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Ignition and transition to flame spread over a thermally thin cellulosic sheet in a microgravity environment
Abstract
An axisymmetric, time-dependent model is developed describing auto-ignition and subsequent transition to flame spread over a thermally-thin cellulosic sheet heated by external radiation in a quiescent microgravity environment. Due to the unique combination of a microgravity environment and low Reynolds number associated with the slow, thermally induced flow, the resulting velocity is taken as a potential flow. A one-step global gas phase oxidation reaction and three global degradation reactions for the condensed phase are used in the model. A maximum external radiant flux of 5 W/cm2 (Gaussian distribution) with 21%, 30%, and 50% oxygen concentrations is used in the calculations. The results indicate that autoignition is observed for 30% oxygen concentrations but the transition to the flame spread does not occur. For 50% oxygen the transition is achieved. A detailed discussion of the transition from ignition to flame spread is given as an aid to understanding this process. Also, a comparison is made between the axisymmetric configuration and a two-dimensional (line source) configuration.
... For thick fuels, the heat-up term dominates the ignition delay, whereas the second term and third terms dominate for thin fuels. Ignition delay has been extensively studied, with excellent overviews written on the subject [1,[3][4][5][6], specific studies with thick fuels [7][8][9][10][11], radiative ignition of thin fuels [12][13][14], and models of the ignition processes [2,[14][15][16][17][18][19][20]. ...
... Thin fuel radiative ignition experiments have found that ignition delay decreases with increasing oxygen concentration [12][13][14] and increasing pressure [12] and ignition delay times can be seconds long even for thin fuels [12][13][14]. Thin fuel radiative ignition models have predicted that ignition delay decreases with increasing oxygen [15][16][17] but have not examined pressure variations. Slow flow affected experimental mixing times and thus ignition delay at lower oxygen concentration [13]. ...
... Kinetic theory states that for an mth order reaction, the reaction rate increases with both reactant concentration and pressure to the mth power [22]. Theoretical models of ignition of thin solid fuels generally assume a one step irreversible Arrhenius reaction [2,15,17,19] in the form ...
Piloted ignition delay of thermally-thin cellulose as a function of forced flow, oxygen concentration, and pressure was studied in a low-speed flow tunnel mounted on a NASA Zero Gravity Research Facility drop vehicle. Fuel heat-up times are very short and invariant due to the rapid heating of the pilot igniter and thermally-thin fuel used in this study. The effect of flow was found to be negligible for the test conditions studied, where mixing times were very rapid. Ignition delay in these tests was found to be dominated by the induction time, which was found to be inversely proportional to the oxygen concentration and the total pressure, or equivalently, the oxygen partial pressure. The reaction rate (the inverse of the induction time) is thus linear with the oxygen partial pressure, in agreement with kinetic theory for simple thermal reactions.
... The thermal degradation of the cellulosic sample is modeled by three exothermic global reactions governing pyrolysis (slightly exothermic), thermal oxidation, and char oxidation. The evolution of the thermal degradation process is governed by conservation equations for solid mass, cellulose, char, and energy with Arrhenius-type reaction rates [8]. The global kinetic constants and heats of reaction were obtained by thermogravimetric analysis and are identical to those used in Refs. ...
... The gas-phase model [8] consists of the conservation equations for total mass, momentum (full Navier-Stokes), energy, and species mass fractions (fuel and oxygen) formulated in the low Mach number limit. A global one-step reaction ...
... A finite difference control volume approach on a staggered grid is used to solve the gasphase equations. A time-splitting algorithm is used for the species and temperature equations since the characteristic time scale of the chemical reaction is relatively small compared to that for diffusion and convection (high Damkö hler number) [8]. This method involves first solving the species and temperature equations without the convective and diffusive terms using a stiff ordinary differential equation solver over small time steps. ...
In most microgravity studies of flame spread, the flame is assumed to be two-dimensional, and two-dimensional models are used to aid data interpretation. However, since limited space is available in microgravity facilities, the flames are limited in size. It is important, therefore, to investigate the significance of three-dimensional effects. Three-dimensional and two-dimensional simulations of ignition and subsequent transition to flame spread were performed on a thermally thin cellulosic sample, Ignition occurred by applying a radiant heat flux in a strip across the center of the sample. The sample was bounded by an inert sample holder. Heat loss effects at the interface of the sample and the sample holder were tested by varying the thermal-physical properties of the sample holder. Simulations were also conducted with samples of different widths and with different ambient wind speeds (i.e., different levels of oxygen supply).The width of the sample affected both the duration of the flame transition period and the post-transition flame spread rate. Finite width effects were most significant when the ambient wind was relatively small (limited oxygen supply). In such environments, the velocity due to thermal expansion reduced the net inflow of oxygen enough to significantly affect flame behavior. For a given sample width, the influence of thermal expansion on the net incoming oxygen supply decreased as the ambient wind speed increased. Thus, both the transition and flame spread behavior of the three-dimensional flame (along the centerline) tended to that of the two-dimensional flame with increasing ambient wind speed. Heat losses to the sample holder were found to affect the flame spread rate in the case of the narrowest sample with the slowest ambient wind.
... The following equations were considered, maintaining conservation of mass, momentum, energy, and species i for compressible flow [45][46][47][48][49][50]. In the momentum equation, the effect of the gravity was not included. ...
In this study, we focused on a fuel reforming technology by applying ultrafine oxygen bubble as the pretreatment for in-cylinder combustion s. It is assumed that oxygen is dissolved in the droplets in the form of ultrafine bubbles, and released into air when the decane fuel evaporates. A numerical simulation of the spray combustion was conducted using a PSI-CELL model. We changed the oxygen concentration of the droplets, the initial droplet diameter, and the number of injected droplets per unit time to discuss the ignition time and the temperature field. When there is no oxygen in the fuel droplet, most of the flames are diffusion flames. On the other hand, when oxygen exists in the droplets, premixed flames are formed at the upstream edge of the fuel spray. Due to the effects of ultrafine oxygen bubbles, the ignition time is shortened. However, on the condition that there is only a small amount of oxygen in the fuel droplets, as more fuel is supplied by enlarging the droplet diameter or increasing the number of injected droplets per unit time, the ignition time increases. Thus, when discussing ignition time, the balance between evaporated fuel and oxygen in the gas phase is important.
... Nakabe et al. 199 developed a model for the study on ignition and transition to flame spread on a thermally thin cellulosic sheet. The material was considered to be exposed to an external radiative heat flux under a quiescent microgravity atmosphere. ...
