The growing demand for the development of efficient miniature cooling systems has led to stimulating numerous investigations on two-phase flow boiling in small to microscale tubes. Because of the variation in properties of synthetic cooling fluids, this causes an inaccuracy of existing flow boiling prediction models or correlations in the literature to interpolate or extrapolate the two-phase flow boiling heat transfer and pressure drop. The purpose of this investigation was to study experimentally the parametric aspects of flow boiling characteristics inside vertical stainless-steel tubes with four different internal diameters (1.1, 2.01, 2.88 and 4.26 mm). The R245fa (1,1,1,3,3-pentafluoropropane, HFC-245fa) was used as working fluid. The experiments were carried out under a system pressure range of 185 - 310 kPa (which correspond to a saturation temperature range of 31 - 46 °C), mass flux range of 200 - 500 kg/m²s, heat flux range of 3 - 188.5 kW/m², vapour quality up to the onset of dryout and 5 K inlet subcooling. Flow pattern visualisations, two-phase pressure drops and saturated flow boiling heat transfer coefficients were presented. The experimental data of R134a employed for comparison is acquired from the previous studies of Huo et al. (2007), Shiferaw et al. (2011) and Mahmoud et al. (2014a). These studies were carried out in the same experimental facility and under the similar operating conditions. The Two-phase flow regimes inside four tubes were visualised in a borosilicate glass tube located at the heated section outlet to capture the dominant flow patterns which assist to elucidate the heat transfer results. The flow boiling visualisation was recorded by a high-speed camera with experiments of increasing and decreasing heat flux. The four observed flow regimes are identified as bubbly flow, slug flow, churn flow and annular flow. In increasing heat flux experiments, the churn and annular flows were only the dominant patterns in all four tubes. The slug flow was often discerned at lower mass flux except for the tube of 1.1 mm where it was not observed at all. This is contrary to decreasing heat flux experiments where all flow patterns including the bubble flow were observed in all the tubes. This shows a strong impact of hysteresis, which is a result of nucleation sites remained active as the heat flux is reduced. The flow patterns and transition boundaries for R245fa are affected by mass flux, system pressure, and tube diameter. The vapour quality corresponding to flow pattern transition boundary tends to decrease with increasing mass flux and tends to increase with increasing system pressure and decreasing tube diameter. Except for the bubbly-slug boundary, its vapour quality decreases with decreasing tube diameter. The experimental flow pattern maps of R245fa were fairly predicted with the predictive models developed for mini- and micro-channels by Tibiriçá et al. (2017). The two-phase pressure drop of R245fa is affected by mass flux, heat flux, system pressure, tube diameter and surface topography. The two-phase pressure drop increases with increasing mass flux and heat flux (vapour quality) and decreases with increasing system pressure and tube diameter. The two-phase pressure drop of the coated tube is higher than that of the uncoated tube. This is attributed to the coated tube having a higher surface roughness compared to the uncoated tube. The comparison between R245fa and R134a shows that the measured two-phase pressure drop of R245fa is dramatically higher than that of R134a. This arises from the difference in physical properties of the two fluids. The experimental data of two-phase pressure drop for 4.26 mm tube were reasonably predicted by Müller-Steinhagen and Heck (1986) correlation. Further, the experimental data of 2.88 mm and 2.01 mm tubes were well predicted by Chisholm (1973a), and Kim and Mudawar (2013), respectively. The experimental data of 1.1 mm tube were not well predicted by any of the selected predictive methods. The local heat transfer coefficient of all tubes increases with increasing heat flux for low and intermediate vapour qualities. After this vapour qualities, the heat flux effect diminishes. Then, the local heat transfer coefficient increased slightly with vapour quality, especially for higher heat flux near the outlet of the tube. However, the dryout inception in the 1.1 mm tube occurs after the intermediate vapour quality value and expands along the high vapour quality region. The behaviour of the local heat transfer coefficients of 1.1 and 2.88 mm tube is slightly dependent on the mass flux and vapour quality. Contrarily, there is insignificant effect of mass flux along 2.01 and 4.26 mm tube. This gives an indication of the contribution of nucleate boiling in the heat transfer process at lower and medium heat fluxes and nucleate boiling plus convective evaporation at higher heat fluxes near the tube outlet. Further, the local heat transfer coefficient increases as the system pressure increases. The tube diameter has a strong influence on the enhancement of local heat transfer coefficient. The enhancement in average heat transfer coefficient approaches 83% when the tube diameter is reduced from 4.26 to 1.1 mm. The trend of the local heat transfer coefficient of R134a was almost similar to that of R245fa with the exception of local dryout. The average heat transfer coefficient of R134a is about 106-151% larger than that of R245fa for the operational range studied. The dominant heat transfer mechanism is also represented by nucleate boiling for both fluids, particularly for 4.26 mm tube tested in this study. Also, the average heat transfer coefficient was enhanced by 33% when the inner tube surface coated with a copper coating. Finally, the correlation of Fang et al. (2017) predicted all experimental data for the four tubes with fair and similar accuracy.

