Browsing by Subject "Porous Media"

Now showing 1 - 5 of 5
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    Microfluidic Experiments and Numerical Simulations of Inertia-induced Mixing and Reaction Maximization in Laminar Porous Media Flows
    (2024-10-10) Chen, Michael; Lee, Sanghyun; Kang, Peter; pkkang@umn.edu; Kang, Peter; Kang Research Group
    Solute transport and biogeochemical reactions in porous and fractured media flows are controlled by mixing, as are subsurface engineering operations such as contaminant remediation, geothermal energy production, and carbon sequestration. A porous media flow is generally regarded as slow, so the effects of fluid inertia on mixing and reaction are typically ignored. Here, we demonstrate through microfluidic experiments and numerical simulations of mixing-induced reaction, that inertial recirculating flows readily emerge in laminar porous media flows and dramatically alter mixing and reaction dynamics. An optimal Reynolds number that maximizes the reaction rate is observed for individual pore throats of different sizes. This reaction maximization is attributed to the effects of recirculation flows on reactant availability, mixing, and reaction completion, which depend on the topology of recirculation relative to the boundary of the reactants or mixing interface. Recirculation enhances mixing and reactant availability, but a further increase in flow velocity reduces the residence time in recirculation, leading to a decrease in reaction rate. The reaction maximization is also confirmed in a flow channel with grain inclusions and a randomized porous media. Interestingly, the domain-wide reaction rate shows a dramatic increase with increasing Re in the randomized porous media case. This is because fluid inertia induces complex three-dimensional flows in a randomized porous media, which significantly increases transverse spreading and mixing. This study shows how inertial flows control reaction dynamics at the pore scale and beyond, thus having major implications for a wide range of environmental systems.
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    Microscale Modeling of Porous Thermal Protection System Materials
    (2015-05) Stern, Eric
    Ablative thermal protection system (TPS) materials play a vital role in the design of entry vehicles. Most simulation tools for ablative TPS in use today take a macroscopic approach to modeling, which involves heavy empiricism. Recent work has suggested improving the fidelity of the simulations by taking a multi-scale approach to the physics of ablation. In this work, a new approach for modeling ablative TPS at the microscale is proposed, and its feasibility and utility is assessed. This approach uses the Direct Simulation Monte Carlo (DSMC) method to simulate the gas flow through the microstructure, as well as the gas-surface interaction. Application of the DSMC method to this problem allows the gas phase dynamics -- which are often rarefied -- to be modeled to a high degree of fidelity. Furthermore this method allows for sophisticated gas-surface interaction models to be implemented. In order to test this approach for realistic materials, a method for generating artificial microstructures which emulate those found in spacecraft TPS is developed. Additionally, a novel approach for allowing the surface to move under the influence of chemical reactions at the surface is developed. This approach is shown to be efficient and robust for performing coupled simulation of the oxidation of carbon fibers. The microscale modeling approach is first applied to simulating the steady flow of gas through the porous medium. Predictions of Darcy permeability for an idealized microstructure agree with empirical correlations from the literature, as well as with predictions from computational fluid dynamics (CFD) when the continuum assumption is valid. Expected departures are observed for conditions at which the continuum assumption no longer holds. Comparisons of simulations using a fabricated microstructure to experimental data for a real spacecraft TPS material show good agreement when similar microstructural parameters are used to build the geometry. The approach is then applied to investigating the ablation of porous materials through oxidation. A simple gas surface interaction model is described, and an approach for coupling the surface reconstruction algorithm to the DSMC method is outlined. Simulations of single carbon fibers at representative conditions suggest this approach to be feasible for simulating the ablation of porous TPS materials at scale. Additionally, the effect of various simulation parameters on in-depth morphology is investigated for random fibrous microstructures.
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    Natural convection in horizontal fluid-superposed porous layers heated locally from below.
    (2010-12) Bagchi, Aniruddha
    Natural convection in a horizontal fluid-superposed porous layer heated locally from below is studied in this thesis. This problem occurs in numerous engineering and geophysical systems, such as fibrous and granular thermal insulations, water reservoirs, grain storage installations, solid-matrix heat exchangers, solidifying castings, and post-accident cooling of nuclear reactors. In nature, thermal circulation in lakes, shallow coastal areas and other reservoirs occurs in a system having a fluid layer superposed on top of a porous matrix that is heated locally at the base. In spite of its fundamental nature, there are virtually no studies in the literature pertaining to this problem. To address this shortcoming, the fundamental aspects of this problem are studied in this work. The goal of the thesis is to study how free convective heat transfer in a composite layer with a localized bottom heat source is affected by parameters such as the size of the heat source, the fluid-to-porous layer height ratio, the Darcy number, the aspect ratio, the solid-to-fluid conductivity ratio, and the fluid Prandtl number. To that end, two particular aspects of the problem are studied: (a) the steady-state and transient flow and temperature fields, and (b) measurement and prediction of the overall heat transfer characteristics of the system. A numerical approach has been used to study the development of the temperature and flow fields in the system and predict overall heat transfer rates. A one-domain formulation, which uses a single set of governing equations to model fluid flow and heat transfer throughout the entire composite domain, is used. To solve the governing equations, a control volume based numerical solution technique is used. Results show that the nature of convective motion in a composite system with a localized bottom heat source is identical to that observed when the base is uniformly heated. A cellular flow pattern is observed with flow penetration occurring from the overlying fluid layer to the underlying porous layer. Penetration, however, is significant only near the fluid-porous layer interface. The overall heat transfer coefficients are found to depend strongly on the heater length, height ratio, and the fluid-to-porous layer conductivity ratio, while the effects of the aspect ratio and the fluid Prandtl number are not very significant. The effect of the Darcy number is moderate up to Darcy numbers of 10-3 beyond which there is a sudden increase in heat transfer coefficients Experiments are performed to validate the numerical solutions and develop empirical Nusselt-versus-Rayleigh number correlations. Experiments are conducted in a cubical chamber with 3 mm DIA glass beads as the porous layer and distilled water as the saturating fluid. Two different heater lengths and three different height ratios are investigated. Experimental results confirm the numerical predictions that the Nusselt number increases with a decrease in the heater length and an increase in the height ratio. However, significant numerical differences are seen when experimental and numerical results are compared. The most likely reason for the observed discrepancy is the implementation of the one-domain model for the numerical solution. This discrepancy calls into question the results of prior numerical studies for the fully heated bottom which have used a one-domain formulation and emphasizes the necessity of validation via experiment.