Over the past years, pyrolysis models have moved from thermal models to comprehensive models with great flexibility including multi-step decomposition reactions. However, the downside is the need for a complete set of input data such as the material properties and the parameters related to the decomposition kinetics. Some of the parameters are not directly measurable or are difficult to determine and they carry a certain degree of uncertainty at high temperatures especially for materials that can melt, shrink, or swell. One can obtain input parameters by searching through the literature; however, certain materials may have the same nomenclature but the material properties may vary depending on the manufacturer, thereby inducing uncertainties in the model. Modelers have resorted to the use of optimization techniques such as gradient-based and direct search methods to estimate input parameters from experimental bench-scale data. As an integral part of the model, a sensitivity study allows to identify the role of each input parameter on the outputs. This work presents an overview of pyrolysis modeling, sensitivity analysis, and optimization techniques used to predict the fire behavior of combustible solids when exposed to an external heat flux.
... The kinetic parameters for the solid decomposition [13] and gas-phase reaction [14] are listed in Table 1 , and the major physical parameters are listed in Table 2 . ...
The transition from smoldering to flaming is basically a homogeneous gas-phase ignition reaction that has a high potential to engulf the entire fuel bed in flames. However, the transition mechanism, especially under windy environment, is poorly understood due to lack of theoretical study. This work proposes a two- dimensional computational model with multiphase formulations to simulate the forward horizontal smolder- ing and spontaneous transition to flaming of cellulosic insulation. A chemical scheme of three-step solid decomposition reactions combined with one global gas-phase reaction is used in the model. The multi-physical coupling of heat, mass and momentum fields is implemented to reveal the interactions between the packed insulation bed and the surrounding free flow. This model shows that the hotter surface char layer acts as an ignition source for heating the gas mixture, and both the surface char oxidation and sample pyrolysis reactions behave like a gaseous fuel supplier for gas-phase reaction. With favorable conditions of high-temperature combustible fuel gas and sufficient oxygen supply under windy environment, the transition ultimately occurs at 2-3 cm above the leading edge of smolder front on the top surface, which is totally different from the tran- sition within the porous fuel bed under windless condition. The predicted effects of wind velocity and bulk density of fuel bed on the smoldering velocity agree well with literature data. The modeling results for the first time give an interpretation of the spontaneous smoldering to flaming transition over the fuel bed surface.
... Understanding the burning behavior of wildland fuels is essential to develop a complete understanding of wildfire spread. For the last decades, numerous studies have been attempted to better describe the fire dynamics of solid fuels [1][2][3] , including polymers [4][5][6][7][8] , cellulosic materials [9,10] , and different species of wood [11][12][13][14] . The transfer of this knowledge to describe the burning behavior of wildland fuels, which are natural polymers, is not trivial because they differ from classical solid fuels, as they often consist in highly porous media where the solid phase is very small compared to the gas phase. ...
An experimental and numerical study was carried out to assess the performance of the different submodels and parameters used to describe the burning dynamics of wildfires. A multiphase formulation was used and compared to static fires of dried pitch pine needles of different bulk densities. The samples were exposed to an external heat flux of 50 kW/m² in the FM Global Fire Propagation Apparatus and subjected to different airflows, providing a controlled environment and repeatable conditions. Submodels for convective heat transfer, drag forces, and char combustion were investigated to provide mass loss rate, flaming duration, and gas emissions. Good agreement of predicted mass loss rates and heat release rates was achieved, where all these submodels were selected to suit the tested conditions. Simulated flaming times for different flow conditions and different fuel bulk densities compared favorably against experimental measurements. The calculation of the drag forces and the heat transfer coefficient was demonstrated to influence greatly the heating/cooling rate, the degradation rate, and the flaming time. The simulated CO and CO2 values compared well with experimental data, especially for reproducing the transition between flaming and smoldering. This study complements a previous study made with no flow to propose a systematic approach that can be used to assess the performance of the submodels and to better understand how specific physical phenomena contribute to the wildfire dynamics. Furthermore, this study underlined the importance of selecting relevant submodels and the necessity of introducing relevant subgrid-scale modelling for larger scale simulations.
... Since a more detailed description of the mathematical model has been given in Refs. [10,11], only a brief summary is given here. The gas phase was formulated with the conservation equations of mass, full Navier-Stokes form of the momentum (without gravity), energy, and species (fuel and oxygen) under the low Mach number limit. ...
The effects of imposed flow velocity on flame spread along open edges of a thermally thin cellulosic sample in microgravity were studied experimentally and theoretically. In this study, the sample was ignited locally at the middle of the 4 cm wide sample, and subsequent flame spread reached both open edges of the sample along the direction of the flow. The following flame behaviors were observed in the experiments and predicted by the numerical calculation, in order of increased imposed flow velocity: (1) ignition but subsequent flame spread was not attained, (2) flame spread upstream (opposed mode) without any down-stream flame, and (3) the upstream flame and two separate downstream flames traveled along the two open edges (concurrent mode). Generally, the upstream and downstream edge flame spread rates were faster than the central flame spread rate for an imposed flow velocity of up to 5 cm/s. This was due to greater oxygen supply from the outer free stream to the edge flames and more efficient heat transfer from the edge flames to the sample surface than the central flames. For the upstream edge flame, flame spread rate was nearly independent of, or decreased gradually with, the imposed flow velocity. The spread rate of the downstream edge, however, increased significantly with the imposed flow velocity.
... This model focuses on the preheating feedback flux from flame until pyrolysis temperatures, or so-called ignition temperatures, are achieved. Because the overall effect of the cellulose degradation is only slightly endothermic [18,19], it is neglected in this analysis. This energy balance provides a simple method to evaluate the heat feedback rate from the leading edge of the flame to the surface. ...
