Microbubble contrast agents for ultrasound are widely used in numerous medical applications, both diagnostic and therapeutic. Due to their size, similar to that of red blood cells, microbubbles are able to traverse the entire vascular bed, enabling their utilization for applications such as tumour diagnosis. Vaporizable submicron droplets of liquid perfluoro- carbon potentially represent a new generation of extravascular contrast agents for ultrasound. Droplets of a few hundred nanometers in diameter have the ability to extravasate selectively in regions of tumour growth while staying intravascular in healthy tissues. Upon extravasation, these droplets may be vaporized with ultrasound and converted into gas bubbles. In this thesis we argue that vaporized submicron perfluorocarbon droplets possess the necessary stability and acoustic characteristics to be potentially applicable as a new gener- ation of extravascular ultrasound contrast agents. We examine, separately, the ultrasound conditions necessary for vaporization of the droplets into microbubbles, the size and stability of these bubbles following vaporization, on timescales ranging from nanoseconds to minutes, and the bubbles’ acoustic response to incident diagnostic ultrasound. We show that submicron droplets may be vaporized into bubbles of a few microns in diameter using single ultrasound pulse within the diagnostic range. The efficiency of conversion is shown to be on the order of at least 10% of the exposed droplets converting into stable microbubbles. The bubbles are shown to be stabilized by the original coating material encapsulating the droplet precursors, and be stable for at least minutes following vaporization. Finally, vaporized droplets are shown to be echogenic, with acoustic characteristics comparable to these of the commercially available ultrasound contrast agents. The results presented here show that vaporized droplets possess the necessary stability properties and echogenicity required for successful application as contrast agents, suggesting potential for their future translation into clinical practice.

Microbubble contrast agents for ultrasound are widely used in numerous medical applications, both diagnostic and therapeutic. Due to their size, similar to that of red blood cells, microbubbles are able to traverse the entire vascular bed, enabling their utilization for applications such as tumour diagnosis. Vaporizable submicron droplets of liquid perfluoro- carbon potentially represent a new generation of extravascular contrast agents for ultrasound. Droplets of a few hundred nanometers in diameter have the ability to extravasate selectively in regions of tumour growth while staying intravascular in healthy tissues. Upon extravasation, these droplets may be vaporized with ultrasound and converted into gas bubbles. In this thesis we argue that vaporized submicron perfluorocarbon droplets possess the necessary stability and acoustic characteristics to be potentially applicable as a new gener- ation of extravascular ultrasound contrast agents. We examine, separately, the ultrasound conditions necessary for vaporization of the droplets into microbubbles, the size and stability of these bubbles following vaporization, on timescales ranging from nanoseconds to minutes, and the bubbles’ acoustic response to incident diagnostic ultrasound. We show that submicron droplets may be vaporized into bubbles of a few microns in diameter using single ultrasound pulse within the diagnostic range. The efficiency of conversion is shown to be on the order of at least 10% of the exposed droplets converting into stable microbubbles. The bubbles are shown to be stabilized by the original coating material encapsulating the droplet precursors, and be stable for at least minutes following vaporization. Finally, vaporized droplets are shown to be echogenic, with acoustic characteristics comparable to these of the commercially available ultrasound contrast agents. The results presented here show that vaporized droplets possess the necessary stability properties and echogenicity required for successful application as contrast agents, suggesting potential for their future translation into clinical practice.