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    Net Flow Through Soft Porous Media Generated by a Periodic Mean-Zero Pressure Gradient
    (2023-04) Stein, Jacob
    In this thesis, a soft porous media experiment is developed to investigate the effect of applying periodic mean-zero pressure gradients to a soft porous medium. These periodic mean-zero pressure gradients are composed of square waves, with a high magnitude positive pressure drop applied for 1/3 of the period and a negative pressure drop with half the magnitude applied for 2/3 of the period. We investigate the effect of varying the pressure magnitude and period. This setup uses a novel tracking method to quantify the porous media and fluid velocities simultaneously, and operates with high levels of accuracy and repeatability. The periodic mean-zero pressure gradients we investigate are shown to generate substantial net flow, in the direction associated with the smaller pressure magnitude portion of the period. The system is also shown to demonstrate hysteretic behavior, where complex packing features form and persist in the solid phase, influencing the results of the system for subsequent experiments. Results from this experiment have important implications for fluid flow in biological tissues, such as interstitial transport in the brain and body, as it demonstrates that small scale, periodic mean-zero pressure gradients can drive significant amounts of net transport through a deformable porous medium.
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    Transport and Chemical Phenomena in a Solar Thermochemical Reactor to Split Carbon Dioxide and Water to Produce Synthesis Gas
    (2015-10) Bala Chandran, Rohini
    Numerical study of a solar thermochemical reactor for isothermal reduction and oxidation of cerium dioxide is presented with the goal of accomplishing effective gas phase heat recovery and efficient gas utilization — the two most important attributes for achieving high fuel conversion efficiency in the reactor. Models were applied in tandem with and/or compared to experiments to interpret physical processes in the reactor and for validation. To achieve gas phase heat recuperation, a counterflow, ceramic heat exchanger filled with alumina reticulated porous ceramic was designed to operate at 1773 K. The focus of the modeling was selection of the morphology of the reticulated porous ceramic/ foam to balance heat transfer and pressure drop. Foam morphologies of 5–30 pores per inch (PPI) with porosities of 65–90% were considered. Large pore sizes, or equivalently low PPI, augment radiative penetration and reduce pressure drop. Solid phase conduction is the dominant mode of heat transfer over a majority of the heat exchanger length. Consequently, lower porosity improves the overall heat exchange at the penalty of increased pressure drop. The tradeoff in heat transfer performance and pressure drop point to use of higher porosity, 85–90%, and large pore sizes to optimize the solar-to-fuel efficiency. The final design is a 1.4 m long heat exchanger filled with 10 and 5 PPI, 85% porous, alumina foam in the annulus and center tube. This design recuperates more than 90% of the sensible energy of the reactant and product gases with less than 15 kPa pressure drop through the bed of ceria and heat exchanger. Motivated by the need to investigate the influence of transport processes on reactor performance, especially on the temperature distribution and the reaction rates, a transient, three-dimensional computational model of the reactor was developed. A hybrid Monte Carlo/finite volume approach was used to model radiative transport in the reactor surfaces and in the participating media. The chemical kinetics of the cyclic, gas-solid reactions were modeled by obtaining the best fit reaction rate coefficients from global rate data from a bench top reactor at 1773 K. For a solar input of 4.2 kW and gas flow rates of 0.67×10-4 mol s-1 gceria-1, the model predicts nearly isothermal cycling at an average temperature of 1791 K. Results elucidate that spatial variations in temperature, species concentration and reaction rate are interrelated and more pronounced along the gas flow direction. Carbon monoxide is produced continuously at 3.6×10-4 mol s-1, translating to 100 W of stored chemical energy. The overall reaction rates are driven by gas phase advection and the intrinsic material thermodynamics, rather than surface kinetics. The numerical modeling framework developed in this dissertation is robust and conducive to study other thermochemical processes in a high temperature solar reactor.