Non-piloted radiative ignition and transition to flame spread over thin cellulose fuel samples was studied aboard the USMP-3 STS-75 Space Shuttle mission, and in three test series in the 10 second Japan Microgravity Center (JAMIC). A focused beam from a tungsten/halogen lamp was used to ignite the center of the fuel sample while an external air flow was varied from 0 to 10 cm/s. Non-piloted radiative ignition of the paper was found to occur more easily in microgravity than in normal gravity. Ignition of the sample was achieved under all conditions studied (shuttle cabin air, 21%–50% O2 in JAMIC), with transition to flame spread occurring for all but the lowest oxygen and flow conditions. Although radiative ignition in a quiescent atmosphere was achieved, the flame quickly extinguished in air. The ignition delay time was proportional to the gas-phase mixing time, which is estimated by using the inverse flow rate. The ignition delay was a much stronger function of flow at lower oxygen concentrations. After ignition, the flame initially spread only upstream, in a fan-shaped pattern. The fan angle increased with increasing external flow and oxygen concentration from zero angle (tunneling flame spread) at the limiting 0.5 cm/s external air flow, to 90 degrees (semicircular flame spread) for external flows at and above 5 cm/s, and higher oxygen concentrations. The fan angle was shown to be directly related to the limiting air flow velocity. A surface energy balance reveals that the heat feedback rate from the upstream flame to the surface decreases with decreasing oxygen mass transport via either imposed flow velocity or ambient oxygen concentration. Quenching extinction occurs when the heat feedback rate from the flame is no longer sufficient to offset the ongoing surface radiative heat losses. Despite the convective heating from the upstream flame, the downstream flame was inhibited due to the ‘oxygen shadow’ of the upstream flame for the air flow conditions studied. Downstream flame spread rates in air, measured after upstream flame spread was complete and extinguished, were slower than upstream flame spread rates at the same flow. The quench regime for the transition to flame spread was skewed toward the downstream, because of the augmenting role of diffusion for opposed flow flame spread, versus the canceling effect of diffusion at very low cocurrent flows.
... The physical model is similar to that used earlier for the study of axi-symmetric ignition processes [21]. It was found that the two-dimensional flame quenched more easily than the three-dimensional flame during the transition period. ...
A three-dimensional model of concurrent flame spread over a thin solid in a low-speed flow tunnel in microgravity was formulated and numerically solved. In a parametric study varying the flow velocity, oxygen level, and tunnel and solid fuel widths, two distinctive types of flame behavior were noted. In high-oxygen-percentage and/or higher-speed flows, the flames were long and far away from the quenching limit. In such cases, three-dimensional effects were dominated by heat loss to the wall in the downstream portion of the flame. In low-oxygen and low-speed flows, the flames were short and in the region near the quenching limit. These near-limit flames were controlled by the oxygen supply rate. Oxygen-side diffusion in the cross-wind direction became a dominant mechanism exhibiting large effects on the narrow three-dimensional flames. A number of trend reversals on spread rates and extinction limits were discovered for these nearlimit flames. Aided by the detailed flame profiles obtained in the computations, an explanation for the reversal phenomena is offered in the paper.
... The present configuration is more realistic than the one flame front case because there is interaction between the two flames during the ignition and transition stages. A complete description of the mathematical model is given in Ref. [8]. ...
A two-dimensional, time-dependent model is developed describing ignition and the subsequent transition to flame spread over a thermally thin cellulosic sheet heated by external radiation in a microgravity environment. The effects of a slow external wind (0–5 cm/s), and of the flux distribution of the external radiation on the transition are studied mainly in an atmosphere of 30% oxygen concentration. The ignition is initiated along the width of a sample strip, giving rise initially to two flame fronts spreading in opposite directions. The calculated results are compared with data obtained in the 2.2-s drop tower. Both experimental and calculated results show that with a slow, imposed wind, the upstream flame front (opposed mode) is stronger and slightly faster than the quiescent counterpart due to a greater supply of oxygen. However, the downstream flame front (concurrent mode) tends to die during the transition period. For all calculated cases studied in this work using the selected kinetic constants for the global one-step gas phase reaction, the downstream flame front dies out in oxygen concentrations up to 50% and wind velocity up to 5 cm/s. This is caused by the “oxygen shadow” cast by the upstream flame. The ignition delay time depends mainly on the peak flux of external radiation, whereas the transition time to steady state flame spread depends mainly on the broadness of the flux distribution. The broader the radiative flux distribution, the greater the transient flame spread rate due to the preheating of the sample ahead of the flame front by the external radiation and thus the greater the delay to steady state flame spread.
Effects of confined spaces on flame spread over thin solid fuels in a low-speed opposing flow is investigated by combined use of microgravity experiments and computations. The flame behaviors are observed to depend strongly on the height of the flow tunnel. In particular, a non-monotonic trend of flame spread rate versus tunnel height is found, with the fastest flame occurring in the 3 cm high tunnel. The flame length and the total heat release rate from the flame also change with tunnel height, and a faster flame has a larger length and a higher heat release rate. The computation analyses indicate that a confined space modifies the flow around the spreading flame. The confinement restricts the thermal expansion and accelerates the flow in the streamwise direction. Above the flame, the flow deflects back from the tunnel wall. This inward flow pushes the flame towards the fuel surface, and increases oxygen transport into the flame. Such a flow modification explains the variations of flame spread rate and flame length with tunnel height. The present results suggest that the confinement effects on flame behavior in microgravity should be accounted to assess accurately the spacecraft fire hazard.
Upstream (opposed) flame spread over a thermally thin cellulosic sheet in various imposed flow speed and ambient oxygen concentration under microgravity environment is studied numerically. Two-dimensional time-dependent phenomena, from spontaneous ignition to the flame spread in a fully-developed velocity boundary layer formed over a flat cellulosic sheet, are simulated with a well-developed numerical model. The model predicts that the upstream flame spread in the boundary layer is essentially time-dependent phenomena, and the unsteadiness is pronounced when the imposed flow speed is very slow/high and lower oxygen concentration. This unsteady flame spread behavior is due to the boundary layer effect; when the flame spreads to upstream (i.e. thin boundary layer zone), local flow speed at the flame front is not constant but increased. The local flow speed has excellent correlation with the local flame spread rate. Plots of the flame temperature vs. the local flame spread rate show their linear relationship for all conditions considered in the present study, indicating that classical deRis's theory still be valid for present unsteady (quasi-steady) flame spread mode.
The objective of this work is to provide information about the details of the effect of light wavelength on radiative ignition of solid material. An experiment of radiative ignition has been performed in microgravity to determine the events occurring in ignition processes in a quiescent atmosphere. Filter papers were irradiated by infrared radiation (CO2 laser 10.6 μm) or near infrared radiation (Diode laser, 800.1 nm). The ignition delay was measured for various experimental conditions, and the density change of gas phase before ignition were observed by a Mach-Zehnder interferometer. The results showed that the ignition delay time of the infrared radiation was much shorter than that of near infrared radiation, and it was observed that the difference of light wavelength of radiations had a strong effect on the ignition process. It was suggested that one of main reasons to give the difference was the difference of radiative absorption in gas phase as well as the difference of absorptivity by solid surface according to the Mach-Zehnder interferometer.
