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Hasan Bedir

Bio: Hasan Bedir is an academic researcher from Case Western Reserve University. The author has contributed to research in topics: Radiative transfer & Flammability limit. The author has an hindex of 6, co-authored 6 publications receiving 168 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the effects of gas-phase radiative effects on the burning and extinction of a solid fuel in a stagnation-point flow geometry were investigated using a statistical narrowband model with carbon dioxide and water vapor as the radiative participating media.
Abstract: Gas-phase radiative effects on the burning and extinction of a solid fuel in a stagnation-point flow geometry are investigated using a statistical narrow-band model with carbon dioxide and water vapor as the radiative participating media. The model, coupled to other flame conservation equations with a one-step overall gas-phase chemical reaction and Arrhenius solid pyrolysis relation, is solved numerically. Flame temperature, solid burning rate, and heat fluxes are examined as functions of stretch rate. Using ambient oxygen percentage and stretch rate as coordinates, A U-shaped extinction boundary is identified. The extinction behavior at low stretch rates is qualitatively similar to that predicted by earlier theory with only surface radiation loss. However, gas radiation introduces additional heat loss from the system and shrinks the solid flammable domain. In addition, gas radiation can cause a substantial decrease of flame temperature and constitutes a significant portion of the heat feedback to the solid at low stretch rates. In the second part of the paper, a computationally less intensive gray gas radiation model is tested. As with a number of earlier investigations, the use of Planck mean absorption coefficient is found to overpredict net emission and flame radiative loss. By multiplying a correction factor (less than 1) in front of the Planck mean absorption coefficient, it is possible to compute many global flame properties with reasonable accuracy. An empirically determined formula is given to find the value of this correction factor for a given flame. This is offered as an engineering approach for the flame radiation treatment.

68 citations

Journal ArticleDOI
TL;DR: In this paper, a comparison of several radiative heat transfer models is made for a stagnation-point diffusion flame at low stretch rate, with CO2 and H2O as the participating media.
Abstract: A comparison of several radiative heat transfer models is made for a stagnation-point diffusion flame at low stretch rate, with CO2 and H2O as the participating media. Computed results of the radiative source distribution for wideband, narrowband and SLWSGG show reasonable agreement with each other. Results from the optically thin and grey gas models with Planck mean absorption coefficient are shown to underestimate the self-absorption and overestimate the emission substantially for the low stretch flame. The relative computation times of using the various radiation models are also given.

64 citations

Journal ArticleDOI
TL;DR: In this article, a narrowband radiation model is coupled to the OPPDIF program to enable the study of one-dimensional hydrogen oxygen diffusion e ame over the entire range of e ammable stretch rates.
Abstract: A narrowband radiation model is coupled to the OPPDIF program to enable the study of one-dimensional hydrogen‐ oxygen diffusion e ame over the entire range of e ammable stretch rates. The e ame characteristics and the extinction limits at a low pressure of 1.013 kPa are sought, with the amount of carbon dioxide dilution level and stretch rate as parameters. The conditions studied are particularly relevant to Mars exploration. In addition, a e ammability map is presented using these two parameters as coordinates. Both the high-stretch blowoff and the low-stretch quenching limits are found. Theexistence of an absolute carbon dioxide dilution limit, above which the diffusion e ame is not possible, is demonstrated. Low-stretch diffusion e ames at low pressures are unusually thick, with e ame temperatures substantially below those of the adiabatic e ames. This large temperature drop results from the combined effect of e ame radiation and limited gas residence time in the e ame, and may be particular to the hydrogen‐ oxygen chemical kinetics. One of the novel features of the application of the narrowband radiation model is the inclusion of Doppler broadening, which is shown to be important in low-pressure e ames.

