Theses and Dissertations from UMD
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Item HEAT AND MASS TRANSFER ANALYSIS AND PERFORMANCE IMPROVEMENT FOR AIR GAP MEMBRANE DISTILLATION(2022) Kim, Gyeong Sung; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Seawater desalination method can be largely divided into evaporation- and membrane-based techniques. From decades ago, the global installation capacity of reverse-osmosis membrane-based seawater desalination (SWRO) started outgrowing that of the evaporative desalination plant due to its higher energy efficiency and it became the mainstream technology in the 20th century. However, small-scale SWRO facilities installed on South Korean islands are not competitive compared to the thermally driven evaporation method as their specific energy consumption (SEC) values are highly ranging in 9 – 19 kWh∙m^(-3) and there have been frequent maintenance events.By taking the advantages of direct utilization of renewable and thermal energy, air gap membrane distillation (AGMD) is investigated in this study as an improved approach. From the preliminary experimental study, it was found that the lower air-gap pressure of AGMD helps to increase its water productivity. However, most of the heat and mass transfer models in AGMD used the constant atmospheric pressure for the air gap. Therefore, new models considering the pressure effect of the air gap is needed. Since maintaining a vacuum pressure in the gap requires additional energy, a vacuum technique consuming less energy is also needed. In addition to controlling the total pressure of the gap, condensation augmentation on the cooling surface on one side of the gap is critical since the vapor flux is dependent on the vapor pressure in the gap. As the preliminary experimental study showed that the dropwise condensation mode dominates the condensation of AGMD, the effect of gap size between the condensation surface and hydrophobic membrane is needed to be investigated. Therefore, this research was performed with the following objectives: (i) experimental investigation and mass transfer model development for vacuum applied AGMD (V-AGMD), (ii) development of a wave-powered desalination system using V-AGMD, (iii) experimental investigation of condensation in AGMD, and (iv) development of condensation enhancement technology for AGMD. From the modeling and experimental research, this study made the following major research outcomes and observations. First, a straightforward mass transfer model was developed by using the concept of Kinetic Theory of Evaporation and temperature fraction value between the fluid temperatures of feed and coolant, based on the AGMD experimental results. This model was evaluated experimentally and showed an excellent prediction of water flux in various air-gap pressures without measuring each temperature of the interface of the feed-membrane-air-cooling surface-coolant. Second, considering that the air gap of AGMD can be operated in a vacuum state using wave power, a novel wave-powered AGMD desalination device was proposed and evaluated for the island’s dwellers. Third, during the whole AGMD tests, only dropwise condensation (DWC) modes were observed on the stainless-steel condensing wall. Therefore, experiments were conducted to understand the physical pattern of DWC from nucleation to departure. After testing under various temperature and humidity conditions, it was confirmed that the average size of the water droplets followed the power law for each case. Fourth, as the periodic cleaning of the condensate wall of AGMD could improve the production of condensate, an experimental study was subsequently performed for the condensation augmentation using an electrohydrodynamic (EHD) method. By both cleaning periodically and applying 2.5 kV and 5.0 kV fields on the condensing surface in a thermos-hygrostat chamber, the water production rate was increased by 32% and 88%, respectively. This study concluded that the performance of an AGMD desalination system can be improved by applying a vacuum or an EHD device in its air gap. Therefore, pilot-scale experiments will be conducted as future studies to evaluate the commercial viability of the improved system.Item COMPACT ABSORBER FOR ADVANCED ABSORPTION HEAT PUMPS(2018) Bangerth, Stefan; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Almost half of all energy contained in primary energy carriers is discarded as low temperature waste heat. One of few application areas for low temperature waste heat recovery is to drive absorption cooling systems for conversion of waste heat to cooling energy. However, absorption chillers are often not economical due to their bulky, and hence expensive, heat and mass exchangers; the absorber heat/mass exchanger being the largest among them. This dissertation introduces original contributions to advance next generation, more economical absorption chillers by utilizing a novel, highly compact absorber. The novel absorber designed in this work enhances absorption performance by combining rotation of the heat transfer surface for solution-side heat and mass transfer enhancement with innovative high-performance heat transfer technology on the water-side. A numerical model was developed to describe the absorption process and promote design optimization. The replacement of gravitation force by the stronger centrifugal acceleration thins and mixes the solution film and thereby decreases