The use of ultrasound contrast agents increases the information available for reconstruction during ultrasound imaging. Previously studied microbubbles, consisting of a gas core, are subject to limitations such as a short lifetime and a large size. Droplets with a liquid perfluorocarbon core that is stabilized by cellulose nanofibers eliminate these drawbacks, but require further investigation. By studying the frequency-dependent attenuation profile of the cellulose nanofiber coated perfluorocarbon droplets within an ultrasound field, information about the droplets as oscillators can be retrieved, enabling characterization of their physical properties. In this study, the frequency-dependent attenuation profile was experimentally acquired and compared between two concentrations, using flat transducers covering the frequency range of 1-15 MHz. The data collected in the time domain was processed and transformed into the frequency domain and the attenuation was calculated across the entire frequency range. Among the frequencies studied, the attenuation increases with frequency and covers the range of approximately 0.25-8.3 dB/cm and 0.01-3.3 dB/cm at the concentrations of 50 million droplets/ml and 10 million droplets/ml respectively. The attenuation reaches a minimum at 3 MHz within the highest concentration, compared to 4.43 MHz within the lowest. The increase of the attenuation with frequency is explained by the droplets not exhibiting large oscillations within the range covered. It is probable that the droplets do exhibit oscillations, due to a viscosity lower than that of water, but a resonance frequency is not found within the spectrum studied. This could be explained by a shell elasticity or a small droplet radius placing the resonance frequency outside of the spectrum studied, or high levels of damping broadening the resonance peak. Localizing the resonance frequency would enable characterization of these physical properties of the cellulose nanofiber shell as well as the perfluorocarbon liquid core of the droplets. The increase of the attenuation with frequency demonstrates that the droplets do not produce a maximized amount of scattering at a specific frequency within the range studied, which is observed among other oscillating particles implemented as ultrasound contrast agents. The attenuation is, however, larger than that of blood across all frequencies except for those among which the attenuation reaches its minimum. Potential errors that are affecting the results include droplet vaporization, the formation of flocs after the mechanical agitation has ceased, the experimental setup, the settings on the pulse generator, the sensitivity of the transducers and the processing code.

No description available.

<p>Understanding the drying of droplets is of interest for processes such as spray drying, where particulate materials are produced by evaporating moisture. Even though spray-drying is a widely used method, there are still challenges, such as undesired agglomeration or controlling the morphology and size of the final dried product. This dissertation develops a physics based model that is used to examine the droplet dynamics and drying kinetics at large and small scales.  In addition, the model simulates the internal motion of insoluble particles and  is used to better understand particle formation during spray drying type processes.</p> <p><br></p> <p>The first part of this work examines the effect of droplet-droplet collisions on evaporation and the size distribution at a large scale. Droplet collision dynamics are implemented into an Eulerian-Lagrangian framework, where droplets are tracked in the Lagrangian frame, and the background gas is modeled as a continuum. The modeling framework includes fully coupled interphase heat and momentum transfer between the droplet and gas phases. Binary collision of droplets could result in coalescence, reduction in surface area, or separation of droplets, resulting in the generation of satellite droplets and an increase in total surface area. By capturing the change in size distribution due to the collision of particles, our results show a linear relationship between the Weber number and the evaporation rate at low droplet number densities. Further, it is shown that droplet number density is a critical factor influencing the evaporation rate. At high droplet number densities, the relationship between the evaporation rate and the Weber number becomes non-linear, and at extremely high droplet number densities, the evaporation rate decreases even at high Weber numbers.</p> <p><br></p> <p>In the next part of this dissertation, the drying of a single droplet containing insoluble solid particles is investigated. Using a volume-of-fluid framework coupled with the Lagrangian phase, we study the particle transport within a droplet, and how it is affected by airflow, phase properties (e.g., viscosity and density of each phase), surface tension, and evaporation. Unlike the traditional one-dimensional modeling approach, our multi-dimensional model can capture the generation of internal flow patterns due to shear flow and the accumulation of solid particles on the surface of the drying droplet. Our results show that the surface tension effect is more pronounced at larger droplet diameters and low airflow velocities. Our approach also provides a quantitative method for modeling crust growth and formation. </p> <p>Our results show that increasing solids mass fraction, and decreasing particle diameter, slow down the internal transport of solid particles, leading to a more quick accumulation near the surface of the droplet. Further, despite the droplet undergoing a constant-rate drying stage, the accumulation of solids near the surface is non-linear. In addition, the inclusion of solids within the droplet drastically reshapes the formation of internal vortices compared to the uncoupled case, which determines solids distribution.</p>

