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• 1. [Article] Modification of Lower Plenum Structure for Mixing Promotion during Normal Operation of the High Temperature Gas Cooled Reactor

  Since High Temperature Gas-cooled Reactors are being considered as most promising design of upcoming IV Gen reactors, thus key research areas were identified to address safety aspects of this design. Number ...

  Citation × Citation Citation Abstract Copy Title: Modification of Lower Plenum Structure for Mixing Promotion during Normal Operation of the High Temperature Gas Cooled Reactor Author: Gradecka, Malwina Since High Temperature Gas-cooled Reactors are being considered as most promising design of upcoming IV Gen reactors, thus key research areas were identified to address safety aspects of this design. Number of simulations and experiments need to be conducted in this field. In this study thermal-hydraulics aspects of coolant flow through Lower Plenum (LP) of HTGR were considered. In specific: flow characteristics, risk of temperature stratification in LP and hot streaking. As power profile is non-uniform across the core, jets of coolant exit the core region at different temperatures and enter the LP impinging on LP floor creating possibility of hot spots at LP structure and temperature stratification in the plenum itself. Large local temperature gradients can cause material degradation effects. To address those issues numerical simulation and an experiment were developed. The numerical simulation provides coolant flow velocity and temperature fields. The purpose of this study is developing a method for gas mixing promotion in the Lower Plenum of HTTF to reduce risk of the hot streaking and thermal stratification phenomena during normal operation. Following aspect are being examined: identification of gas flow behavior in lower plenum of HTTF based on CFD simulations, identification of hot streaking issue in the HTTF lower plenum using CFD tools, scooping study for available methods for mixing promotion and computational investigation of efficiency of selected method. Study includes description of experimental setup of HTTF, guidance for LP CFD modeling, results and analysis of CFD simulations and proposition of thermal mixing enhancement using available methods. Title: Modification of Lower Plenum Structure for Mixing Promotion during Normal Operation of the High Temperature Gas Cooled Reactor Author: Gradecka, Malwina Since High Temperature Gas-cooled Reactors are being considered as most promising design of upcoming IV Gen reactors, thus key research areas were identified to address safety aspects of this design. Number of simulations and experiments need to be conducted in this field. In this study thermal-hydraulics aspects of coolant flow through Lower Plenum (LP) of HTGR were considered. In specific: flow characteristics, risk of temperature stratification in LP and hot streaking. As power profile is non-uniform across the core, jets of coolant exit the core region at different temperatures and enter the LP impinging on LP floor creating possibility of hot spots at LP structure and temperature stratification in the plenum itself. Large local temperature gradients can cause material degradation effects. To address those issues numerical simulation and an experiment were developed. The numerical simulation provides coolant flow velocity and temperature fields. The purpose of this study is developing a method for gas mixing promotion in the Lower Plenum of HTTF to reduce risk of the hot streaking and thermal stratification phenomena during normal operation. Following aspect are being examined: identification of gas flow behavior in lower plenum of HTTF based on CFD simulations, identification of hot streaking issue in the HTTF lower plenum using CFD tools, scooping study for available methods for mixing promotion and computational investigation of efficiency of selected method. Study includes description of experimental setup of HTTF, guidance for LP CFD modeling, results and analysis of CFD simulations and proposition of thermal mixing enhancement using available methods. Close

• 2. [Article] Pressure effects on density-difference driven stratified flow : CFD model of DCC event in the HTGR

  In a Modular High Temperature Gas Reactor (MHTGR), the Depressurized Conduction Cooldown (DCC) event can be separated into three distinct stages: 1) depressurization, 2) air ingress and, 3) natural circulation. ...

