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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

The Effects of Marangoni Convection on the Rate of Condensation of Pure Water

Fernando, Nilendri L. 04 December 2012 (has links)
A series of steady-state water condensation experiments were conducted to determine the effects of Marangoni convection on the condensation flux. The interface was flat so that the results of the interfacial temperature discontinuities could be compared to past condensation experiments conducted under similar experimental conditions using a spherical interface. Two experimental methods were used. Method 1 was to vary the temperature in the cooling pipes (Tcp ) with the position of the interface relative to the cooling pipes fixed. Method 2 was to vary the position of the interface while Tcp was held constant. The interfacial temperature discontinuities in this study were approximately 2.3-9 times smaller in magnitude, than those measured using a spherical liquid-vapour interface. The experimental results showed that the condensation flux increased as thermocapillary convection increased (increase in interfacial temperature gradients and speed). This increase resulted in a 1.37-12.5 times enhancement in the condensation flux of pure water.
2

The Effects of Marangoni Convection on the Rate of Condensation of Pure Water

Fernando, Nilendri L. 04 December 2012 (has links)
A series of steady-state water condensation experiments were conducted to determine the effects of Marangoni convection on the condensation flux. The interface was flat so that the results of the interfacial temperature discontinuities could be compared to past condensation experiments conducted under similar experimental conditions using a spherical interface. Two experimental methods were used. Method 1 was to vary the temperature in the cooling pipes (Tcp ) with the position of the interface relative to the cooling pipes fixed. Method 2 was to vary the position of the interface while Tcp was held constant. The interfacial temperature discontinuities in this study were approximately 2.3-9 times smaller in magnitude, than those measured using a spherical liquid-vapour interface. The experimental results showed that the condensation flux increased as thermocapillary convection increased (increase in interfacial temperature gradients and speed). This increase resulted in a 1.37-12.5 times enhancement in the condensation flux of pure water.
3

The Onset of Marangoni Convection for Evaporating Liquids

MacDonald, Brendan D. 30 August 2012 (has links)
The stability of evaporating liquids is examined. The geometries investigated are semi-infinite liquid sheets, bounded liquid sheets, sessile droplets, and funnels. Stability parameters are generated to characterize the stability of evaporating semi-infinite liquid sheets, and bounded liquid sheets. The derivation is made possible by introducing evaporation as the specific heat transfer mechanism at the interface, and using the statistical rate theory expression for evaporation flux so there are no fitting parameters. It is demonstrated that a single parameter can be used to predict the onset criterion instead of two parameters. A linear stability analysis is performed for spherical sessile droplets evaporating on substrates constructed of either insulating or conducting materials. A stability parameter is generated to characterize the stability of sessile droplets evaporating on insulating substrates and conducting substrates. The results indicate that spherical sessile droplets evaporating on insulating substrates are predicted to transition to Marangoni convection. Since there are currently no experimental results to compare the theory with, another analysis is performed for liquids evaporating from funnels, which can be compared with existing experimental observations. A linear stability analysis predicts stable evaporation for funnels constructed of insulating materials, in contrast to the sessile droplet case, and generates a new stability parameter for funnels constructed of conducting materials. The stability parameter is free of fitting variables since the statistical rate theory expression for the evaporation flux is used. The theoretical predictions are found to be consistent with experimental observations for water evaporating from a funnel constructed of poly(methyl methacrylate) (PMMA) and for water and heavy water evaporating from a funnel constructed of stainless steel. A parametric analysis is performed on the new stability parameter for liquids evaporating from funnels constructed of conducting materials, indicating that smaller interfacial temperature discontinuities, higher evaporation rates, and smaller radii correspond to less stable systems. It is also illustrated that calculations using statistical rate theory predict an instability, which is consistent with experimental observations, whereas using the Hertz-Knudsen theory does not predict any instability.
4