Yes / The tribological performance of the piston assembly of an automotive engine is highly influenced by the complex flow mechanisms that supply lubricant to the upper piston rings. As well as affecting friction and wear, the oil consumption and emissions of the engine are strongly influenced by these mechanisms. There is a significant body of work that seeks to model these flows effectively. However, these models are not able to fully describe the flow of lubricant through the piston assembly. Some experimental studies indicate that droplets of lubricant carried in the gas flows through the piston assembly may account for some of this. This work describes an investigation into the nature of lubricant misting in a fired gasoline engine. Previous work in a laboratory simulator showed that the tendency of a lubricant to form mist is dependent on the viscosity of the lubricant and the type and concentration of viscosity modifier. The higher surface area-to-volume ratio of the lubricant if more droplets are formed or if the droplets are smaller is hypothesised to increase the degradation rate of the lubricant. The key work in the investigation was to measure the size distribution of the droplets in the crankcase of a fired gasoline engine. Droplets were extracted from the crankcase and passed through a laser diffraction particle sizer. Three characteristic droplet size ranges were observed: Spray sized (250–1000 μm); Major mist (30–250 μm); and Minor mist (0.1–30 μm). Higher base oil viscosity tended to reduce the proportion of mist-sized droplets. The viscoelasticity contributed by a polymeric viscosity modifier reduced the proportion of mist droplets, especially at high load.

The phenomenon of spray in crossflow is of relevance in gas turbine combustor development. The current work focuses on spray in crossflow rather than liquid jet in crossflow from the standpoint of enhancing fuel dispersion and mixing. Specifically, the first part of the work involves study of spray structure, droplet sizing, and velocimetry for sprays of water and ethanol in a crossflow under ambient conditions. Laser-based diagnostic techniques such as Particle/Droplet Image Analysis (PDIA) and Particle Tracking Velocimetry (PTV) are utilized. Using spray structure images, trajectory equations are derived by multi-variable regression. It is found that the spray trajectory depends only on the two-phase momentum ratio and is independent of other flow parameters. A generalized correlation for the spray trajectory is proposed incorporating the liquid surface tension, which is found to be effective for our data, with water and ethanol, as well as data on Jet-A from the literature for a wide variety of operating conditions. An interesting phenomenon of spatial bifurcation of the spray is observed at low Gas-to-Liquid ratios (GLRs). The reason for this phenomenon is attributed to the co-existence of large and highly deformed ligaments along with much smaller droplets at low GLR conditions. The smaller droplets lose their vertical momentum rapidly leading to lower penetration, whereas the larger ligaments/droplets penetrate much more due to their larger momentum leading to a spatial separation of the two streams. The second part of the study focuses on evaporating sprays in preheated crossflow. Experiments are conducted using ethanol, decane, Jet-A1 fuel, and a two-component surrogate for Jet-A1 fuel. The crossflow air is heated up to 418 K and the effect of evaporation is studied on spray trajectory and droplet sizes. Measured droplet sizes and velocities at two successive locations are used to estimate droplet evaporation lifetimes. Evaporation constant for the d2 law derived from the droplet lifetimes represents the first-ever data for the above-mentioned liquids under forced convective conditions. This data can be used to validate multi-component droplet evaporation models. The last part of the study focuses on Large Eddy Simulations (LES) of the spray in crossflow. The near-nozzle spray structure is investigated experimentally to obtain droplet size and velocity distributions that are used as inputs to the computational model. For the spray in crossflow under ambient conditions, trajectory and droplet sizes at different locations are compared with experimental results. While the predicted trajectory is found to be in good agreement with data, the predicted droplet sizes are larger than the measured values. This is attributed to the implicit assumption in the secondary breakup model that the droplets are spherical, whereas the experimental data in the near-nozzle region clearly shows presence of mostly ligaments and non-spherical droplets, especially for the low GLR cases. A modified breakup model is found to lead to improved agreement in droplet sizes between predictions and measurements. Overall, the experiments and computations have provided significant insight into spray in crossflow phenomenon, and have yielded useful results in terms of validated spray trajectory correlations, droplet evaporation lifetimes under forced convective conditions, and a methodology for simulation of airblast sprays.

