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Gravitational effects on flame spread through non-homogeneous gas layers
Proceedings of the Combustion Institute, Volume 29, 2002/pp. 2561–2567
GRAVITATIONAL EFFECTS ON FLAME SPREAD THROUGH NON-HOMOGENEOUS GAS LAYERS FLETCHER J. MILLER,1 JOHN W. EASTON,1 ANTHONY J. MARCHESE2 and HOWARD D. ROSS1 1
National Center for Microgravity Research NASA Glenn Research Center Cleveland, OH 44135-3191, USA 2 Rowan University 201 Mullica Hill Road Glassboro, NJ 08028-1700, USA
Flame propagation through non-uniformly premixed gases occurs in several common combustion situations. Compared with the more usual limiting cases of diffusion or uniformly premixed flames, the practical concern of non-uniform premixed gas flame spread has received scant attention, especially regarding the potential role of gravity. This research examines a system in which a fuel concentration gradient exists normal to the direction of flame propagation and parallel with the gravitational vector. This paper presents experimental and numerical results for flame spread through alcohol/air layers formed by diffusive evaporation of liquid fuel at temperatures between the flash-point temperature and the stoichiometric temperature. A gallery, which had either the top and/or one end open to maintain constant pressure, surrounded the test section. The numerical simulations and experiments conducted include normal and microgravity cases. An interferometer was used, in normal gravity only, to determine the initial fuel layer thickness and fuel concentration distribution before and during flame spread. Both the model and experimental results show that the absence of gravity results in a faster spreading flame, by as much as 80% depending on conditions. This is the opposite effect to that predicted by an independent model reported earlier in this symposium series. Determination of the flame height showed that the flame was taller in microgravity, an effect also seen in the results of the numerical model reported here. Having a gallery lid results in faster flame spread, an effect more pronounced at normal gravity, demonstrating the importance of enclosure geometry. The interferometry and numerical model both indicated a redistribution of fuel vapor ahead of the flame. Numerical simulations show that, despite the rapid flame spread in these systems, the presence of gravity strongly affects the overall flow field in the gallery.
Introduction Flame spread through uniformly premixed gas systems is a frequently studied problem in combustion science. In contrast, flame propagation through non-uniformly premixed gas systems (also called ‘‘layered systems’’) has been the subject of relatively few studies. Layered mixtures (see Fig. 1), however, are ubiquitous in terrestrial fire hazards, such as chemical spills, underground mining operations, and automobile and aircraft crashes. The flames in such systems have been shown to carry over fences, and to propagate past the ends of the fuel spill, thus representing a hazardous area beyond that associated with the original fuel location. They also are a potential fire hazard aboard long-duration spacecraft, such as the International Space Station, because flammable gases may accumulate near waste storage, laboratory fluids, fuel cells, lasers, etc. Our hypothesis is that gravity can influence flammability and the rate of flame propagation in a layered system in at least three ways: through a hydrostatic pressure gradient, through buoyantly induced
Fig. 1. Schematic of flame spread through a non-homogeneous mixture contained in a gallery (not drawn to scale). Darker areas indicate region of higher fuel vapor concentration near surface. Also labeled on the figure are the gallery size and boundary conditions used in the numerical model and experiment.
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flows during spread, or by affecting the initial distribution of fuel vapor. The first effect has been professed by previous researchers as important since agreement between predicted and observed flame spread rates in normal gravity tests improves with the inclusion of the hydrostatic pressure term in a simple model [1]. The second gravitational influence on the flame propagation in a stratified layer is the effect on the flows responsible for the redistribution of the fuel vapor, oxygen, and combustion products once a propagating flame is established. Previous research indicates that convective or aerodynamic effects can induce flows ahead of the flame that alter the fuel vapor concentration distribution and assist the flame spread [1–3]. The high propagation speeds in systems that maintain a stoichiometric concentration at some height in the layer suggest buoyant flow might not develop rapidly enough to affect spread; however, in the substoichiometric regime, this may not be the case. A third effect, how gravity affects the initial distribution of fuel from a source and how that affects flame spread, is relatively unexplored in the flame spread literature. In normal gravity, a leaking buoyant fuel (e.g., hydrogen) will rise from its source until it encounters a ceiling where it may collect and form a flammable layer. Just the opposite may occur with point sources of heavier-than-air fuels. In microgravity, the fuel would not be buoyant and instead will collect around its source or be redistributed by the slow (5–10 cm/s) flows due to ventilation systems and crew movements. These differences in fuel vapor distribution may lead to very different flame propagation behavior and thus different fire hazards. Review of Previous Research That Included Gravitational Effects Ishida showed convincingly in experiments that flame spread over a liquid-saturated fuel bed with the initial temperature (T0) less than the stoichiometric temperature (Tst) is affected by its orientation relative to the gravity vector [4]. If T0 ⬎ Tst, the flame appearance and velocity (Vf) are similar to a system comprising layered gases without a liquid surface; in this range, Vf is two to five times greater than the laminar burning velocity for a stoichiometric, homogeneous mixture. The motion can be driven by the expansion of the low-density products, which displaces and redistributes the unburned gas layers ahead of the flame into a broader, curved area [2]. The reported upstream distance covered by this disturbance ranges from a few centimeters [3] to 10– 15 cm [1]. Researchers at the Bureau of Mines investigated this problem both for heavy gases sitting on floors [5] and for lighter gases trapped under ceilings [3]. They reported that the flame speed depended on the