  Citation × Citation Citation Abstract Copy Title: Pressure effects on density-difference driven stratified flow : CFD model of DCC event in the HTGR Author: Magnusson, Jill E. In a Modular High Temperature Gas Reactor (MHTGR), the Depressurized Conduction Cooldown (DCC) event can be separated into three distinct stages: 1) depressurization, 2) air ingress and, 3) natural circulation. During normal operations, the HTGR utilizes forced convection to move the helium coolant through the reactor core. Thus, during normal operations the helium is pressurized to about 7 MPa. As a result of the high pressure system, when a DCC event is initialized, there exists a pressure difference between the helium coolant inside the vessel and the ambient air outside the vessel. After depressurization there exist an air-helium mixture of higher density than the helium in the reactor vessel. This results in the air-helium mixture ingressing into the reactor vessel through stratified flow. The High Temperature Test Facility (HTTF), under construction at Oregon State University, is a scaled model of the MHTGR. The HTTF is being built for code validation purposes. This study is to utilize CFD methods to simulate a DCC event in the HTTF. With this CFD simulation the effects of the depressurization stage on the rest of the model will be quantified. Title: Pressure effects on density-difference driven stratified flow : CFD model of DCC event in the HTGR Author: Magnusson, Jill E. In a Modular High Temperature Gas Reactor (MHTGR), the Depressurized Conduction Cooldown (DCC) event can be separated into three distinct stages: 1) depressurization, 2) air ingress and, 3) natural circulation. During normal operations, the HTGR utilizes forced convection to move the helium coolant through the reactor core. Thus, during normal operations the helium is pressurized to about 7 MPa. As a result of the high pressure system, when a DCC event is initialized, there exists a pressure difference between the helium coolant inside the vessel and the ambient air outside the vessel. After depressurization there exist an air-helium mixture of higher density than the helium in the reactor vessel. This results in the air-helium mixture ingressing into the reactor vessel through stratified flow. The High Temperature Test Facility (HTTF), under construction at Oregon State University, is a scaled model of the MHTGR. The HTTF is being built for code validation purposes. This study is to utilize CFD methods to simulate a DCC event in the HTTF. With this CFD simulation the effects of the depressurization stage on the rest of the model will be quantified. Close

• 3. [Article] A study of opposing mixed convection in the GRTS and in downward pipe flows using the FLUENT CFD code

  Downward flow opposing mixed convection conditions have been studied using the FLUENT CFD code. This work was comprised of two primary objectives. The first was to study core channel flow reversal susceptibility ...