The Onset of Marangoni Convection for Evaporating Liquids

MacDonald, Brendan D. 30 August 2012 (has links)
The stability of evaporating liquids is examined. The geometries investigated are semi-infinite liquid sheets, bounded liquid sheets, sessile droplets, and funnels. Stability parameters are generated to characterize the stability of evaporating semi-infinite liquid sheets, and bounded liquid sheets. The derivation is made possible by introducing evaporation as the specific heat transfer mechanism at the interface, and using the statistical rate theory expression for evaporation flux so there are no fitting parameters. It is demonstrated that a single parameter can be used to predict the onset criterion instead of two parameters. A linear stability analysis is performed for spherical sessile droplets evaporating on substrates constructed of either insulating or conducting materials. A stability parameter is generated to characterize the stability of sessile droplets evaporating on insulating substrates and conducting substrates. The results indicate that spherical sessile droplets evaporating on insulating substrates are predicted to transition to Marangoni convection. Since there are currently no experimental results to compare the theory with, another analysis is performed for liquids evaporating from funnels, which can be compared with existing experimental observations. A linear stability analysis predicts stable evaporation for funnels constructed of insulating materials, in contrast to the sessile droplet case, and generates a new stability parameter for funnels constructed of conducting materials. The stability parameter is free of fitting variables since the statistical rate theory expression for the evaporation flux is used. The theoretical predictions are found to be consistent with experimental observations for water evaporating from a funnel constructed of poly(methyl methacrylate) (PMMA) and for water and heavy water evaporating from a funnel constructed of stainless steel. A parametric analysis is performed on the new stability parameter for liquids evaporating from funnels constructed of conducting materials, indicating that smaller interfacial temperature discontinuities, higher evaporation rates, and smaller radii correspond to less stable systems. It is also illustrated that calculations using statistical rate theory predict an instability, which is consistent with experimental observations, whereas using the Hertz-Knudsen theory does not predict any instability.
5

Melting of Ice and Formation of Lateral Cavity during In Situ Burning in Ice-Infested Waters