solution-side thermal and mass transfer resistance. The development of an original adaptation of manifold-microchannel technology leads to significant water-side heat transfer enhancement. This dissertation includes the first publication of an experimental characterization of exothermic absorption on a spinning disk. The overall and film-side heat transfer coefficients were 4.7 and 5.5 times higher, respectively, than conventional horizontal tube banks. The absorption rate increased by a factor of 4 to 10 folds over those of the conventional tube absorbers. The power required for spinning the disk was modest and ranged between 1.1% and 2.3% of the cooling capacity. The results suggest that a spinning disk absorber could substantially reduce the size of absorber in the absorption machines. The technology developed in this dissertation can lead to more compact and hence more economical absorption chillers, thereby easing higher market penetration of absorption chillers which in turn can reduce the amount of primary energy spent on cooling applications. Spinning disk absorbers may be especially useful if combined with a new generation of absorbents that promise improved system efficiency and/or expanded application range but exhibit challenging thermophysical properties.Item Thermal Characterization of Firebrand Piles(2017) Hakes, Raquel Sara Pilar; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past several decades, the severity of wildland-urban interface (WUI) fires has increased drastically, resulting in thousands of structures lost globally each year. The cause of the majority of structure losses is ignition via firebrands, small pieces of burning material which are generated from burning vegetation and structures. In this thesis, a methodology for studying the heating to recipient fuels by firebrands is developed. Small-scale experiments designed to capture heating from firebrand piles and the process of ignition were conducted using laboratory-fabricated cylindrical wooden firebrands. The methodology compares two heat flux measurement methods. Experimental results compare the effects of varying firebrand diameter, pile mass, and wind speed. An ignition condition is described using temperature and heat flux.Item DIRECT NUMERICAL SIMULATION OF INCOMPRESSIBLE MULTIPHASE FLOW WITH PHASE CHANGE(2015) Lee, Moon Soo Soo; Riaz, Amir; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase change problems arise in many practical applications such as air-conditioning and refrigeration, thermal energy storage systems and thermal management of electronic devices. The physical phenomenon in such applications are complex and are often difficult to be studied in detail with the help of only experimental techniques. The efforts to improve computational techniques for analyzing two-phase flow problems with phase change are therefore gaining momentum. The development of numerical methods for multiphase flow has been motivated generally by the need to account more accurately for (a) large topological changes such as phase breakup and merging, (b) sharp representation of the interface and its discontinuous properties and (c) accurate and mass conserving motion of the interface. In addition to these considerations, numerical simulation of multiphase flow with phase change introduces additional challenges related to discontinuities in the velocity and the temperature fields. Moreover, the velocity field is no longer divergence free. For phase change problems, the focus of developmental efforts has thus been on numerically attaining a proper conservation of energy across the interface in addition to the accurate treatment of fluxes of mass and momentum conservation as well as the associated interface advection. Among the initial efforts related to the simulation of bubble growth in film boiling applications the work in \cite{Welch1995} was based on the interface tracking method using a moving unstructured mesh. That study considered moderate interfacial deformations. A similar problem was subsequently studied using moving, boundary fitted grids \cite{Son1997}, again for regimes of relatively small topological changes. A hybrid interface tracking method with a moving interface grid overlapping a static Eulerian grid was developed \cite{Juric1998} for the computation of a range of phase change problems including, three-dimensional film boiling \cite{esmaeeli2004computations}, multimode two-dimensional pool boiling \cite{Esmaeeli2004} and film boiling on horizontal cylinders \cite{Esmaeeli2004a}. The handling of interface merging and pinch off however remains a challenge with methods that explicitly track the interface. As large topological changes are crucial for phase change problems, attention has turned in recent years to front capturing methods utilizing implicit interfaces that are more effective in treating complex interface deformations. The VOF (Volume of Fluid) method was adopted in \cite{Welch2000} to simulate the one-dimensional Stefan problem and the two-dimensional film boiling problem. The approach employed a specific model for mass transfer across the interface involving a mass source term within cells containing the interface. This VOF based approach was further coupled with the level set method in \cite{Son1998}, employing a smeared-out Heaviside function to avoid the numerical instability related to the source term. The coupled level set, volume of fluid method and