A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mixture and is almost independent of the gravity level and the ambient oxygen index. The ignition delay time increases nearly linearly with an increase in solid-fuel thickness within the range of 0.005\,{\rm cm}\le {\bar \tau}\le 0.02\,{\rm cm} and is proportional to ({\bar Q}_{\max})^{-1.11} within 2\,{\rm W/cm}^2\le\bar Q_{\max}\le 8\,{\rm W/cm}^2 . The steady downward flame-spread rate decreases with increases in the gravity level or fuel thickness and with decreases in the ambient oxygen index but is independent of the incident peak heat flux. The blowoff limit is around 6.7\,{\bar g}_{\rm e} and the extinction limit is found to be Y O X = 0.131.
A time-dependent model was developed and solved numerically to study a purely buoyant downward flame spread over a thermally thin solid fuel in various gravity environments. According to the specified burn-out solid density and no retained ash, the flame propagation behavior over a thin solid fuel surface could be simulated. At ignition, the flame is premixed. After several transition burnouts the flame transitions into a self-sustained steady spread diffusion flame. When the gravity level was varied, the Damkohler number effects were verified. An unstable flame spread was noted near the extinction limit at which the flame spread rate decelerated. Blow-off extinction was predicted after the flame spread a short distance. The ignition delay time increased with increasing gravity level. Compared with the experimental measurements, the predicted blow-off extinction limit was closer than that predicted by the steady combustion model.
This investigation applies a four-step chemical kinetics mechanism to implement the original combustion model developed by Chen and Weng ( Combustion Science and Technology , Vol. 73, pp. 427-446, 1990). Comparing the blowoff curve of Tsuji ( Progress in Energy and Combustion Science , Vol. 8, p.93, 1982) with that of Chen and Weng reveals that this study yields a much better prediction than that in the latter reference. Also, the data in this study has an excellent agreement with the measured data of Dreier et al. ( Berichtc der Bunsen-Gesellschaft--Physical Chemistry , Vol. 90, pp. 1010-1015, 1986). The interested parameter is the inflow velocity U in . As U in increases, the envelope diffusion flame, wake flame, liftoff flame, and another wake flame appear in that order before complete extinction. The formal wake flame is transformed from the envelope diffusion flame and the other is from the liftoff flame. The existence of a liftoff flame is verified by a corresponding experimental observation (Tsa et al., 2003). The maximal liftoff height is 1.7 D when U in is 1.05 m/s, and this height is maintained up to U in =1.09 m/s. Then the height declines gradually as the inflow velocity increases, which is a process that can be regarded as flashback. No recirculation flow exists behind the cylindrical burner for these liftoff flames. A transition from liftoff to wake flame occurs between 1.13 to 1.15 m/s. The wake flame reappears at U in =1.16 m/s. Finally, the flame is extinguished completely when U in > 2.12 m/s. The flame's lifting and dropping back is explained.
We have investigated the downward flame spread over a thin solid fuel. Hydrogen, methane, or propane, included in the gaseous product of pyrolysis reaction, is added in the ambient air. The fuel concentration is kept below the lean flammability limit to observe the partially premixing effect. Both experimental and numerical studies have been conducted. Results show that, in partially premixed atmospheres, both blue flame and luminous flame regions are enlarged, and the flame spread rate is increased. Based on the flame index, a so-called triple flame is observed. The heat release rate ahead of the original diffusion flame is increased by adding the fuel, and its profile is moved upstream. Here, we focus on the heat input by adding the fuel in the opposed air, which could be a direct factor to intensify the combustion reaction. The dependence of the flame spread rate on the heat input is almost the same for methane and propane/air mixtures, but larger effect is observed for hydrogen/air mixture. Since the deficient reactant in lean mixture is fuel, the larger effect of hydrogen could be explained based on the Lewis number consideration. That is, the combustion is surely intensified for all cases, but this effect is larger for lean hydrogen/air mixture (Le
We consider the transition from smoldering to flaming. Though there have been numerous experimental studies of this topic, it has been virtually untouched theoretically. Rather than investigating the details of either the transition process or the subsequent flaming, we focus on determining the mechanism and the conditions that trigger the transition. We consider a planar, forward smolder wave driven by a constant forced flow of gas containing oxidizer. The kinetic scheme consists of three reactions: pyrolysis, fuel oxidation and char oxidation. There have been a number of speculations about the nature of the triggering mechanism, including the gaseous reactions, the char oxidation reaction, destruction of the porous matrix through which the smolder wave propagates, and others. However, no mechanism has, as yet, been shown to be capable of acting as the triggering mechanism. We show that the char oxidation reaction hardly affects the characteristics of smolder wave propagation due to its small reaction rate, though under appropriate conditions, it can act as the trigger for the transition to flaming due to its ability to self-accelerate. Specifically, we introduce the concept of, and then compute, a quantity which we term the flaming distance LF. This is the distance that a propagating smolder wave initiated at the gas flux inlet, travels before the char oxidation reaction spontaneously self-accelerates, resulting in an eruption of the temperature in the smolder front, i.e., the smolder wave propagates for a long time until it reaches LF. A transition to flaming then occurs. LF depends on the physicochemical parameters of the fuel and the products and external parameters, e.g., the velocity and composition of the incoming gas, heat loss. We show that smolder waves propagating in samples of length L do (do not) exhibit a transition to flaming if LFL).
To understand the mechanism of the transition from smoldering to flaming combustion of a flexible polyurethane foam, experiments were conducted with horizontally oriented foam layers under natural convection; this is a common scenario for a real fire. Temperature measurement, gas sampling, and thermal analysis were used to explore the transition’s mechanism. It is seen that the exothermicity of the oxidation of the residual char left by the passage of the smolder wave front is much larger than that of the oxidation of the original foam material. The transition is determined mainly by the exothermic oxidation of the residual char and the availability of oxygen inside the foam. Furthermore, from the aspect of fire safety during the use of flexible polyurethane foam in upholstered furniture, a series of experiments was done on how the foam length, ignition power, moisture content in the foam and flame retardant influence the transition in the flexible polyurethane foam. The results show that these factors can affect the occurrence of transition from smoldering to flaming and also the transition time because of the oxidation of the residual char left by the smoldering.