  Citation × Citation Citation Abstract Copy Title: A study of opposing mixed convection in the GRTS and in downward pipe flows using the FLUENT CFD code Author: Jackson, R. Brian Downward flow opposing mixed convection conditions have been studied using the FLUENT CFD code. This work was comprised of two primary objectives. The first was to study core channel flow reversal susceptibility in the Gas Reactor Test Section (GRTS) at low flow conditions. The second objective was the development of a general stability correlation for opposing mixed convection conditions in downward pipe flows as a function of pipe radius, gas material, inlet velocity and elevated wall temperatures. The traditional criterion that buoyancy forces must be considered in mixed convection flows when the Gr/Re² value is on the order of 1.0 was used to identify low flow conditions in the GRTS when core channel flow reversal may occur. The corresponding GRTS power levels and flowrates were studied using FLUENT. An initial FLUENT model was used to model the solid core GRTS components to obtain the individual coolant channel wall temperatures for each axial level of each coolant channel of the GRTS associated with the low flow conditions susceptible to flow reversal. These coolant channel wall temperatures were then used as boundary conditions in a more refined second FLUENT model containing just the fluid regions of the GRTS that was used to obtain the flow characteristics. It was found that flow reversal did not occur in the GRTS even when the traditional criterion predicted that buoyancy forces could not be neglected. When FLUENT was used to study flow reversal for downward opposing mixed convection as a function of pipe radius, gas material, inlet velocity and elevated wall temperatures, it was found that while the traditional criterion that buoyancy forces must be considered when the Gr/Re² value is on the order of 1.0 is a good estimate, it is insufficient to give a good prediction for when flow reversal occurs. The ability of the flow to reverse was largely dependent on pipe radius and gas material. This thesis presents two correlations, one for pipe radii below 2 cm, and one for pipe radii 2 cm and above, predicting when flow reversal will occur as a function of Grashof and Reynolds numbers. Title: A study of opposing mixed convection in the GRTS and in downward pipe flows using the FLUENT CFD code Author: Jackson, R. Brian Downward flow opposing mixed convection conditions have been studied using the FLUENT CFD code. This work was comprised of two primary objectives. The first was to study core channel flow reversal susceptibility in the Gas Reactor Test Section (GRTS) at low flow conditions. The second objective was the development of a general stability correlation for opposing mixed convection conditions in downward pipe flows as a function of pipe radius, gas material, inlet velocity and elevated wall temperatures. The traditional criterion that buoyancy forces must be considered in mixed convection flows when the Gr/Re² value is on the order of 1.0 was used to identify low flow conditions in the GRTS when core channel flow reversal may occur. The corresponding GRTS power levels and flowrates were studied using FLUENT. An initial FLUENT model was used to model the solid core GRTS components to obtain the individual coolant channel wall temperatures for each axial level of each coolant channel of the GRTS associated with the low flow conditions susceptible to flow reversal. These coolant channel wall temperatures were then used as boundary conditions in a more refined second FLUENT model containing just the fluid regions of the GRTS that was used to obtain the flow characteristics. It was found that flow reversal did not occur in the GRTS even when the traditional criterion predicted that buoyancy forces could not be neglected. When FLUENT was used to study flow reversal for downward opposing mixed convection as a function of pipe radius, gas material, inlet velocity and elevated wall temperatures, it was found that while the traditional criterion that buoyancy forces must be considered when the Gr/Re² value is on the order of 1.0 is a good estimate, it is insufficient to give a good prediction for when flow reversal occurs. The ability of the flow to reverse was largely dependent on pipe radius and gas material. This thesis presents two correlations, one for pipe radii below 2 cm, and one for pipe radii 2 cm and above, predicting when flow reversal will occur as a function of Grashof and Reynolds numbers. Close

• 4. [Article] Conversion of biomass constituents to hydrogen-rich gas by supercritical water in a microchannel reactor

  Microreactors offer high rates of heat transfer able to intensify highly endothermic biomass reforming reactions in supercritical water. In this study, two continuous flow microreactor configurations were ...

