Numerical modeling of limiting oxygen index apparatus for film type fuels
01 Dec 2012-International Journal of Spray and Combustion Dynamics (SAGE Publications)-Vol. 4, Iss: 4, pp 299-322
TL;DR: In this article, a detailed three-dimensional numerical model is used to compute the flow pattern and the flame behavior of thin solid fuels in a rectangular column that resembles a standard Limiting Oxygen Index (LOI) device.
Abstract: A detailed three-dimensional numerical model is used to compute the flow pattern and the flame behavior of thin solid fuels in a rectangular column that resembles a standard Limiting Oxygen Index (LOI) device. The model includes full Navier-Stokes equations for mixed buoyant-forced flow and finite rate combustion and pyrolysis reactions so that the sample LOI can be computed to study the effect of feeding flow rate, sample width and gravity levels. In addition to the above parameters, the sample location in the column and the column cross-sectional area are also investigated on their effect on the ambient air entrainment from the top.
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01 Jan 2015
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.
74 citations
TL;DR: In this article, the authors studied the near-limiting behavior of various thicknesses of thermoplastic materials under a candle-like burning configuration; followed by an ISO 4589-2 protocol.
Abstract: We studied the near-limiting behavior of various thicknesses of thermoplastic materials under a candle-like burning configuration; followed by an ISO 4589-2 protocol. The motivation for this work is to understand the sensitivity of the sample thickness on the limiting oxygen concentration in the range from 0.5 mm to 10 mm. In the first place, the effect of heating time on successive ignition was discussed. Through a simple analysis, it was suggested that a 30 s heating time, regulated in ISO 4589-2, might be insufficient to achieve a successful ignition when the specimen becomes thicker. Second, the effect of the thickness of the test specimen (PMMA, ABS) on the limiting oxygen concentration was examined. Flames formed over thicker PMMA (>4.0 mm thickness in this study) at near-limiting condition displayed a flickering motion, which then suddenly extinguished when the critical condition was achieved due to temporal acceleration of the surrounding flow. While the flame behavior with a thinner sample (<4.0 mm thickness in this study) at the limiting condition was found to be stationary, a gentle extinction was experienced as approached to the limit. This fact suggests that the key to leading extinction is different between thicker and thinner sample. Third, the temperature distribution over the 4.0 mm PMMA at the near-limiting condition was measured and a strategy to model/predict the limiting behavior is then proposed.
18 citations
TL;DR: In this article, a mass balance coupled with a variable oxygen flow tunnel/nozzle was used to measure the burning rate of solid and liquid fuels as a function of ambient oxygen percentage all the way to the extinction limit.
Abstract: A precision mass balance coupled with a variable oxygen flow tunnel/nozzle enables us to measure the burning rate of solid and liquid fuels as a function of ambient oxygen percentage all the way to the extinction limit. Two sample configurations have been studied. The first is a liquid fueled wick flame (ethanol tea lamp). The total burning rate (mass/time) is measured as a function of wick length and oxygen percentage. Near the low oxygen limit, limit-cycle flame oscillation has been found that can last for many minutes to hours. The averaged mass burning rate of the oscillatory flame is about one-half that of the steady flame occurring at slightly higher oxygen. In the second configuration, local burning rate (mass/area/time) of poly(methyl methacrylate) spherical shell samples have been measured in the flame stabilization zone. Each sample has a different amount of heat loss and a different oxygen limit. The critical burning rate at their respective oxygen limits are different. This implies that critical burning rate is not a property of the material alone and it should not be used as the only criterion to judge the extinction or the ignition of materials.
12 citations
TL;DR: In this paper, a wick-LOC method is used to evaluate the fire-retardant effectiveness of organophosphorus compounds (OPC) added to Li-ion battery electrolyte solvents, and the flame stability limits are measured as a function of OPC addition for both flame modes.