Droplet microfluidics is an emerging technology for analysis of single cells and biomolecules at high throughput. The controlled encapsulation of particles along with the surrounding microenvironment in discrete droplets, which acts as miniaturized reaction vessels, allows millions of particles to be screened in parallel. By utilizing the unit operations developed to generate, manipulate and analyze droplets, this technology platform has been used to miniaturize a wide range of complex biological assays including, but not limited to, directed evolution, rare cell detection, single cell transcriptomics, rare mutation detection and drug screening. The aim of this thesis is to develop droplet microfluidics based methods for analysis of single cells and nucleic acids. In Paper I, a method for time-series analysis of mammalian cells, using automated fluorescence microscopy and image analysis technique is presented. The cell-containing droplets were trapped on-chip and imaged continuously to assess the viability of hundreds of isolated individual cells over time. This method can be used for studying the dynamic behavior of cells. In Paper II, the influence of droplet size on cell division and viability of mammalian cell factories during cultivation in droplets is presented. The ability to achieve continuous cell division in droplets will enable development of mammalian cell factory screening assays in droplets. In Paper III, a workflow for detecting the outcome of droplet PCR assay using fluorescently color-coded beads is presented. This workflow was used to detect the presence of DNA biomarkers associated with poultry pathogens in a sample. The use of color-coded detection beads will help to improve the scalability of the detection panel, to detect multiple targets in a sample. In Paper IV, a novel unit operation for label-free enrichment of particles in droplets using acoustophoresis is presented. This technique will be useful for developing droplet-based assays that require label-free enrichment of cells/particles and removal of droplet content. In general, droplet microfluidics has proven to be a versatile tool for biological analysis. In the years to come, droplet microfluidics could potentially be used to improve clinical diagnostics and bio-based production processes. / <p>QC 20160926</p>

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

Η διδακτορική διατριβή πραγματεύεται την πειραματική διερεύνηση της αλληλεπίδρασης δέσμης εκροής νέφους σταγονιδίων νερού με το θερμικά στρωματοποιημένο,τυρβώδες ρευστο-θερμικό πεδίο που αναπτύσσεται πάνω από οριζόντια θερμαινόμενη επίπεδη επιφάνεια. Τα φαινόμενα και οι φυσικοί μηχανισμοί που διέπουν την ως άνω αλληλεπίδραση πέρα από τη σημασία τους σε σχέση με τη βασική έρευνα στο πεδίο των ρευστοθερμικών φαινομένων, παρουσιάζουν ενδιαφέρον σε πληθώρα πρακτικών προβλημάτων όπως κατά τον ψεκασμό καυσίμου σε κινητήρες εσωτερικής καύσης και καυστήρες, στη διφασική ψύξη ηλεκτρονικών, στην μετεωρολογία, στην πυρόσβεση, σε διεργασίες απόθεσης και επικάλυψης κ.