# Browsing by Subject "Two-phase"

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Item A Search for Dark Matter with the ZEPLIN II Detector(2010-01-14) Gao, JiantingShow more Galaxies and clusters of galaxies are believed to be dominated by non-luminous non-baryonic dark matter. A favored candidate is a new type of Weakly Interacting Massive Particle (WIMP) with a mass of order 100 GeV/c^2. The ZEPLIN II experiment is a WIMP search experiment that attempts to directly detect WIMP interactions using the two-phase xenon approach. The detector measures both scintillation and ionization generated by interactions in a 31 kg liquid xenon target. This approach provides a powerful discrimination between nuclear recoils, as expected from WIMPs, and background electron recoils. In this work, we develop a new X^2 approach to determine the three dimensional event positions in an attempt to improve the background rejection. The optical properties of the PTFE reflectors and the grids of the detector were determined using the Geant4 simulation, and event positions were obtained by finding the best match to the amount of light in each photomultiplier. This was found to greatly improve the position resolution. The approach was then applied to the WIMP search data. It was found that one of the dominating background sources was events from the gas above the anode grid and not from the PTFE walls caused by the small signals as previously thought. WIMP search results were then obtained from the first 31 days of stable ZEPLIN II data using two methods. Although the X^2 method greatly improved position resolution, the number of background events was not significantly altered and the new limit agreed well with the limit published by the collaboration.Show more Item Analytical and Experimental Study of Annular Two-Phase Flow Friction Pressure Drop Under Microgravity(2011-02-22) Nguyen, Ngoc ThanhShow more Two-phase liquid-gas flow has a wide variety of applications in space, including active thermal control systems, high-power communications satellites, heat pumps and space nuclear reactors. Two-phase systems have many potential advantages over current single-phase systems due to reductions in system size, weight and power consumption. The mechanisms of pressure drop, heat transfer coefficients, void fractions, and flow regimes must be well understood under microgravity conditions in order to design reliable two-phase systems. The main objective of this present research is to develop a new mathematical model that can accurately predict the annular two-phase friction pressure drop to optimize the design of two-phase systems. The two-phase flow tests were conducted aboard the NASA KC-135 aircraft by the Interphase Transport Phenomena (ITP) group from Texas A&M University. The two-phase flow pressure drops were measured across a single transparent test section 12.7 mm ID and 1.63 m long in annular regimes under microgravity conditions during two flight campaigns. Different from previous work, this was the first time both the void fraction and the film thickness were measured under microgravity conditions. The empirical correlations for the interfacial friction factor and void fraction were developed from 57 experimental data using a linear least squares regression technique. The annular two-phase friction pressure drop can be predicted by the new mathematical model requiring only knowledge of the length and diameter of the tube, liquid and vapor mass flow rates, and properties of the working fluid. In addition, the new mathematical model was validated using Foster-Miller & ITP data collected over twelve flights aboard the KC-135 with working fluid R-12 (77 data points), Sundstrand data collected aboard the KC-135 with working fluid R-114 (43 data points) and Zhao and Rezkallah data aboard the KC-135 with working fluid water and air (43 data points). Compared with the LockhartMartinelli model, Wheeler model, Chen model and homogeneous model, the new mathematical model is the optimal model for predicting the two-phase friction pressure drop in annular regimes. The majority of the data falls within +-20% of the proposed correlation and the average error is 12%.Show more Item Electrohydrodynamic induction and conduction pumping of dielectric liquid film: theoretical and numerical studies(Texas A&M University, 2007-04-25) Al Dini, Salem A. S.Show more Electrohydrodynamic (EHD) pumping of single and two-phase media is attractive for terrestrial and outer space applications since it is non-mechanical, lightweight, and involves no moving parts. In addition to pure pumping purposes, EHD pumps are also used for the enhancement of heat transfer, as an increase in mass transport often translates to an augmentation of the heat transfer. Applications, for example, include two-phase heat exchangers, heat pipes, and capillary pumping loops. In this research, EHD induction pumping of liquid film in annular horizontal and vertical configurations is investigated. A non-dimensional analytical model accounting for electric shear stress existing only at the liquid/vapor interface is developed for attraction and repulsion pumping modes. The effects of all involved