O modelo F–SAC (Functional–Segment Activity Coefficient), recentemente introduzido, combina a ideia de contribuição de grupos com a teoria de superfícies de contato COSMO–RS. Este modelo tem uma dependência reduzida dos parâmetros de interação binária quando comparado com as variantes do modelo UNIFAC e precisão melhorada quando comparada com modelos baseados em COSMO. No presente trabalho uma modificação na formulação do modelo F–SAC é proposta para a inclusão de interações dispersivas. Para testar esta modificação, foram considerados os dados experimentais de alcanos, ciclo–alcanos, alcenos, aromáticos e perfluorocarbonetos. O modelo proposto foi capaz de correlacionar entalpias de vaporização de substâncias puras, não consideradas em versões anteriores do modelo. Uma vez que a capacidade calorífica de líquidos está intimamente relacionada com a entalpia de vaporização, o modelo também pode prever a capacidade calorífica de substâncias puras. Em relação ao coeficiente de atividade em diluição infinita e dados de equilíbrio líquido–vapor, o modelo modificado manteve o bom desempenho do modelo original, também semelhante a outros modelos similares, como variantes do modelo UNIFAC. Além disso, o modelo modificado pode agora calcular valores consistentes para a entalpia e entropia de excesso para sistemas onde as interações são principalmente dispersivas. Para estes sistemas, a maioria dos modelos de coeficiente de atividade prevê entalpia de excesso zero, contrabalançando valores de energia de Gibbs de excesso confiáveis com entropias de excesso distorcidas. / The recently introduced F–SAC (Functional–Segment Activity Coefficient) model combines the group contribution idea with a COSMO–RS surface contacting theory. This model has a reduced dependency on binary interaction parameters when compared to classical UNIFAC type models and improved accuracy when compared with COSMO based models. In the present work a modified F–SAC formulation is proposed for including dispersive interactions. For testing the modification, experimental data of alkanes, cycloalkanes, alkenes aromatics and perfluorocarbons were considered. The proposed model was able to correlate pure compound enthalpies of vaporization, not considered in previous versions of the model. Since the heat capacity of liquids is closely related to the enthalpies of vaporization, the model also can predict pure compound heat capacity of liquids. Regarding mixture infinite dilution activity coefficient and vapor–liquid equilibrium data, the modified model maintained the good performance of the original model, also similar to other competing models such as UNIFAC variants. Additionally, the modified model now can compute consistent values for the excess enthalpy and entropy for systems where the interactions are mainly dispersive. For these systems, most activity coefficient models predict zero excess enthalpies, counterbalancing that with distorted excess entropies.

O modelo F–SAC (Functional–Segment Activity Coefficient), recentemente introduzido, combina a ideia de contribuição de grupos com a teoria de superfícies de contato COSMO–RS. Este modelo tem uma dependência reduzida dos parâmetros de interação binária quando comparado com as variantes do modelo UNIFAC e precisão melhorada quando comparada com modelos baseados em COSMO. No presente trabalho uma modificação na formulação do modelo F–SAC é proposta para a inclusão de interações dispersivas. Para testar esta modificação, foram considerados os dados experimentais de alcanos, ciclo–alcanos, alcenos, aromáticos e perfluorocarbonetos. O modelo proposto foi capaz de correlacionar entalpias de vaporização de substâncias puras, não consideradas em versões anteriores do modelo. Uma vez que a capacidade calorífica de líquidos está intimamente relacionada com a entalpia de vaporização, o modelo também pode prever a capacidade calorífica de substâncias puras. Em relação ao coeficiente de atividade em diluição infinita e dados de equilíbrio líquido–vapor, o modelo modificado manteve o bom desempenho do modelo original, também semelhante a outros modelos similares, como variantes do modelo UNIFAC. Além disso, o modelo modificado pode agora calcular valores consistentes para a entalpia e entropia de excesso para sistemas onde as interações são principalmente dispersivas. Para estes sistemas, a maioria dos modelos de coeficiente de atividade prevê entalpia de excesso zero, contrabalançando valores de energia de Gibbs de excesso confiáveis com entropias de excesso distorcidas. / The recently introduced F–SAC (Functional–Segment Activity Coefficient) model combines the group contribution idea with a COSMO–RS surface contacting theory. This model has a reduced dependency on binary interaction parameters when compared to classical UNIFAC type models and improved accuracy when compared with COSMO based models. In the present work a modified F–SAC formulation is proposed for including dispersive interactions. For testing the modification, experimental data of alkanes, cycloalkanes, alkenes aromatics and perfluorocarbons were considered. The proposed model was able to correlate pure compound enthalpies of vaporization, not considered in previous versions of the model. Since the heat capacity of liquids is closely related to the enthalpies of vaporization, the model also can predict pure compound heat capacity of liquids. Regarding mixture infinite dilution activity coefficient and vapor–liquid equilibrium data, the modified model maintained the good performance of the original model, also similar to other competing models such as UNIFAC variants. Additionally, the modified model now can compute consistent values for the excess enthalpy and entropy for systems where the interactions are mainly dispersive. For these systems, most activity coefficient models predict zero excess enthalpies, counterbalancing that with distorted excess entropies.