α. Το ρευστο-θερμικό πεδίο αναπτύσσεται πάνω από οριζόντια θερμαινόμενη επιφάνεια μεταλλικής πλάκας μέσα σε ορθογωνική κοιλότητα με ανοικτή οροφή. Το νέφος σταγονιδίων δημιουργείται σε έναν νεφελοποιητή και εκτοξεύεται μέσα από ένα ακροφύσιο κυλινδρικού σωλήνα εσωτερικής διαμέτρου 4mm, κάθετα προς την οριζόντια επίπεδη επιφάνεια και σε σημαντική απόσταση από αυτήν (50cm), δημιουργώντας αρχικά μια διφασική δέσμη εκροής σταγονιδίων. Για την δημιουργία βάσης δεδομένων αναφοράς, σημαντικό μέρος της εργασίας αναφέρεται στην καταγραφή και μελέτη του θερμικού πεδίου ελεύθερης συναγωγής που δημιουργείται πάνω από τη θερμαινόμενη πλάκα χωρίς την παρουσία σταγονιδίων. Αντίστοιχα μελετήθηκε η ανάπτυξη της δέσμης εκροής σταγονιδίων σε ισοθερμοκρασιακές συνθήκες. Η παραμετρική μελέτη της αλληλεπίδρασης δέσμης εκροής σταγονιδίων και θερμικά στρωματοποιημένου πεδίου πραγματοποιήθηκε για δέσμες σε δύο αριθμούς Reynolds και για δύο ρυθμούς ροής θερμότητας από την πλάκα. Οι αριθμοί Reynolds που επιβάλλονται στη ροή δέσμης του νέφους (mistjet) είναι σχετικά χαμηλοί έτσι ώστε να ισχυροποιηθεί σχετικά η επίδραση του τοιχώματος και κυρίως οι ροϊκές δομές που δημιουργούνται λόγω των ανωστικών δυνάμεων κοντά στη θερμαινόμενη επιφάνεια. Η σύνθετη ροή που παράγεται λόγω της σημαντικής απόστασης μεταξύ του ακροφυσίου και της επίπεδης επιφάνειας - στόχου κατατάσσεται στην κατηγορία των δεσμών εκροής ασθενούς πρόσκρουσης. Οι παράμετροι που εξετάζονται περιλαμβάνουν μέσα και τυρβώδη χαρακτηριστικά τόσο των σταγόνων (μέγεθος και ταχύτητα), όσο και της θερμοκρασιακής κατανομής πάνω από την θερμαινόμενη πλάκα. Το θερμικό πεδίο σε συνθήκες ελεύθερης μεταφοράς και υπό την επίδραση της ροής των σταγονιδίων καταγράφτηκε με τη βοήθεια θερμοζεύγους πολύ μικρών διαστάσεων ώστε να αλλοιώνει όσο το δυνατόν λιγότερο τη ροή. Οι διαστάσεις του αισθητηρίου είναι σημαντικά μικρότερες της κλίμακας μήκους Kolmogorov και επομένως το αισθητήριο κρίθηκε ικανό για την ανάλυση όλων των σχετικών κλιμάκων της ροής. Το πεδίο νέφους σταγονιδίων μελετήθηκε με την τεχνική Ανεμομετρίας Φάσης Doppler (PhaseDopplerAnemometry - PDA) η οποία επιτρέπει την μέτρηση τόσο της ταχύτητας όσο και του μεγέθους των σταγονιδίων παρέχοντας τη δυνατότητα συσχέτισης των δύο μεγεθών για τον ενδελεχή χαρακτηρισμό της συμπεριφοράς των σταγονιδίων. Η τεχνική αυτή είναι μια από τις λίγες μη παρεμβατικές μεθόδους σημειακών μετρήσεων σε διφασικά ροϊκά πεδία η οποία δίνει πληροφορία για τη συγκέντρωση και την παροχή, με την τελευταία να αποτελεί σημαντικό εργαλείο στον υπολογισμό της εξάτμισης. Τα αποτελέσματα των μετρήσεων κατά την αλληλεπίδραση των πεδίων (θερμικού και σταγονιδίων) μελετώνται και συγκρίνονται με τις συνθήκες αναφοράς της στρωματοποιημένης ροής του θερμοκρασιακού πεδίου και της ισόθερμης ροής του πεδίου ταχυτήτων δίνοντας πληροφορία για την επίδραση του νέφους σταγονιδίων στους μηχανισμούς τυρβώδους μεταφοράς. Αναγνωρίστηκε ως κυρίαρχη δομή του πεδίου συναγωγής το κινούμενο πλούμιο συχνά μορφής μανιταριού. Η μορφή των κατανομών πυκνότητας πιθανότητας (PDF) ερμηνεύθηκε σε σχέση με το ιστορικό δημιουργίας και ανταλαγών θερμότητας των επί μέρους αερίων μαζών που διέρχοτναι από τη θέση μέτρησης. Στις κατανομές φασματικής ισχύος (PSD) του σήματος της θερμοκρασίας αναγνωρίσθηκε ιδιοσυχνότηταπου υποδεικνύει την παρουσία δομής μεγάλης κλίμακας. Αναφορικά με το πεδίο σταγονιδίων παρατηρήθηκε