flammable zone thickness and the concentration gradients, and interestingly, that the flame propagated through regions that were below the lean flammability limit for homogeneous mixtures. They attribute this to ‘‘the proximity of the stabler [richer] burning mixture’’ [5]. In their data analysis of methane layers, they found that inclusion of gravitational effects through the Richardson number (which they defined as the ratio of buoyant forces tending to stabilize a layer to shear forces tending to mix it) was useful for the closed-gallery case. Extension of their correlation to 0 g predicts infinitely fast spread, which is not possible, but for 10ⳮ3g, the non-dimensional flame speed increase is approximately a factor of 2.6 over 1 g conditions. Feng et al. [2] treated both the open and bounded gallery configurations through experiments and modeling using a stream tube approach. The model of Feng et al., however, overpredicted by about 50% the Vf observed in Kaptein and Hermance’s experiments [1]. The latter authors demonstrated that agreement with their experimental results improved by modifying the Feng model to include a gravitational potential energy term (essentially hydrostatic pressure) accounting for the different heights of the combustible and burned gas layers. The Current Research In this paper, we present the results of experimentally and numerically determining flame spread rates in layered systems in normal and microgravity. In particular, we focus on cases where the maximum system equivalence ratio is stoichiometric or fuel lean prior to ignition. We achieved the fuel layers by allowing a liquid fuel to diffuse for a predetermined time upward into a gallery in normal gravity, and then igniting the layer in either normal or reduced gravity. Diffusion time and liquid temperature controlled the fuel layer thickness and concentration, respectively. Of the three reasons cited earlier regarding the influence of gravity on the flame spread, the first two are considered here; the effect of gravity on the initial fuel distribution could not be determined experimentally because of insufficient microgravity time for diffusion. The experiments were simulated using a two-dimensional, transient, chemically reacting flow numerical model. Experimental Apparatus A drop rig at the NASA Glenn Research Center 2.2 Second Drop Tower provided the platform for collecting both the normal and reduced gravity data presented here. As seen in Fig. 2, the rig contains an 80 cm long gallery with a 10 cm square cross section. The gallery has one Lexan and one aluminum sidewall with a removable Lexan top. The end
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in the drop tower, a delay of 0.1 s allows vibration transients to dampen, then the fuel/air mixture is ignited, and the flame spreads toward the open end. At the end of the test, the gallery is flooded with nitrogen and the frit lid closes as a safety precaution, extinguishing any flames remaining after impact. For normal gravity tests, the same procedure is followed, except a substitute drop signal begins the test. Numerical Model Fig. 2. Experimental flame spread gallery 80 cm long with 10 cm square cross section. The ignitor is a Kanthal wire stretched a few millimeters above the fuel surface across one end.
nearest the hot-wire igniter is closed, while the end farthest from the igniter is open, matching the boundary conditions of the numerical model. A bronze frit with a porosity of 20% acts as the floor of the gallery and is saturated with liquid fuel. A retractable lid covers the frit prior to a test, preventing fuel vapor from diffusing into the gallery. A fan on the open end exhausts vapors that accumulate during the filling process, but it is not on during the experiment. The frit is mounted to a movable stage that has water channels running through it to maintain the temperature of the fuel prior to the test. Two stainless steel sheathed, type T thermocouples embedded in the frit flush with its top surface near each end measure the initial temperature. Two CCD cameras image the frit surface from above at 30 frames/s, with each field-of-view overlapping in the center of the frit. Additionally, a mirror along the side of the gallery at 45⬚ provides a side view of the upper portion of the flame. Because of the time needed to move the drop rig into position at the top of the tower, the frit is slightly overheated and then allowed to cool to the desired test temperature while in the drop position. Once the fuel frit reaches the desired temperature as indicated by the two thermocouples (which are normally within 0.1 to 0.2 ⬚C of each other), the experimenter sends a signal to the computer on the drop rig. The computer then opens the frit lid, and begins counting a predetermined amount of time, ranging in these tests from 5 to 60 s, here referred to as the ‘‘diffusion time.’’ During this time, the fuel in the frit evaporates into the gallery, forming a non-homogeneous mixture with the air above the frit, while the flow generated by the lid rapidly decays. (We characterized the decay of the flow field by performing flow visualization tests using dry ice condensation trails. Within approximately 3 s, the motion caused by the lid removal could no longer be detected visually.) Note that all of the diffusion of fuel vapor occurs in normal gravity. After release into free fall
The flame spread experiments were simulated utilizing a two-dimensional, transient numerical model previously developed for studying flame spread across subflash liquids [6]. As detailed in Schiller et al. [6], the numerical model uses the SIMPLE algorithm [7] and a hybrid-differencing scheme to solve the two-dimensional gas-phase continuity, species, energy, and x–y momentum equations and the liquid-phase energy equation. For the major results discussed here—gravitational effects on flame spread rate, flame height, etc.—two dimensions are sufficient to provide a good comparison between the model and the experiment. As shown in Fig. 1, the liquid tray was modeled as an 80 cm long pool with a fuel depth of 2 mm. The thermal properties of the liquid were modified to account for the bronze frit by averaging the properties using the porosity as a weighting factor. The height of the gas phase above the liquid pool was 10 cm. A constant Schmidt number of 1.5 was used for all species. The rectangular numerical domain used in this study consisted of 112 grid points in the x-direction, with 82 grid points in the gas-phase y-direction and 32 grid points in the porous bronze y-direction. The grid is adaptive in the x-direction and follows the flame along the gallery. To simulate some of the experiments, the gallery was modeled as closed at the ignition end of the domain and open at the top and right-hand sides of the domain (see Fig. 1). The model initially ran with normal gravity for a specified time (the diffusion time) without introducing the ignition source. During this period, the time step was 5 ms. This allowed the fuel to vaporize at the pool surface and diffuse into the gas phase including the effect of gravity, setting up initial conditions consistent with experiments. The resulting fuel vapor distribution then became an input to the reacting case, for which the time step was 0.05 ms. Comparisons of the fuel vapor distribution just prior to the end of a 60 s diffusion period with those 1 s into the zero-gravity period showed no difference, indicating that the transition to 0 g does not affect the fuel vapor layer from the perspective of the model. Results and Discussion Using the test apparatus described above, we conducted experiments with propanol, methanol, and
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1 cm
Hf
Lean Limit
Frit Level Fig. 3. Side-view image of a flame spreading over npropanol at 27 ⬚C for a 60 s diffusion time in normal gravity (flame spread is left to right). The lean-limit height is the height of the flammable layer prior to ignition; Hf indicates the flame height measurement (Fig. 5).