A two-dimensional ignition model is examined for two different flow patterns, one with a Navier–Stokes calculation and one with an Oseen approximation. The physical phenomena include channel flow and combustion reaction in the gas phase, pyrolysis and melting in the condensed phase, and radiation heat loss and fuel injection flow at the interface. Ignition is studied by means of an integral energy balance analysis, where various heat transfer mechanisms are evaluated. It is found that the flow pattern has an influence on the ignition process though the qualitative dependence is unchanged except for some isolated (though important) features of the problem. Generally, the Oseen approximation results in a shorter ignition delay. The influence of condensed phase material properties such as latent heat, conductivity, and heat capacity are investigated based on the energy barrier concept. Based on the numerical observations, a simple theory is derived to determine the ignition delay time by employing the energy balance principle. The predicted ignition delay agrees well with the direct numerical results. The external radiation is studied by changing its magnitude over a wide range. It influences the ignition behavior by mainly changing the ignition delay, whose results indicate that the polymer under investigation belongs to in the category of thermally thick polymers. In addition, the surface temperature is observed to increase with increasing external radiation heat flux.
A one dimensional numerical model is proposed to study the counter-current fire spread in a pine needles fuel bed confined in a tube. The physical mechanisms which control the fire spread and the interactions between the gas flow and the solid fuel particles are described using a multiphase formulation. The set of partial differential equations which governs the physical behavior of the gas flow and of the solid fuel bed is solved using a finite volume method. To describe accurately the small area where the thermal degradation (drying, pyrolysis, char combustion) of the solid fuel occurs, the calculations are carried out using an adaptative grid algorithm consisting in refining locally the grid on both sides of the pyrolysis front. The computation results have not been confronted with experimental measurements, therefore the objective of this study is to highlight the behavior of this physical system according to the rate of air supply. While varying the inlet flow velocities ranging from 2 to 30 cm/s, the numerical results show the existence of two modes of propagation, respectively limited by the arrival of oxygen and the production of combustible gas.
Experiments with radiative ignition were performed in microgravity to elucidate the events arising during the ignition process in a quiescent field. Filter paper was irradiated by Diode laser light (800.1nm), which is little absorbed in the gas phase, at various oxygen concentrations (0-50%). The ignition delay was measured for various experimental conditions. The density changes of the gas phase before ignition was observed in detail by a Mach-Zehnder interferometer. The results showed that heat conduction from the sample surface induced a weak chemical reaction in the vicinity of the sample surface, and this propagated outward to achieve combustion. The ignition delay decreased with increases in O2 concentration because the mixture near the sample surface contained more oxygen causing an immediate transition from the weak chemical reactions to the strong reactions of the combustion.
This study numerically investigates the ignition behaviors of vertically oriented cellulosic materials subjected to a radiant heat flux in a normal gravitational field. The entire process is delineated into two distinct stages. In the heating up stage, the maximum temperature increases with time but at a decreasing rate because of the pyrolysis reaction. The flame development stage consists of ignition and transition processes. In the ignition process, the maximum temperature in gas phase increases dramatically within a short period of time because a large amount of heat is generated from chemical reaction of the accumulative, flammable mixture. The flame is in a transition from a premixed flame to a diffusion one, except for the small region around the flame front. For the effect of varying the heating duration on ignition behavior, prolonging the imposed radiative heat time leads the ignition from a transition one to a persisting ignition. The effect of varying the solid fuel thickness indicates that the ignition delay lime increases with an increase of solid fuel thickness while 5S<1.802. For ].802<6S<3.244, the ignition delay times remain constant. Finally, if the external heating rate is the same order as the heal diffuse rate, the ignition delay time increases with an increase in solid fuel thickness.
This study numerically investigates the ignition behaviors of horizontal-oriented cellulosic materials subjected to radiant heat flux under a natural convective environment. This process can be divided into two stages: (1) Ihe heating-up stage, during which the maximum temperature increases with time and (2) the flame development stage, consisting of the ignition and transition processes. The ignition process is marked with a sharp increase in maximum temperature. Meanwhile, the flame is in a transition from a premixed flame into a diffused one. In the transition process, the flame propagates upstream, forming a so-called opposed flame spread. There is a time lag between solid fuel pyrolysis and the gas-phase chemical reaction. The pyrolysis front overtakes the flame front in the initial heating-up process. As soon as the flammable mixture ahead of the flame front is ignited, the flame moves forward quickly, moving ahead of the pyrolysis front. The ignition delay time is shorter for a horizontal solid fuel than for a vertical solid fuel under the same environment, because the resultant high-temperature region is not cooled and the flammable mixture is not diluted by the induced flow in a horizontal solid fuel. For the effect of changing gravity, the ignition delay time decreases with a decrease in gravity level, owing to the smaller induced flow velocity.
This study numerically investigated spontaneous ignition in the middle of a vertically placed solid thin fuel heated by external radiation in a normal-gravity quiescent environment. The ambient oxygen mass fraction varied from 35% down to the nonignition limit. Numerical results indicate that, in the nonignition case, the maximum reaction rate and maximum temperature in the gas phase were too weak to achieve the combustion reaction. For ambient oxygen mass fraction exceeding 17%, the ignition and transition to flame spread are all achievable. The ignited flame lengthens and strengthens with decreasing ambient oxygen mass fraction. The rate of flame growth was found to exceed that of the purely opposed and purely concurrent spread flames. Although the ignited flame is smaller, the propagating flame in high ambient oxygen mass fraction becomes longer and stronger in structure than the low-oxygen flame after 2 s of flame growth time. Our results further demonstrate that the flame growth rate in transition increases with the mass fraction of ambient oxygen.
We consider the transition from smoldering to flaming. Though there have been numerous experimental studies of this topic, it has been virtually untouched theoretically. Rather than investigating the details of either the transition process or the subsequent flaming, we focus on determining the mechanism and the conditions that trigger the transition. We employ both computation and approximate analytical approaches. We consider a planar, forward smolder wave driven by a constant forced flow of gas containing oxidizer. The chemical kinetic scheme employed consists of three reactions, namely, pyrolysis, fuel oxidation, and char oxidation. There have been a number of speculations about the nature of the triggering mechanism, including the gaseous reactions, the char oxidation reaction, destruction of the porous matrix through which the smolder wave propagates, and others. However, no mechanism has, as yet, been theoretically demonstrated to be capable of acting as the triggering mechanism. We show that the char oxidation reaction hardly affects the characteristics of smolder wave propagation due to its small reaction rate, though under appropriate conditions, it can act as the trigger for the transition to flaming due to its ability to self-accelerate. Specifically, we introduce the concept of, and then compute, a quantity that we term the flaming distance, LF. This is the distance that a steadily propagating smolder wave initiated at the gas flux inlet travels inside the porous medium before the char oxidation reaction spontaneously self-accelerates, resulting in an eruption of the temperature at the smolder front. That is, the smolder wave propagates for a relatively long latent period of time until it reaches LF. A transition to flaming then occurs. The flaming distance LF depends on the physicochemical parameters of the fuel and the products as well as external parameters: the velocity and composition of the incoming gas, heat loss, etc. We show that smolder waves propagating in porous samples of length L do (do not) exhibit a transition to flaming if LF<L (LF>L).