ότι η παρουσία της επιφάνειας – στόχου επιβραδύνει τις δέσμες πρόσπτωσης ελαττώνοντας και τις διακυμάνσεις στην περιοχή του τοιχώματος. Η θέρμανση επηρεάζει τις μέσες και κυμαινόμενες ταχύτητες σε σημαντική απόσταση από την οριζόντια πλάκα, ενώ το μέγεθος των σταγονιδίων μειώνεται κατάντη της ροής με διαφορετικές τάσεις ως προς τον Re. Αυξημένες τιμές της μέσης διαμέτρου Sauter (D32) παρατηρούνται κοντά στην επιφάνεια ειδικά για την περίπτωση ισόθερμου ψεκασμού του υψηλότερου Re. Η θέρμανση της επιφάνειας έχει σημαντική επίπτωση στον περιορισμό της αυξητικής τάσης της παροχής αέρα λόγω συμπαράσυρσης, που είναι πιο εμφανής στις περιπτώσεις χαμηλού Re. / The interaction of a water mist jet with the thermally stratified turbulent field developing over a horizontal heated flat plate, in an open top cavity, is investigated experimentally. The physical phenomena dominating this interaction, besides their importance for theoretical thermo-fluids, influence a wide spectrum of applications including fuel injection in internal combustion engines and burners, electronics cooling, meteorology, fire extinguishing, deposition and coating processes etc. Water mist, generated in a nebulizer, is sprayed through a cylindrical orifice of internal diameter 4mm, vertically, towards the horizontal surface and at considerable distance from it (50cm). In order to establish a reference case data base, a significant part of the work refers to data collection and study on the thermal convection field characteristics, in the absence of the spray mist, as well as on the isothermal mist jet development. The interaction field has been investigated for mist jets of two Reynolds numbers and at two flat plate heating rates. The mist jet Reynolds numbers were rather low in order to enhance the influence of the plate and the flow structures generated due to buoyancy. Mean and turbulent characteristics of the spray (velocity and size) and the temperature distribution were monitored. A small thermocouple, significantly smaller than the Kolmogorov length scale, was used for temperature measurements. Droplet velocities and sizes were measured with Phase Doppler Anemometry, which also provided concentration and flux measurements. Moving plumes, often in the form of mushrooms, were identified as the dominant structures in the convection field. The form of temperature probability density functions was related to the past history of formation and heat exchange of air masses crossing the measuring point. An eigenfrequency, identified in the temperature power spectra indicates the presence of a large scale structure. The presence of the target plate decelerates the mist jets reducing also turbulent fluctuations close to the surface. Heating influences the mean and fluctuating velocities at considerable distance from the plate, reducing the rate of jet mass flux growth due to entrainment, more evidently for the low Re jet. Droplet sizes decrease downstream, presenting different trends in relation to Re. Increased values of the Sauter mean diameter (D32) are observed very close to the plate surface, particularly for the high Re number, isothermal jet.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.