Fig. 4. Flame spread rate as a function of diffusion time for two temperatures (27 ⬚C and 35 ⬚C) in normal and reduced gravity, for numerical and experimental cases. The diffusion times correspond to flammable layer thicknesses of 0.4, 0.7, and 1.2 cm at 27 ⬚C and 0.7, 1.3, and 2.2 cm at 35 ⬚C. The numerical model results are shown as lines; the experimental point error bars represent the high and low values obtained for each condition.
ethanol to determine the effect of initial temperature, diffusion time (i.e., layer thickness), and gravity level on the flame spread rate through layered systems. We chose these fuels because the numerical model had originally been developed using them [6], and because they have flashpoints in a convenient temperature range. This paper presents only the 1propanol results; ethanol results did not show as great an effect of gravitational level and are reported in Ref. [8], and for methanol, only 1 g tests have been completed [9]. For a variety of the experimental conditions, we used the numerical model to obtain comparative data regarding flame spread rate, flame height, fuel vapor concentration, and gasphase velocity, allowing comparisons with experiment to be given below.
A side-view image of a 1-propanol flame in 1 g is displayed in Fig. 3 to show the flame structure and size. The flame consists of two branches: an upper lean, premixed branch and a lower diffusion flame where fuel vapor and oxygen that penetrated the lean flame above meet and burn. As discussed below, the flame can burn much higher above the surface than the initial lean limit height as shown on the figure. Figure 4 shows the predicted and experimental flame spread rates for tests conducted in normal and microgravity at 27 ⬚C and 35 ⬚C with diffusion times of 5, 20, and 60 s. The flame spread rate was constant (i.e., not pulsating or acceleratory) in each experimental and numerical case studied, regardless of temperature or diffusion time. The model slightly over- or underpredicts the flame spread rate depending on the values of the kinetic constants that are chosen. In this work, we used an activation energy of 25,500 cal/gmol, and a pre-exponential constant of 9.38 ⳯ 1011 m3/(kmol s), which gave approximate agreement with the experiments over the temperature range we tested. We have not found one set of pre-exponential factor and activation energy that are valid at all temperatures for this simple, single-step model. Therefore, rather than seeking to use the model to obtain absolute flame spread rate predictions, it is more appropriate to use it to explore some of the other features and trends of the process. Each experimental point on the graph represents an average of at least four microgravity tests (and sometimes five), and two normal gravity tests, with the error bars indicating the highest and lowest values for each condition. The flame spread rate increases markedly with increasing temperature, due to the increase in vapor pressure of the fuel. It is well known for uniformly premixed gases that small increases in the equivalence ratio of lean mixtures dramatically increase the flame speed, and the same effect occurs here. There is some variation in the data that we have been unable to eliminate, perhaps owing to being at slightly different temperatures from test to test. The variability may also be due to how saturated the frit is with fuel when we commence the test, a factor we try to control but with difficulty. An increase in diffusion time also led to a slight increase in flame spread rate between 5 and 20 s, with the effect being most pronounced at 35 ⬚C. This effect is more evident in reduced gravity than in normal gravity. At longer times, neither the model nor the experiments show much effect of diffusion time. The fact that the effect is small indicates that it is the maximum fuel concentration in the layer (as determined by the temperature), and not the layer thickness (as determined by the length of the diffusion time) that most influences how fast the flame will spread. Note that this may not be true for very thin layers (on the order of the quench distance), but our apparatus requires diffusion times of at least
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Fig. 5. Flame height as a function of diffusion time (or lean-limit layer height as shown on the upper axis) at 35 ⬚C for normal and reduced gravity, experiment and numerical results. Experimental results are with gallery lid on, while numerical results are with gallery lid off.
5 s to allow air motion induced by the lid to cease, which limited us to thicker layers. The gravitational influences are perhaps the most significant results presented here because they have not been explored previously. Once averaged, the results clearly show a large effect of gravity level, with the flame spreading faster in microgravity, especially at longer preignition diffusion times. This is true regardless of the presence of a lid on the gallery, which has the effect of increasing the flame spread rate in both 1 g and microgravity, most likely by channeling gas expansion in the direction of flame spread [10]. At 27 ⬚C, for a 60 s diffusion time, the microgravity (lg) flame spread 81% faster with the lid off (shown), and 63% faster with it on, compared to 1 g. At 35 ⬚C, the 1 g/lg flame spread rates were much closer, varying by about 15% with the lid off, and being essentially identical with the lid on. Even with the cautions expressed earlier about the kinetic constants, the model captures the trends of increased speed at microgravity and little increase in speed with diffusion time. Gravitational level also affects the height of the flame above the fuel source. Fig. 5 shows the height of the flame, measured behind the flame front (where it becomes essentially horizontal; see Fig. 3), as a function of diffusion time for both normal and reduced gravity at 35 ⬚C. The flame height in reduced gravity is larger than the corresponding normal gravity case for both the experimental and numerical results, although the model predicts a larger gravitational effect. The most likely causes of this gravitational effect are the two factors, change in the hydrostatic pressure gradient and buoyant induced flows, described earlier. Without the hydrostatic
Fig. 6. Fuel mole fraction contours at 35 ⬚C and 60 s diffusion time as the flame passes. (a) Measured with interferometry; (b) numerically predicted. In both cases the upward movement of the fuel vapor due to the flame can be seen. The lean limit is 2.2% for 1-propanol.
pressure present, the low-density products can expand further upward. In microgravity, there is also no buoyant flow that brings air to the flame so that the flame must rise higher off the surface as it seeks oxygen. Fig. 5 also demonstrates the effect of fuel layer thickness. For both the normal and reduced gravity experimental cases, the flame height increased with thicker layers. The model predicts a leveling off of the flame height with layer thickness in both cases, an effect not seen in the experiments. Results from the measurement of the fuel layer thickness in normal gravity using a Michelson interferometer demonstrate how the flame front redistributes the upstream fuel vapor [9,10]. Prior to ignition, the mole fraction contours are essentially horizontal lines (except near the ends of the gallery where vapor spillover occurs). In Fig. 6a, we show the fuel mole fraction as the flame passes, deduced from interferometric measurements for a 1 g case. The molar refractivity of 1-propanol was not directly
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Fig. 7. Numerically predicted U component of velocity in laboratory coordinates at 35 ⬚C. (a) 1 g; (b) microgravity. The white lines represent the U component of velocity 1 mm above the fuel surface (shown on the right-hand axis), the location of the maximum velocity.
