Browsing by Subject "heat transfer"

Now showing 1 - 5 of 5
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    CFD Simulations and Thermal Design for Application to Compressed Air Energy Storage
    (2015-06) Zhang, Chao
    The present computational research focuses on fluid flow analysis and heat transfer enhancement in support of the design of a hydraulic Compressed Air Energy Storage (CAES) system. A CAES system compresses air to high pressure during high power generation periods, stores the compressed air, and expands it to generate power during high power demand periods. The main benefit of using CAES is that it overcomes the mismatch between power generation and power demand. An innovative liquid piston method is used in the present research, where liquid (water) is pumped into the lower section of a compression chamber, and the gas (air) is compressed by the rising liquid-gas interface. Important to the efficient operation of CAES is reducing the temperature rise during compression. The work input process for compressing the air requires two main steps. In the first step, compression work compresses the air to a high pressure. The air temperature rises during compression, and this leads to a second step, where the compressed air cools. In order to maintain the work potential, the pressure of the compressed air is maintained during cooling, volume decreases and cooling work is done. As a result, higher temperature rise during compression requires greater amount of total work input for a given amount of air and a given pressure ratio. For similar reasons, it is also desirable to reduce the temperature drop during expansion of compressed air. The use of a liquid piston offers opportunities to insert heat exchanger matrices into the compression chamber to improve heat transfer. Computational Fluid Dynamics (CFD) and design analyses on the heat exchangers are done in the present research. Two main types of heat exchangers, a commercially available, open-cell metal foam and an in-house designed interrupted plate matrix, are investigated, mainly through computational methods, and with experimental validations. CFD modeling of the liquid piston chamber inserted with exchanger matrices requires closure models that characterize the heat transfer and flow resistance characteristics of the heat exchanger elements. For the metal foam matrix, characterization is done by measuring the flow pressure drop and comparing existing heat transfer models to a liquid piston experiment. For the interrupted plate matrix, large numbers of CFD simulations on the unit cells of the exchanger and experiments are applied for developing correlations for these terms. Based on the unit cell simulations, models for three-dimensionally anisotropic heat transfer behavior of porous media have been developed. Using these models, 3-D global-scale CFD simulations of the liquid piston chamber have been done. As the liquid chamber represents an application of two-phase flow through porous media, the simulation combines a VOF (Volume of Fluid) method and a two-energy-equation modeling method for porous media. The choice of the interrupted plate matrix offers flexibility to vary the shape (e.g. plate height, thickness and separation distance) based on an optimum design to further improve CAES efficiency. Design sensitivity analyses typically require large numbers of computational runs and would demand extraordinary computational resources if combined with 2-D or 3-D CFD simulations. Developed in the present research is a simplified, one-dimensional (1-D) code that is much less computationally expensive, and preserves the main physics of the two-phase flow in porous media. The 1-D code is used for the design analysis of the heat exchanger shape distribution along the axial direction of the chamber. CFD Simulations have been done to also compare a no-insert chamber to chambers with exchanger inserts, for both compression and expansion processes. The expansion process allows the expanding compressed gas to push the liquid out of the chamber to generate power. It is shown that in both the compression and expansion processes, using a heat exchanger matrix in the liquid piston chamber can significantly reduce the rise or drop in gas temperature, thus reducing the losses. Using the CFD modeling tools, a design exploration is also done to investigate the effect of changing the profile of the chamber's cross sectional radius along the axial direction to design a gourd-like shaped chamber to agitate flow and enhance heat transfer.
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    Going Deeper into Laser Damage: Experiments and Methods for Characterizing Materials in High Power Laser Systems
    (2016-05) Taylor, Lucas
    Laser damage is a primary limiting factor to the design of high-power laser systems. This is true for short-pulse systems as well as long-pulse and continuous-wave (CW) systems. Unlike short-pulse laser damage, CW laser damage has been much less studied. This work comprises a background of laser damage and laser heating theory, a CW laser damage experiment and an imaging technique for monitoring laser heating. The damage experiment was performed on 100 nm thick hafnia coatings deposited on fused silica. Uniformly grown films were compared to hafnia-alumina nanolaminates. While the nanolaminates are known to perform better for 1 ns pulses, we found they had worse laser damage performance in the CW regime. We found the nanolaminates reduced crystallinity. The polycrystalline uniform films are thought to have increased absorption. We measured the thermal conductivity of the nanolaminates to be approximately 1/2 that of the uniform films. A theoretical model including the absorption and thermal conductivity of the nanolaminate and uniform film agreed with the experimental data for 1 ns pulses and CW tests. During laser damage experiments, anomalous damage morphologies were observed that we were unable to explain with theoretical techniques. We then developed an experimental method to observe high-speed laser damage events at the ms time-scale. We imaged laser heating and compared it to a theoretical model with good agreement. Our measurement method captured image data from a Mach- Zender interferometer that had do be processed ex-situ. We desired a system capable of providing real-time thermal data. We developed an image processing technique at least 66 times faster than the original method.