the diffused interface approach was used for film boiling with water and R134a at the near critical pressure condition \cite{Tomar2005}. The effect of superheat and saturation pressure on the frequency of bubble formation were analyzed with this approach. The work in \cite{Gibou2007} used the ghost fluid and the level set methods for phase change simulations. A similar approach was adopted in \cite{Son2008} to study various boiling problems including three-dimensional film boiling on a horizontal cylinder, nucleate boiling in microcavity \cite{lee2010numerical} and flow boiling in a finned microchannel \cite{lee2012direct}. The work in \cite{tanguy2007level} also used the ghost fluid method and proposed an improved algorithm based on enforcing continuity and divergence-free condition for the extended velocity field. The work in \cite{sato2013sharp} employed a multiphase model based on volume fraction with interface sharpening scheme and derived a phase change model based on local interface area and mass flux. Among the front capturing methods, sharp interface methods have been found to be particularly effective both for implementing sharp jumps and for resolving the interfacial velocity field. However, sharp velocity jumps render the solution susceptible to erroneous oscillations in pressure and also lead to spurious interface velocities. To implement phase change, the work in \cite{Hardt2008} employed point mass source terms derived from a physical basis for the evaporating mass flux. To avoid numerical instability, the authors smeared the mass source by solving a pseudo time-step diffusion equation. This measure however led to mass conservation issues due to non-symmetric integration over the distributed mass source region. The problem of spurious pressure oscillations related to point mass sources was also investigated by \cite{Schlottke2008}. Although their method is based on the VOF, the large pressure peaks associated with sharp mass source was observed to be similar to that for the interface tracking method. Such spurious fluctuation in pressure are essentially undesirable because the effect is globally transmitted in incompressible flow. Hence, the pressure field formation due to phase change need to be implemented with greater accuracy than is reported in current literature. The accuracy of interface advection in the presence of interfacial mass flux (mass flux conservation) has been discussed in \cite{tanguy2007level,tanguy2014benchmarks}. The authors found that the method of extending one phase velocity to entire domain suggested by Nguyen et al. in \cite{nguyen2001boundary} suffers from a lack of mass flux conservation when the density difference is high. To improve the solution, the authors impose a divergence-free condition for the extended velocity field by solving a constant coefficient Poisson equation. The approach has shown good results with enclosed bubble or droplet but is not general for more complex flow and requires additional solution of the linear system of equations. In current thesis, an improved approach that addresses both the numerical oscillation of pressure and the spurious interface velocity field is presented by featuring (i) continuous velocity and density fields within a thin interfacial region and (ii) temporal velocity correction steps to avoid unphysical pressure source term. Also I propose a general (iii) mass flux projection correction for improved mass flux conservation. The pressure and the temperature gradient jump condition are treated sharply. A series of one-dimensional and two-dimensional problems are solved to verify the performance of the new algorithm. Two-dimensional and cylindrical film boiling problems are also demonstrated and show good qualitative agreement with the experimental observations and heat transfer correlations. Finally, a study on Taylor bubble flow with heat transfer and phase change in a small vertical tube in axisymmetric coordinates is carried out using the new multiphase, phase change method.Item TRANSIENT TEMPERATURE MEASUREMENTS OF COMBUSTOR WALLS ENCLOSING A 2-D MODEL COAXIAL INJECTOR(2013) Lee, Hak Seung; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct measurements of combustor inner wall temperatures are difficult due to harsh flow conditions. A novel approach was used to obtain the combustor wall temperature as a function of time and location in a H2-O2 model injector, enclosing acoustically forced flames. The emphasis was to obtain thermal boundary conditions for various injector operation. The new approach combined a series of experimental measurements on the outer wall with a transient heat transfer analysis applicable for low Biot number and low Fourier number conditions. Infrared thermometry technique was applied to obtain outer wall temperature distribution at three different wall thicknesses, and these measurements were combined with the transient analysis to calibrate the amount of heat transfer and the corresponding inner wall temperature. The results showed that the combustor inner wall temperature distribution evolved much differently for acoustically forced flames, suggesting a different thermal boundary condition should be used in those cases.Item On The Computation Of Buoyancy Affected Turbulent Wall Flows Using Large Eddy Simulation(2013) Ojofeitimi, Ayodeji; Trouvé, Arnaud; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A high fidelity object-oriented C++ solver