Farmahini Farahani, Hamed 12 February 2018 (has links)
The ice melting and lateral cavity formation caused by in situ burning (ISB) of liquid fuels in ice-infested waters was studied in order to improve predictions on the removal efficiency of this oil spill mitigation method. For this purpose, several experimental studies were conducted to increase the fundamental understanding of the mechanisms that lead to ice melting and lateral cavity formation. The findings of the experimental studies provided the required knowledge to mathematically formulate the ice melting problem. Mathematical scaling analysis of ice melting during burning of oils in the vicinity of ice was performed to create a tool to estimate the extent of melting that occurs during ISB in ice-infested waters. A series of lab-scale experiments were designed to systematically investigate the ice melting problem. The first set of experiments were conducted in cylindrical shaped ice cavities with a 5.7 cm diameter. Burning of n-octane from ignition to natural extinction and the subsequent geometry change of the ice, fuel thickness, and fuel temperature were measured. The preliminary experimental observations showed that the melting of the ice walls was higher in areas where the fuel layer was in contact with ice compared with places of flame exposure. Based on these observations, a hypothesis that suggested the convective flows in the liquid fuel (driven mainly by surface tension and buoyancy) were contributing in melting of the ice was proposed to explain the origins of the lateral cavity. To evaluate this hypothesis, two dimensionless numbers (Marangoni and Rayleigh) were calculated as the indicators of the mechanisms of convection in the fuel layer. The comparison between the melting speed and these dimensionless numbers indicated surface tension driven flow was dominant while the role of buoyancy was negligible. In another set of experiments, Particle Image Velocimetry (PIV) was used to study the flow structure within the liquid-phase of n-octane pool fire bound on one side by an ice wall. Experiments were conducted in a square glass tray (9.6 cm × 9.6 cm × 5 cm) with a 3 cm thick ice wall placed on one side of the tray. Burning rate, flame height, and melting front velocity were measured to analyze the effect of heat feedback on melting of the ice. The melting rate of the ice increased from 0.6 cm/min for the first 50 seconds after ignition to 1 cm/min for the rest of burning period. Meanwhile, the measurement of the burning rates and flame heights showed two distinctive behaviors; a growth period from self-sustained ignition to the peak mass loss rate (first 50 seconds after ignition) followed by a steady phase from the peak of mass loss rate until the manual extinguishment. Similarly, the flow field measurements by a 2-dimensional PIV system indicated the existence of two different flow regimes. In the moments before ignition of the fuel, coupling of surface tension and buoyancy forces led to a combined one roll structure in the fuel. This was when a single large vortex was observed in the flow field. After ignition the flow field began transitioning toward an unstable flow regime (separated) with an increase in number of vortices around the ice wall. As the burning rate/flame height increased the velocity and evolving flow patterns enhanced the melting rate of the ice wall. Experimentally determined temperature contours showed that a hot zone with thickness of approximately 3 mm was present below the free surface, corresponding to the multi-roll location. The change in the flow field behavior was found to relate to the melting front velocity of ice. To further study the lateral cavity phenomena, a parametric experimental study on melting of ice adjacent to liquids exposed from above to various heat fluxes was conducted in order to understand the role of liquid properties in formation of cavities in ice. Multiple liquids with wide variety and range of thermophysical properties were used in order to identify the key influential properties on melting. The melting rate of the ice and penetration speed of the liquid in a transparent glass tray (70 mm × 70 mm × 45 mm) with a 20 mm thick ice wall (70 mm × 50 mm × 20 mm) was measured. The melting front velocities obtained from experiments were then compared to surface flow velocities of liquids obtained through a scaling analysis of the surface flow to elucidate the influence of the various thermophysical properties of the liquids on ice melting. The surface velocity of the liquids correlated well to the melting front velocities of the ice which showed a clear relationship between the flow velocity and melting front velocity. As the final step of this work, to extend the findings of the experimental studies conducted herein to larger sizes comparable to realistic situations in the Arctic, an order of magnitude scaling analysis was performed to obtain the extent of ice melting. The scaling considered the heat feedback from the flame to fuel surface, the convective heat transfers toward the ice, and the melting energy continuity of ice. The existing experimental data on the size of lateral cavity were also collected and were correlated to the results of the scaling analysis using a nonlinear regression fitting technique. The mathematical correlation that was obtained by the scaling analysis can be used to predict the size of the lateral cavity for a given fuel, pool fire diameter, and burning time. This correlation will provide a predictive tool to estimate the size of a potential lateral cavity formed during ISB of a given spill scenario. In general, the ability to predict the ice melting caused by burning of spilled oil in ice-infested waters is of great practical importance for assessment of the response outcome. This would assist with quantifying the geometry change of the burning medium which in turn will define oil burning rate and extinction condition. Knowledge of burning behavior and extinction condition indicate the burned volume which can directly be used to define the removal effectiveness of ISB. Nevertheless, this analysis was conducted on a generic interaction of oil and ice and the specific details that are observed in actual application of ISB in ice-infested waters were neglected for simplicity. Extending the outcome of this study to more specific (scenario-based) oil-in-ice situation and improving the predictability of the melting correlation with large-scale experiments are the next steps to develop this work.
6

Computer simulations of evaporation of sessile liquid droplets on solid substrates

Semenov, Sergey January 2012 (has links)
Present work is focused on the numerical study of evaporation of sessile liquid droplets on top of smooth solid substrates. The process of evaporation of a sessile liquid droplet has lots of different applications both in industry and research area. This process has been under study for many years, and still it is an actual problem, solution of which can give answers on some fundamental and practical questions. Instantaneous distribution of mass and heat fluxes inside and outside of an evaporating sessile droplet is studied in this research using computer simulations. The deduced dependences of instantaneous fluxes are applied for self-consistent calculations of time evolution of evaporating sessile droplets. The proposed theory of evaporating sessile droplets of liquid has been validated against available experimental data, and has shown a good agreement. Evaporation of surfactant solution droplets is studied experimentally. The theory, proposed for two stages of evaporation, fits experimental data well. An additional evaporation stage, specific for surfactant solutions, is observed and described. Mathematical modelling of this stage requires further research on surfactant adsorption and its influence on the value of receding contact angle. Numerical study of the evaporation of microdroplets is conducted in order to evaluate the significance of different evaporation mechanisms (diffusive and kinetic models of evaporation) and different physical phenomena (Kelvin s equation, latent heat of vaporization, thermal Marangoni convection, Stefan flow).
7

Scaling Weld or Melt Pool Shape Affected by Thermocapillary Convection with High Prandtl number