Este trabalho apresenta os resultados de um estudo experimental sobre a ocorrência do atraso de vaporização no escoamento de misturas de fluidos refrigerantes em tubos capilares adiabáticos, com o intuito de aprimorar modelos de simulação do desempenho desse componente do ciclo de refrigeração previamente desenvolvidos. Foi realizada uma série de levantamentos experimentais para o R410A, uma mistura quase azeotrópica composta de 50% de HFC 32 e 50% de HFC 125 em base mássica, utilizando a unidade laboratorial para estudo de tubos capilares do Laboratório de Máquinas Térmicas do PMEEPUSP. Os sensores de temperatura foram posicionados adequadamente ao longo do tubo capilar, com maior concentração na região onde o atraso de vaporização tende a ocorrer, de forma a permitir uma determinação acurada do ponto onde se inicia a mudança de fase para diversas condições operacionais e geométricas. Primeiramente foram realizados estudos para caracterização de alguns parâmetros geométricos, como a medição dos diâmetros dos tubos capilares e a determinação das curvas de fator de atrito em função do n° de Reynolds para cada tubo capilar. A seguir foram realizados 27 ensaios, nos quais se obteve um total de 44 pontos experimentais, caracterizando o efeito de vários parâmetros geométricos e operacionais na diferença de pressão de saturação e no comprimento do trecho metaestável. Os dados obtidos foram correlacionados com os parâmetros experimentais no intuito de obter uma correlação para previsão da diferença de pressão de saturação no escoamento do R 410A em tubos capilares adiabáticos. A correlação obtida prevê 67% dos valores experimentais dentro da faixa de ± 26,4%, com um erro médio de 3,2%, resultado semelhante a outros trabalhos existentes na literatura. / This work presents the results of an experimental study on the occurrence of the delay of vaporization in the flow of refrigerant blends in adiabatic capillary tubes in order to improve previously developed models that simulate this component of refrigeration cycle. Experiments were carried out for R410A, a near azeotropic mixture composed of 50% of HFC 32 and 50% of HFC 125 on mass basis, using the capillary tubes laboratorial unit of the Mechanical Engineering Department of University of São Paulo. The temperature sensors was properly positioned along the capillary tubes, concentrated in the region where the delay of vaporization is expected to happen, to allow an accurate determination of the flashing point inception for a several operational conditions and geometries. Preliminary studies for characterization of some geometric parameters like capillary tubes diameters and determination of friction factor curves as function of the Reynolds number were performed. A total of 44 experimental points, collected from 27 runs, allowed the characterization of the effect of geometric and operational parameters on the underpressure of vaporization and metastable length. A correlation was developed for predicting the underpressure of vaporization as function of operational parameters and capillary tube geometry. The obtained correlation predicts 67% of experimental data within a ± 26,4% range, with an average error of 3,2%. This result is similar to other works in the literature.