Abstract: To evaluate the fire-retardant effectiveness of organophosphorus compounds (OPC) added to Li-ion battery electrolyte solvents, the limiting oxygen concentration (LOC) method is used in conjunction with a wick combustion system, called as wick-LOC method. With the wick-LOC method, two modes of stabilized flame are found, namely, wake flame and full flame. When OPC is added to the electrolyte, two distinct branches of extinction processes occur according to the different flame modes near extinction with no transition from the full flame to the wake flame in the case of higher OPC addition. The flame stability limits are measured as a function of OPC addition for both flame modes. The wake flame is shown to be consistently more stable at low levels of OPC addition. However, once the OPC addition exceeds a critical amount, the full flame shows higher stability with a lower LOC than the wake flame. These phenomena in the two regimes are also found in other cases of high OPC addition (different type of OPC and electrolyte solvent). In the most stable flame mode, the regime switches from the wake flame to the full flame with increasing OPC addition, and they are defined correspondingly as “blow-off regime” and “quenching regime”. To explain the presence of these two regimes, the thermal balance effect is considered in the discussion of flame extinction mechanisms. The difference in flame volume near the extinction limit shows that the quenching mechanism dominates flame extinction under higher OPC addition. The thermal balance effect on flame stabilization or extinction can be the additional impact on the fire retardation abilities of OPC itself.
10 citations
01 Jan 2017
TL;DR: In this paper, the authors studied the extinction conditions for diffusion flames in inert porous media and analyzed the effects of the heat exchange between gas and solid phases on the flame structure and found that when the heat removed from the flame by the solid matrix is large, the flame can extinguish because the lowering in the flame temperature leads to increasingly large leakage of reactants through the flame sheet.
Abstract: Diffusion flames established in inert porous media have been reported to present temperatures lower than a comparable gaseous mixture. Therefore, the study of the flame structure, temperature and extinction limits of confined diffusion flames is of importance. In the present work we discuss extinction conditions for such flames. Using an asymptotic model that accounts for the excess/deficient enthalpy at the reaction region, we study the multiscale problem and analyze the effects of the heat exchange between gas and solid phases on the flame structure. When the heat removed from the flame by the solid matrix is large, the flame can extinguish because the lowering in the flame temperature leads to increasingly large leakage of reactants through the flame sheet. We show that this occurs when the porosity or the mass injection rate is small enough. The extinction limit associated with a small value of the mass injection rate adds to the kinetic extinction limit (which is associated with a large value of the mass injection rate) to characterize a dual-extinction-point behavior for this problem. When the porosity of the medium reaches a minimum critical value, these two distinct extinction points collapse, such that for porosities lower than the critical porosity no flame can be established inside the porous chamber. Then, it is possible to construct a flammability map for the confined diffusion flame, where the critical porosity defines an absolute flammability limit.
7 citations
References
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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...
35 citations
"Numerical modeling of limiting oxyg..." refers background in this paper
...In solid combustion, surface radiative loss normally has more impact on flame extinction than gas radiation [19, 20]....
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...The two effects are seen to balance each other approximately [19, 20]....
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TL;DR: In this paper, the authors investigated the effect of surface heat flux on downward burning of PMMA fuel rods in an apparatus similar to that used in the Limiting Oxygen Index flammability test and found that extinction is not caused by reduction of heat transfer from the flame to the burning region.
Abstract: Measurements are reported on downward burning of PMMA fuel rods in an apparatus similar to that used in the Limiting Oxygen Index flammability test. As the ambient oxygen concentration is reduced the flame propagation rate decreases. The propagation rate V does not go to zero as the oxygen concentration Y ox is reduced, rather extinction occurs at a finite value of V . Although the propagation rate decreases as Y ox is reduced, the surface regression rate is not reduced. In agreement with this finding, the calculated heat flux per unit area to the pyrolysis region does not decrease as Y ox decreases. Thus, extinction is not caused by reduction of heat transfer from the flame to the burning region. To investigate the cause of extinction a previously developed analysis for the effect of surface heat flux on flame propagation is applied to the present problem. It is found that flame propagation is sensitive to the amount of heat transferred to the fuel surface ahead of the flame, and that extinction occurs (at a finite propagation rate) for sufficiently low values of forward heat flux. Comparisons of predicted and measured values of V are made using heat flux distributions measured at values of Y ox for atmospheric and extinction conditions and reasonable agreement is found. A transient measurement near the extinction limit confirms that the Limiting Oxygen Index is a measure of the critical condition for downward flame propagation rather than a more general limit for burning.
32 citations
"Numerical modeling of limiting oxyg..." refers background in this paper
...In thick samples, the flame can also be stabilized in the wake region behind the top end of the specimen [23, 24]....
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TL;DR: In this article, a steady-state flame spread model has been used to study the effect of side-edge burning on flame spread over thin solid fuel strips of finite width, and simulations have been carried out for fuel strips with both inhibited (by metallic strips) and uninhibited side edges.