  Citation × Citation Citation Abstract Copy Title: Conversion of biomass constituents to hydrogen-rich gas by supercritical water in a microchannel reactor Author: Goodwin, Aaron K. Microreactors offer high rates of heat transfer able to intensify highly endothermic biomass reforming reactions in supercritical water. In this study, two continuous flow microreactor configurations were used to gasify biomass constituents in supercritical water. The first reactor configuration was a micro diameter stainless steel or Hastelloy tube (508 µm – 762 µm) imbedded into a reactor block. The reactor provided high rates of heat transfer to the reacting fluid to sustain an isothermal reaction temperature and was used to elucidate interactions between phenol, a lignin model compound, and xylose, a hemicellulose model compound, during co-gasification. In addition, a reaction mechanism and kinetics for phenol and xylose gasification by supercritical were estimated. The second microreactor configuration studied was a parallel channel Hastelloy microreactor. The reactor consisted of 14 parallel rectangular microchannels (1000 µm by 127 µm) integrated into a single contiguous reactor block by diffusion bonding a series of shims between two header plates. Fabrication of the reactor from Hastelloy C-276, a high nickel content alloy, substantially intensified biomass gasification reactions and promoted the water gas shift and methanation reactions due to its high catalytic activity and the large surface area to volume ratio in the microchannel. The dispersion and channel flow distribution in the microchannel reactor was characterized experimentally and modeled using computational fluid dynamics by a tracer pulse study. Furthermore, computational fluid dynamics were used to investigate the effect of endothermic biomass reforming reactions and the average fluid Reynolds number on the reaction temperature. Lastly, xylose was completely gasified to hydrogen and carbon dioxide in the parallel channel microreactor at 650°C and 250 bar within a 1.4 second residence time. Title: Conversion of biomass constituents to hydrogen-rich gas by supercritical water in a microchannel reactor Author: Goodwin, Aaron K. Microreactors offer high rates of heat transfer able to intensify highly endothermic biomass reforming reactions in supercritical water. In this study, two continuous flow microreactor configurations were used to gasify biomass constituents in supercritical water. The first reactor configuration was a micro diameter stainless steel or Hastelloy tube (508 µm – 762 µm) imbedded into a reactor block. The reactor provided high rates of heat transfer to the reacting fluid to sustain an isothermal reaction temperature and was used to elucidate interactions between phenol, a lignin model compound, and xylose, a hemicellulose model compound, during co-gasification. In addition, a reaction mechanism and kinetics for phenol and xylose gasification by supercritical were estimated. The second microreactor configuration studied was a parallel channel Hastelloy microreactor. The reactor consisted of 14 parallel rectangular microchannels (1000 µm by 127 µm) integrated into a single contiguous reactor block by diffusion bonding a series of shims between two header plates. Fabrication of the reactor from Hastelloy C-276, a high nickel content alloy, substantially intensified biomass gasification reactions and promoted the water gas shift and methanation reactions due to its high catalytic activity and the large surface area to volume ratio in the microchannel. The dispersion and channel flow distribution in the microchannel reactor was characterized experimentally and modeled using computational fluid dynamics by a tracer pulse study. Furthermore, computational fluid dynamics were used to investigate the effect of endothermic biomass reforming reactions and the average fluid Reynolds number on the reaction temperature. Lastly, xylose was completely gasified to hydrogen and carbon dioxide in the parallel channel microreactor at 650°C and 250 bar within a 1.4 second residence time. Close

• 5. [Article] CFD simulation of single-phase and flow boiling in confined jet impingement with in-situ vapor extraction using two kinds of multiphase models

  With continued development of the electronic industry, the demand for highly efficient heat removal solutions requires innovative cooling technologies. A computational fluid dynamic (CFD) study, including ...