Liu, Han-Jen 08 August 2011 (has links)
The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a thinner thickness of the thermal boundary layer than that of momentum boundary layer. Since Marangoni number is usually very high, the domain of scaling is divided into the hot, intermediate and cold corner regions, boundary layers on the solid-liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary surface velocity, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The scaled results agree with numerical data, different combinations among scaled equations, and available experimental data.
8

Scaling molten pool shape induced by thermocapillary force in melting

Lin, Chao-lung 05 August 2009 (has links)
The molten pool shape and thermocapillary convection in melting or welding of metals or alloys having negative surface tension coefficients and Prandtl number greater than unity are determined from a scale analysis. Negative surface tension coefficient indicates that the surface flow is in outward direction, while Prandtl number greater than unity represents that boundary layer thickness of conduction is less than that of momentum. Determination of the molten pool shape is crucial due to its close relationship with the strength, microstructure and properties of the fusion zone. Since Marangoni and Reynolds number are usually greater than ten thousands, transport processes can be determined by scale analysis. In this work, the molten pool is divided into the hot, intermediate and cold corner regions on the flat free surface, boundary layers on the solid-liquid interface and ahead of the melting front for analysis. The results find that the pool shape, surface speed and temperature profiles can be self-consistently evaluated as functions of Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The predictions agree with numerical computations and experimental data in the literature.
9

Mise en oeuvre de méthodes optiques de vélocimétries 2D et 3D appliquées à l’étude de l’effet Marangoni autour d’une bulle unique / Using optical velocimetry methods applied to the study of the Marangoni effect around a single bubble

Carvalho, Victor 18 December 2014 (has links)
La convection de Marangoni est un phénomène hydrodynamique qui apparaît en présence d'un gradient de tension de surface le long d'une interface entre deux fluides non miscibles. Il est possible de voir apparaître cette convection, dans les échangeurs de chaleur avec changement de phase, autour des bulles de vapeur. Cependant, la convection de Marangoni a longtemps été négligée devant les autres phénomènes intervenant dans le transfert de chaleur. A l'ère de la miniaturisation, il devient impossible de négliger cette micro convection. Le but de la thèse est donc de caractériser la dynamique d'écoulement de la convection de Marangoni autour d'une bulle. La première partie présente la résultats 2D obtenus autour d'une bulle d'air en présence d'un gradient de température. Ce cas est plus simple à mettre en oeuvre et permet ainsi de se familiariser avec la convection de Marangoni. La seconde partie porte cette fois-ci sur l'étude bidimensionnelle de cette convection autour d'une bulle de vapeur. Les résultats ont montré que le phénomène devenait très rapidement tridimensionnel . La dernière partie présente donc une méthode de mesure optique 3D innovatrice qui permet de connaître la dynamique de l'écoulement dans les trois dimensions et les trois composantes. / The Marangoni convection is a phenomenon that appears in the presence of a tension surface gradient along an interface between two immiscible fluids. It is possible to observe that appear convection around vapor bubbles in the heat exchangers with the phase change. However, the Marangoni convection has been neglected to other phenomena involved in the heat transfer. In the age of miniaturization, it becomes impossible to overlook this micro convection. The aim of this thesis si to characterize the dynamics of Marangoni convection around a bubble. The first part deals with the 2D results around an air bubble in the presence of a temperature gradient. This case is easier to implement and allows having a better knowledge with the Marangoni convection. The second part focuses on the two-dimensional study of the convection around a vapor bubble The results showed that the phenomenon quickly became three-dimensional. The last section therefore presents a method for measuring optical innovative 3D3C.
10

Svařování oceli technologií PATIG / Steel welding by PATIG technology

Nogol, Petr January 2012 (has links)
The effect of activating flux PATIG on welding arc was investigate in master thesis, A-TIG welding with flux PATIG. The test was performed on six materials with different chemical and mechanical properties. TIG and A-TIG welding was carried out after application flux. Performed require layer of PATIG flux was quite difficult. Result was compared between conventional TIG and experimental A-TIG method of welding. The best reached goals were for high-alloy steels. Typical and 3times deeper penetration was reached for A-TIG compared to conventional TIG welding.

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