An experimental investigation was performed to study the influence of the bubble size on an effervescent atomization. Experiments were conducted in horizontal facility with a 25.4mm diameter feeding pipe using water and air as the working fluids that were sprayed through an effervescent nozzle. Water flow rates from 113 to 189 kg/min and air to liquid mass ratios from 1% to 4% were selected. High speed photographs, of the bubbles in the feeding conduit and of the resulting droplets on the spray, were taken to use the particle projected areas to estimate their sizes. A monotonic positive correlation was found between the bubble size and the droplet size, in a fairly narrow range of feed flow void fractions. A bubble size sensitivity parameter was defined. Knowledge of the droplet behaviour provides data to enhance the design and operating conditions of the atomization process and a means to control droplet size.

An experimental investigation was performed to study the influence of the bubble size on an effervescent atomization. Experiments were conducted in horizontal facility with a 25.4mm diameter feeding pipe using water and air as the working fluids that were sprayed through an effervescent nozzle. Water flow rates from 113 to 189 kg/min and air to liquid mass ratios from 1% to 4% were selected. High speed photographs, of the bubbles in the feeding conduit and of the resulting droplets on the spray, were taken to use the particle projected areas to estimate their sizes. A monotonic positive correlation was found between the bubble size and the droplet size, in a fairly narrow range of feed flow void fractions. A bubble size sensitivity parameter was defined. Knowledge of the droplet behaviour provides data to enhance the design and operating conditions of the atomization process and a means to control droplet size.

This thesis is based on observations of the physical properties of wheat flour dough using ultrasonic measurements. Three frequency ranges were used in the study, low frequencies (near 40 kHz), intermediate frequencies (1 to 5 MHz, where bubble resonance effects are apparent), and high frequencies (near 20 MHz). Doughs mixed under different head space air pressures, from vacuum to atmospheric pressure, as well as under nitrogen, were studied at low frequency to investigate their relaxation behavior. Subsamples from ambient dough and vacuum dough displayed differences in the dependence of velocity and attenuation on time after compression, but no post mixing relaxation effect was apparent. A critical headspace pressure of approximately 0.16 atmospheres determined whether vacuum-like or ambient-like relaxation was observed. A peak in attenuation and changes in ultrasonic velocity were observed around the bubble resonance frequency, and these ultrasonic parameters changed substantially as a function of time. A bubble resonance model was used to interpret the results around the bubble resonance frequency, and bubble size distributions were estimated for ambient and vacuum dough from the ultrasonic data. For the high frequency range, a molecular relaxation model was used to interpret the results. Different fast relaxation times were observed for ambient dough (5 ns) and vacuum dough (1 ns). This relaxation time may be associated with conformational rearrangements in glutenin inside the dough matrix. These experiments have enabled dough relaxation to be probed over a very wide range of time scales (from ns to hours), and will lead to a better understanding of the role of dough matrix and gas cell effects on the physical properties of wheat flour doughs. / October 2007

No description available.

This thesis is based on observations of the physical properties of wheat flour dough using ultrasonic measurements. Three frequency ranges were used in the study, low frequencies (near 40 kHz), intermediate frequencies (1 to 5 MHz, where bubble resonance effects are apparent), and high frequencies (near 20 MHz). Doughs mixed under different head space air pressures, from vacuum to atmospheric pressure, as well as under nitrogen, were studied at low frequency to investigate their relaxation behavior. Subsamples from ambient dough and vacuum dough displayed differences in the dependence of velocity and attenuation on time after compression, but no post mixing relaxation effect was apparent. A critical headspace pressure of approximately 0.16 atmospheres determined whether vacuum-like or ambient-like relaxation was observed. A peak in attenuation and changes in ultrasonic velocity were observed around the bubble resonance frequency, and these ultrasonic parameters changed substantially as a function of time. A bubble resonance model was used to interpret the results around the bubble resonance frequency, and bubble size distributions were estimated for ambient and vacuum dough from the ultrasonic data. For the high frequency range, a molecular relaxation model was used to interpret the results. Different fast relaxation times were observed for ambient dough (5 ns) and vacuum dough (1 ns). This relaxation time may be associated with conformational rearrangements in glutenin inside the dough matrix. These experiments have enabled dough relaxation to be probed over a very wide range of time scales (from ns to hours), and will lead to a better understanding of the role of dough matrix and gas cell effects on the physical properties of wheat flour doughs.