15 citations

Journal ArticleDOI
01 Jan 1998
TL;DR: In this article, a numerical model of a polymethylmethacrylate (PMMA) diffusion flame in a stagnation-point geometry is solved with flame radiation, and the contribution from carbon dioxide and water vapor to emission increase with decreasing stretch rate, because the thicker flames at low stretch rate produce a longer optical path.
Abstract: A numerical model of a polymethylmethacrylate (PMMA) diffusion flame in a stagnation-point geometry is solved with flame radiation. A narrow-band radiation model is used with carbon dioxide, water vapor (combustion products), and methylmethacrylate (fuel) vapor as the gaseous participating species. A detailed account of the emission and absorption from these species as well as the net radiative heat flux are given as a function of the flame stretch rate. Flame radiation is more important at low stretch rates. The contributions from carbon dioxide and water vapor to emission increase with decreasing stretch rate, because the thicker flames at low stretch rate produce a longer optical path. Because of the differnet concentrations at the fuel surface, the optical path length of the MMA vapor stays approximately the same with varying stretch rate, therefore contributing to a smaller fraction of the total emission at low stretch as compared with that at high stretch. Radiative absorption by these gaseous species is substantial and dominated by self-absorption. Consequently, the radiative contributin by MMA vapor is mainly as an emitter.

15 citations

01 May 1999
TL;DR: In this article, the effect of low-speed, concurrent flow on the spreading and extinction processes of flames over solid fuels was investigated, using a custom-made fabric consisting of a 1:1 blend of cotton and fiberglass.
Abstract: This research program is concerned with the effect of low-speed, concurrent flow on the spreading and extinction processes of flames over solid fuels. The primary objective is to verify the theoretically predicted extinction boundary, using oxygen percentage and flow velocity as coordinates. Of particular interest are the low-speed quenching limits and the existence of the critical oxygen flammability limit. Detailed flame spread characteristics, including flame spread rate, flame size, and flame structure are sought. Since the predicted flame behavior depends on the inclusion of flame and surface radiation, the measured results will also be used to assess the importance of radiative heat transfer by direct comparison to a comprehensive numerical model. The solid fuel used in this experiment is a custom-made fabric consisting of a 1:1 blend of cotton and fiberglass. This choice was made following an extensive search to yield a material with favorable properties, namely, rollability, non-cracking behavior during combustion, strength after combustion, and flammability in a range of oxygen limits permissible within the Combustion Integrated Rack (CIR) on the International Space Station. At the present time, an effort is being made to characterize both the radiative properties of the fuel and the flame spreading behavior in normal gravity at reduced pressure. These will provide a basis for comparison with the microgravity results as well as aid in bracketing the anticipated flammability boundary for the flight experiment. An overview of recent work, with emphasis on theoretical results, is presented.

6 citations


Cited by
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Journal ArticleDOI
TL;DR: There has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations as mentioned in this paper, however, the focus is narrower, namely on quasisteady flamelets and therefore the considerations of flamelet extinction that are presented in this paper are not the dynamics of extinction.
Abstract: In the past 25 years there has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations. The term “flamelet” as used here really would mean “laminar flame” to most readers and is employed only because a major motivation is for ultimate use in connection with more complex flows, mainly turbulent. There is, however, no consideration here of how the knowledge reviewed may be employed in flamelet modeling of turbulent combustion. Not even time-dependent flamelets are addressed, although a few related references are provided. The focus is narrower, namely on quasisteady flamelets, and therefore the considerations of flamelet extinction that are presented concern quasisteady extinction, that is, not the dynamics of extinction. Even in this narrow context, it will be seen that a great deal has been accomplished. When such a long-term view is taken, it is found remarkable how much progress has been made. The progress is addressed separately for premixed, nonpremixed and partially premixed systems. Suggested directions of future research also are indicated. Despite the limited scope of the topic and the extensive advancement that has occurred, much more research remains to be done.

