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George W. Woodruff School of Mechanical Engineering
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ItemThermo-Fluids Perfromance of Helium Cooled Divertors Emphasizing Plate-Type Concepts(Georgia Institute of Technology, 2023-08-17) Lee, Daniel SeokjunThe magnetic fusion energy (MFE) tokamak reactor, which confines a high temperature plasma within a torus shaped chamber by means of magnetic fields, is one of the most promising and best developed concepts for making nuclear fusion energy possible. One of the challenges in current long-pulse MFE reactors is overcoming the extreme conditions at the first wall of the reactor chamber, where plasma facing surfaces are subject to very high heat fluxes. The divertor is a magnetic field configuration that enables removal of impurities and fusion products from the core plasma by impingement on solid surfaces. The solid target plates of the divertor, which are directly exposed to the plasma and very high heat fluxes of at least 10 MW/m^2, must therefore be cooled so that they do not melt and contaminate the plasma. In addition, about 20% of the total energy produced by the fusion reaction is absorbed by plasma facing surfaces including the divertor targets, and therefore it is important to have a cooling system that can recover this energy. Several designs for cooling the divertor targets have been proposed, and most use helium to cool the back side of the target plates with impinging jets. This doctoral thesis details experimental and numerical studies to estimate the cooling capabilities and required pumping power under prototypical conditions of a number of helium cooled divertor designs, specifically the helium-cooled flat plate (HCFP) divertor, helium-cooled modular divertor with multiple jets (HEMJ), and a “flat design” which is a simplified variant of the HEMJ. Experiments were performed over He mass flow rate ranging from 3 to 10 g/s in helium loops operating at the prototypical pressure of 10 MPa, and elevated helium inlet temperatures as high as 400 °C and incident heat fluxes as high as 8.1 MW/m^2. Numerical simulations using commercial software, validated by these experimental data, complement these experimental studies and are used to extrapolate the thermo-fluids performance to fully prototypical conditions. The results are used to develop “parametric design curves” to allow designers of He-cooled divertors to estimate the maximum allowable heat flux and the corresponding pumping power requirements at different coolant flow rates for a specified limit on the maximum surface temperature of the pressure boundary.
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ItemTechno-Economic Optimization and Market Potential Study of Small-Scale Particle Heating Receiver Based Central Receiver Power Tower Plants in MENA Region(Georgia Institute of Technology, 2021-05-04) Khatti, Shakir ShakoorReceiver designs using different heat transfer mediums such as salt, gas, and particulate material are under development. High-temperature operation characteristics of particle heating receiver (PHR) based central receiver power tower (CRPT) systems with low-cost thermal energy storage (TES) provide an advantage over the existing salt heating systems. Therefore, this study proposes to investigate and identify the best sites for small-scale PHR based CRPT system deployment to provide electricity to off-grid remote locations in the Middle East and North Africa (MENA) region due to higher direct normal irradiation (DNI) in the region throughout the year. This research aims to develop and apply a methodology for the techno-economic analysis optimization and geographic information system (GIS) based market potential study of PHR based CRPT systems in the region. For sizing the system, optimum solar multiple (SM) and TES hours are found by performing techno-economic optimization in SolarPILOT and SAM. To compare the economic feasibility, a useful economic criterion, Levelized Savings for Electricity (LSFE) is calculated by taking the difference between Levelized Cost of Electricity (LCOE) of PHR based CRPT system and LCOE of stand-alone diesel generators. Suitable regions for PHR-CRPT sites are identified by excluding regions with higher slopes, extensive urbanization, proximity to electrical transmission lines, and other negative features. Favorable locales within the feasible region are identified by mapping the density of potential consumers, per capita electricity consumption, and LSFE. Overall results of GIS-based multi-criteria analysis are presented as maps showing favorable (higher savings) locales for investment in PHR-CRPT systems.
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ItemThe development and application of design and optimization methods for energy intensive mechanical systems for challenging environments as applied to a concentrated solar power particle lift system(Georgia Institute of Technology, 2019-03-21) Repole, Kenzo K.This dissertation investigated improved methods and tools for the design and optimization of energy intensive mechanical systems with specific application to high temperature particle transport for use in a particle heating receiver based concentrated solar power system. This form of concentrated solar power uses solid particles to capture the solar energy and then use it for power generation or store it as thermal energy for later use. The particle lift system is a critical component which must transport the particles from the lower temperature storage bin back to the particle heating receiver. This research is the integration and development of design and analysis tools for such energy-intensive mechanical systems and their demonstration in the conceptual design followed by the design development and optimization. The conceptual design employs an innovative multi-stage structured design process. For optimization, a unique performance and cost model based on first principles and standard cost engineering is used to generate efficiency and cost estimates. The design development, modeling, and optimization methods developed herein, while demonstrated for a particular system, are generally applicable to any energy intensive materials handling system, especially one developed for operation in a challenging environment such as the high temperature particle-laden environment in this application. This research furthers the development of design and analysis tools and the methods available for developing such energy intensive systems and the development of basic design methods. It helps ensure that potentially effective conceptual design approaches are not overlooked and that the most promising concepts are selected and developed and implemented with a minimum investment in the design and engineering effort.