This thesis is based on observations of the physical properties of wheat flour dough using ultrasonic measurements. Three frequency ranges were used in the study, low frequencies (near 40 kHz), intermediate frequencies (1 to 5 MHz, where bubble resonance effects are apparent), and high frequencies (near 20 MHz). Doughs mixed under different head space air pressures, from vacuum to atmospheric pressure, as well as under nitrogen, were studied at low frequency to investigate their relaxation behavior. Subsamples from ambient dough and vacuum dough displayed differences in the dependence of velocity and attenuation on time after compression, but no post mixing relaxation effect was apparent. A critical headspace pressure of approximately 0.16 atmospheres determined whether vacuum-like or ambient-like relaxation was observed. A peak in attenuation and changes in ultrasonic velocity were observed around the bubble resonance frequency, and these ultrasonic parameters changed substantially as a function of time. A bubble resonance model was used to interpret the results around the bubble resonance frequency, and bubble size distributions were estimated for ambient and vacuum dough from the ultrasonic data. For the high frequency range, a molecular relaxation model was used to interpret the results. Different fast relaxation times were observed for ambient dough (5 ns) and vacuum dough (1 ns). This relaxation time may be associated with conformational rearrangements in glutenin inside the dough matrix. These experiments have enabled dough relaxation to be probed over a very wide range of time scales (from ns to hours), and will lead to a better understanding of the role of dough matrix and gas cell effects on the physical properties of wheat flour doughs.

Bread owes its appeal to its aerated structure which directly relies on the bubbles entrained into the dough during mixing. If the bubble size distribution (BSD) in the dough can be determined at the end of mixing, then the resulting loaf quality could be predicted before bread is fully manufactured. However, non-invasively monitoring the structure of a fragile opaque soft solid such as dough is challenging. This thesis addressed the challenge by determining dough’s BSD and its evolution using ultrasound and X-ray microtomography. Using a resonant scattering model and the frequency dependence of the ultrasonic parameters measured in the dough, the change in the BSD in dough (made without yeast) with time as a result of disproportionation was determined. At 30 min after mixing, the median radius (R0) of the lognormal BSD was 6.5 microns. Converting the BSD to the radius dependence of bubble volume fraction (BVF(R)), R0V (the median radius of BVF(R)) was 66.4 microns and increased 18 % in the succeeding 90 min. In order to validate the bubble sizes determined ultrasonically, X-rays from a synchrotron source were utilized to examine dough’s microstructure. Large numbers of very small bubbles were discovered and it was apparent that lognormality did not describe the BSDs. Nevertheless, lognormal characterization of the BVF(R) was appropriate. At 30 min after mixing R0V of the BVF(R) was 32.5 microns and it increased by 20 % in the succeeding 90 min, supporting the ultrasonic quantification of bubble volume changes due to disproportionation. Changes in the mode, median and mean of the BVF(R) with time after mixing had the same trend for ultrasound and for X-ray microtomography. The time evolution of the mode of the BVF(R) obtained by ultrasound and X-ray microtomography matched very well; both increasing linearly as a function of time. Ultrasonic assessments of bubble sizes and their changes with time are very encouraging, but the ultrasonic model should use distribution functions that precisely define the empirical data, perhaps not making ‘pre-assumptions’ of lognormality for the BSD data. / February 2015

Measurements of gas volume fraction, bubble velocity, chord length and bubble size distributions were performed in the research vessel Athena II operating in Saint Andrew Bay in the gulf coast near Panama City, FL. Double tipped sapphire optical local phase-detection probes were used to acquire indicator functions downstream of the breaking bow wave, behind the masker and at the stern. These indicator functions were also taken at different depths, distances from the hull, operating speeds and headings respect to the waves. The data processing includes the computation of velocity of individual bubbles and chord lengths, resulting in chord length distributions. These chord length distributions are used to obtain bubble size distributions using a novel procedure described in detail. Uncertainty analysis is performed for gas volume fraction, average bubble velocity and chord length. The results indicate that air entrainment increases with ship speed and sailing against the waves at all positions. The bow wave exhibits unsteady breaking that creates bubble clouds, which were characterized and identified by signal processing. At the stern a very strong dependence of bubble size with depth was found, with evidence that bubbles smaller than 500 micrometers are transported through the bottom of the hull and reach the transom. The roller present at the transom, the associated strong unsteadiness and bubble entrainment are well captured, as indicated by the stern results, showing the frothy nature of the upper layer.