203 citations

Journal ArticleDOI
01 Jan 2007
TL;DR: In this paper, the effects of spectral radiation absorption on the flame speed at normal and elevated pressures were experimentally and numerically investigated using the CO 2 diluted outwardly propagating CH 4 -O 2 -He flames.
Abstract: The effects of spectral radiation absorption on the flame speed at normal and elevated pressures were experimentally and numerically investigated using the CO 2 diluted outwardly propagating CH 4 –O 2 –He flames. Experimentally, the laminar burning velocities of CH 4 –O 2 –He–CO 2 mixtures at both normal and elevated pressures (up to 5 atm) were measured by using a pressure-release type spherical bomb. The results showed that radiation absorption with CO 2 addition increases the flame speed and extends the flammability limit. In addition, it was also shown that the increase of pressure augments the effect of radiation absorption. Computationally, a fitted statistical narrow-band correlated- k (FSNB-CK) model was developed and validated for accurate radiation prediction in spherical geometry. This new radiation scheme was integrated to the compressible flow solver developed to simulate outwardly propagating spherical flames. The comparison between experiment and computation showed a very good agreement. The results showed that the flame geometry have a significant impact on radiation absorption and that the one-dimensional planar radiation model was not valid for the computation of the flame speed of a spherical flame. An effective Boltzmann number is extracted from numerical simulation. Furthermore, the FSNB-CK model was compared with the grey band SNB model. It was shown that the grey band SNB model over-predicts the radiation absorption. It is concluded that quantitative prediction of flame speed and flammability limit of CO 2 diluted flame requires accurate spectral dependent radiation model.

161 citations

Journal ArticleDOI
TL;DR: In this article, the exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method using a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and I atm, (ii) an inhomogeneous H 2 O/N 2 mixture at 1000 k and I k, and (iii) a nonisothermal and homogeneous mixture of CO 2 /H 2 O /N 2 2 at 1 atm.
Abstract: Three-dimensional non-grey gas radiation analyses were conducted using the statistical narrow-band model along with updated band parameters. The exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method. Accurate numerical results were presented for non-grey real gas radiative transfer in a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and I atm, (ii) an isothermal and inhomogeneous H 2 O/N 2 mixture at 1000 K and I atm, and (iii) a nonisothermal and homogeneous mixture of CO 2 /H 2 O/N 2 at 1 atm

88 citations

Journal ArticleDOI
01 Jan 1998
TL;DR: A review of research on the effects of gravity on combustion processes is presented, with an emphasis on a discussion of the ways in which reduced-gravity experiments and modeling has led to new understanding as discussed by the authors.
Abstract: A review of research on the effects of gravity on combustion processes is presented, with an emphasis on a discussion of the ways in which reduced-gravity experiments and modeling has led to new understanding. Comparison of time scales shows that the removal of buoyancy-induced convection leads to manifestations of other transport mechanisms, notably radiative heat transfer and diffusional processes such as Lewis number effects. Examples from premixed-gas combustion, non-premixed gas-jet flames, droplet combustion, flame spread over solid and liquid fuels, and other fields are presented. Promising directions for new research are outlined, the most important of which is suggested to be radiative reabsorption effects in weakly burning flames.

86 citations

01 Jan 1999
TL;DR: In this paper, the exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method using a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and I atm, (ii) an inhomogeneous H 2 O/N 2 mixture at 1000 k and I k, and (iii) a nonisothermal and homogeneous mixture of CO 2 /H 2 O /N 2 2 at 1 atm.
Abstract: Three-dimensional non-grey gas radiation analyses were conducted using the statistical narrow-band model along with updated band parameters. The exact narrow-band averaged radiative transfer equation was solved using a ray-tracing method. Accurate numerical results were presented for non-grey real gas radiative transfer in a three-dimensional rectangular enclosure containing (i) an isothermal pure water vapor at 1000 K and I atm, (ii) an isothermal and inhomogeneous H 2 O/N 2 mixture at 1000 K and I atm, and (iii) a nonisothermal and homogeneous mixture of CO 2 /H 2 O/N 2 at 1 atm

85 citations