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ItemExperimental verification of numerical models of granular flow through wire mesh screens(Georgia Institute of Technology, 2017-11-10) Sandlin, Matthew J.A proposed design for concentrating solar power receiver uses a granular material - such as sand, which is inert, inexpensive, and able to operate at relatively high temperatures, thereby increasing thermodynamic efficiency - as the heat transfer and energy storage medium. An early design of particle heating receivers (PHR) utilizes a falling curtain of particles which directly absorbs the concentrated solar radiation. However, falling curtain receivers have several disadvantages including significant heat and particle losses and short residence time within the irradiation zone. One design proposal which overcomes these challenges is the so called impeded flow PHR design, in which the particles flow over, around, or through a series of obstacles in the flow path. This reduces the average velocity of the particles, thereby increasing residence time in the irradiation zone of the receiver. It also reduces heat and particle losses from the receiver. However, the hydrodynamics of complex granular flows are not well understood, rendering a priori design of impeded flow PHR geometries difficult. This investigation had two main goals. First, a series of representative impeded flow PHR geometries were constructed, instrumented and tested, allowing detailed quantitative measurement of such parameters such as mass flux and particle velocity distribution within the receiver geometry. This allowed the development of performance envelopes for the various receiver geometries, which may be useful for future receiver designers. Second, numerical models of the receiver designs were developed using two different approaches - the discrete element method (DEM), which tracks individual particles and models particle collisions as small overlaps, and a two-fluid finite volume method (FVM), in which a granular flow is modeled using typical computational fluid dynamics methods. Predictions of both models were compared against experimental data. It was found that the DEM models generally described the granular flow characteristics better than the FVM models, and were generally able to run faster on parallel computing resources. However, inclusion of heat transfer may be more easily accomplished in future FVM models.
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ItemOptimization of the thermal-hydraulic performance of the helium-cooled modular divertor with multiple jets(Georgia Institute of Technology, 2017-11-07) Zhao, BaileyThe divertor is a key plasma-facing component of future commercial magnetic fusion energy (MFE) reactors that helps sustain fusion reactions by removing helium ash particles and impurities from the core plasma. The divertor target plates are therefore subject to high steady-state incident heat fluxes, expected to be at least 10 MW/m^2 in the international demonstration (DEMO) fusion reactor. The helium-cooled modular divertor with multiple jets (HEMJ), which uses 25 impinging jets of helium to cool plasma-facing tungsten tiles, is a leading candidate for DEMO. Experiments were performed on a single HEMJ module to characterize its thermal-hydraulic performance at coolant inlet temperatures up to 425 °C, inlet pressures of 10 MPa, and incident heat fluxes up to 6.6 MW/m^2 using a closed helium loop. The effect of the jets-to-impingement surface separation distance was experimentally investigated. A numerical model was developed with a commercial software package, and validated against experimental data. The model was used to evaluate the thermo-mechanical performance of the HEMJ, and to optimize the divertor geometry toward a design that is more favorable to fabricate. The optimized HEMJ variant was fabricated and tested in the helium loop. The experimental results were used to develop parametric design charts that predict the HEMJ thermal performance at prototypical inlet temperatures of 600 °C and heat fluxes of 10 MW/m^2. The simulation results provide estimates of thermally-induced stresses and expansion. The results suggest that the HEMJ can accommodate 10 MW/m^2 while keeping the pumping power requirements within specified limits. The simpler HEMJ variant can accommodate 8 MW/m^2 under the same conditions, which could simplify manufacturing and reduce fabrication costs for O(10^6) modules.
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ItemThermal-hydraulic performance evaluation and optimization of the T-tube divertor design using numerical simulations(Georgia Institute of Technology, 2017-07-27) Akdeniz, SercanA key technology issue in magnetic fusion energy is plasma-materials interactions (PMI). Because our understanding of how materials change when exposed to burning plasmas over extended periods is limited, Oak Ridge National Laboratory (ORNL) has proposed a linear plasma simulator, the Material-Plasma Exposure eXperiment (MPEX), to test various fusion-relevant materials. Since this facility will expose various materials to conditions similar to those in a burning plasma (except for fusion-relevant neutrons) at steady state, effectively cooling the target plate which will be exposed to steady-state heat fluxes of several MW/m2 is a major challenge. The objective of this Master's thesis is to use numerical simulation to evaluate and improve the thermal-hydraulic performance of a helium-cooled divertor design adapted for cooling the target plate in a linear plasma simulator. The T-tube divertor design, originally developed by the Advanced Reactor Innovations and Evaluations Compact Stellarator Study (ARIES-CS), was used as the starting point for these simulations because it can withstand a uniform heat flux of 10 MW/m2 over an area of several cm2 when cooled by helium (He) at 10 MPa and 600 °C. The T-tube was adapted for cooling the proposed target plate design for the MPEX using He at 4 MPa and room temperature. Given the much lower coolant temperatures, the simulations considered a target plate consisting of a copper chromium zirconium (CuCrZr) alloy, vs. the tungsten alloy proposed for the original T-tube. The simulations considered two different He flow configurations; a number of modified geometries were evaluated in an attempt to improve the thermal-hydraulic performance of both configurations. The simulations also compared the performance of two different target plate materials, namely the original CuCrZr and a titanium zirconium molybdenum (TZM) alloy.
