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A. Hamins - One of the best experts on this subject based on the ideXlab platform.
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The two-dimensional structure of Low Strain Rate counterfLow nonpremixed-methane flames in normal and microgravity
Combustion Theory and Modelling, 2008Co-Authors: A. Hamins, Matthew F. Bundy, Jeong ParkAbstract:The structure and extinction of Low Strain Rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinctio...
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effect of buoyancy on the radiative extinction limit of Low Strain Rate nonpremixed methane air flames
Combustion and Flame, 2007Co-Authors: A. Hamins, Matthew F. Bundy, Sung Chan KimAbstract:The structure and extinction of nonpremixed flames were investigated through comparison of experiments and calculations using a counterfLow configuration. Experiments were conducted at the NASA Glenn Research Center’s 2.2-s drop tower to attain suppression and temperature measurements in Low-Strain nonpremixed methane–air microgravity flames. Suppression measurements using nitrogen added to the fuel stream were performed for global Strain Rates from 7 to 50 s −1 . Judicious hardware selection and an optimized experimental procedure facilitated rapid, controllable, and repeatable flame extinction measurements. The minimum nitrogen volume fraction in the fuel stream needed to ensure suppression for all Strain Rates in microgravity was measured to be 0.855 ± 0.016, associated with the turning point, which occurred at a global Strain Rate of 15 s −1 . This value was higher than the analogous value in normal gravity. Flame temperature measurements were attained in the high-temperature region of the fl ame ( T> 1200 K) using visible emission from a SiC filament positioned axially along the burner centerline. The suppression and temperature measurements were used to validate a two-dimensional flame simulation developed here, which included buoyancy effects and finite-Rate kinetics. The simulations yielded insight into the differences between microgravity and normal gravity suppression results and also explained the inadequacy of the one-dimensional model results to explain the microgravity suppression results. Published by Elsevier Inc. on behalf of The Combustion Institute.
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SUPPRESSION AND STRUCTURE OF Low Strain Rate NONPREMIXED FLAMES
2003Co-Authors: A. Hamins, Matthew F. Bundy, Woe Chul Park, Ki Yong Lee, Jennifer LogueAbstract:National Institute of Standards and Technology Gaithersburg, Maryland 20899-8663 The agent concentration required to achieve suppression of Low Strain Rate nonpremixed flames is an important fire safety consideration. In a microgravity environment such as a space platform, unwanted fires will likely occur in near quiescent conditions where Strain Rates are very Low. Diffusion flames typically become more robust as the Strain Rate is decreased. When designing a fire suppression system for worst-case conditions, Low Strain Rates should be considered The first comprehensive extinction measurements of very Low Strain non-premixed flames in microgravity were reported by Maruta
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Suppression limits of Low Strain Rate non-premixed methane flames
Combustion and Flame, 2003Co-Authors: Matthew F. Bundy, A. Hamins, Ki Yong LeeAbstract:The suppression of Low Strain Rate non-premixed flames was investigated experimentally in a counterfLow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global Strain Rate of 20 s 1 . The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N2 ,C O 2, and CF3Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global Strain Rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global Strain Rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N 2 concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstRate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global Strain Rates was measured to be 0.841 0.01 for N2, 0.773 0.009 for CO2, and 0.437 0.005 for CF3Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global Strain Rates was measured as 0.299 0.004 for N2, 0.187 0.002 for CO2, and 0.043 0.001 for CF3Br. © 2003 The Combustion Institute. All rights reserved.
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Suppression of Low Strain Rate Nonpremixed Flames by an Agent
2001Co-Authors: Sandra L. Olson, A. Hamins, M. Bundy, Joodong Park, I. K. PuriAbstract:The agent concentration required to achieve the suppression of Low Strain Rate nonpremixed flames is an important consideration for fire protection in a microgravity environment such as a space platform. Currently, there is a lack of understanding of the structure and extinction of Low Strain Rate (
Young Won Chang - One of the best experts on this subject based on the ideXlab platform.