La vaporisation rapide du sodium liquide surchauffé est supposée être à l’origine des arrêts automatiques pour réactivité négative du réacteur Phénix.Un dispositif expérimental a été mis en œuvre pour reproduire la détente d'un gaz pressurisé, repoussant un liquide dans un canal de section rectangulaire très allongée.L’interface qui sépare les deux fluides, initialement plate, ondule du fait d'instabilités de Rayleigh-Taylor dont le caractère 2D est garanti par le rapport d'aspect de la section du canal. L’aire interfaciale augmente d'un facteur 50.L’expansion du gaz peut être divisée en deux phases principales : une phase dite « de Rayleigh-Taylor » (linéaire et non-linéaire) et une phase dite « à multi-structures » (transitionnelle et chaotique). La première est caractérisée par la dynamique de l'interface et l’aire interfaciale qui en résulte est proportionnelle à l’amplitude des ondulations. La deuxième est influencée par le comportement des structures liquides, dispersées dans la matrice gazeuse et l’aire interfaciale est alors proportionnelle au nombre de structures.La distribution de fraction volumique suggère un modèle d’écoulement composé de trois régions : une région où la frontière des bulles est clairement définie et régulière, une région compartimentée par des membranes liquides issues des frontières des bulles, une région diphasique formée de la queue de ces structures. L’analyse de sensibilité à la tension superficielle confirme que plus la tension est faible, plus les interfaces sont instables. Les ondes sont plus prononcées et plus de structures sont produites, ce qui conduit à une majoration du taux de production de l’aire interfaciale. / The sharp vaporization of superheated liquid sodium is investigated. It is suspected to be at the origin of the automatic shutdown for negative reactivity, occurred in the Phénix reactor at the end of the eighties.An experimental apparatus has been designed and operated to reproduce the expansion of overpressurized air, superposed to water in a narrow vertical rectangular section channel.When expansion begins, the initial flat interface separating the two fluids becomes corrugated under the development of two-dimensional Rayleigh-Taylor instabilities. The interface area increases significantly and becomes even 50 times larger than the initial value. Since the channel is very narrow, instabilities along the channel depth do not develop.The gas expansion in a narrow channel can be divided into two main phases: Rayleigh-Taylor (linear and non-linear) and multi-structures (transition and chaotic) phases. The former is characterized by the dynamic of corrugated profile and the interface area results proportional to the amplitude of corrugation The latter is influenced by the behavior of the liquid structures dispersed in gas matrix and the interface area is mainly proportional to the number of liquid structures.The distribution of volume fraction suggests a model of channel flow consisting of three regions: the regular profile of peaks, the spike region and the structures tails. The analysis of sensibility to surface tension confirms that, with a lower surface tension, the fluids configuration is more unstable. The interface corrugations are more pronounced and more structures are produced, leading to a higher increment of the interface area.

No description available.

Ultrasound-mediated drug delivery has been proposed as a safe and non-invasive method to achieve localized drug release. Drug-loaded microbubbles are injected in the vascular system and ultrasound waves are then used to localize and burst the microbubbles at a specific targeted area. The relatively large size of microbubbles however limits both their lifetime and their reach in the human body. Phase-change liquid droplets can extend the use of ultrasound contrast agents for localized drug delivery. Their smaller size provides several advantages. The droplets can reach smaller capillaries, such as those in tumors vasculature. Their lifetime is also considerably prolonged. Through the phenomenon of Acoustic Droplet Vaporization (ADV), triggered by ultrasound stimulation, the liquid-filled droplets experience a phase change and are converted into gas-filled microbubbles. The newly created microbubbles can then be disrupted by further stimulation and release their drug load in the tumor tissue. In this project, a protocol to image and burst perfluoropentane-based micro-sized droplets using a single transducer is developed using the Verasonics Ultrasound System. The pulse sequences are developed to allow close monitoring of the drug delivery by capturing a series of images before and after the vaporization or destruction of the droplets. The droplets response was assessed for different pulse voltages and durations. Mean pixel value was calculated for the regions of interest, using the images captured before and after delivery of the ultrasound pulse. Vaporization of the droplets can be achieved with low voltage (10V), whereas high voltage (50V) triggers their destruction. Combined with high voltage, pulse duration affects the rate at which droplets can be destructed.