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ItemNumerical simulation of water-cooled sample holders for high-heat flux testing of low-level irradiated materials(Georgia Institute of Technology, 2014-12-04) Charry León, Carlos HumbertoThe promise of a vast source of energy to power the world and protect our planet using fusion technology has been the driving force for scientists and engineers around the globe for more than sixty years. Although the materialization of this ideal still in the distance, multiple scientific and technological advances have been accomplished, which have brought commercial fusion power closer to a reality than it has ever been. As part of the collaborative effort in the pursuit of realizable fusion energy, the International Thermonuclear Experimental Reactor (ITER) is being developed by a coalition of nations of which the United States is a part of. One critical technological challenge for ITER is the development of adequate plasma facing materials (PFMs) that can withstand the strenuous conditions of operation. To date, high heat flux (HHF) testing has been conducted mainly on non-irradiated specimens due to the difficulty of working with radioactive specimens, such as instrument contamination. In this thesis, the new Irradiated Material Target Station (IMTS) facility for fusion materials at Oak Ridge National Laboratory (ORNL), in which the HHFs are provided by water-wall plasma-arc lamps (PALs), is considered for neutron-irradiated specimens, especially tungsten. The facility is being used to test irradiated plasma-facing components materials for magnetic fusion reactors as part of the US-Japan plasma facing components evaluation by tritium plasma, heat and neutron irradiation experiments (PHENIX). In order to conduct HHF testing on the PFMs various sample holders designs were developed to accommodate radioactive specimens during HHF testing. As part of the effort to design sample holders that are compatible with the IMTS facility, numerical simulations were performed for different water-cooled sample holder designs with the commercial computational fluid dynamics (CFD) software package, ANSYS™ FLUENT®. The numerical models are validated against experimental temperature measurements obtained from the IMTS facility. These experimentally validated numerical models are used to assess the thermal performance of two sample holder designs and establish safe limits for HHF testing under various operating conditions. The limiting parameter for the current configuration was determined for each sample holder design. For the Gen 1 sample holder, the maximum temperature reached within the Copper rod limits the allowable incident heat flux to about 6 MW/m². In the case of the Gen 2 sample holder, the maximum temperature reached within the Molybdenum clamping disk limits the allowable incident heat flux to about 5 MW/m². In addition, the numerical model are used to parametrically investigate the effect of the operating pressure, mass flow rate, and incident heat flux on the local heat flux distributions and peak surface temperatures. Finally, a comparative analysis is conducted to evaluate the advantages and disadvantages associated with the main design modifications between the two sample holder models as to evaluate their impact in the overall thermal performance of each sample holder in order to provide conclusive recommendations for future sample holder designs.
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ItemOn the use of dynamically similar experiments to evaluate the thermal performance of helium-cooled tungsten divertors(Georgia Institute of Technology, 2014-06-24) Mills, BrantleyMany technological hurdles remain before a viable commercial magnetic fusion energy reactor can be constructed, including the development of plasma-facing components with long lifetimes that can survive the harsh environment inside a reactor. One such component, the divertor, which maintains the purity of the plasma by removing fusion byproducts from the reactor, must be able to accommodate very large incident heat fluxes of at least 10 MW/m^2 during normal operation. Modular helium-cooled tungsten divertors are one of the leading divertor designs for future commercial fusion reactors, and a number of different candidates have been proposed including the modular He-cooled divertor concept with pin array (HEMP), the modular He-cooled divertor concept with multiple-jet-cooling (HEMJ), and the helium-cooled flat plate (HCFP). These three designs typically operate with helium coolant inlet temperatures of 600 °C and inlet pressures of 10 MPa. Performing experiments at these conditions to evaluate the thermal performance of each design is both challenging and expensive. An alternative, more economical approach for evaluating different designs exploits dynamic similarity. Here, geometrically similar mockups of a single divertor module are tested using coolants at lower temperatures and pressures. Dynamically similar experiments were performed on an HEMP-like divertor with helium and argon at inlet temperatures close to room temperature, inlet pressures below 1.4 MPa, and incident heat fluxes up to 2 MW/m^2. The results are used to predict the maximum heat flux that the divertor can accommodate, and the pumping power as a fraction of incident thermal power, for a given maximum tungsten temperature. A new nondimensional parameter, the thermal conductivity ratio, is introduced in the Nusselt number correlations which accounts for variations in the amount of conduction heat transfer through the walls of the divertor module. Numerical simulations of the HCFP divertor are performed to investigate how the thermal conductivity ratio affects predictions for the maximum heat flux obtained in previous studies. Finally, a helium loop is constructed and used to perform dynamically similar experiments on an HEMJ module at inlet temperatures as high as 300 °C, inlet pressures of 10 MPa, and incident heat fluxes as great as 4.9 MW/m^2. The correlations generated from this work can be used in system codes to determine optimal designs and operating conditions for a variety of fusion reactor designs.
