# On the role of radiation and dimensionality in predicting flow opposed flame spread over thin fuels

TL;DR: In this paper, a flame spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels, coupled with a three-dimensional gas radiation model to assess the role of radiation and effect of dimensionality on the prediction of the flame spread phenomena.

Abstract: In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radi...

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TL;DR: In this paper, the authors introduce fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed.

Abstract: This paper introduces fire safety standards for flammability evaluation of solid material intended for use in a spacecraft habitat. Two types of existing standards include material evaluation by pass/fail criteria corresponding to Test 1 of NASA STD 6001B and evaluation by a flammability index such as maximum oxygen concentration (MOC) corresponding to the improved Test 1. The advantage of the latter is the wide applicability of the MOC index to different atmospheres in spacecraft. Additionally, the limiting oxygen index (LOI) method is introduced as a potential alternative index for the evaluation using the improved Test 1 method. When criteria based on an index such as MOC or LOI are applied for material screening, the discrepancy of the index to the actual flammability limit in microgravity such as minimum limiting oxygen concentration (MLOC) is essential information for guaranteeing fire safety in space because material flammability can be higher in microgravity. In this paper, the existing research on the effects of significant parameters on material flammability in microgravity are introduced, and the difference between the limiting value in microgravity and the indices given by the standard test methods on the ground is discussed. Finally, on-going efforts to develop estimation methods of material flammability in microgravity according to normal gravity tests are summarized.

54 citations

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TL;DR: In this work, single-port, cylindrical grain laboratory-scale hybrid rocket motors are numerically simulated to study the effect of diaphragms.

Abstract: In this work, single-port, cylindrical grain laboratory-scale hybrid rocket motors are numerically simulated to study the effect of diaphragms. The effect of single and multiple diaphragms is investigated by varying diaphragm height, its axial location (for a single diaphragm), and spacing (for multiple diaphragms) at selected inlet GOX. A single diaphragm increases the local regression rate and its influence is prominent only in the region immediate downstream. Therefore, a tangible increment in average regression rate with a single diaphragm can only be realized for motors with small L/D( 10), use of multiple diaphragms are required to increase the regression rate and combustion effici...

31 citations

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TL;DR: In this paper, the effect of swirl on regression rate (r b ) in a hybrid rocket motor is investigated numerically, and it is shown that swirl is more effective in improving the average r b for short grains (L / D 5 ) and large diameter grains.

Abstract: In this work, the effect of swirl on regression rate ( r b ) in a hybrid rocket motor is investigated numerically. The r b increased monotonically with inlet swirl number and was also found to depend on the inlet swirl profile. The swirl velocity profiles with the peak closer to the axis yielded higher r b . Parametric study on fuel grains of various lengths and diameters ( L / D ⩽ 25 ) shows that swirl is more effective in improving the average r b for short grains ( L / D 5 ) and large diameter grains.

25 citations

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TL;DR: In this article, the authors compared the spread rates in the three regimes of opposed-flow flame spread (i.e., radiative, thermal, and kinetic) in a single set of experiments for the first time.

Abstract: The three regimes of opposed-flow flame spread – radiative, thermal, and kinetic regimes – are well known. For thermally thin fuels, the spread rate is independent of opposing flow velocity in the thermal regime. It decreases with an increase in the flow velocity in the kinetic regime, leading to blow off extinction. In the radiative regime which occurs mostly in a buoyancy-free environment of microgravity, the spread rate decreases with a decrease in flow velocity leading to radiative extinction unless the oxygen level is very high. In a recent experiment aboard the International Space Station, thin sheets of PMMA were ignited in a flow tunnel with the opposing flow varying over a wide range. All three regimes of flame spread were captured in a single set of experiments for the first time. Instantaneous spread rates were obtained from digital video processing and compared with a computational model in all three regimes along with the evolution of flame shapes. Spread rates in the radiative and thermal regimes are also compared with existing theories of flame spread in the thermal and the radiative regime producing remarkable qualitative agreement.

22 citations

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TL;DR: In this paper, the effect of opposing flow on flame spread rate over thin solid fuel is investigated with the help of scaling theory, a comprehensive computational model, and experiments conducted aboard the International Space Station.