Abstract: A steady-state flame spread model has been used to study the effect of side-edge burning on flame spread over thin solid fuel strips of finite width. Simulations have been carried out for fuel strips with both inhibited (by metallic strips) and uninhibited side edges. The effect inhibition on both normal- and microgravity flame spread along with several intermediate gravity levels has been investigated. Such a study is important for understanding the physiochemical processes controlling the flame spread in low gravity where human experience is limited. Although simulations have shown an overall increase in spread rate for uninhibited cases for both normal- and microgravity flames, some effects such as flame spread variation with external imposed velocity and flame extinction limits show different behavior for microgravity and normal gravity flames. The heat and mass transport processes in the flame have been discussed in detail to explain the observed trends.
21 citations
"Numerical modeling of limiting oxyg..." refers background in this paper
...Also, it should be noted that the standard testing procedure requires fuel specimen to be placed in between metallic strips, however it was shown in [26] that in case the fuel is allowed to burn along the edges the LOI values can get affected drastically and even reverse the trend between 0ge and 1ge reported here....
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TL;DR: In this paper, a two-dimensional numerical model was used to study the flame stabilization and extinction characteristics over a thick model solid fuel, which resembles a specimen burning near extinction limit in an LOI testing procedure.
Abstract: A preventive approach to fire safety is proper material selection based on the flammability characteristics. One such measure of flammability is the limiting oxygen index (LOI). This is a commonly used numerical flammability index for relative grading and selecting materials. The test measures the extinction limit of a downward spreading flame over a finite size (rectangular slab or rod) fuel specimen. In this work, a two-dimensional numerical model was used to study the flame stabilization and extinction characteristics over a thick model solid fuel, which resembles a specimen burning near extinction limit in an LOI testing procedure. The computations were preformed for a mixed buoyant-forced-flow environment prevailing in the LOI test at normal gravity. At sufficiently high oxygen level (far away from flame extinction limit) the flame is anchored on the sides of the fuel slab (side-stabilized flame), but as the oxygen level is reduced, at a certain value the flame stabilization point abruptly shifts from the sides of the specimen to the wake region (wake flame). The structural details of these two modes of flame stabilization are discussed. The computations also show that the shift in the flame-anchoring position with oxygen level exhibits hysteresis; i.e., multiple flame solutions may exist for a given environmental condition of flow and oxygen. The flame extinction limits and flame-shift limits (side-stabilized flame to wake flame and vice versa) in ambient oxygen percentage are presented for different forced-flow velocities. Over the velocity range studied here, the side-stabilized flame (similar to flames over of thin fuels) becomes less flammable (higher LOI) at higher velocity, whereas the wake-stabilized flame becomes more flammable (lower LOI) with increase in forced-flow velocity. To understand the implication of this earth-based measurement to material selection for space application, additional computations were performed for pure forced-flow environment in zero gravity. Stabilization and extinction behavior of the flames at normal gravity are compared with those in zero gravity. The computations in a pure forced-flow environment show that the two modes are also present in a zero-gravity environment but only above certain minimum flow velocity. Below this velocity only the single flaming solution was obtained. In the present computations, the LOI values at zero gravity were lower than those at normal gravity. For zero gravity the LOI based on side-stabilized flame (for thin fuels) shows a nonmonotonic behavior: there is a minimum LOI which for the present set of property and kinetic parameter values occurs at a forced velocity of about 3–5 cm/s. For zero gravity the LOI based on wake flame extinction decreases rapidly with flow velocity for small velocities but with further increase in velocity the LOI value stays approximately constant.
17 citations
"Numerical modeling of limiting oxyg..." refers background in this paper
...In a previous numerical study [25] it was shown that whether the flame will stabilize on the side or on the top of a thick fuel depends on the flow and oxygen conditions....
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15 citations
"Numerical modeling of limiting oxyg..." refers methods in this paper
...While the higher sensitivity of the microgravity flame and non-monotonic flame spread rate trend with velocity have been demonstrated and explained previously by both experiments [10, 15] and numerical simulations [9, 13], it is interesting to note that the maximum Vf occurs for narrow width fuel strips at higher inflow velocities....
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...The system of coupled elliptic partial differential equations for the flow and combustion in the gas phase is solved numerically by an in-house code using SIMPLER algorithm [9]....
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...Grid independence check has been performed [9]....
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