available to us, but we calculated a value of 18 on the basis of the method reported in Ref. [11] and used this for the interpretation of the interferograms. The value of 18 also agrees well with extrapolated values of methanol/methane and ethanol/ethane given in Ref. [12]. As can be seen, the flame pushes the fuel vapor upward as it approaches, increasing the effective flammable layer thickness. Because of the limited field of view, we did not record the flame burning below the lean flammability limit of 2.2% for 1-propanol, although we have seen this effect for methanol and ethanol [9]. The work of Ref. [3] reports a similar displacement of fuel for methane layers along ceilings. In Fig. 6b we show the predicted fuel mole fraction after ignition for a 1 g case. The agreement is good between model and experiment, with both showing roughly the same flammable layer thickness and the effect of fuel layer displacement by the flame. This plot also shows the predicted and observed double flame (see Fig. 3). (The diffusion flame is the dark line along the surface in Fig. 6b.) As shown in Fig. 7, the modeling results verify the hypothesis that aerodynamic effects induce flows ahead of the flame, thereby altering the fuel vapor concentration distribution and assisting the flame spread. Specifically, the model shows that expansion of the hot gas upstream of the propagating flame creates a convective velocity (Fig. 7) in the direction
of the propagating flame. This increased velocity can carry the flame at speeds that in laboratory coordinates well exceed the laminar burning velocity. Furthermore, behind the flame front there is a much larger flow velocity away from the flame in microgravity (see dark region) due to a higher reaction rate. This thrust drives the flame forward faster than at normal gravity. Of the three effects of gravity we identified earlier, the flow field changes due to buoyancy seem most likely to be responsible for the gravitational effect seen here. Up to the stoichiometric temperature, the predicted difference in velocity at the flame front between 1 g and 0 g exceeds the laminar burning velocity (see lines in Fig. 7), so that buoyancy appears to have an effect even at the flame leading edge. In addition, numerical simulations (not shown) predict that in normal gravity flow induced from buoyancy well behind the flame affect the flame spread by changing the global flow pattern in the gallery. Using the numerical model, we have been unable to identify any systematic change in the pressure field due to the flame spread in normal vs. microgravity. Although the simple model of Kaptein and Hermance predicts an effect of hydrostatic pressure due to gravity, it is opposite to that observed here [1]. In their accounting for the hydrostatic pressure change the flame should spread faster in normal gravity by up to 50%. Their model did not, however, include any flow field effects, which may be stronger than any pressure differences.
Summary and Conclusions We have shown that flame spread through nonhomogeneous gas layers is faster in microgravity than in 1 g for every condition we tested, up to 80% faster in the most extreme case. Both experiments and a numerical model have demonstrated this phenomenon, which is the opposite trend to that surmised in Ref. [1] via a simple analytical model. Flame heights were also higher in microgravity than in 1 g. We obtained good agreement in the fuel vapor mole fraction predictions and measurements in 1 g. The numerical model showed little difference between 1 g and microgravity predictions of the reactant mixture concentrations. Therefore, the faster flame spread rate in microgravity is likely not due to a different redistribution of flammable gases ahead of the flame. (Microgravity experiments are needed to verify this fully.) Although we don’t yet have flow field measurements, the model indicates a substantial difference in the velocity field around the spreading flame depending on the gravitational level. Therefore, it appears that modification of this field by buoyancy is responsible for the difference in flame spread rate between 1 g and microgravity.
FLAME SPREAD THROUGH NON-HOMOGENEOUS GAS LAYERS Acknowledgments NASA funded this research under contract to the National Center for Microgravity Research and through Grant NAG 3 2521 to Rowan University. We gratefully acknowledge Prof. William Sirignano for the baseline numerical model used here, and Ron Mileto and Frank Zaccaro for help with the experimental apparatus. References 1. Kaptein, M., and Hermance, C. E., Proc. Combust. Inst. 16 (1976). 2. Feng, C. C., Lam, S. H., and Glassman, I., Combust. Sci. Technol. 10:59–71 (1975). 3. Leibman, I., Corry, J., and Perlee, H. E., Combust. Sci. Technol. 1:257–267 (1970). 4. Ishida, H., Fire Safety J. 13:115 (1988). 5. Liebman, I., Perlee, H., and Corry, J., Investigation of Flame Propagation Characteristics in Layered Gas Mixtures, U.S. Bureau of Mines 7078, Pittsburgh, PA, 1968.
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6. Schiller, D. N., Ross, H. D., and Sirignano, W. A., Combust. Sci. Technol. 118:205 (1996). 7. Patankar, S. V., Numerical Heat Transfer and Fluid Flow, McGraw-Hill, New York, 1980. 8. Miller, F. J., Easton, J. W., Marchese, A., and Ross, H. D., ‘‘Flame Spread Through Non-Homogeneous Gas Layers,’’ paper 005-039, Western States Section of the Combustion Institute, Golden, CO, March 13–14, 2000. 9. White, E. B., ‘‘Flame Propagation Through Fuel Vapor Concentration Gradients,’’ Master’s thesis, Case Western Reserve University, Cleveland, OH, 1997. 10. Miller, F. J., Easton, J., Marchese, A., Ross, H. D., Perry, D., and Kulis, M., ‘‘Gravitational Influences on Flame Propagation Through Non-Uniform Premixed Gas Systems,’’ paper 81, Sixth International Microgravity Symposium, Cleveland, OH, 2001. 11. Weinberg, F., Optics of Flames, Butterworths, Washington, DC, 1963. 12. Gardiner, W. C., Hidaka, Y., and Tanzawa, T., Combust. Flame 40:213–219 (1981).
