid cross-flow direction, on the bubbly flow characteristics such as bubble formation, detached bubble size and frequency at different gas and liquid flow rates. The experiments were conducted in a Plexiglas two-dimensional rig using a high speed camera. High speed imaging and an image processing algorithm were used to track each individual bubble and to quantify the bubble growth as well as the detachment frequency and the bubble velocity. Back light shadowgraphy which utilizes a low intensity diffuse light source to illuminate the background was used to image bubbles. Nozzles were mounted in the test section which was designed such that the flow in this section has a two-dimensional profile. The results showed that the bubble size increases with an increase in GLR (gas to liquid flow rates ratio). Furthermore, the bubble formations and detached bubble size were strongly influenced by the nozzle shape and configuration. Copyright © 2012 by ASME., ASME 2012 Fluids Engineering Division Summer Meeting, FEDSM 2012 Collocated with the ASME 2012 Heat Transfer Summer Conf. and the ASME 2012 10th International Conference on Nanochannels, Microchannels, and M, FEDSM 2012, 8 July 2012 through 12 July 2012, Rio Grande