This thesis focuses on the direct numerical simulation of the combustion of a single aluminum droplet with phase change. For this purpose, the Volume of Fluid (VOF) method is employed for the direct numerical simulation of two distinct phases. To model the droplet combustion, the phase change (evaporation) and chemical reactions are modeled by setting source terms for each governing equation. This work proposes a new form of species source term by phase change, derived using the local instant formulation of two-phase flow. The Stefan problem is used to verify the modified source term. Evaporation fluxes calculated with both modified and conventional sources are compared, demonstrating that the modified species source term yielded mass flow rates closer to theoretical values, with an error rate of less than 20%. The instabilities of source terms in the droplet case are also analyzed, revealing that surface tension and chemical reactions cause numerical errors arising from the sharp discontinuities at the interfacial cells. The model’s validation includes a comparison with a benchmark case, assessing the temporal evolution of droplet diameter change and temperature fields. While the diameter change aligns reasonably with the benchmark, the temperature fields do not reach the benchmark’s flame temperature due to numerical diffusions. Furthermore, the molar fraction of aluminum gas at the interface closely matches experimental values, although the overall spatial distribution of molar fraction of species does not align with the benchmark.

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Evaporation and wetting of droplets are a phenomena present in everyday life and in many industrial, biological or medical applications; thus controlling and understanding the underlying mechanisms governing this phenomena becomes of paramount importance. More recently, breakthroughs in the fabrication of new materials and nanomaterials have led to the synthesis of novel nanoscale particulates that dispersed into a base fluid modify the properties of this latter. Enhancement in heat transfer or the self-assembly of the particles in suspension during evaporation, are some of the areas in which nanofluids excel. Since it is a relatively new area of study, the interplay particle-particle, particle-fluid or particle-substrate at the macro-, micro-, and nanoscale is yet poorly understood. This work is an essay to elucidate the fundamental physics and mechanisms of these fluids during free evaporation, of great importance for the manipulation and precise control of the deposits. The evaporative behaviour of pure fluids on substrates varying in hydrophobicity has been studied and an unbalance Young’s force is proposed to explain the effect of substrate hydrophilicity on the pinning and the depinning forces involved during droplet evaporation. On other hand, the addition of nanoparticles to a base fluid modifies the evaporative behaviour of the latter and: a more marked “stick-slip” behaviour is observed when increasing concentration on hydrophobic substrates, besides the longer pinning of the contact line reported on hydrophilic ones when adding nanoparticles. A deposition theory to explain the final deposits observed, for the outermost ring, after the complete vanishing of a 0.1% TiO2-ethanol nanofluid droplet has also been developed. In addition, the evaporation of pinned nanofluid droplets on rough substrates at reduced pressures has been systematically studied. A revisited Young-Lippmann equation is proposed as one of the main findings to explain the enhancement on electrowetting performance of nanoparticle laden fluid droplets when compared to the pure fluid case. On the other hand, of relevant importance is the absence of “stick-slip” behaviour and the more homogeneous deposits found after the complete evaporation of a nanofluid droplet under an external electric field applied when compared to free evaporation of these fluids.

This study explores an innovative approach to control the droplet size distribution produced by an electrospray with the intention of eventually being able to deliver precisely controlled quantities of precursor materials for nanofabrication. The technique uses a thermionic cathode to charge the droplets in excess of the Rayleigh limit, leading to droplet breakup or fission. The objective of these experiments was to assess whether the proposed technique could be used to produce a new droplet size distribution with a smaller mean droplet diameter without excessively broadening the distribution. An electrospray was produced in a vacuum chamber using a dilute mixture of ionic liquid. During their transit from the capillary source to a diagnostic instrument, the resulting droplets were exposed to an electron stream with controlled flux and kinetic energy. The droplets were sampled in an inductive charge detector to characterize changes in the size distribution. A positively biased anode electrode was used to collect electron current during droplet exposure. This collected current was used as the primary control variable and used as a measure of the electron flux. The anode bias voltage was a secondary control variable and used as a measure of the electron energy. In a series of seven tests, two sets showed evidence of fission having occurred resulting in the formation of two droplet populations after electron bombardment. Three sets of results showed evidence of a single droplet population after electron bombardment, but shifted to a smaller mean diameter, and one set of results was inconclusive. Because of the large standard deviation in the droplet diameter distributions, the two cases in which a second population was evident were the strongest indication that droplet fission had occurred.

The effects of liquid superheat on the disruption of liquid droplets accelerated in a supersonic flow were examined experimentally in a drawdown supersonic wind tunnel. Monodisperse 60 ìm diameter droplets of two test fluids (methanol and ethanol) were generated upstream of the entrance to the tunnel and accelerated with the supersonic flow such that their maximum velocities relative to the air flow were transonic. Droplets were imaged by shadowgraphy and by multiple-exposure direct photography using planar laser sheet illumination. In addition to providing information on droplet lifetime, the latter technique allows measurement of the droplet downstream distance versus time, from which the velocity and acceleration during disruption can be inferred. All droplets were unheated upon injection. Depending on the vapor pressure of the liquid, the droplets achieved varying levels of liquid superheat as they experienced low static pressure in the supersonic flow. Histograms of the droplet population downstream of the supersonic nozzle throat indicate that the lifetime of droplets in supersonic flow decreases with an increasing amount of droplet superheat. The shorter lifetime occurs even as the droplet Weber number (based on initial droplet size) decreases initially due to the lower relative velocity of the methanol droplets to that of ethanol droplets. This is due to a higher acceleration than ethanol droplets of comparable initial size. This is consistent with the more rapid disruption and the faster decrease in mass for the methanol droplets. The droplets, depending on the level of superheating, in some cases underwent disruption modes different than those expected for the corresponding values of Weber number.

We present an experimental study on the behavior of bubbles captured in a Taylor vortex. The gap between a rotating inner cylinder and a stationary outer cylinder is filled with a Newtonian mineral oil. Beyond a critical rotation speed (ω[subscript c]), Taylor vortices appear in this system. Small air bubbles are introduced into the gap through a needle connected to a syringe pump. These are then captured in the cores of the vortices (core bubble) and in the outflow regions along the inner cylinder (wall bubble). The flow field is measured with a two-dimensional particle imaging velocimetry (PIV) system. The motion of the bubbles is monitored by using a high speed video camera. It has been found that, if the core bubbles are all of the same size, a bubble ring forms at the center of the vortex such that bubbles are azimuthally uniformly distributed. There is a saturation number (N[subscript s]) of bubbles in the ring, such that the addition of one more bubble leads eventually to a coalescence and a subsequent complicated evolution. Ns increases with increasing rotation speed and decreasing bubble size. For bubbles of non-uniform size, small bubbles and large bubbles in nearly the same orbit can be observed to cross due to their different circulating speeds. The wall bubbles, however, do not become uniformly distributed, but instead form short bubble-chains which might eventually evolve into large bubbles. The motion of droplets and particles in a Taylor vortex was also investigated. As with bubbles, droplets and particles align into a ring structure at low rotation speeds, but the saturation number is much smaller. Moreover, at high rotation speeds, droplets and particles exhibit a characteristic periodic oscillation in the axial, radial and tangential directions due to their inertia. In addition, experiments with non-spherical particles show that they behave rather similarly. This study provides a better understanding of particulate behavior in vortex flow structures. / Singapore-MIT Alliance (SMA)

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.