Characteristics of Laminar Diffusion Flames in a Quiescent Microgravity Environment

Transcription

1 Paper # 070LT-0365 Topic: Laminar Flames 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Characteristics of Laminar Diffusion Flames in a Quiescent Microgravity Environment Fumiaki Takahashi 1*, Nicholas Hennigan 2, Dennis P. Stocker 3, Paul V. Ferkul 1, Viswanath R. Katta 4 1 National Center for Space Exploration Research, Cleveland, OH NASA Undergraduate Student Research Program (USRP) 3 NASA Glenn Research Center, Cleveland, OH Innovative Scientific Solutions, Inc., Dayton, OH Burning characteristics of low-speed (~1 mm/s order) methane and ethylene laminar diffusion flames formed over a porous cup burner in a quiescent microgravity environment were studied experimentally and computationally. The experiment was conducted at the NASA 2.2 s drop tower. The video images showed that the luminosity of the hemispherical blue flame zone peaked near the flame base (edge) and gradually decreased toward the centerline. The temporal radiometer output signal reflected the transient flame size variation and captured the partial flame extinction. A time-dependent, axisymmetric, full-chemistry computation with radiation and transport processes was performed to reveal the diffusion flame structure. The calculated heat-release rate peaked in the flame base consistent with the observed flame luminosity. Although the initial fuel injection velocity was small, the flow accelerated by thermal expansion passed through the flame zone. Therefore, the calculated flame zone continued to expand outwardly, except for very low fuel injection velocity cases, for which nearly stationary flames were obtained. 1. Introduction The Burning Rate Emulator (BRE) experiment [Zhang et al., 2013] is in development to be conducted on board the International Space Station to improve spacecraft fire safety. The purpose of the BRE experiment is to gain fundamental understanding of spacecraft materials flammability by simulating condensed fuels combustion using gaseous fuels in a quiescent microgravity ( g) environment. By using fuels that share similar properties, gaseous flames can emulate the burning of condensed fuels such as paper, plastic, and alcohol. Currently, the selection of materials intended for use in the habitable environments of US spacecraft is based on pass-or-fail flammability testing based on upward flame spread in a quiescent normal earth-gravity (1g) environment [Anon., 1998]. However, microgravity testing has shown that some fuels burn in microgravity but not in normal gravity. Laminar diffusion flames over a porous plate, through which a gaseous fuel is injected, have long been used to simulate the gas phase part of condensed fuel burning experimentally, analytically, and numerically [Emmons, 1956; de Ris, 1969; Hirano et al., 1974; Ohki and Tsuge, 1974; Fernandez-Pello and Williams, 1977; Wichman and Williams, 1983; Quintiere, 1981; Ramachandra and Raghunandan, 1984; Chen and T ien, 1986]. Fires on Earth are always associated with surrounding air flow (wind or natural convection) and, in spacecraft, there is an air current (typically up to 20 cm/s) due to the environmental control and life support system. Therefore, the majority of investigations in the past have considered flame-flow interactions (e.g., opposed or concurrent flame spread), while studies on flames in a purely quiescent g environment are limited [Ross, 2001]. Brahmi et al. [2005] burned ethane in a porous burner with relatively low-velocity oxidizer flow ( mm/s) in g and observed three flame shape regimes: elliptical, parabolic, and flat flames. * Corresponding author. fumiaki.takahashi-1@nasa.gov