A general formulation based on a weighting average procedure is developed for describing the fire-induced behaviour of a multiphase, reactive and radiative medium. The complete set of the resulting equations should be used as the basic one for later studies, especially in the framework of wildland fires. For the moment, in order to demonstrate the capability of the general formulation, a simplified model, named zeroth-order model, in which some physical phenomena (such as char combustion, second-order terms, particle motion) are neglected is presented. In the frame of this simplified model, reverse and forward one-dimensional fire propagations through an heterogeneous medium composed of fixed fuel particles are studied numerically.
Enclosure effects on the transition from a localized ignition to subsequent flame growth over a thermally thin solid fuel in microgravity are numerically investigated by solving the low Mach number time-dependent Navier-Stokes equations. The numerical model solves the two and three dimensional, time-dependent, convective/diffusive mass, and heat transport equations with a one-step global oxidation reaction in the gas phase coupled to a three-step global pyrolysis/oxidative reaction system in the solid phase. Cellulosic paper is used as the solid fuel and is placed in a slow imposed flow parallel to the surface. Ignition is initiated across the width of the sample or at a small circular area by an external thermal radiation source. Two cases are examined; an open configuration (i.e., without any enclosure) and the case with the test chamber used in our previous microgravity experiments. Numerical results show that the upstream centerline flame spread rate for the case with the enclosure is faster than that for the case without any enclosure. This is due to the confinement of the flow field and also thermal expansion initiated by heat and mass addition in the chamber. The confinement accelerates the flow in the chamber, which enhances oxygen transport into the flame. In the three-dimensional configuration with the spot ignition, the flame growth in the direction perpendicular to the flow is significantly enhanced by the confinement effects. The effect of the enclosure is most significant at the slowest flow condition investigated and the effect becomes less important with an increase in imposed flow velocity. The total heat release rate from the flame during a flame growth period increases significantly with the confinement and the enclosure effects should be accounted to avoid underestimating fire hazard in a spacecraft.
A numerical study was made on the time-dependent ignition process of a horizontally placed solid fuel heated by external radiation. As soon as the solid starts to be heated, a hot plume with combustible fuel is ejected into the oxygen-containing atmosphere to be mixed and to react leading to a spontaneous ignition. The effects of gravity and ambient oxygen concentration on the ignition behavior were studied. The numerical model is a two-dimensional axisymmetric configuration with time-dependent heat and mass transport process and one-step exothermic reaction for gas phase, and three-step degradative reactions for solid phase. An appropriate ignition criterion was introduced to define ignition delay time and position. It was found that an increase in the gravity tends to prevent the ignition by increasing heat loss from the hot fuel-gas plume, whereas an increase in the ambient oxygen concentration enhances the ignition by accelerating chemical reaction in the plume. The two distinct types of ignition were identified in gravity vs. ambient oxygen concentration plot; the first one occurs when the oxygen concentration is relatively high and is ignited at the tip of the plume with a short ignition delay time, while the second one occurs when the oxygen concentration is low and is ignited at the inside of the plume with a relatively long delay time. The former type of ignition was found to be controlled basically by 1-D heat and mass transport process, whereas the latter type is controlled by the 2-D process caused by buoyancy-induced flow.
Three-dimensional (3-D) and two-dimensional (2-D) simulations of the transition from radiative ignition on a solid fuel to flame spread in an imposed wind were performed in microgravity. Two-dimensional flames were found to quench (due to poor oxygen supply) more easily (i.e., at larger wind speeds) than 3-D flames. Results from the 2- and 3-D simulations were compared during the transition phase at wind speeds that ultimately lead to quenching of the 2-D flame but survival of the 3-D flame. In all locations near and in the flame, oxygen mass flux was larger in the 3-D flames and dominated by diffusion (as opposed to convection). Oxygen was supplied to the core of the 3-D flame due to diffusion from the sides of the flame (in a cross-wind direction). Diffusion in the 2-D flame was limited to directions parallel to the wind. This cross-wind diffusion was most significant at early times during transition when the flame was small and had a relatively large curvature. The 3-D flame, therefore, required less oxygen supply from an external wind to undergo transition to flame spread. Once flame spread was established there was little difference between the 3-D flames (in the centerline plane) and 2-D flames, due to the decreased curvature of the three-dimensional flame relative to the curvature during ignition and transition.
Typescript. Department of Mechanical and Aerospace Engineering. Thesis (Ph. D.)--Case Western Reserve University, 2000. Includes bibliographical references (leaves 156-159).
The ultimate goal of this research is to extend the current understanding of the characteristics of spherical diffusion flames in microgravity. In particular, one of the key objectives is to assess the effects of gas radiation as a means to promote flame extinction. To investigate these phenomena, a one-dimensional computational model was developed to simulate the evolution of a spherical diffusion flame with consideration of detailed chemistry and transport properties. Various levels of radiation models were implemented and the results were compared with experimental measurements of flame radius and temperature profiles. It was shown that the statistical narrow band model (SNB) combined with the discrete ordinate method (DOM) reproduced the experimental results with highest accuracy, and this combination of the radiation models were adopted in the subsequent parametric studies.
The parametric studies explored the relative effectiveness of fuel- and oxidizer-side dilution on the flame radius and temperature behavior, with nitrogen, CO2, and helium as diluents. In the spherical configuration considered in this study, the oxidizer-side dilution has a stronger effect on flame transient behavior than the fuel-side dilution, thereby suggesting a more effective means to induce flame extinction by dilution.
Study on various oxidizer-side dilution cases shows that CO2 has a larger suppression effect than helium or nitrogen with the same dilution level. CO2 dilution has multiple effects on flame behavior including radiation, thermodynamic, diffusion, and chemical effects. Quantitative analysis shows that the radiation effect is the primary factor accounting for flame temperature drop by approximately 60%, as compared to the thermal/diffusion (30%), and chemical effect (10%).