GRAVITATIONAL EFFECTS ON FLAME SPREAD THROUGH NON-HOMOGENEOUS GAS LAYERS FLETCHER J. MILLER,1 JOHN W. EASTON,1 ANTHONY J. MARCHESE2 and HOWARD D. ROSS1 1
National Center for Microgravity Research NASA Glenn Research Center Cleveland, OH 44135-3191, USA 2 Rowan University 201 Mullica Hill Road Glassboro, NJ 08028-1700, USA
Flame propagation through non-uniformly premixed gases occurs in several common combustion situations. Compared with the more usual limiting cases of diffusion or uniformly premixed flames, the practical concern of non-uniform premixed gas flame spread has received scant attention, especially regarding the potential role of gravity. This research examines a system in which a fuel concentration gradient exists normal to the direction of flame propagation and parallel with the gravitational vector. This paper presents experimental and numerical results for flame spread through alcohol/air layers formed by diffusive evaporation of liquid fuel at temperatures between the flash-point temperature and the stoichiometric temperature. A gallery, which had either the top and/or one end open to maintain constant pressure, surrounded the test section. The numerical simulations and experiments conducted include normal and microgravity cases. An interferometer was used, in normal gravity only, to determine the initial fuel layer thickness and fuel concentration distribution before and during flame spread. Both the model and experimental results show that the absence of gravity results in a faster spreading flame, by as much as 80% depending on conditions. This is the opposite effect to that predicted by an independent model reported earlier in this symposium series. Determination of the flame height showed that the flame was taller in microgravity, an effect also seen in the results of the numerical model reported here. Having a gallery lid results in faster flame spread, an effect more pronounced at normal gravity, demonstrating the importance of enclosure geometry. The interferometry and numerical model both indicated a redistribution of fuel vapor ahead of the flame. Numerical simulations show that, despite the rapid flame spread in these systems, the presence of gravity strongly affects the overall flow field in the gallery.
Introduction Flame spread through uniformly premixed gas systems is a frequently studied problem in combustion science. In contrast, flame propagation through non-uniformly premixed gas systems (also called ‘‘layered systems’’) has been the subject of relatively few studies. Layered mixtures (see Fig. 1), however, are ubiquitous in terrestrial fire hazards, such as chemical spills, underground mining operations, and automobile and aircraft crashes. The flames in such systems have been shown to carry over fences, and to propagate past the ends of the fuel spill, thus representing a hazardous area beyond that associated with the original fuel location. They also are a potential fire hazard aboard long-duration spacecraft, such as the International Space Station, because flammable gases may accumulate near waste storage, laboratory fluids, fuel cells, lasers, etc. Our hypothesis is that gravity can influence flammability and the rate of flame propagation in a layered system in at least three ways: through a hydrostatic pressure gradient, through buoyantly induced
Fig. 1. Schematic of flame spread through a non-homogeneous mixture contained in a gallery (not drawn to scale). Darker areas indicate region of higher fuel vapor concentration near surface. Also labeled on the figure are the gallery size and boundary conditions used in the numerical model and experiment.
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flows during spread, or by affecting the initial distribution of fuel vapor. The first effect has been professed by previous researchers as important since agreement between predicted and observed flame spread rates in normal gravity tests improves with the inclusion of the hydrostatic pressure term in a simple model [1]. The second gravitational influence on the flame propagation in a stratified layer is the effect on the flows responsible for the redistribution of the fuel vapor, oxygen, and combustion products once a propagating flame is established. Previous research indicates that convective or aerodynamic effects can induce flows ahead of the flame that alter the fuel vapor concentration distribution and assist the flame spread [1–3]. The high propagation speeds in systems that maintain a stoichiometric concentration at some height in the layer suggest buoyant flow might not develop rapidly enough to affect spread; however, in the substoichiometric regime, this may not be the case. A third effect, how gravity affects the initial distribution of fuel from a source and how that affects flame spread, is relatively unexplored in the flame spread literature. In normal gravity, a leaking buoyant fuel (e.g., hydrogen) will rise from its source until it encounters a ceiling where it may collect and form a flammable layer. Just the opposite may occur with point sources of heavier-than-air fuels. In microgravity, the fuel would not be buoyant and instead will collect around its source or be redistributed by the slow (5–10 cm/s) flows due to ventilation systems and crew movements. These differences in fuel vapor distribution may lead to very different flame propagation behavior and thus different fire hazards. Review of Previous Research That Included Gravitational Effects Ishida showed convincingly in experiments that flame spread over a liquid-saturated fuel bed with the initial temperature (T0) less than the stoichiometric temperature (Tst) is affected by its orientation relative to the gravity vector [4]. If T0 ⬎ Tst, the flame appearance and velocity (Vf) are similar to a system comprising layered gases without a liquid surface; in this range, Vf is two to five times greater than the laminar burning velocity for a stoichiometric, homogeneous mixture. The motion can be driven by the expansion of the low-density products, which displaces and redistributes the unburned gas layers ahead of the flame into a broader, curved area [2]. The reported upstream distance covered by this disturbance ranges from a few centimeters [3] to 10– 15 cm [1]. Researchers at the Bureau of Mines investigated this problem both for heavy gases sitting on floors [5] and for lighter gases trapped under ceilings [3]. They reported that the flame speed depended on the