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structural superplasticity of an al alloy in Low Strain Rate regime an internal variable approach
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005Co-Authors: J E Park, S L Semiatin, Young Won ChangAbstract:Abstract Most of the conventional approaches for structural superplasticity have utilized the external variables such as total stress and Strain. The effect of grain size and test temperature has not, however, been clarified quantitatively yet from these approaches. A quantitative analysis for structural superplasticity has recently been progressed considerably with the use of the internal variable concept. Nevertheless the precise role of grain size and test temperature on fLow characteristics has not been elucidated especially in Low Strain Rate region. Several fundamental characteristics, such as the validity of threshold stresses and the role of grain size, are yet to be resolved or still in controversy in the superplasticity community. In this study, a series of load relaxation and tensile tests has been conducted in the above regard to obtain the fLow curves, which were consequently analyzed based on the internal variable theory for structural superplasticity, focusing especially on the Low Strain Rate region.
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Structural superplasticity of an Al alloy in Low Strain Rate regime—An internal variable approach
Materials Science and Engineering: A, 2005Co-Authors: Park Jeongmin, S L Semiatin, C.s. Lee, Young Won ChangAbstract:Abstract Most of the conventional approaches for structural superplasticity have utilized the external variables such as total stress and Strain. The effect of grain size and test temperature has not, however, been clarified quantitatively yet from these approaches. A quantitative analysis for structural superplasticity has recently been progressed considerably with the use of the internal variable concept. Nevertheless the precise role of grain size and test temperature on fLow characteristics has not been elucidated especially in Low Strain Rate region. Several fundamental characteristics, such as the validity of threshold stresses and the role of grain size, are yet to be resolved or still in controversy in the superplasticity community. In this study, a series of load relaxation and tensile tests has been conducted in the above regard to obtain the fLow curves, which were consequently analyzed based on the internal variable theory for structural superplasticity, focusing especially on the Low Strain Rate region.
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On the Low Strain Rate Regime of Structural Superplasticity - an Internal Variable Approach
Materials Science Forum, 2005Co-Authors: Won Kyu Bang, C.s. Lee, Jaeyeong Park, Yong Nam Kwon, Young Won ChangAbstract:The superplastic deformation behavior of a fine-grained 7075 Al has been investigated to clarify the issue of threshold stress. A series of mechanical tests has been conducted at various temperatures for the specimens with various grain sizes. The quantitative constitutive parameters have been determined from load relaxation test by applying the internal variable theory of structural superplaticity (SSP) proposed by Chang et al. The GBS fLow could be formulated as a viscosity-type equation, characterized by the Newtonian exponent of 1.0. The unresolved issue of threshold stress is clarified and identified as a critical stress required for the GBS. The micro-mechanical roll of grain size refinement has also been manifested in terms of proposed constitutive parameters.
Jay P. Gore - One of the best experts on this subject based on the ideXlab platform.
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computed structure of Low Strain Rate partially premixed ch4 air counterfLow flames implications for no formation
Combustion and Flame, 1999Co-Authors: Linda G. Blevins, Jay P. GoreAbstract:Results from computations of Low Strain Rate, partially premixed methane/air counterfLow flames are reported. The Oppdif computer code was used with GRI-Mech 2.11 to obtain the results. When the fuel-side equivalence ratio (ΦB) is above 2.5, the present flame structure can be described as a CH4/air premixed flame merged with a CO/H2/air nonpremixed flame. When ΦB is beLow 2.5, the two flame zones exist on opposite sides of the stagnation plane, and the CO/H2/air nonpremixed flame is characterized by hydrocarbon concentration peaks on its fuel-side edge. Broad NO destruction regions, caused primarily by CHi + NO reactions, exist between the resulting double hydrocarbon concentration peaks. The fuel-side equivalence ratio is the most important indicator of how rapidly NO is destroyed relative to how rapidly it is formed, and NO destruction reactions are more important in pure diffusion flames than in partially premixed flames for the present Low Strain Rate computations.
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Computed Structure of Low Strain Rate Partially Premixed CH4/Air CounterfLow Flames: Implications for NO Formation.