  Citation × Citation Citation Abstract Copy Title: CFD simulation of single-phase and flow boiling in confined jet impingement with in-situ vapor extraction using two kinds of multiphase models Author: He, Xiaoliang With continued development of the electronic industry, the demand for highly efficient heat removal solutions requires innovative cooling technologies. A computational fluid dynamic (CFD) study, including heat transfer, is performed for an axisymmetric, confined jet impingement experiencing boiling and coupled with vapor extraction. Boiling occurs at the target surface while extraction occurs at the wall confining the radial flow. The region between the target and confining wall is defined as a confined gap. Extraction is employed to enhance heat transfer and to minimize the potential negative influence of flow instabilities resulting from two-phase flow within a confined region. A three-dimensional sector of the confined jet is employed in the simulation. A single circular impinging jet with a constant jet diameter (4 mm) and variable gap height (0.5, 1.0 and 1.5 mm), also known as nozzle-to-target spacing, is considered. The effect of mass flux at the confined gap entrance is also investigated (200, 400 and 800 kg/m²-s) for a range of heat flux (5 to 50 W/cm²). Fluid flow and heat transfer are simulated using the Volume of Fluid (VOF) model and the wall-boiling sub-model within the Multiphase Segregated Flow (MSF) model. The boiling sub-model in the VOF model applies the Rohsenow boiling correlation, while in the MSF model, the Kurul-Podowski boiling sub-model is used. Also, vapor extraction is realized by different mechanisms for these two models. For the VOF model, a specific phase "wall porosity" can be assigned to a wall to make it porous. Over a range of pressure differentials across this porous wall such that the inertial transport influence is negligible, vapor transport should agree with Darcy's law. For the MSF model, a wall can be made permeability to one substance or phase while remaining impermeable to the other substance or phase. However, a portion of the substance or phase reaching the boundary allowed to pass through the surface must be specified. A pressure drop cannot be applied across the wall, thereby prohibiting Darcy flow modeling. The solutions of both models are at steady state. The boiling curves without vapor extraction from both models are provided and compared to experiments. Simulations matching experimental wall temperatures under-predict theoretical vapor generation and those matching vapor generation over-estimate wall superheat. For cases with no extraction, local temperature and velocity profiles from the VOF model are provided at several radial locations within the confined gap. Scalar temperature and pressure distributions and velocity vectors are presented to explain observations in profiles. Title: CFD simulation of single-phase and flow boiling in confined jet impingement with in-situ vapor extraction using two kinds of multiphase models Author: He, Xiaoliang With continued development of the electronic industry, the demand for highly efficient heat removal solutions requires innovative cooling technologies. A computational fluid dynamic (CFD) study, including heat transfer, is performed for an axisymmetric, confined jet impingement experiencing boiling and coupled with vapor extraction. Boiling occurs at the target surface while extraction occurs at the wall confining the radial flow. The region between the target and confining wall is defined as a confined gap. Extraction is employed to enhance heat transfer and to minimize the potential negative influence of flow instabilities resulting from two-phase flow within a confined region. A three-dimensional sector of the confined jet is employed in the simulation. A single circular impinging jet with a constant jet diameter (4 mm) and variable gap height (0.5, 1.0 and 1.5 mm), also known as nozzle-to-target spacing, is considered. The effect of mass flux at the confined gap entrance is also investigated (200, 400 and 800 kg/m²-s) for a range of heat flux (5 to 50 W/cm²). Fluid flow and heat transfer are simulated using the Volume of Fluid (VOF) model and the wall-boiling sub-model within the Multiphase Segregated Flow (MSF) model. The boiling sub-model in the VOF model applies the Rohsenow boiling correlation, while in the MSF model, the Kurul-Podowski boiling sub-model is used. Also, vapor extraction is realized by different mechanisms for these two models. For the VOF model, a specific phase "wall porosity" can be assigned to a wall to make it porous. Over a range of pressure differentials across this porous wall such that the inertial transport influence is negligible, vapor transport should agree with Darcy's law. For the MSF model, a wall can be made permeability to one substance or phase while remaining impermeable to the other substance or phase. However, a portion of the substance or phase reaching the boundary allowed to pass through the surface must be specified. A pressure drop cannot be applied across the wall, thereby prohibiting Darcy flow modeling. The solutions of both models are at steady state. The boiling curves without vapor extraction from both models are provided and compared to experiments. Simulations matching experimental wall temperatures under-predict theoretical vapor generation and those matching vapor generation over-estimate wall superheat. For cases with no extraction, local temperature and velocity profiles from the VOF model are provided at several radial locations within the confined gap. Scalar temperature and pressure distributions and velocity vectors are presented to explain observations in profiles. Close

• 6. [Article] Numerical design of a high-flux microchannel solar receiver

  This thesis discusses the design of several microchannel solar receiver devices for use in CSP (concentrated solar power) using CFD (computational fluid dynamics) simulations. The goal is to demonstrate ...