flammable zone thickness and the concentration gradients, and interestingly, that the flame propagated through regions that were below the lean flammability limit for homogeneous mixtures. They attribute this to ‘‘the proximity of the stabler [richer] burning mixture’’ [5]. In their data analysis of methane layers, they found that inclusion of gravitational effects through the Richardson number (which they defined as the ratio of buoyant forces tending to stabilize a layer to shear forces tending to mix it) was useful for the closed-gallery case. Extension of their correlation to 0 g predicts infinitely fast spread, which is not possible, but for 10ⳮ3g, the non-dimensional flame speed increase is approximately a factor of 2.6 over 1 g conditions. Feng et al. [2] treated both the open and bounded gallery configurations through experiments and modeling using a stream tube approach. The model of Feng et al., however, overpredicted by about 50% the Vf observed in Kaptein and Hermance’s experiments [1]. The latter authors demonstrated that agreement with their experimental results improved by modifying the Feng model to include a gravitational potential energy term (essentially hydrostatic pressure) accounting for the different heights of the combustible and burned gas layers. The Current Research In this paper, we present the results of experimentally and numerically determining flame spread rates in layered systems in normal and microgravity. In particular, we focus on cases where the maximum system equivalence ratio is stoichiometric or fuel lean prior to ignition. We achieved the fuel layers by allowing a liquid fuel to diffuse for a predetermined time upward into a gallery in normal gravity, and then igniting the layer in either normal or reduced gravity. Diffusion time and liquid temperature controlled the fuel layer thickness and concentration, respectively. Of the three reasons cited earlier regarding the influence of gravity on the flame spread, the first two are considered here; the effect of gravity on the initial fuel distribution could not be determined experimentally because of insufficient microgravity time for diffusion. The experiments were simulated using a two-dimensional, transient, chemically reacting flow numerical model. Experimental Apparatus A drop rig at the NASA Glenn Research Center 2.2 Second Drop Tower provided the platform for collecting both the normal and reduced gravity data presented here. As seen in Fig. 2, the rig contains an 80 cm long gallery with a 10 cm square cross section. The gallery has one Lexan and one aluminum sidewall with a removable Lexan top. The end
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in the drop tower, a delay of 0.1 s allows vibration transients to dampen, then the fuel/air mixture is ignited, and the flame spreads toward the open end. At the end of the test, the gallery is flooded with nitrogen and the frit lid closes as a safety precaution, extinguishing any flames remaining after impact. For normal gravity tests, the same procedure is followed, except a substitute drop signal begins the test. Numerical Model Fig. 2. Experimental flame spread gallery 80 cm long with 10 cm square cross section. The ignitor is a Kanthal wire stretched a few millimeters above the fuel surface across one end.
nearest the hot-wire igniter is closed, while the end farthest from the igniter is open, matching the boundary conditions of the numerical model. A bronze frit with a porosity of 20% acts as the floor of the gallery and is saturated with liquid fuel. A retractable lid covers the frit prior to a test, preventing fuel vapor from diffusing into the gallery. A fan on the open end exhausts vapors that accumulate during the filling process, but it is not on during the experiment. The frit is mounted to a movable stage that has water channels running through it to maintain the temperature of the fuel prior to the test. Two stainless steel sheathed, type T thermocouples embedded in the frit flush with its top surface near each end measure the initial temperature. Two CCD cameras image the frit surface from above at 30 frames/s, with each field-of-view overlapping in the center of the frit. Additionally, a mirror along the side of the gallery at 45⬚ provides a side view of the upper portion of the flame. Because of the time needed to move the drop rig into position at the top of the tower, the frit is slightly overheated and then allowed to cool to the desired test temperature while in the drop position. Once the fuel frit reaches the desired temperature as indicated by the two thermocouples (which are normally within 0.1 to 0.2 ⬚C of each other), the experimenter sends a signal to the computer on the drop rig. The computer then opens the frit lid, and begins counting a predetermined amount of time, ranging in these tests from 5 to 60 s, here referred to as the ‘‘diffusion time.’’ During this time, the fuel in the frit evaporates into the gallery, forming a non-homogeneous mixture with the air above the frit, while the flow generated by the lid rapidly decays. (We characterized the decay of the flow field by performing flow visualization tests using dry ice condensation trails. Within approximately 3 s, the motion caused by the lid removal could no longer be detected visually.) Note that all of the diffusion of fuel vapor occurs in normal gravity. After release into free fall
The flame spread experiments were simulated utilizing a two-dimensional, transient numerical model previously developed for studying flame spread across subflash liquids [6]. As detailed in Schiller et al. [6], the numerical model uses the SIMPLE algorithm [7] and a hybrid-differencing scheme to solve the two-dimensional gas-phase continuity, species, energy, and x–y momentum equations and the liquid-phase energy equation. For the major results discussed here—gravitational effects on flame spread rate, flame height, etc.—two dimensions are sufficient to provide a good comparison between the model and the experiment. As shown in Fig. 1, the liquid tray was modeled as an 80 cm long pool with a fuel depth of 2 mm. The thermal properties of the liquid were modified to account for the bronze frit by averaging the properties using the porosity as a weighting factor. The height of the gas phase above the liquid pool was 10 cm. A constant Schmidt number of 1.5 was used for all species. The rectangular numerical domain used in this study consisted of 112 grid points in the x-direction, with 82 grid points in the gas-phase y-direction and 32 grid points in the porous bronze y-direction. The grid is adaptive in the x-direction and follows the flame along the gallery. To simulate some of the experiments, the gallery was modeled as closed at the ignition end of the domain and open at the top and right-hand sides of the domain (see Fig. 1). The model initially ran with normal gravity for a specified time (the diffusion time) without introducing the ignition source. During this period, the time step was 5 ms. This allowed the fuel to vaporize at the pool surface and diffuse into the gas phase including the effect of gravity, setting up initial conditions consistent with experiments. The resulting fuel vapor distribution then became an input to the reacting case, for which the time step was 0.05 ms. Comparisons of the fuel vapor distribution just prior to the end of a 60 s diffusion period with those 1 s into the zero-gravity period showed no difference, indicating that the transition to 0 g does not affect the fuel vapor layer from the perspective of the model. Results and Discussion Using the test apparatus described above, we conducted experiments with propanol, methanol, and
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1 cm
Hf
Lean Limit
Frit Level Fig. 3. Side-view image of a flame spreading over npropanol at 27 ⬚C for a 60 s diffusion time in normal gravity (flame spread is left to right). The lean-limit height is the height of the flammable layer prior to ignition; Hf indicates the flame height measurement (Fig. 5).