was developed in OpenFOAM® for the solution of low Mach number variable density Navier Stokes equations. Employing the Large Eddy Simulation (LES) methodology to compute the turbulent flowfield, the filtered LES equations were subsequently utilized to study buoyancy affected spatially developing boundary layers in natural and mixed convection spatially developing boundary layer flows. For the subgrid scale (SGS) closure, a locally dynamic Smagorinsky SGS model was implemented into OpenFOAM® to enable the backscatter phenomenon intrinsic to transitioning boundary layers. As a precursor to simulating the intricate aero-thermal flowfield of an in-flight aircraft engine pool fire due to a fuel leak, detailed investigations of two canonical problems in the absence of flames were conducted to assess the robustness of the C++ solver and to elucidate the turbulent flow physics; these test cases consisted of a natural convection turbulent boundary layer over an isothermal vertical plate without any forced flow and the mixed convection turbulent boundary layer over an isothermal vertical plate where the effects of a gradually increasing forced flow in the direction opposite to the gravitational vector were assessed. A third canonical case, the mixed convection over an isothermal horizontal plate, was also investigated as an extension of this thesis. For the first two cases, wall-resolved LES computations were compared with experimental data for first and second order turbulent statistics, along with available experimental frequency spectra of temperature and streamwise velocity fluctuations. In an effort to reduce the computational cost, wall-layer modeled LES computations were performed by implementing new wall models into OpenFOAM®. The fidelity of the wall-resolved and wall-layer modeled LES successfully confirmed the ability of the solver in computing high Grashof number transitioning natural and mixed convection spatially developing boundary layers. As it pertains to the third case, while experimental measurements in air of mixed convection over an isothermal horizontal plate is lacking in the literature, the fundamental structure of the boundary layer was qualitatively validated by examining the near-wall vortical flow topology and employing available empirical data. The accuracy of the results acquired for this flow configuration was deemed reliable due to the excellent agreement attained with the prior two test cases. Overall, the level of fidelity illustrated in this thesis has not been previously demonstrated for spatially developing turbulent boundary layers in natural and mixed convection wall flows, especially for LES. Thus, with the establishment of the methodology employed in this work, it can be further utilized as a reliable tool in computing buoyancy affected flame spread problems aboard in-flight aircraft engine fires to shed light upon the complex flow physics inherent to such flows.Item Investigation of Enhanced Surface Spray Cooling(2006-11-29) Silk, Eric A; Kiger, Ken; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase change technology is a science that is continually finding new applications, from passive refrigeration cycles to semiconductor cooling. The primary heat transfer techniques associated with phase change heat transfer are pool boiling, flow boiling, and spray cooling. Of these techniques, spray cooling is the least studied and the most recent to receive attention in the scientific community. Spray cooling is capable of removing large amounts of heat between the cooled surface and the liquid, with reported heat flux capabilities of up to 1000 W/cm2 for water. Many previous studies have emphasized heat flux as a function of spray parameters and test conditions. Enhanced spray cooling investigations to date have been limited to surface roughness studies. These studies concluded that surface tolerance (i.e. variations in machined surface finish) had an impact upon heat flux when using pressure atomized sprays. Analogous pool boiling studies with enhanced surfaces have shown heat flux enhancement. A spray cooling study using enhanced surfaces beyond the surface roughness range may display heat flux enhancement as well. In the present study, a group of extended and embedded surfaces (straight fins, cubic pin fins, pyramids, dimples and porous tunnels) have been investigated to determine the effects of enhanced surface structure on heat flux. The surface enhancements were machined on the top surface of copper heater blocks with a cross-sectional area of 2.0 cm2. Measurements were also obtained on a flat surface for baseline comparison purposes. Thermal performance data was obtained under saturated (pure fluid at 101 kPa), nominally degassed (chamber pressure of 41.4 kPa) and gassy conditions (chamber with N2 gas at 101 kPa). The study shows that both extended and embedded structures (beyond the surface roughness range) promote heat flux enhancement for both degassed and gassy spray cooling conditions. The study also shows that straight fins provide the best utilization of surface area added for heat transfer. An Energy conservation based CHF correlation for flat surface spray cooling was also developed. CHF predictions were compared against published and non-published studies by several researchers. Results for the correlations performance show an average mean error of ±17.6% with an accuracy of ±30% for 88% of the data set compared against.