  Citation × Citation Citation Abstract Copy Title: Numerical design of a high-flux microchannel solar receiver Author: Rymal, Charles This thesis discusses the design of several microchannel solar receiver devices for use in CSP (concentrated solar power) using CFD (computational fluid dynamics) simulations. The goal is to demonstrate that, by taking advantage of the higher heat transfer coefficienct of microchannels, solar receivers can achieve higher efficiency than current receiver technology, reducing the cost of solar thermal power. Design using CFD simulations is necessary in order to estimate the performance of different designs and identify potential issues before investing in a real device. The lack of previous research into such devices is most likely due to challenges concerning (a) the manufacturing of microchannels in materials that are suited to the high temperature and stress of the application and (b) the design of a headering system for a large scale implementation. Both supercritical carbon-dioxide and molten salt are used as heat-transfer fluids. The required inlet and outlet temperatures of the fluid are 773 K and 923 K for carbon-dioxide and 573 K and 873 K for molten salt. These values are determined by the CSP application and the properties of the fluids. Designs presented range in size from 1 cm² to 4 cm² and in heat transfer rates from 200 W to 400 W. These values are determined by the capacity of the solar simulator, which will be used for testing. For carbon-dioxide, three designs are developed with varying manufacturability. The high risk design features a circular micro-pin-fin array created using chemical etching and is constructed using diffusion bonding. The low risk design features machined and welded parts and parallel circular channels. The medium risk design features machined and diffusion bonded parts and parallel rectangular channels. For molten salt, two designs are developed: one using parallel rectangular channels and one using a circular pin-fin array. Conjugate CFD simulations of each design are used to evaluate pressure drop, receiver efficiency, and flow distribution. Two- and three-dimensional structural analyses are used to ensure that the devices will withstand the mechanical and thermal stress. An efficiency of 89.7%, pressure drop of 0.2 bar, and structural safety factor of 1.3 was achieved for carbon-dioxide. An efficiency of 92.1%, pressure drop of 0.5 bar, and structural safety factor of 2.5 was achieved for molten salt. The results demonstrate that microchannel devices can withstand the high flux, temperature, and stress of a CSP appliction and have high efficiency. However, additional work is needed before these designs can be implemented on a large scale. Title: Numerical design of a high-flux microchannel solar receiver Author: Rymal, Charles This thesis discusses the design of several microchannel solar receiver devices for use in CSP (concentrated solar power) using CFD (computational fluid dynamics) simulations. The goal is to demonstrate that, by taking advantage of the higher heat transfer coefficienct of microchannels, solar receivers can achieve higher efficiency than current receiver technology, reducing the cost of solar thermal power. Design using CFD simulations is necessary in order to estimate the performance of different designs and identify potential issues before investing in a real device. The lack of previous research into such devices is most likely due to challenges concerning (a) the manufacturing of microchannels in materials that are suited to the high temperature and stress of the application and (b) the design of a headering system for a large scale implementation. Both supercritical carbon-dioxide and molten salt are used as heat-transfer fluids. The required inlet and outlet temperatures of the fluid are 773 K and 923 K for carbon-dioxide and 573 K and 873 K for molten salt. These values are determined by the CSP application and the properties of the fluids. Designs presented range in size from 1 cm² to 4 cm² and in heat transfer rates from 200 W to 400 W. These values are determined by the capacity of the solar simulator, which will be used for testing. For carbon-dioxide, three designs are developed with varying manufacturability. The high risk design features a circular micro-pin-fin array created using chemical etching and is constructed using diffusion bonding. The low risk design features machined and welded parts and parallel circular channels. The medium risk design features machined and diffusion bonded parts and parallel rectangular channels. For molten salt, two designs are developed: one using parallel rectangular channels and one using a circular pin-fin array. Conjugate CFD simulations of each design are used to evaluate pressure drop, receiver efficiency, and flow distribution. Two- and three-dimensional structural analyses are used to ensure that the devices will withstand the mechanical and thermal stress. An efficiency of 89.7%, pressure drop of 0.2 bar, and structural safety factor of 1.3 was achieved for carbon-dioxide. An efficiency of 92.1%, pressure drop of 0.5 bar, and structural safety factor of 2.5 was achieved for molten salt. The results demonstrate that microchannel devices can withstand the high flux, temperature, and stress of a CSP appliction and have high efficiency. However, additional work is needed before these designs can be implemented on a large scale. Close