The use of peruorocarbon lled droplets for use as Phase Changing Contrast Agents (PCCAs) is a promosing eld. These capsules also have potential to be used for mediated drug delivery. The phase change, which has given the capsules their name, is the process when the capsule transforms from a droplet into a bubble. This process is referred to as Acoustic Droplet Vaporization (ADV) and can be induced with the use of ultrasonic waves. In this study a new type of Perfluorpentane (PFC5) capsules which are stabilized with Cellulose Nano Fibers (CNF) have been evaluated for its potential as a PCCA. To investigate this potential a setup was designed in which the capsules could be exposed to ultrasound waves. Following the ultrasound exposure the capsules were visualized under a light transmission microscope. The experiments were conducted for dierent combinations of ultrasound parameters. For each combination eight volume distributions were created, in which two of them as reference cases were not exposed to ultrasound waves. Six cases with the ultrasound ring with different levels of acoustic power, resulting in peak negative pressures ranging from 0.144 to 0.291 MPa. The results showedfthat ADV could be observed when the frequency of the acoustic wave is 3.5 MHz, the pulse repetition frequency is 500 and the burst width is set to 12 cycles. The Peak Negative Pressure (PNP)-threshold for ADV is about 0.200 MPa. When the burst width is set to 8, ADV is also observed however to a lesser extent then when it is set to 12. These results indicate that the CNF-stabilized PFC5 capsules are promising droplets with a potential future as an alternative to currently used PCCAs.

In order to follow increasingly strict regulation of pollutant emissions, a new concept of Lean Premixed pre-vaporized (LPP) combustion has been proposed for turbines. In LPP combustion, controlled atomization, dispersion and vaporization of different types of liquid fuel in the premixer are the key factors required to stabilize the combustion process and improve the efficiency. A numerical study is conducted for the fundamental understanding of the liquid fuel dispersion and vaporization process in pre-mixers using both cross-flow and co-flow injection methods. First, the vaporization model is validated by comparing the numerical data to existing experiments of single droplet vaporization under both low and high convective air temperatures. Next, the dispersion and vaporization process for biofuels and conventional fuels injected transversely into a typical simplified version of rectangular pre-mixer are simulated and results are analyzed with respect to vaporization performance, degree of mixedness and homogeneity. Finally, collision model has been incorporated to predict more realistic vaporization performance. Four fuels, Ethanol, Rapeseed Methyl Esters (RME), gasoline and jet-A have been investigated. For mono-disperse spray with no collision model, the droplet diameter reduction and surface temperature rise were found to be strongly dependent on the fuel properties. The diameter histogram near the pre-mixer exit showed a wide droplet diameter distribution for all the fuels. In general, pre-heating of the fuels before injection improved the vaporization performance. An improvement in the drag model with Stefan flow correction showed that a low speed injection and high cone angle improved performance. All fuels achieved complete vaporization under a iv spray cone angle of 140°. In general, it was found that cross-flow injection achieved better vaporization performance than co-flow injection. A correlation is derived for jet-A‟s total vaporization performance as a function of non-dimensional inlet air temperature and fuel/air momentum flux ratio. This is achieved by curve-fitting the simulated results for a broad range of inlet air temperatures and fuel/air momentum flux ratios. The collision model, based on no-time-counter method (NTC) proposed by Schmidt and Rutland, was implemented to replace O‟Rourke‟s collision algorithm to improve the results such that the unphysical numerical artifact in a Cartesian grid was removed and the results were found to be grid-independent. The dispersion and vaporization processes for liquid fuel sprays were simulated in a cylindrical pre-mixer using co-flow injection method. Results for jet-A and Rapeseed Methyl Esters (RME) showed acceptable grid independence. At relatively low spray cone angle and injection velocity, it was found that the collision effect on the average droplet size and the vaporization performance were very high due to relatively high coalescence rate induced by droplet collisions. It was also found that the vaporization performance and the level of homogeneity of fuel-air mixture could be significantly improved when the dispersion level is high, which can be achieved by increasing the spray cone angle and injection velocity. In order to compare the performance between co-flow and cross-flow injection methods, the fuels were injected at an angle of 40° with respect to the stream wise direction to avoid impacting on the wall. The cross-flow injection achieved similar vaporization performance as co-flow because a higher coalescence rate induced by droplet collisions cancelled off its higher heat transfer efficiency between two phases for cross-flow injections.