Abstract: The effect of opposing flow on flame spread rate over thin solid fuel is investigated with the help of scaling theory, a comprehensive computational model, and experiments conducted aboard the International Space Station. While spread rate over thin fuels is independent of the opposing flow velocity in the thermal regime, in the microgravity regime, where the opposing flow can be very mild or even completely absent in the absence of buoyancy induced flow, the spread rate is known to decrease as the opposed flow is reduced. Under certain conditions, this can lead to flame extinguishment at a low enough flow velocity. This paper combines scaling arguments with computational results to predict a critical flow velocity for such flame extinction. Results from the recently conducted limited number of space based tests, presented in this paper, seem to confirm the prediction validating the closed-form formula for the critical extinction velocity.

18 citations

##### References

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01 Jan 1980

TL;DR: In this article, the authors focus on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms.

Abstract: This book focuses on heat and mass transfer, fluid flow, chemical reaction, and other related processes that occur in engineering equipment, the natural environment, and living organisms. Using simple algebra and elementary calculus, the author develops numerical methods for predicting these processes mainly based on physical considerations. Through this approach, readers will develop a deeper understanding of the underlying physical aspects of heat transfer and fluid flow as well as improve their ability to analyze and interpret computed results.

21,638 citations

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01 Jan 1991

TL;DR: In this article, asymptotics and computational singular perturbation for simplified kinetics modelling were used to predict turbulent non-premixed methane jet flames using full and reduced kinetic mechanisms.

Abstract: Formulation of the premixed and nonpremixed test problems- Premixed and nonpremixed test problem results- Reducing mechanisms- Overview of asymptotics for methane flames- On reduced mechanisms for methane-air combustion- Structure of the oxidation layer for stoichiometric and lean methane-air flames- Asymptotic analysis of methane-air diffusion flames- Sensitivity analysis of laminar premixed CH4-air flames using full and reduced kinetic mechanisms- Application of reduced chemical mechanisms for prediction of turbulent nonpremixed methane jet flames- Conventional asymptotics and computational singular perturbation for simplified kinetics modelling

513 citations

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TL;DR: In this article, a theoretical description of a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed is presented, where both a thin sheet and a semi-infinite fuel bed are considered.

Abstract: A theoretical description is presented for a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed. Both a thin sheet and a semi-infinite fuel bed are considered. The burning process is described as follows: The hot flame heats the unburned fuel bed, which subsequently vaporizes. The resulting fuel vapor reacts with the oxygen supplied by the incoming air, thereby producing the heat that maintains the flame-spread process. The formulated model treats the combustion as a diffusion flame, for which the details of the reaction kinetics can be ignored by assuming infinite reaction rates. The model includes the chemical stoichiometry, heat of combustion, gas-phase conductive heat transfer, radiation, mass transfer, fuel vaporization, and fuel-bed thermal properties. The radiation is mathematically treated as a heat loss at the flame sheet and a heat gain at the fuel-bed surface. The calculated flame-spread formulas are not inconsistent with available experimental data. These results reveal much of the physics involved in a spreading, flame. For instance, the flame-spread rate is strongly influenced by (1) the adiabatic stoichiometric flame temperature, and (2) the fuel-bed thermal properties, except for the fuel-bed conductivity parallel to the propagation direction.

324 citations

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TL;DR: In this article, a review of the literature in this area is presented, with a focus on gaseous radiation properties of gases and their applications in engineering applications, where the assumption is that the radiating gas under consideration is at the state of complete or local thermodynamic equilibrium and of negligible scattering effect.

Abstract: Publisher Summary This chapter aims to systematically develop the background information needed to formulate and evaluate thermal radiation properties of gases for engineering applications, and to review the literature of present works and approaches for future research in this area. The scope of the chapter is limited by the assumption that the radiating gas under consideration is at the state of complete or local thermodynamic equilibrium and of negligible scattering effect. The chapter introduces the general concepts concerning gaseous radiation and presents a review of the physics of atomic and molecular spectra. The radiation resulting from transitions of electronic, atomic, or molecular states has been discussed; they are line radiation, band radiation, and continuum radiation. The evaluation of total (engineering) emissivity and its applications to radiation from homogeneous gas bodies of complex geometry have been discussed. Consideration has been given to the appropriate absorption coefficients for use in the radiative transport calculations.

250 citations

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