Fig. 4. Flame spread rate as a function of diffusion time for two temperatures (27 ⬚C and 35 ⬚C) in normal and reduced gravity, for numerical and experimental cases. The diffusion times correspond to flammable layer thicknesses of 0.4, 0.7, and 1.2 cm at 27 ⬚C and 0.7, 1.3, and 2.2 cm at 35 ⬚C. The numerical model results are shown as lines; the experimental point error bars represent the high and low values obtained for each condition.
ethanol to determine the effect of initial temperature, diffusion time (i.e., layer thickness), and gravity level on the flame spread rate through layered systems. We chose these fuels because the numerical model had originally been developed using them [6], and because they have flashpoints in a convenient temperature range. This paper presents only the 1propanol results; ethanol results did not show as great an effect of gravitational level and are reported in Ref. [8], and for methanol, only 1 g tests have been completed [9]. For a variety of the experimental conditions, we used the numerical model to obtain comparative data regarding flame spread rate, flame height, fuel vapor concentration, and gasphase velocity, allowing comparisons with experiment to be given below.
A side-view image of a 1-propanol flame in 1 g is displayed in Fig. 3 to show the flame structure and size. The flame consists of two branches: an upper lean, premixed branch and a lower diffusion flame where fuel vapor and oxygen that penetrated the lean flame above meet and burn. As discussed below, the flame can burn much higher above the surface than the initial lean limit height as shown on the figure. Figure 4 shows the predicted and experimental flame spread rates for tests conducted in normal and microgravity at 27 ⬚C and 35 ⬚C with diffusion times of 5, 20, and 60 s. The flame spread rate was constant (i.e., not pulsating or acceleratory) in each experimental and numerical case studied, regardless of temperature or diffusion time. The model slightly over- or underpredicts the flame spread rate depending on the values of the kinetic constants that are chosen. In this work, we used an activation energy of 25,500 cal/gmol, and a pre-exponential constant of 9.38 ⳯ 1011 m3/(kmol s), which gave approximate agreement with the experiments over the temperature range we tested. We have not found one set of pre-exponential factor and activation energy that are valid at all temperatures for this simple, single-step model. Therefore, rather than seeking to use the model to obtain absolute flame spread rate predictions, it is more appropriate to use it to explore some of the other features and trends of the process. Each experimental point on the graph represents an average of at least four microgravity tests (and sometimes five), and two normal gravity tests, with the error bars indicating the highest and lowest values for each condition. The flame spread rate increases markedly with increasing temperature, due to the increase in vapor pressure of the fuel. It is well known for uniformly premixed gases that small increases in the equivalence ratio of lean mixtures dramatically increase the flame speed, and the same effect occurs here. There is some variation in the data that we have been unable to eliminate, perhaps owing to being at slightly different temperatures from test to test. The variability may also be due to how saturated the frit is with fuel when we commence the test, a factor we try to control but with difficulty. An increase in diffusion time also led to a slight increase in flame spread rate between 5 and 20 s, with the effect being most pronounced at 35 ⬚C. This effect is more evident in reduced gravity than in normal gravity. At longer times, neither the model nor the experiments show much effect of diffusion time. The fact that the effect is small indicates that it is the maximum fuel concentration in the layer (as determined by the temperature), and not the layer thickness (as determined by the length of the diffusion time) that most influences how fast the flame will spread. Note that this may not be true for very thin layers (on the order of the quench distance), but our apparatus requires diffusion times of at least
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Fig. 5. Flame height as a function of diffusion time (or lean-limit layer height as shown on the upper axis) at 35 ⬚C for normal and reduced gravity, experiment and numerical results. Experimental results are with gallery lid on, while numerical results are with gallery lid off.
5 s to allow air motion induced by the lid to cease, which limited us to thicker layers. The gravitational influences are perhaps the most significant results presented here because they have not been explored previously. Once averaged, the results clearly show a large effect of gravity level, with the flame spreading faster in microgravity, especially at longer preignition diffusion times. This is true regardless of the presence of a lid on the gallery, which has the effect of increasing the flame spread rate in both 1 g and microgravity, most likely by channeling gas expansion in the direction of flame spread [10]. At 27 ⬚C, for a 60 s diffusion time, the microgravity (lg) flame spread 81% faster with the lid off (shown), and 63% faster with it on, compared to 1 g. At 35 ⬚C, the 1 g/lg flame spread rates were much closer, varying by about 15% with the lid off, and being essentially identical with the lid on. Even with the cautions expressed earlier about the kinetic constants, the model captures the trends of increased speed at microgravity and little increase in speed with diffusion time. Gravitational level also affects the height of the flame above the fuel source. Fig. 5 shows the height of the flame, measured behind the flame front (where it becomes essentially horizontal; see Fig. 3), as a function of diffusion time for both normal and reduced gravity at 35 ⬚C. The flame height in reduced gravity is larger than the corresponding normal gravity case for both the experimental and numerical results, although the model predicts a larger gravitational effect. The most likely causes of this gravitational effect are the two factors, change in the hydrostatic pressure gradient and buoyant induced flows, described earlier. Without the hydrostatic
Fig. 6. Fuel mole fraction contours at 35 ⬚C and 60 s diffusion time as the flame passes. (a) Measured with interferometry; (b) numerically predicted. In both cases the upward movement of the fuel vapor due to the flame can be seen. The lean limit is 2.2% for 1-propanol.
pressure present, the low-density products can expand further upward. In microgravity, there is also no buoyant flow that brings air to the flame so that the flame must rise higher off the surface as it seeks oxygen. Fig. 5 also demonstrates the effect of fuel layer thickness. For both the normal and reduced gravity experimental cases, the flame height increased with thicker layers. The model predicts a leveling off of the flame height with layer thickness in both cases, an effect not seen in the experiments. Results from the measurement of the fuel layer thickness in normal gravity using a Michelson interferometer demonstrate how the flame front redistributes the upstream fuel vapor [9,10]. Prior to ignition, the mole fraction contours are essentially horizontal lines (except near the ends of the gallery where vapor spillover occurs). In Fig. 6a, we show the fuel mole fraction as the flame passes, deduced from interferometric measurements for a 1 g case. The molar refractivity of 1-propanol was not directly
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Fig. 7. Numerically predicted U component of velocity in laboratory coordinates at 35 ⬚C. (a) 1 g; (b) microgravity. The white lines represent the U component of velocity 1 mm above the fuel surface (shown on the right-hand axis), the location of the maximum velocity.
