A Computational Study on Radiation Effects on Flame Spread Over a Thin Fuel in a Quiescent Microgravity Environement [sic] PDF Download
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Author: Kevin D. Tolejko
Publisher:
ISBN:
Category :
Languages : en
Pages : 262
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![A Computational Study on Radiation Effects on Flame Spread Over a Thin Fuel in a Quiescent Microgravity Environement [sic] PDF](https://books.google.com/books/content/images/frontcover/dC_yNwAACAAJ?fife=w80-h100)
Author: Kevin D. Tolejko
Publisher:
ISBN:
Category :
Languages : en
Pages : 262
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Author:
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ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 99
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The purpose of this thesis is to perform radiation computations in opposed-flow flame spread in a microgravity environment. In this work, the flame spread simulations consider a thermally thin, PMMA fuel in a quiescent, microgravity environment or facing low opposed-flow velocities at ambient conditions of 1 atm and 50-50 volumetric mixture of oxygen and nitrogen. The flame spread model, which is a Computational Fluid Dynamics (CFD) model, is used for numerical simulations in combination with a radiation model. The CFD code is written in FORTRAN language, and a Matlab code is developed for plotting results. The temperature and species fields from CFD computations are used as inputs into the radiation model. Radiative quantities are calculated by using a global balance method along with the total transmittance non-homogeneous model. Radiation effect on thermocouple temperature measurement is investigated. Although this topic is well known, performing radiation correction calculations usually considers surface radiation only and not gas radiation. The inclusion of gas radiation is utilized in predicting the gas temperature that a thermocouple would measure. A narrow bed radiation model is used to determine the average incident radiative flux at a specified location from which a thermocouple temperature measurement is predicted. This study focuses on the quiescent microgravity environment only. The effect of parameters such as thermocouple surface emissivity and bead diameter are also studied. For the main part of this thesis, the effect of gas radiation on the mechanism of flame spread over a thermally thin, solid fuel in microgravity is investigated computationally. Generated radiative fields including thermal and species fields are utilized to investigate the nature of the influence of gas radiation on flame structure as well as its role in the mechanism of opposed-flow flame spread. The opposed-flow configuration considers low flow velocities including a quiescent environment where radiation has been shown to be dominant. However, given the fact that gas radiation acts as a loss mechanism, and at the same time, it enhances forward heat transfer through radiation feedback to the fuel surface, there is no definitive work that establishes the role of gas radiation. This thesis explores the role played by gas radiation as a driving versus as a retarding mechanism. In this work, it is found that gas radiation is important in capturing flame images and spread rates. Gas radiation primarily acts as a loss mechanism through its effects on flame temperature which overwhelms the radiation feedback to the surface.
Author:
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Category : Electronic books
Languages : en
Pages : 70
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The purpose of this research is to investigate how oxygen concentration, opposed flow velocity and thickness of a thin PMMA fuel affect the flame spread rate and flame extinction in microgravity. The flame spread rate increases with an increase in oxygen concentration. The critical oxygen level, which is the minimum concentration for a flame to spread, is inversely related to the fuel thickness. For fuel thickness above and below a critical thickness, the flame spread rate increases and decreases with a decrease in fuel thickness, respectively. Also, an unexpected extinction is discovered. The critical fuel thickness is inversely related to the opposed flow velocity. The flame spread rate decreases when the opposed flow velocity decreases. Unexpected extinction is discovered when oxygen level is low and opposed flow is absent or weak. The simulation results are consistent with the available experimental results obtained by NASA. For a quiescent environment in microgravity, the critical oxygen level increases with the fuel thickness while the critical oxygen level decreases with the fuel thickness for environments with an opposed flow. The research on how a flame extinguishes reveals that the flame temperature in the anomaly region is lower than the flame temperature in the normal region. A flame extinguishes when the percentage surface radiation loss, which is the ratio of the surface radiation loss to heat generated from combustion, is higher than 45% with an opposed flow and 48% in quiescent environment.