Combustion and Flame, 1999Co-Authors: Linda G. Blevins, Jay P. GoreAbstract:Abstract Results from computations of Low Strain Rate, partially premixed methane/air counterfLow flames are reported. The Oppdif computer code was used with GRI-Mech 2.11 to obtain the results. When the fuel-side equivalence ratio (Φ B ) is above 2.5, the present flame structure can be described as a CH 4 /air premixed flame merged with a CO/H 2 /air nonpremixed flame. When Φ B is beLow 2.5, the two flame zones exist on opposite sides of the stagnation plane, and the CO/H 2 /air nonpremixed flame is characterized by hydrocarbon concentration peaks on its fuel-side edge. Broad NO destruction regions, caused primarily by CH i + NO reactions, exist between the resulting double hydrocarbon concentration peaks. The fuel-side equivalence ratio is the most important indicator of how rapidly NO is destroyed relative to how rapidly it is formed, and NO destruction reactions are more important in pure diffusion flames than in partially premixed flames for the present Low Strain Rate computations.
Matthew F. Bundy - One of the best experts on this subject based on the ideXlab platform.
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The two-dimensional structure of Low Strain Rate counterfLow nonpremixed-methane flames in normal and microgravity
Combustion Theory and Modelling, 2008Co-Authors: A. Hamins, Matthew F. Bundy, Jeong ParkAbstract:The structure and extinction of Low Strain Rate nonpremixed methane–air flames was studied numerically and experimentally. A time-dependent axisymmetric two-dimensional (2D) model considering buoyancy effects and radiative heat transfer was developed to capture the structure and extinction limits of normal gravity (1-g) and zero gravity (0-g) flames. For comparison with the 2D modelling results, a one-dimensional (1D) flamelet computation using a previously developed numerical code was exercised to provide information on the 0-g flames. A 3-step global reaction mechanism was used in both the 1D and 2D computations to predict the measured extinction limit and flame temperature. Photographic images of flames undergoing the process of extinction were compared with model calculations. The axisymmetric numerical model was validated by comparing flame shapes, temperature profiles, and extinction limits with experiments and with the 1D computational results. The 2D computations yielded insight into the extinctio...
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effect of buoyancy on the radiative extinction limit of Low Strain Rate nonpremixed methane air flames
Combustion and Flame, 2007Co-Authors: A. Hamins, Matthew F. Bundy, Sung Chan KimAbstract:The structure and extinction of nonpremixed flames were investigated through comparison of experiments and calculations using a counterfLow configuration. Experiments were conducted at the NASA Glenn Research Center’s 2.2-s drop tower to attain suppression and temperature measurements in Low-Strain nonpremixed methane–air microgravity flames. Suppression measurements using nitrogen added to the fuel stream were performed for global Strain Rates from 7 to 50 s −1 . Judicious hardware selection and an optimized experimental procedure facilitated rapid, controllable, and repeatable flame extinction measurements. The minimum nitrogen volume fraction in the fuel stream needed to ensure suppression for all Strain Rates in microgravity was measured to be 0.855 ± 0.016, associated with the turning point, which occurred at a global Strain Rate of 15 s −1 . This value was higher than the analogous value in normal gravity. Flame temperature measurements were attained in the high-temperature region of the fl ame ( T> 1200 K) using visible emission from a SiC filament positioned axially along the burner centerline. The suppression and temperature measurements were used to validate a two-dimensional flame simulation developed here, which included buoyancy effects and finite-Rate kinetics. The simulations yielded insight into the differences between microgravity and normal gravity suppression results and also explained the inadequacy of the one-dimensional model results to explain the microgravity suppression results. Published by Elsevier Inc. on behalf of The Combustion Institute.