available to us, but we calculated a value of 18 on the basis of the method reported in Ref. [11] and used this for the interpretation of the interferograms. The value of 18 also agrees well with extrapolated values of methanol/methane and ethanol/ethane given in Ref. [12]. As can be seen, the flame pushes the fuel vapor upward as it approaches, increasing the effective flammable layer thickness. Because of the limited field of view, we did not record the flame burning below the lean flammability limit of 2.2% for 1-propanol, although we have seen this effect for methanol and ethanol [9]. The work of Ref. [3] reports a similar displacement of fuel for methane layers along ceilings. In Fig. 6b we show the predicted fuel mole fraction after ignition for a 1 g case. The agreement is good between model and experiment, with both showing roughly the same flammable layer thickness and the effect of fuel layer displacement by the flame. This plot also shows the predicted and observed double flame (see Fig. 3). (The diffusion flame is the dark line along the surface in Fig. 6b.) As shown in Fig. 7, the modeling results verify the hypothesis that aerodynamic effects induce flows ahead of the flame, thereby altering the fuel vapor concentration distribution and assisting the flame spread. Specifically, the model shows that expansion of the hot gas upstream of the propagating flame creates a convective velocity (Fig. 7) in the direction
of the propagating flame. This increased velocity can carry the flame at speeds that in laboratory coordinates well exceed the laminar burning velocity. Furthermore, behind the flame front there is a much larger flow velocity away from the flame in microgravity (see dark region) due to a higher reaction rate. This thrust drives the flame forward faster than at normal gravity. Of the three effects of gravity we identified earlier, the flow field changes due to buoyancy seem most likely to be responsible for the gravitational effect seen here. Up to the stoichiometric temperature, the predicted difference in velocity at the flame front between 1 g and 0 g exceeds the laminar burning velocity (see lines in Fig. 7), so that buoyancy appears to have an effect even at the flame leading edge. In addition, numerical simulations (not shown) predict that in normal gravity flow induced from buoyancy well behind the flame affect the flame spread by changing the global flow pattern in the gallery. Using the numerical model, we have been unable to identify any systematic change in the pressure field due to the flame spread in normal vs. microgravity. Although the simple model of Kaptein and Hermance predicts an effect of hydrostatic pressure due to gravity, it is opposite to that observed here [1]. In their accounting for the hydrostatic pressure change the flame should spread faster in normal gravity by up to 50%. Their model did not, however, include any flow field effects, which may be stronger than any pressure differences.
Summary and Conclusions We have shown that flame spread through nonhomogeneous gas layers is faster in microgravity than in 1 g for every condition we tested, up to 80% faster in the most extreme case. Both experiments and a numerical model have demonstrated this phenomenon, which is the opposite trend to that surmised in Ref. [1] via a simple analytical model. Flame heights were also higher in microgravity than in 1 g. We obtained good agreement in the fuel vapor mole fraction predictions and measurements in 1 g. The numerical model showed little difference between 1 g and microgravity predictions of the reactant mixture concentrations. Therefore, the faster flame spread rate in microgravity is likely not due to a different redistribution of flammable gases ahead of the flame. (Microgravity experiments are needed to verify this fully.) Although we don’t yet have flow field measurements, the model indicates a substantial difference in the velocity field around the spreading flame depending on the gravitational level. Therefore, it appears that modification of this field by buoyancy is responsible for the difference in flame spread rate between 1 g and microgravity.
FLAME SPREAD THROUGH NON-HOMOGENEOUS GAS LAYERS Acknowledgments NASA funded this research under contract to the National Center for Microgravity Research and through Grant NAG 3 2521 to Rowan University. We gratefully acknowledge Prof. William Sirignano for the baseline numerical model used here, and Ron Mileto and Frank Zaccaro for help with the experimental apparatus. References 1. Kaptein, M., and Hermance, C. E., Proc. Combust. Inst. 16 (1976). 2. Feng, C. C., Lam, S. H., and Glassman, I., Combust. Sci. Technol. 10:59–71 (1975). 3. Leibman, I., Corry, J., and Perlee, H. E., Combust. Sci. Technol. 1:257–267 (1970). 4. Ishida, H., Fire Safety J. 13:115 (1988). 5. Liebman, I., Perlee, H., and Corry, J., Investigation of Flame Propagation Characteristics in Layered Gas Mixtures, U.S. Bureau of Mines 7078, Pittsburgh, PA, 1968.
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6. Schiller, D. N., Ross, H. D., and Sirignano, W. A., Combust. Sci. Technol. 118:205 (1996). 7. Patankar, S. V., Numerical Heat Transfer and Fluid Flow, McGraw-Hill, New York, 1980. 8. Miller, F. J., Easton, J. W., Marchese, A., and Ross, H. D., ‘‘Flame Spread Through Non-Homogeneous Gas Layers,’’ paper 005-039, Western States Section of the Combustion Institute, Golden, CO, March 13–14, 2000. 9. White, E. B., ‘‘Flame Propagation Through Fuel Vapor Concentration Gradients,’’ Master’s thesis, Case Western Reserve University, Cleveland, OH, 1997. 10. Miller, F. J., Easton, J., Marchese, A., Ross, H. D., Perry, D., and Kulis, M., ‘‘Gravitational Influences on Flame Propagation Through Non-Uniform Premixed Gas Systems,’’ paper 81, Sixth International Microgravity Symposium, Cleveland, OH, 2001. 11. Weinberg, F., Optics of Flames, Butterworths, Washington, DC, 1963. 12. Gardiner, W. C., Hidaka, Y., and Tanzawa, T., Combust. Flame 40:213–219 (1981).