To simulate dispersed two-phase flows CFD tools for predicting the local particle number density and the size distribution are required. These quantities do not only have a significant effect on rates of mixing, heterogeneous chemical reaction rates or interfacial heat and mass transfers, but also a direct relevance to the hydrodynamics of the total system, such as the flow pattern and flow regime. The Multiple Size Group (MUSIG) model available in the commercial codes CFX-4 and CFX-5 was developed for this purpose. Mathematically, this model is based on the population balance method and the two-fluid modeling approach. The dispersed phase is divided into N size classes. In order to reduce the computational cost, all size groups are assumed to share the same velocity field. This model allows to use a sufficient number of particle size groups required for the coalescence and breakup calculation. Nevertheless, the assumption also restricts its applicability to homogeneous dispersed flows. We refer to the CFX MUSIG model mentioned above as the homogeneous model, which fails to predict the correct phase distribution when heterogeneous particle motion becomes important. In many flows the non-drag forces play an essential role with respect to the bubble motion. Especially, the lift force acting on large deformed bubbles, which is dominated by the asymmetrical wake, has a direction opposite to the shear induced lift force on a small bubble. This bubble separation cannot be predicted by the homogeneous MUSIG model. In order to overcome this shortcoming we developed an efficient inhomogeneous MUSIG model in cooperation with ANSYS CFX. A novel multiple velocity multiple size group model, which incorporates the population balance equation into the multi-fluid modeling framework, was proposed. The validation of this new model is discussed in this report.

Cette thèse présente les études de sonoluminescence multibulle (MBSL) effectuées à l’ICSM pour compléter de précédents résultats ayant mis en évidence la formation d’un plasma hors équilibre au cours de la cavitation multibulle dans l'eau. La sonoluminescence et la réactivité sonochimique de l’eau sous flux continu de mélanges gaz rare et N2 et d’une solution aqueuse d’ammoniaque sous flux continu de gaz rare sont étudiées par plusieurs techniques expérimentales. Nous avons observé, en plus de l’émission de OH (A2Σ+-X2Πi) et du continuum typique de SL, pour la première fois la sonoluminescence de NH (A3Π – X3Σ-). Les spectres de sonoluminescence, le suivi des rendements de formation des produits de la sonolyse et les résultats de fits de NH (A3Π- X3Σ-) et OH (A2Σ+-X2Πi) en utilisant le logiciel Specair confirment l’atteinte de conditions plus extrêmes au moment de l’implosion des bulles à haute fréquence ultrasonore. D’autre part, ces résultats indiquent clairement l’absence d’équilibre thermique à l’intérieur des bulles de cavitation au moment de l’implosion (Tv > Tr) quelles que soient les conditions expérimentales, et que la température vibrationnelle est plus élevée à haute fréquence US ce qui conduit au non suivi de la loi de Boltzmann des populations des niveaux vibrationnels et/ou rotationnels. En parallèle, l’évolution de la taille des bulles de cavitation a été mesurée par une technique d’ultrasons pulsés, dans le cadre d’une collaboration entre l’ICSM/LSFC et l’université de Melbourne en Australie. Nous avons en particulier mis en évidence un problème de coalescence des bulles sous flux continu de gaz, qui complique grandement l’interprétation des résultats de taille des bulles. Une autre observation est que la présence d’azote dans l’argon conduit à une diminution de la taille des bulles. / This thesis presents the studies on multibubble sonoluminescence (MBSL) performed at ICSM to complete previous results that have shown the formation of a non-equilibrium plasma in the multibubble cavitation in water. Sonoluminescence and sonochemical reactivity of water under continuous flow of noble gas and N2 mixtures and of aqueous ammonia solutions under continuous flow of noble gas are studied by several experimental techniques. In addition to OH (A2Σ+-X2Πi) band and continuum emission usually observed in the SL spectra of water in the presence of noble gases, the sonoluminescence of NH (A3Π) radicals was observed for the first time. Spectroscopy of sonoluminescence, the follow up of the sonochemical products and the spectral fits of NH (A3Π- X3Σ-) and OH (A2Σ+-X2Πi) systems using Specair software indicate more drastic conditions at high US frequency. On the other hand, NH* and OH* radicals generated inside the cavitation bubbles are far from equilibrium (Tv > Tr) whatever the experimental conditions and the vibrational temperatures at high frequency ultrasound are much higher compared to 20 kHz ultrasound which leads to strong deviation from the equilibrium (non-Boltzmann behavior). In parallel, the evolution of the bubble size is measured by a pulsed ultrasound technique, in the framework of a collaboration between ICSM / LSFC and the University of Melbourne in Australia. The problem of the coalescence of bubbles under continuous flow of gas was identified, which greatly complicates the interpretation of results of bubble size. Another interesting observation is that the presence of nitrogen in argon leads to a strong reduction in bubble size.