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Author: Gonzalo Del Álamo Publisher: ISBN: Category : Languages : en Pages : 135
Book Description
The objective of this work is to investigate theoretically two different problems of interest in Combustion. First, autoignition in hydrogen-oxygen systems above crossover temperatures and under various conditions of pressure and composition is addressed computationally and by asymptotic methods. Different descriptions of the detailed chemistry are evaluated through comparison of computed and measured ignition times, and a balance between accuracy and simplicity is struck in selecting rate parameters to be used in investigating reduced chemistry. For isobaric, homogeneous, and adiabatic mixtures, only five elementary steps suffice to describe accurately ignition over the range of conditions addressed, which allows the derivation of explicit formulas for the induction time by neglecting heat release and reactants consumption. Further reduction of the chemistry to two overall steps, is used to include the effect of the energetics in the temperature-dependent initiation and branching rates. In this approach, an asymptotic analysis for high activation energy of the branching step is developed for predicting ignition times of branched-chain explosions on the basis of a criterion of thermal runaway. The second study addresses the vaporization of a droplet in rectilinear motion relative to a stagnant gaseous atmosphere for the limit of low Reynolds numbers and slow variation of the droplet velocity. A formal asymptotic analysis is performed, showing that, under the conditions addressed, there is an inner region in the vicinity of the droplet within which the flow is nearly quasisteady except during shorts periods of time when the acceleration changes abruptly and a fully time-dependent outer region in which departures of velocities and temperatures from those of the ambient medium are small. Matched asymptotic expansions, followed by a Greens-function analysis of the outer region enable expressions to be obtained for the velocity and temperature fields and for the droplet drag and vaporization rate. The results are applied to problems in which the droplet experiences constant acceleration, constant deceleration and oscillatory motion. The results, which identify dependences on the Prandtl number and the transfer number, are intended to be compared with experimental measurements on droplet behaviors in time-varying flows.
Author: Gonzalo Del Álamo Publisher: ISBN: Category : Languages : en Pages : 135
Book Description
The objective of this work is to investigate theoretically two different problems of interest in Combustion. First, autoignition in hydrogen-oxygen systems above crossover temperatures and under various conditions of pressure and composition is addressed computationally and by asymptotic methods. Different descriptions of the detailed chemistry are evaluated through comparison of computed and measured ignition times, and a balance between accuracy and simplicity is struck in selecting rate parameters to be used in investigating reduced chemistry. For isobaric, homogeneous, and adiabatic mixtures, only five elementary steps suffice to describe accurately ignition over the range of conditions addressed, which allows the derivation of explicit formulas for the induction time by neglecting heat release and reactants consumption. Further reduction of the chemistry to two overall steps, is used to include the effect of the energetics in the temperature-dependent initiation and branching rates. In this approach, an asymptotic analysis for high activation energy of the branching step is developed for predicting ignition times of branched-chain explosions on the basis of a criterion of thermal runaway. The second study addresses the vaporization of a droplet in rectilinear motion relative to a stagnant gaseous atmosphere for the limit of low Reynolds numbers and slow variation of the droplet velocity. A formal asymptotic analysis is performed, showing that, under the conditions addressed, there is an inner region in the vicinity of the droplet within which the flow is nearly quasisteady except during shorts periods of time when the acceleration changes abruptly and a fully time-dependent outer region in which departures of velocities and temperatures from those of the ambient medium are small. Matched asymptotic expansions, followed by a Greens-function analysis of the outer region enable expressions to be obtained for the velocity and temperature fields and for the droplet drag and vaporization rate. The results are applied to problems in which the droplet experiences constant acceleration, constant deceleration and oscillatory motion. The results, which identify dependences on the Prandtl number and the transfer number, are intended to be compared with experimental measurements on droplet behaviors in time-varying flows.
Author: Isadore L. Drell Publisher: ISBN: Category : Aeronautics Languages : en Pages : 92
Book Description
This literature survey digest of hydrogen-air combustion fundamentals presents data on flame temperature, burning velocity, quenching distance, flammability limits, ignition energy, flame stability, detonation, spontaneous ignition, and explosion limits. The data are assessed, recommended values are given, and relations among various combustion properties are discussed. New material presented includes: theoretical treatment of variation in spontaneous ignition lag with temperature, pressure, and composition, based on reaction kinetics of hydrogen-air composition range for 0.01 to 100 atmospheres and initial temperatures of 0 degrees to 1400 degrees k.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
This research involved theoretical studies of the chemical and fluid-mechanical phenomena which make turbulent combustion in high-speed flows different from such combustion in low-speed flows. Finite-rate chemistry plays a significant role in high-speed flows because of the small ratios of flow times to chemical times. The studies addressed ignition and extinction phenomena in nonpremixed turbulent combustion of hydrogen-air systems by both numerical and asymptotic methods. Efforts were made to provide a firmer foundation for the modeling of high-speed turbulent reacting flows, to aid in the development of a formulation which gives results that can be compared with experiments on turbulent combustion. It was shown that turbulent combustion occurs in the reaction-sheet regime in hypersonic propulsion, and the correspondingly needed critical ignition and extinction conditions for hydrogen-air systems were determined from fundamentals.
Author: Boris E. Gelfand Publisher: Springer Science & Business Media ISBN: 3642253520 Category : Science Languages : en Pages : 338
Book Description
The potential of hydrogen as an important future energy source has generated fresh interest in the study of hydrogenous gas mixtures. Indeed, both its high caloricity and reactivity are unique properties, the latter underscoring safety considerations when handling such mixtures. The present monograph is devoted to the various aspects of hydrogen combustion and explosion processes. In addition to theoretical and phenomenological considerations, this work also collates the results of many experiments from less well known sources. The text reviews the literature in this respect, thereby providing valuable information about the thermo-gas-dynamical parameters of combustion processes for selected experimental settings in a range of scientific and industrial applications.
Author: Jonathan Frydman Publisher: ISBN: Category : Languages : en Pages : 83
Book Description
In liquid rocket propulsion oxygen normally enters the combustion chamber as a dispersed phase, as the hydrogen fuel more rapidly evaporates into a continuous, vapor phase. The foundational configuration considered here is a single liquid oxygen droplet surrounded by gaseous hydrogen. This project employs computational techniques to model the coupled oxidizer/fuel mixing, ignition, and combustion behaviors of the liquid oxygen/gaseous hydrogen system. For simplicity the problem is modeled with spherical symmetry, consistent with microgravity conditions. Conservation equations are implemented within the OpenFOAM platform to calculate species concentrations, temperatures, and heat release rates as functions of time and space. The OpenFOAM software package allows for calculations to be run in a sophisticated run-time environment. Both single-step and six-step chemical reaction models are employed for simulating oxygen-hydrogen combustion. Numerical simulations of both chemical models show diffusion, ignition, and the subsequent formation of three distinct flame zones. These include premixed flames in both the fuel and oxidizer sides of the diffusion flame at the stoichiometric interface. The premixed flame zones weaken as a quasi-steady combustion state is achieved in both cases. The global-chemistry reaction simulation is run until near total consumption of the oxygen occurs. A number of suggestions are presented based off these results for improving future simulation accuracy and efficiency.
Author: Gonzalo Del Álamo
Author: Gonzalo Del Álamo
Author: Isadore L. Drell
Author: Wallace Chinitz
Author: Charles S. Chen
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Author: 
Author: Boris E. Gelfand
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Author: Jonathan Frydman
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