This thesis presents the continued development of micro optical phase detection instrumentation capable of measuring void fraction, interfacial area density, interfacial velocity and bubble sizes and their application to measurements in a high speed boundary layer. The instrumentation consists of micro sized sapphire tipped probes tailored to measure in the two-phase flow of air bubbles in water. Probe tips with geometries intended to maximize field life while minimizing intrusiveness were designed, fabricated and characterized. The characterization revealed that the active region of a probe tip can go beyond the highly sensitive 45 degree tip. Controlling the active length of the tips can be achieved through a combination of taper angles and 45 degree tip size, with larger tips having shorter active lengths. The full scale bubbly flow measurements were performed on a 6 m flat bottom survey boat. The aforementioned quantities were measured on bubbles naturally entrained at the bow of the boat. Probes were positioned at the bow of the boat, near the entrainment region and at the stern where the bubbles exit after having interacted with the high shear turbulent boundary layer. Experiments were conducted in fresh water, at the Coralville Lake, IA, and salt water, at the St. Andrews Bay and Gulf Coast near Panama City, FL. The results indicate that the bubbles interact significantly with the boundary layer. At low speeds, in fresh water, bubble accumulation and coalescence is evident by the presence of large bubbles at the stern. At high speeds, in both fresh and salt water, bubble breakup dominates and very small bubbles are produced near the hull of the boat. It was observed that salt water inhibits coalescence, even at low boat speeds. Void fraction was seen to increase with boat speeds above 10 knots and peaks near the wall. Bubble velocities show slip with the wall at all speeds and exhibit large RMS fluctuations, increasing near the wall.

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.

Pour augmenter le taux de récupération du pétrole, une des solutions chimiques utilisées consiste à injecter des mousses dans les milieux poreux. En effet, les mousses permettent sous certaines conditions de diminuer la mobilité et ainsi d’améliorer le balayage du réservoir. Cependant, les mécanismes contrôlant la mobilité des mousses ne sont pas bien compris. Nous proposons une approche microfluidique permettant une observation directe de l’écoulement des bulles dans un micromodèle de milieux poreux. Nous observons que l’écoulement n’est pas homogène dans le milieu poreux: il se fait uniquement dans quelques chemins. Le nombre de chemins préférentiels dépend de la qualité de la mousse et du nombre capillaire. Si nous simplifions le milieu poreux à une boucle, nous montrons que la formation des chemins préférentiels dépend de la taille de la boucle. En effet, les bulles sont bloquées dans la boucle uniquement quand la taille de la boucle est de l’ordre de grandeur de la taille des bulles. / Crude oil is already usually trapped into heterogeneous porous media. In order to increase the recovery efficiency, one of the chemical solutions consists in injecting foams in porous media to expel oil from the rock. Foam is indeed able in some cases to greatly decrease the mobility, leading to a better sweeping of the reservoir. However, the mechanisms controlling the foam mobility are not well known. We propose a microfluidic approach allowing a direct observation of the flow of bubbles in a model of porous media. We observe that the flow is not homogeneous in the porous medium: it is concentrated in some paths. The number of these preferential paths depends of the foam quality and the capillary number. If we simplify the geometry of the porous medium to a loop, we prove that the formation of preferential paths depends of the size of the loop. Indeed we can only immobilize the bubbles if the size of the loop is around the size of the bubbles.

Advances in electronics fabrication, coupled with the demand for increased computing power, have driven the demand for innovative cooling solutions to dissipate waste heat generated by these devices. To meet future demands, research and development has focused on robust and stable two-phase heat transfer devices. A confined impinging jet is explored as means of utilizing two-phase heat transfer while minimizing flow instabilities observed in microchannel devices. The test configuration consists of a 4 mm diameter jet of water that impinges on a 38 mm diameter heated aluminum surface. Experimental parameters include inlet mass flow rates from 150 to 600 g/min, nozzle-to-surface spacing from 1 to 8 mm, and input heat fluxes from 0 to 90 W/cm2. Results were used to assess the influence of the testing parameters on the heat transfer performance and stability characteristics of a two-phase confined impinging jet. Stability characteristics were explored using power spectral densities (PSDs) of the inlet pressure time series data. Confined impinging jets, over the range of conditions tested, were found to be stable and an efficient means of removing large amounts of waste heat. The radial geometry of the confined jet allows the fluid to expand as it flows radially away from the nozzle, which suppresses instabilities found in microchannel array geometries. Conditions of the heater surface were found to strongly influence two-phase performance. Analysis of PSDs, for stable operation, showed dominate frequencies in the range of 1-4 Hz, which were attributed to generated vapor expanding in the outlet plenum and the subsequent collapse as it condensed. A stability indicator was developed by inducing artificial instabilities into the system by varying the amount of cross sectional area available for outlet vapor removal and compared to the results for stable operation. / Graduation date: 2012