Author: Linton K. Honda
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ISBN:
Category :
Languages : en
Pages :
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Author: Prashant Attibele Ramachandra
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 158
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Author: Yang Long
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ISBN:
Category : Cellulose
Languages : en
Pages : 348
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Author: Rohit J. Nagarkar
Publisher:
ISBN:
Category :
Languages : en
Pages : 58
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The purpose of this thesis is to simulate flame spread in zero gravity and identify different factors that can impact the flame spread rate. This was possible using the CFD code written in FORTRAN developed by Bhattacharjee. The fuel studied in this thesis is Poly (methyl methacrylate) (PMMA). A mathematical model that shows how spread rate is being calculated is explained. The importance of grids used in CFD was shown by choosing appropriate number of grids for a given domain and a rule for choosing the domain was established. Impact of boundary layer or flow development distance was deeply understood and a formula for flame tip velocity or equivalent velocity was developed. Computational spread rate was then non-dimensionalized by dividing it with spread rate obtained from de- Ris formulae and plotted against Damkohler number which was calculated based on opposed flow velocity and equivalent velocity. A large variation of opposed flow for different fuel thicknesses was plotted against spread rate to show how fuel-half thickness affects the spread rate and the impact of radiation was understood. A critical fuel-thickness up to which flame existed in a quiescent microgravity was computed using this flame code. The impact of oxygen level was also studied in detail for a given fuel thickness. Pressure was varied in the microgravity regime to see its impact on flame spread rate.
Author: Ama R. Carney
Publisher:
ISBN:
Category : Aerospace engineering
Languages : en
Pages : 125
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Microgravity experiments are performed to study wind-assisted flame spread over discrete fuel elements. Ultra-thin cellulose-based fuel segments are distributed uniformly in a low-speed flow and flame spread is initiated by igniting the most upstream fuel segment. Similar to continuous fuels, flame spread over discrete fuels is a continual process of ignition. Flame propagation across a gap only occurs when a burning fuel segment, before it burns out, ignites the subsequent segment. During this process, gaps between samples reduce the fuel load, increasing the apparent flame spread rate and decreasing the heat transfer between adjacent segments. The reduction in heat transfer decreases the solid burning rate. In this study, sample segment length, gap size, and imposed flow velocity are varied to study the impacts on burning characteristics, including propensity of flame spread, flame spread rate, and solid burning rate. Detailed profiles of the transient flame spread process are also presented.
Author: Sandra Lee Olson
Publisher:
ISBN:
Category :
Languages : en
Pages : 308
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Author: National Aeronautics and Space Administration NASA
Publisher:
ISBN: 9781731262950
Category :
Languages : en
Pages : 111
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The research performed at the Center for Reactive Flow and Dynamical Systems in the Laboratory for Computational Physics and Fluid Dynamics, at the Naval Research Laboratory, in support of the NASA Microgravity Science and Applications Program is described. The primary focus was on investigating fundamental questions concerning the propagation and extinction of premixed flames in Earth gravity and in microgravity environments. The approach was to use detailed time-dependent, multispecies, numerical models as tools to simulate flames in different gravity environments. The models include a detailed chemical kinetics mechanism consisting of elementary reactions among the eight reactive species involved in hydrogen combustion, coupled to algorithms for convection, thermal conduction, viscosity, molecular and thermal diffusion, and external forces. The external force, gravity, can be put in any direction relative to flame propagation and can have a range of values. A combination of one-dimensional and two-dimensional simulations was used to investigate the effects of curvature and dilution on ignition and propagation of flames, to help resolve fundamental questions on the existence of flammability limits when there are no external losses or buoyancy forces in the system, to understand the mechanism leading to cellular instability, and to study the effects of gravity on the transition to cellular structure. A flame in a microgravity environment can be extinguished without external losses, and the mechanism leading to cellular structure is not preferential diffusion but a thermo-diffusive instability. The simulations have also lead to a better understanding of the interactions between buoyancy forces and the processes leading to thermo-diffusive instability. Oran, Elaine S. and Kailasanath, K. Unspecified Center ALGORITHMS; EARTH GRAVITATION; EXTINGUISHING; FLAME PROPAGATION; MATHEMATICAL MODELS; MICROGRAVITY; PREMIXED FLAMES; TIME DEPENDENCE; BUOYANCY; CONVECTION; GAS MIX...