Considering the dominance of the radiative heat loss on flame extinction, a unified extinction criterion that applies to a wide range of parametric conditions was sought. The compiled computational results indicated that a critical flame temperature of 1130 K at extinction appears to be valid for most of the conditions under study. Therefore, it is concluded that extinction of spherical diffusion flame is primarily dictated by the local condition in the flame zone rather than by the volumetric radiative heat transfer in the surrounding gases.
A new flammability apparatus and protocol, FIST (Forced Flow Ignition and Flame Spread Test), is under development. Based on the LIFT (Lateral Ignition and Flame Spread Test) protocol, FIST better reflects the environments expected in spacebased facilities. The final objective of the FIST research is to provide NASA with a test methodology that complements the existing protocol and provides a more comprehensive assessment of material flammability of practical materials for space applications. Theoretical modeling, an extensive normal gravity data bank and a few validation space experiments will support the testing methodology. The objective of the work presented here is to predict the ignition delay and critical heat flux for ignition of solid fuels in microgravity at airflow velocities below those induced in normal gravity. This is achieved through the application of a numerical model previously developed of piloted ignition of solid polymeric materials exposed to an external radiant heat flux. The model predictions will provide quantitative results about ignition of practical materials in the limiting conditions expected in space facilities. Experimental data of surface temperature histories and ignition delay obtained in the KC-135 aircraft are used to determine the critical pyrolysate mass flux for ignition and this value is subsequently used to predict the ignition delay and the critical heat flux for ignition of the material. Surface temperature and piloted ignition delay calculations for Polymethylmethacrylate (PMMA) and a Polypropylene/Fiberglass (PP/GL) composite were conducted under both reduced and normal gravity conditions. It was found that ignition delay times are significantly shorter at velocities below those induced by natural convection.
A theoretical model describing radiative ignition of a solid fuel is constructed and is numerically analyzed. The model includes the effects of gas phase reaction and a finite value of the absorption coefficient of the solid (in-depth absorption of incident radiation). It is found that the gas phase reaction must be included in the model in order to understand radiative ignition of a solid fuel and to find its ignition boundary. The in-depth absorption of the incident radiation by a solid fuel significantly affects the ignition delay time. The results indicate that there is a finite range of values for pyrolysis or gas phase reaction activation energy for which ignition will occur. This finding has a direct bearing on efforts to reduce material ignitability.
Attention is given to a theoretical model describing the behavior of a thermally thin cellulosic sheet heated by external thermal radiation in a quiescent microgravity environment. This model describes thermal and oxidative degradation of the sheet and the heat and mass transfer of evolved degradation products from the heated cellulosic surface into the gas phase. Two calculations are carried out: heating without thermal degradation, and heating with thermal degradation of the sheet with endothermic pyrolysis, exothermic thermal oxidative degradation, and highly exothermic char oxidation. It is shown that pyrolysis is the main degradation reaction. Self-sustained smoldering is controlled and severely limited by the reduced oxygen supply.
A numerical analysis is conducted of the initiation and evolution of a combustion reaction generated by fuel vapor absorption of radiation over an evaporating combustible surface in an oxidizer stagnation point flow. The combustible is initially evaporating due to an externally applied irradiance, and is assumed to be in equilibrium vaporization. The transient, stagnation point, gas conservation equations, including one-step Arrhenius type kinetics and fuel vapor absorption of radiation, are solved numerically for the case of PMMA as combustible evaporating in air. Detailed calculations are presented, for a specific case of irradiance and flow velocity, of the evolution of the velocity field, and temperature and species concentration distributions during the ignition of the mixture, and subsequent establishment of a diffusion flame over the combustible surface. Ignition is characterized by thermal run-away of the gas and it is considered to have occurred if, after discontinuing the external irradiance, a self-sustained reaction is present. It is predicted that ignition occurs in the fuel-rich side of the mixing layer formed around the dividing streamlines of the fuel and oxidizer opposing stagnation flows. After ignition a premixed reaction front moves toward the lean side separating the fuel from the oxidizer and leaving behind a diffusion flame which eventually reaches steady conditions. A parametric study is also conducted on the effect of the flow velocity on the minimum irradiance for ignition and on the ignition time. It is predicted that the minimum irradiance for ignition increases approximately linearly with the velocity and that for a given velocity the ignition time decreases approximately inversely with the irradiance.
This article describes experimental measurements of the structure of lean, premixed, laminar, flat flames of CH4/NO2/O2 and CH2O/NO2/O2 mixtures at 55 torr. The compositions of stable species in the flames were measured using a cooled quartz sampling microprobe and gas chromatographic analysis. The compositions of the intermediates OH, CN, CH, NH, and NH2 were measured by laser-induced fluorescence spectroscopy using an excimer laser pumped tunable dye laser system. Flames with only O2 as oxidizer were blue/violet due to C2 and CH emission or CO chemiluminescence. Flames with NO2 and O2 have two luminous zones, one yellow and the other blue/violet, separated by distinct, dark nonluminous zones. Nitrogen dioxide is a poor oxidizer in comparison to O2 and, therefore, rich mixtures with NO2 could not be stabilized. With CH4 as the fuel the products contain considerable unreacted NO2, along with NO and N2 whereas with CH2O as the fuel little N2 was formed. A partial reaction mechanism is discussed which accounts for the observations in the flame data.
Radiative ignition experiments were conducted on PMMA and red oak using a CO2 laser with incident flux up to about 20 W/cm2 under autoignition and piloted ignition in air. The laser irradiated perpendicular to the horizontal sample surface. It was observed that there was strong attenuation of the incident laser radiation by the plume consisting of decomposition products in the gas phase. This was also observed using an electric coil heater as a radiant source. It is postulated that, under autoignition, PMMA ignites by the absorption of the incident radiation by the decomposition products in the gas phase, and red oak by a similar absorption at high-incident flux and at medium flux, aided by high surface temperature acting as an induced pilot.
An analytical expression for gas-phase ignition is developed for a diffusion flame in the two-dimensional or axisymmetric stagnation-point boundary-layer flow of a hot oxidizing gas about a vaporizing condensed fuel surface. The analysis is based on the limit of large activation energy for a one-step, irreversible reaction describing the overall combustion process in the gas phase. The approach in this work, following that of our recent analysis on extinction for the same geometry, is to seek an exact correspondence of the parameters of the present problem with those of counterflow diffusion-flame problem of Liñán. Such a correspondence has been found in the frozen-flow regime and as a consequence, the asymptotic structure of the flame in the nearly frozen ignition regime is identical in both the problems. A particular result of this observation is the availability of an analytical criterion for ignition in the present problem. The analysis reveals that contrary to the case of extinction, fluid dynamic details do not have significant effect on the ignition criterion and that Liñán's results may be applied with good accuracy to the condensed fuel problem.