This thesis presents numerical techniques for solving problems of incompressible flow coupled to scalar transport equations using finite element discretizations in space. The two applications considered in this thesis are multi-phase flow, modeled by level set or phase field methods, and planetary mantle convection based on the Boussinesq approximation. A systematic numerical study of approximation errors in evaluating the surface tension in finite element models for two-phase flow is presented. Forces constructed from a gradient in the same discrete function space as used for the pressure are shown to give the best performance. Moreover, two approaches for introducing contact line dynamics into level set methods are proposed. Firstly, a multiscale approach extracts a slip velocity from a micro simulation based on the phase field method and imposes it as a boundary condition in the macro model. This multiscale method is shown to provide an efficient model for the simulation of contact-line driven flow. The second approach combines a level set method based on a smoothed color function with a the phase field method in different parts of the domain. Away from contact lines, the additional information in phase field models is not necessary and it is disabled from the equations by a switch function. An in-depth convergence study is performed in order to quantify the benefits from this combination. Also, the resulting hybrid method is shown to satisfy an a priori energy estimate. For the simulation of mantle convection, an implementation framework based on modern finite element and solver packages is presented. The framework is capable of running on today's large computing clusters with thousands of processors. All parts in the solution chain, from mesh adaptation over assembly to the solution of linear systems, are done in a fully distributed way. These tools are used for a parallel solver that combines higher order time and space discretizations. For treating the convection-dominated temperature equation, an advanced stabilization technique based on an artificial viscosity is used. For more efficient evaluation of finite element operators in iterative methods, a matrix-free implementation built on cell-based quadrature is proposed. We obtain remarkable speedups over sparse matrix-vector products for many finite elements which are of practical interest. Our approach is particularly efficient for systems of differential equations.

In this thesis the two-fluid (Eulerian/Eulerian) formulation for dispersed two-phase flow is considered. Closure laws are needed for this type of models. We investigate both empirically based relations, which we refer to as a nongranular model, and relations obtained from kinetic theory of dense gases, which we refer to as a granular model. For the granular model, a granular temperature is introduced, similar to thermodynamic temperature. It is often assumed that the granular energy is in a steady state, such that an algebraic granular model is obtained. The inviscid non-granular model in one space dimension is known to be conditionally well-posed. On the other hand, the viscous formulation is locally in time well-posed for smooth initial data, but with a medium to high wave number instability. Linearizing the algebraic granular model around constant data gives similar results. In this study we consider a couple of issues. First, we study the long time behavior of the viscous model in one space dimension, where we rely on numerical experiments, both for the non-granular and the algebraic granular model. We try to regularize the problem by adding second order artificial dissipation to the problem. The simulations suggest that it is not possible to obtain point-wise convergence using this regularization. Introducing a new measure, a concept of 1-D bubbles, gives hope for other convergence than point-wise. Secondly, we analyse the non-granular formulation in two space dimensions. Similar results concerning well-posedness and instability is obtained as for the non-granular formulation in one space dimension. Investigation of the time scales of the formulation in two space dimension suggests a sever restriction on the time step, such that explicit schemes are impractical. Finally, our simulation in one space dimension show that peaks or spikes form in finite time and that the solution is highly oscillatory. We introduce a model problem to study the formation and smoothness of these peaks. / QC 20101018

The air seeding cart is an important piece of farming equipment used in the seeding process. Three factors which are necessary to monitor during the seeding process are the seeding rate (material mass flow rate), air flow rate, and blockages. In current practice, there are systems that monitor and report air flow and blockages but not the actual seeding rate. Presently, the seeding rate is based on the metering calibration before the seeding process starts, which requires a lot of time and energy from the operator. If that goes wrong, it not only takes longer, but also costs more money and increases the already significant stress and fatigue which farmers and operators have during the seeding period. Therefore, the development of reliable, and easily calibrated, on-line sensors for flow monitoring would be beneficial. Further, such sensors would facilitate closed-loop control of the flow rate itself. In order to develop a laboratory prototype for mass flow measurement, a model for mass flow estimation was established. This was accomplished by using pressure transducers to determine the pressure drop across an elevation in the primary air cart run (between the air seeding cart and the air hoe drill). An air seeding test station was designed and developed for the study. Three different types of seeds and a granular fertilizer were chosen and tested. These tested materials were canola, wheat, chickpea and urea fertilizer (46-0-0). The general form of the model was developed using data from the canola tests. The input parameters for this mass flow estimation model were pressure drop and air flow information. The average percent error of the material mass flow rate’s full range was under 10%, except for the highest rate which tested up to 20%. Overall, more than 75% of the estimations had percent errors being less than 5%. The form of the model was also applicable to other individual tested materials with the percent error of their full ranges up to 20%. However, their average of their median error was around 5% of their full ranges. The general model was also applied to the combined data from all tested materials. The results were not as accurate as when the model was applied to the individual tested material. The median of the percent error (of material mass flow rate full range) varied from as low as 1% to as high as 30%, depending on the tested materials. Nevertheless, it demonstrated that there were consistencies between the behaviour of the four tested materials.

No description available.

No description available.