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    Mathematical modeling for assorted problems in crystal growth
    (2019-12) Wang, Kerry
    Crystal growth is a field that is ripe with opportunities for mathematical modeling to elucidate interesting phenomena. Important process parameters such as solute concentration, interface shape and location, and temperature field are uniquely difficult to observe \textit{in-situ} for many high temperature melt crystal growth systems. Additionally, the slow process of growing large, industrially-relevant single crystals can be prohibitive in time, material cost, and labor for tedious repeated experimental studies that are likely to be destructive. Modeling provides an efficient way for researchers to quickly gain an understanding of the physics underlying a crystal growth system. In this thesis, we examine three different cases where mathematical modeling can be utilized to interrogate crystal growth systems. First, we investigate the transport of oxygen in Czochralkski-grown silicon by posing a simple lumped-parameter model. The lumped-parameter model tracks transport of oxygen into and out of the melt without specifying its spatial distribution, relying only on estimated fluxes from various surfaces. The lumped-parameter model offers a near-instantaneous way to obtain a coarse estimate of oxygen given process parameters such as crystal/crucible rotation scheme, melt height, and melt overheating. Second, we examine a past experiment involving Europium-doped BaBrCl monitored \textit{in-situ} via Energy-Resolved Neutron Imaging. Europium acts as a strong neutron attentuator, allowing visualization of its migration in both the solid and melt phases. A prior experiment was conducted to perform \textit{in-situ} imaging of a melt crystal growth system, and we realized this presented an opportunity to use modeling to extract additional data from this past experiment. A 1D model of europium migration in both phases was formulated and solve via Finite Fourier Transforms and Finite Difference Method. The Finite Difference Method, being more flexible, allowed us to deduce the apparent solid and liquid diffusion coefficients of Eu as well as its segregation coefficient. This coupling of \textit{in-situ} imaging and modeling presents an exciting new way to measure physical properties and extract additional value from past experiments. Last, we analyze the curious phenomenon of Temperature Gradient Zone Melting (TGZM), whereby a solute-rich liquid particle migrates through a solid crystal under a thermal gradient. While this phenomenon has been studied in the past, prior models failed to give practical predictions in the time-evolution behavior of such migrating particles. We pose analytical and numerical models of 1-dimensional TGZM, which agree well with each other. The numerical model, solved via Finite Element Method, shows reasonable agreement with experimental data on Te-rich second-phase particles migrating in CdTe. It additionally shows excellent agreement with another physical system, NaCl brine particles in water ice, providing a far more accurate description of the particle's migration than previous theoretical models. Considerations are made for extending the model to higher dimensions in order to understand changes in particle morphology during migration. Different types of modeling using various analytical and numerical techniques are employed for each of these case studies. These three example cases show different scenarios in which mathematical modeling can be utilized to help researchers gain insight in crystal growth systems.
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    Numerical Modeling And Optimization Of Thermofluid Systems: Heat Pumps, Turbocompressors, Porous Media
    (2020-03) Goldenberg, Vlad
    In this dissertation, three types of thermofluid systems: an air Brayton cycle heat pump, a centrifugal compressor stage, and a porous media heat pipe, are investigated. In each of the investigations, numerical modeling is used as the basis that underpins the analyses. Furthermore, the goal of each investigation is to develop a framework for the design and optimization of practical engineering systems. The parameterization of each system is explored and defined. A thermodynamic model of a recuperated air cycle heat pump is developed and used to parametrically study the effects of component performance, operating environment, and design parameters. A numerical optimization is conducted to maximize the heating COP of the air cycle heat pump while maintaining robust performance across a wide operating envelope. Comparison is made to a conventional vapor compression cycle heat pump. It is found that a judicious choice of pressure ratio and maximization of component performance enables a recuperated air cycle heat pump to be comparable in COP to a vapor cycle heat pump for high temperature ratio duty. The recommended pressure ratio is determined to be 1.4. Such a heat pump requires high performance compressor, expander, and heat exchangers. A novel method of the flow path synthesis of a centrifugal compressor stage is revealed. A preliminary design procedure that enables fast and efficient candidate designs is reported. Computational fluid dynamics in conjunction with optimization algorithms, surrogate modeling, and machine learning is used to analyze the fundamental fluid mechanics and to automatically optimize the designs. A single stage performance improvement of over 4% points of isentropic efficiency gain is demonstrated using numerical methods. The microstructure of a flat porous media heat pipe consisting of layers of wire mesh is characterized using numerical techniques. The analysis encompasses the characterizations of the flow-induced pressure drop and interfacial heat transfer for liquid and vapor water phases in a 16-gauge and 200-gauge wire mesh porous domain.
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    Understanding growth rate limitations in production of single-crystal cadmium zinc telluride (CZT) by the traveling heater method (THM)
    (2017-03) Peterson, Jeffrey
    Cadmium telluride (CdTe) and cadmium zinc telluride (CZT) are important optoelectronic materials with applications ranging from medical imaging to nuclear materials monitoring. However, CZT and CdTe have long been plagued by second-phase particles, inhomogeneity, and other defects. The traveling heater method (THM) is a promising approach for growing CZT and other compound semiconductors that has been shown to grow detector-grade crystals. In contrast to traditional directional solidification, the THM consists of a moving melt zone that simultaneously dissolves a polycrystalline feed while producing a single-crystal of material. Additionally, the melt is highly enriched in tellurium, which allows for growth at lower temperatures, limiting the presence of precipitated tellurium second-phase particles in the final crystal. Unfortunately, the THM growth of CZT is limited to millimeters per day when other growth techniques can grow an order of magnitude faster. To understand these growth limits, we employ a mathematical model of the THM system that is formulated to realistically represent the interactions of heat and species transport, fluid flow, and interfacial dissolution and growth under conditions of local thermodynamic equilibrium and steady-state growth. We examine the complicated interactions among zone geometry, continuum transport, phase change, and fluid flow driven by buoyancy. Of particular interest and importance is the formation of flow structures in the liquid zone of the THM that arise from the same physical mechanism as lee waves in atmospheric flows and demonstrate the same characteristic Brunt--V ais al a scaling. We show that flow stagnation and reversal associated with lee-wave formation are responsible for the accumulation of tellurium and supercooled liquid near the growth interface, even when the lee-wave vortex is not readily apparent in the overall flow structure. The supercooled fluid is posited to result in morphological instability at growth rates far below the limit predicted by the classical criterion by Tiller et al. for constitutional supercooling.