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SUPPRESSION AND STRUCTURE OF Low Strain Rate NONPREMIXED FLAMES
2003Co-Authors: A. Hamins, Matthew F. Bundy, Woe Chul Park, Ki Yong Lee, Jennifer LogueAbstract:National Institute of Standards and Technology Gaithersburg, Maryland 20899-8663 The agent concentration required to achieve suppression of Low Strain Rate nonpremixed flames is an important fire safety consideration. In a microgravity environment such as a space platform, unwanted fires will likely occur in near quiescent conditions where Strain Rates are very Low. Diffusion flames typically become more robust as the Strain Rate is decreased. When designing a fire suppression system for worst-case conditions, Low Strain Rates should be considered The first comprehensive extinction measurements of very Low Strain non-premixed flames in microgravity were reported by Maruta
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Suppression limits of Low Strain Rate non-premixed methane flames
Combustion and Flame, 2003Co-Authors: Matthew F. Bundy, A. Hamins, Ki Yong LeeAbstract:The suppression of Low Strain Rate non-premixed flames was investigated experimentally in a counterfLow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global Strain Rate of 20 s 1 . The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N2 ,C O 2, and CF3Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global Strain Rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global Strain Rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N 2 concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstRate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global Strain Rates was measured to be 0.841 0.01 for N2, 0.773 0.009 for CO2, and 0.437 0.005 for CF3Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global Strain Rates was measured as 0.299 0.004 for N2, 0.187 0.002 for CO2, and 0.043 0.001 for CF3Br. © 2003 The Combustion Institute. All rights reserved.
Ewald Werner - One of the best experts on this subject based on the ideXlab platform.
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mechanical properties and fracture behavior of hydrogen charged ahss uhss grades at high and Low Strain Rate tests
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014Co-Authors: Johannes Rehrl, Klemens Mraczek, A Pichler, Ewald WernerAbstract:Abstract The present study focuses on the impact of hydrogen on mechanical properties of four typical advanced and ultra-high strength steel (AHSS/UHSS) grades (complex-phase-, dual-phase- and tempered martensitic steel grades with tensile strength between 1200 MPa and 1400 MPa) at two significantly different loading Rates (10 −5 s −1 and 20 s −1 ). At very Low Strain Rates a strong reduction of tensile strength and elongation at fracture is observed for all grades charged with hydrogen and the fracture appearance is typical for hydrogen embrittlement, HE. At the high Strain Rate an impact of hydrogen on the mechanical properties is not detected and the fracture type is ductile. From a comparison between HE-mechanisms in literature and the fracture surfaces of Low Strain Rate samples it is assumed, that hydrogen induced failure is a combination of enhanced dislocation mobility by hydrogen (hydrogen enhanced localized plasticity, HELP) and decohesion promoted by hydrogen transport through dislocations. An estimation of the critical dislocation velocity for hydrogen to move along with dislocations according to Tien et al. [1] reveals, that even at the high Strain Rate applied hydrogen transport by dislocations is still possible. Therefrom the non-existing effect of hydrogen on mechanical properties at high Strain Rates is assumed to be due to the absence of hydrogen accumulation at highly stressed microstructure regions and an insufficient increase of hydrogen concentration through hydrogen transport by dislocations to initiate decohesion.
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Mechanical properties and fracture behavior of hydrogen charged AHSS/UHSS grades at high- and Low Strain Rate tests
Materials Science and Engineering: A, 2014Co-Authors: Johannes Rehrl, Klemens Mraczek, A Pichler, Ewald WernerAbstract:Abstract The present study focuses on the impact of hydrogen on mechanical properties of four typical advanced and ultra-high strength steel (AHSS/UHSS) grades (complex-phase-, dual-phase- and tempered martensitic steel grades with tensile strength between 1200 MPa and 1400 MPa) at two significantly different loading Rates (10 −5 s −1 and 20 s −1 ). At very Low Strain Rates a strong reduction of tensile strength and elongation at fracture is observed for all grades charged with hydrogen and the fracture appearance is typical for hydrogen embrittlement, HE. At the high Strain Rate an impact of hydrogen on the mechanical properties is not detected and the fracture type is ductile. From a comparison between HE-mechanisms in literature and the fracture surfaces of Low Strain Rate samples it is assumed, that hydrogen induced failure is a combination of enhanced dislocation mobility by hydrogen (hydrogen enhanced localized plasticity, HELP) and decohesion promoted by hydrogen transport through dislocations. An estimation of the critical dislocation velocity for hydrogen to move along with dislocations according to Tien et al. [1] reveals, that even at the high Strain Rate applied hydrogen transport by dislocations is still possible. Therefrom the non-existing effect of hydrogen on mechanical properties at high Strain Rates is assumed to be due to the absence of hydrogen accumulation at highly stressed microstructure regions and an insufficient increase of hydrogen concentration through hydrogen transport by dislocations to initiate decohesion.