parameters including the external load (i.e. pressure gradient) and gravitational force on the nondimensional interfacial velocity are presented. A non-dimensional stability analysis of EHD induction pumping of liquid film in a vertical annular configuration in the presence of external load for repulsion mode is carried out. A general non-dimensional stability criterion is presented. Stability maps are introduced allowing classification of pump operation as stable or unstable based on the input operating parameters. An advanced numerical model accounting for the charges induced throughout the bulk of the fluid due to the temperature gradient for EHD induction pumping of liquid film in a vertical annular configuration is derived. A non-dimensional parametric study including the effects of external load is carried out for different entrance temperature profiles and in the presence of Joule heating. Finally, a non-dimensional theoretical model is developed to investigate and to understand the EHD conduction phenomenon in electrode geometries capable of generating a net flow. It is shown that with minimal drag electrode design, the EHD conduction phenomenon is capable of providing a net flow. The theoretical model is further extended to study the effect of EHD conduction phenomenon for a two-phase flow (i.e. a stratified liquid/ vapor medium). The numerical results presented confirm the concept of liquid film net flow generation with the EHD conduction mechanism.Show more Item Isothermal Gas-liquid Flow Using the Lattice Boltzmann Method(2012-10-19) Kim, DonghoonShow more As the operating conditions of the pressurized water reactor (PWR) have been increased towards the thermal limits of the core for economics, the subcooled boiling heat transfer performance of the rod bundles under normal operating conditions has become an increasingly important design focus. Effective field models such as two-fluid model, on which most previous numerical studies in the nuclear fields have focused, cannot predict detailed phenomenon of subcooled boiling because it involves complex multiphase dynamics, such as nucleation, growth, detachment bubbles from a wall, deformation, break-up, coalescence, and condensation. It also requires numerous, additional closure relations. On the other hand, direct numerical simulations with interfacial tracking enable us to capture specific two-phase flow and do not require additional empirical closure relations. In this thesis, we simulate isothermal, two-dimensional bubble dynamics as a starting point toward direct simulation of the subcooled boiling. We adopt a lattice Boltzmann method with the phase-field model. The lattice Boltzmann method is a mesoscopic approach well-adapted to the simulation of complex fluids and is simple to implement. The phase field model can capture complex topological deformation, such as coalescence and break-up, with better numerical stability than other interfacial tracking methods like Volume of Fluid (VOF) and level set methods. We validate the present method for stationary and moving two-phase interfaces by comparing with theoretical solutions for a single static bubble in a stationary liquid and a capillary wave, respectively. In addition, the capability of the current method to simulate the coalescence of two bubbles and droplets is validated by comparing with experimental data. To see the applicability of the method to problems involving complex bubble behaviors and interactions with a high-density ratio as in subcooled boiling water, we simulate rising single and double bubbles in a viscous fluid. For a single bubble problem, the bubble shapes and terminal velocity agreed well with the experimental results for different fluid dynamic conditions. For a double bubble case, the current method can capture the interaction and dynamics of the bubbles. Thus, it is expected that this study can serve as a stepping-stone extension to convective subcooled boiling heat transfer in the nuclear reactor core.Show more Item Multiscale mortar mixed finite element methods for flow problems in highly heterogeneous porous media(2013-12) Xiao, Hailong; Arbogast, Todd James, 1957-Show more We use Darcy's law and conservation of mass to model the flow of a fluid through a porous medium. It is a second order elliptic system with a heterogeneous coefficient. We consider the equations written in mixed form. In the heterogeneous case, we define a new multiscale mortar space that incorporates purely local information from homogenization theory to better approximate the solution along the interfaces with just a few degrees of freedom. In the case of a locally periodic heterogeneous coefficient of period epsilon, we prove that the new method achieves both optimal order error estimates in the discretization parameters and good approximation when epsilon is small. Moreover, we present numerical examples to assess its performance when the coefficient is not obviously locally periodic. We show that the new mortar method works well, and better than polynomial mortar spaces. On the other hand, we also propose to use multiscale mortars as a coarse component to construct a two-level preconditioner for the saddle