Transmittance of external radiation from a CO2 laser through a boundary layer of decomposition products over a vertical sample surface is measured during the ignition period. The results indicate that there is significant absorption of the external radiation for PMMA, and a lesser but still not negligible amount, for red oak. An increase in gas phase temperature over surface temperature is observed over much of the ignition interval. Using the experimentally measured incident flux at the sample surface, surface temperature history was calculated from a model that included re-radiation and convection losses from the surface, endothermic decomposition and conduction into the material. The results confirm the significant effect of gas phase absorption on surface temperature. Steady-state-derived surface regression rate expression was used for PMMA in this model. The results raise questions about the validity of such data for the dynamic heating conditions during the ignition period. Further studies needed to understand the radiative ignition mechanism are identified.
In an earlier paper an asymptotic analysis was presented for the radiant ignition of a solid fuel that gasifies endothermically then reacts exothermically in the gas phase through a one-step Arrhenius process. The theory was restricted to surface absorption of the incoming radiation and to ignitions occurring during the stage of transport-controlled gasification which follows the stage of transition to gasification. These two restrictions are removed in the present paper. Emphasis is placed on determining the ignition time, defined as the interval between initiation of irradiation and thermal runaway. It is shown that if the gas is sufficiently reactive then ignition can occur early in the stage of transition to gasification and that decreasing the absorptivity of the solid increases the ignition time. Comparisons are made with results of earlier numerical integrations, demonstrating the existence of regimes in which the ignition time depends strongly on the ignition criterion adopted. Graphs are given containing results of a parametric study for the ignition time.
Flame spread over a thermally thick slab of PMMA at several angles of sample orientation from θ = −90° (vertically downward flame spread) to θ = +90° (vertically upward flame spread) in air was investigated by measuring temperature distributions within the PMMA sample in the vicinity of the leading edge of the flame front using holographic interferometry. Samples with widths of 0.32, 0.47, 1.0, and 2.5 cm, a thickness of 2.5 cm, and a length of 30 cm were used. The measured net heat flux from the gas phase to the sample surface at the vaporization front of the sample is about 7 W/cm2 for downward flame spread (θ < 0δ), 6.5 W/cm2 at θ = + 10°, and 2.8 W/cm2 at θ = + 90°. However, the total net heat transfer rate increases with an increase in the angle of sample orientation, because the characteristic heating length, defined as the distance from the adiabatic point on the sample surface to the vaporization point, increases with an increase in the orientation angle of the sample. The total net heat transfer rate into the sample from the gas phase is about 56% of the total net heat transfer input to the sample at θ = − 90°, 78% at θ = 0°, 87% at θ = + 10°, and 99% at θ = + 90°. Therefore, heat transfer from the gas phase into the unburnt fuel ahead of the vaporization point is the dominant heat tranfer path for all angles of orientation. This was clearly demonstrated by the net heat flow vector patterns within the sample. The streamwise conductive heat transfer rate through the sample decreases with an increase in flame spread rate (increase in the sample orientation angle) due to insufficient time being available for the slow thermal wave to travel through the sample.
Values of global kinetic constants for pyrolysis, thermal oxidative degradation, and char oxidation of a cellulosic paper were determined by a derivative thermal gravimetric study. The study was conducted at heating rates of 0.5, 1, 1.5, 3, and 5°C/min in ambient atmospheres of nitrogen, 0.28%, 1.08%, 5.2% oxygen concentrations, and air. Sample weight loss rate, concentrations of CO, CO2, and H2O in the degradation products, and oxygen consumption were continuously measured during the experiment. Values of activation energy, pre-exponential factor, orders of reaction, and yields of CO, CO2, H2O, total hydrocarbons, and char for each degradation reaction were derived from the results. Heat of reaction for each reaction was determined by differential scanning calorimetry. A comparison of the calculated CO, CO2, H2O, total hydrocarbons, sample weight loss rate, and oxygen consumption was made with the measured results using the derived kinetic constants and accuracy of the values of kinetic constants was discussed.
Several steady state and time-dependent solutions to the compressible conservation laws describing direct one-step near-equilibrium irreversible exothermic burning of initially unmixed gaseous reactants, with Lewis-Semenov number unity, are presented. The quantitative investigation first establishes the Burke-Schumann thin-flame solution using the Shvab-Zeldovich formulation. Real flames do not have the indefinitely thin reaction zone associated with the Burke-Schumann solution. Singular perturbation analysis is used to provide a modification of the thin-flame solution which includes a more realistic reaction zone of small but finite thickness. The particular geometry emphasized is the un bounded counterflow such that there exists a spatially constant rate of strain along the flame. While the solutions
for diffusion flames under a finite tangential strain rate may be of interest in and of themselves for laminar flow, the problems are motivated by the authors' belief that they are pertinent to the study of diffusion-flame burning in transitional and turbulent shear flows.
Expert systems are problem-solving programs that combine a knowledge base and a reasoning mechanism to simulate a human expert. The development of an expert system to manage fire safety in spacecraft, in particular the NASA Space Station Freedom, is difficult but clearly advantageous in the long-term. Some needs in low-gravity flammability characteristics, ventilating-flow effects, fire detection, fire extinguishment, and decision models, all necessary to establish the knowledge base for an expert system, are discussed.
A numerical investigation of the transonic twodegree -of-freedom bending/torsion flutter characteristics of the NLR 7301 section is presented using a time domain method. An unsteady, two-dimensional, compressible, thin-layer Navier-Stokes flow-solver is coupled with a two-degree-of-freedom structural model. Furthermore, the Baldwin-Lomax, the Baldwin-Barth and the Spalart-Allmaras turbulence models are implemented, each in conjunction with the transition model of Gostelow et al. The transition onset location can either be predicted with Michel's criterion or specified as an input parameter. Computations of the steady transonic aerodynamic characteristics show good agreement with the experimental results using the BaldwinBarth or the Spalart-Allmaras model in combination with transition modeling. The aeroelastic computations predict limit-cycle flutter in agreement with the experiment. However, the computed flutter amplitudes are an order of magnitude larger than the measured ones. NOM...