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ItemThermal performance of gas-cooled divertors(Georgia Institute of Technology, 2013-06-28) Rader, Jordan D.A significant factor in the overall efficiency of the balance of plant for a future magnetic fusion energy (MFE) reactor is the thermal performance of the divertor. A significant fraction of the reactor power is delivered to the divertor as plasma impurities and fusion products are deposited on its surface. For an advanced MFE device, an average divertor heat load of 10 MW/m² is expected at steady-state operating conditions. Helium cooling of the divertors is one of the most effective ways to accommodate such a heat load. Several helium-cooled divertor designs have been proposed and/or studied during the past decade including the T-Tube divertor, the helium-cooled flat plate (HCFP) divertor, the helium-cooled multi-jet (HEMJ) divertor, the helium-cooled modular divertor with integral fin array (HEMP), and the helium-cooled modular divertor with slot array (HEMS). All of these designs rely on some form of heat transfer enhancement via impinging jets or cooling fins to help improve the heat removal capability of the divertor. For all of these designs very large heat transfer coefficients on the order of 50-60 kW/m²-K have been predicted. As the conditions of a fusion reactor and associated helium flow conditions (600 °C and 10 MPa) are difficult to achieve safely in a controlled laboratory environment, the study of these divertors often relies on computer simulations and experimental modeling at non-prototypical, albeit dynamically similar, conditions. Earlier studies were based on the assumption that, for geometrically similar divertor test modules, dynamic similarity can be achieved by matching only the Reynolds number. Experiments conducted in this investigation using different coolants and test module materials have shown this assumption to be false. Modified correlations for the Nusselt number and loss coefficients for the HEMJ and HEMP-like divertor modules have been developed. These have been used to develop generalized performance curves to predict the divertor performance, i.e. the maximum allowable heat flux and corresponding pumping power fraction, at prototypical conditions. Additionally, a numerical study has been performed to optimize the fin array geometry of the HEMP-like divertor module. The generalized correlations and performance curves developed in this investigation can be incorporated into system design codes, thereby allowing system designers to optimize the divertor geometry and operating conditions.
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ItemApplication of convection heat transfer in near-wall jets to electron-beam-pumped gas lasers(Georgia Institute of Technology, 2010-07-07) Lu, BoHeating of the transmission foil separating the vacuum diodes from the laser cell in electron-beam-pumped gas lasers due to electron beam attenuation necessitates an active cooling scheme to prevent its failure under repetitively pulsed operating conditions. Attenuation of the electron beam (typically 500kV, 100kA and 100ns) produces a strong and pulsed volumetric heat source in the relatively thin (~25μm thick) stainless-steel foil causing it to fail. An experimental and numerical investigation has been conducted to study the cooling effectiveness of high-speed near-wall jets for a single stainless-steel foil strip simulating the geometry between two hibachi ribs in the Electra KrF gas laser developed by the Naval Research Laboratory. The foil is placed inside a channel with continuous gas flow simulating the circulating laser gas. Detailed studies include two jet types (planar and circular) and two injection methods (parallel and impinging) for two designs of hibachi (flat and scalloped). The planar jet flows parallel to the circulating laser gas along the entire foil span. The other configuration uses small diameter (0.8, 1.2 and 1.6 mm) circular jets positioned in two staggered rows located on the foil's two edges along the height of the foil (~30 cm). The jets are issued obliquely towards the foil. For both jet configurations, experiments are conducted at different jet velocities, impingement angles and jet-foil spacing to identify the optimal parameters to be used in the actual hibachi foil cooling. Experimental results are also compared to the predictions from CFD simulations using FLUENT®. The results of this research show that near-wall impinging circular jets can effectively cool the foil separating the vacuum diodes from the laser cell in an electron beam pumped KrF laser under prototypical pulsed (5Hz) operating conditions, thereby assuring the foil's survival, while minimizing the impact on electron beam quality and laser efficiency.