point linear system arising from the fine scale discretization of the mixed finite element system. The two-level preconditioners are constructed based on the interfaces. We propose a framework to define the interpolation operators for the face based two-level preconditioners for different combination of coarse and fine scale mortar spaces for matching and nonmatching grids. In this dissertation, we show that for quasi-homogeneous problems and matching grids, the condition number of the preconditioned interface operator is bounded by (log(H/h))², which is the same as the traditional two-level preconditioners, for quasi-homogeneous problems. We show several numerical examples to demonstrate that for the strongly heterogeneous porous media, it is often desirable and even necessary to use a higher dimensional coarse mortar space to construct the coarse preconditioner to achieve convergence. We apply our ideas to study slightly compressible single phase and two-phase flow in a porous medium. We find that for the nonlinear single phase problem, the two-level preconditioners could be successfully applied to the symmetrized linear system. For the two-phase problem, using the fine scale, instead of multiscale, velocity solutions from the flow problem can greatly benefit the transport problem.Show more Item A predictive model for sand production in poorly consolidated sands(2010-12) Kim, Sung Hyun, 1983-; Sharma, Mukul M.; Prodanovic, MasaShow more This thesis presents a model for the process of sand production that allows us to predict the stability of wellbores and perforation tunnels as well as mass of sand produced. Past analytical, numerical, and empirical models on material failure and erosion mechanisms were analyzed. The sand production model incorporates shear and tensile failure mechanisms. A criterion for sand erosion in failed sand was proposed based on a force balance calculation on the sand face. It is shown that failure, post failure sand mechanics and flow-dominated erosion mechanisms are important in the sand production process. The model has a small number of required input parameters that can be directly measured in the lab and does not require the use of empirical correlations for determining sand erosion. The model was implemented in a numerical simulator. Three different experiments using different materials were simulated and the results were compared to test the model. The model-generated results successfully matched the sand production profiles in experiments. When the post-failure behavior of materials was well-known, the match between the simulation and experiment was excellent. Sensitivity studies on the effect of mechanical stresses, flow rates, cohesion, and permeability show qualitative agreement with experimental observations. In addition, the effect of two-phase flow was presented to emphasize the importance of the water-weakening of the sand. These results show that catastrophic sand production can occur following water breakthrough. Finally the impact of increasing sand cohesion by the use of sand consolidation chemicals was shown to be an effective strategy for preventing sand production.Show more Item A quadrature Eulerian-Lagrangian WENO scheme for reservoir simulation(2015-08) Pool, Jamie Micayla; Arbogast, Todd James, 1957-; Balhoff, Matthew T; Chen, Thomas; Gonzalez, Oscar; Vasseur, Alexis F; Wheeler, Mary FShow more This dissertations focuses on solving the advection problem with the motivation of simulating transport in porous media. A quadrature based Eulerian-Lagrangian scheme is developed to solve the nonlinear advection problem in multiple spatial dimensions. The schemes combines the ideas of Lagrangian traceline methods with high order WENO reconstructions to compute the mass that flows into a given cell over a time step. These schemes are important since they have a relaxed CFL constraint, and can be run in parallel. In this thesis we provide two improvements to Eulerian-Lagrangian schemes. To do this an integration based WENO (IWENO) interpolation technique is derived by reconstructing the primitive function and differentiating. This technique gives a high order reconstruction of the mass at an arbitrary point. This WENO scheme is used to solve the linear advection problem. A scheme is derived by backwards tracing of quadrature points located on mesh elements. The mass at these tracepoints is used to compute the mass in the trace region, without resolving its boundary. This process defines a high order quadrature Eulerian-Lagrangian WENO (QEL-WENO) scheme that solves the multi-dimensional problem without the need for a spatial splitting technique. The second improvement is for solving the nonlinear advection problem using an approximate velocity field. The velocity field is used to transport mass in the manner of a standard Eulerian-Lagrangian scheme. Then a flux correction is applied to compute the flow across the tracelines. The contribution is to use a variation of the IWENO technique to reduce the stencil size of this computation. Numerical results are presented demonstrating the capabilities of the scheme. An application to two-phase flow in porous media is provided.Show more