2 In a quiescent g environment, incipient fires (or diffusion flames) are weak because the relatively fast buoyancy-driven convective flow is removed and only the slow diffusional transport processes remain. The flame on a thick sheet or rod tends to self-extinguish under quiescent g conditions [Olson, 1991; Egorov et al., 1995; Ivanov, et al., 1999]. The only condensed fuels observed to burn in a quiescent g environment for an extensive period of time are thin solid sheets, fuel droplets, and candles [Dietrich et al., 2000; Ross, 2001]. In support of the BRE experiment, this paper reports the characteristics of methane and ethylene laminar diffusion flames formed over a porous cup burner in a quiescent g environment. The experiment was conducted at the NASA Glenn 2.2 s Drop Tower, and the computation was performed using a time-dependent, axisymmetric code with full chemistry, radiation, and transport processes. Of particular interest is the effect of very low fuel velocity (~1 mm/s order) on the flame behavior and whether or not a stationary and steady gaseous flame can be formed in a quiescent g environment. 2. Experimental Method The 2.2-s Drop Tower experimental apparatus consists of a cylindrical test chamber (25.5 cm inner diameter x 53.3 cm length, open to the atmosphere on top), in which a cup burner is coaxially positioned. The apparatus includes a gas flow system, a control/data acquisition system, a radiometer, and an imaging system. The stainless-steel cup burner (28 mm i.d., 31 mm o.d., 45º-chamfered inside the rim) is equipped with a flat stainlesssteel mesh screen at 3 mm below its exit plane. To speed up the initial warm-up process, the cup-burner can be preheated with an electric heating disk (29 AWG Kanthal wire, 39 cm length, wrapped around a mica sheet; maximum power: 43 W) installed underneath the cup burner head. The heating element is connected to relay switch terminals of a temperature controller (Omega CN R1-R2-F3-RSP-LV ), which adjusts the burner temperature in response to a K-type thermocouple (Omega CO2-K, mm wire diameter) attached to the burner outer surface 0.5 mm below the burner tip. The burner temperature was set at 348 K (75 C) and 473 K (200 C) in addition to the room temperature (no heating) case. The fuel (high purity methane or ethylene) flow rate is measured by a critical orifice (O Keefe, type BLP, size 1E) which is calibrated and has uncertainty within 1 % of indicated flow. A generally hemispherical and blue flame was formed above the burner. The fuel flow rate was varied as follows: methane: 50 ccm, 100 ccm, and 120 ccm at 294 K (which correspond to mean fuel speeds of 1.35 mm/s, 2.71 mm/s, and 3.25 mm/s at 294 K, respectively); and ethylene: 13.5 ccm, 23.3 ccm, 37.7 ccm, 53.3 ccm and 60 ccm at 294 K (0.37 mm/s, 0.63 mm/s, 1.02 mm/s, 1.44 mm/s, and 1.62 mm/s at 294 K, respectively). The experimental control and data acquisition system consists of an onboard controller (National Instruments CompactRIO) and a wirelessly connected laptop computer, running LabVIEW software. Data signals from thermocouples, pressure transducers, an accelerometer (Crossbow, CXL02TG3-S), and a thermopile radiometer (Dexter, ST150, nitrogen filled, KRS-5 window) were digitized and recorded at 15 Hz. The radiometer is placed approximately at the burner exit plane with its optical axis perpendicular to the burner axis. The fuel was pre-flowed (3 s to7 s) to establish a steady flow rate and ignited by activating a hot-wire igniter (coiled 29- gauge Kanthal, 11 cm length) for < 0.3 s immediately after the microgravity condition was achieved. A rotary solenoid is used to position the igniter near the burner for ignition and away from it otherwise. The flame behavior was observed using two CCD video color cameras (Prosilica/AVT Model GC1390 color gigabit Ethernet camera with a Navitar 7000 zoom lens and Hitachi model KP-D20b NTSC analog camera). Camera output signals are fed through hardwire to fiber-optic media converters, then transmitted via the multi-channel drop fiber to the digital recording workstation (HP Z800, running 64-bit Window 7 and Norpix StreamPix 5 video recording software), placed on the top floor of the drop tower. The signals are converted back to hardwire by the appropriate media converter and connected to the recording workstation inputs. Certain commercial equipment, instruments, or materials are identified in this paper to adequately specify the procedure. Such identification does not imply recommendation or endorsement by NASA, nor does it imply that the materials or equipment are necessarily the best available for the intended use. 2

3 3. Computational Method A time-dependent, axisymmetric numerical code (UNICORN) [Katta and Roquemore, 1994; Roquemore and Katta, 2000] is used for the simulation of laminar diffusion flames established over the cup burner in zero-gravity (0g). The code solves the axial and radial (z and r) full Navier-Stokes momentum equations, continuity equation, and enthalpy- and species-conservation equations on a staggered-grid system (301 x 151). A clustered mesh system is employed to trace the gradients in flow variables near the flame surface. The thermo-physical properties such as enthalpy, viscosity, thermal conductivity, and binary molecular diffusion of all of the species are calculated from polynomial curve fits developed for the temperature range 300 K to 5000 K. Mixture viscosity and thermal conductivity are then estimated using the Wilke and Kee expressions, respectively. Molecular diffusion is assumed to be of the binary-diffusion type, and the diffusion velocity of a species is calculated using Fick's law and the effective-diffusion coefficient of that species in the mixture. A simple radiation model based on the optically thin-media assumption was incorporated into the energy equation. Only radiation from CH 4, CO, CO 2, and H 2 O was considered in the present study. The finite-difference forms of the momentum equations are obtained using an implicit QUICKEST scheme [Katta and Roquemore, 1994], and those of the species and energy equations are obtained using a hybrid scheme of upwind and central differencing. The fourcarbon hydrocarbon reaction mechanism of Wang and co-workers [Wang et al., 2007; Sheen et al., 2009] (110 species and 1568 one-way elementary reactions) is integrated into the UNICORN code for the simulation of methane and ethylene flames in air (21 % O 2 in N 2 ). The boundary conditions are treated in the same way as that reported in earlier papers [Takahashi et al., 2008]. The computational domain is bounded by the axis of symmetry, a chimney wall, and the inflow and outflow boundaries. The burner inner diameter is 28 mm, outer diameter is 30.8 mm and the chimney inner diameter is 96 mm. The fuel is injected from the burner surface at 3 mm below the burner exit plane. The burner wall surface is under the no-slip velocity condition. The fuel flow rate was varied to compare to the experiment as follows: methane: 50 ccm at 294 K (which converts to the mean fuel velocity of 1.35 mm/s at 294 K and 2.18 mm/s at 473 K) and 120 ccm at 294 K (3.25 mm/s at 294 K); and ethylene: 23.3 ccm at 294 K (0.63 mm/s at 294 K), 37.3 ccm at 294 K (1.02 mm/s at 294 K and 1.64 mm/s at 473 K), and 60.0 ccm at 294 K (1.62 mm/s at 294 K). The surrounding is nearly still air (0.1 mm/s at 294 K) for all cases. 4. Results and Discussion 4.1 Experiment Figure 1 shows the video images of methane and ethylene laminar diffusion flames in g at the elapsed time after drop of t = 1.9 s. For a low methane flow rate (Fig. 1a), a stable, convex disk shaped, blue flame was formed immediately after an initial flash at ignition, and the flame base (edge) rose approximately 5 mm above the burner rim until the impact at t = 2.2 s. For lower fuel flow rates (50 ccm and 100 ccm) in 1g (not shown), a blue flame was unstable (oscillating) if the burner temperatures was low (no heating or 75 C). For higher methane flow rates (100 ccm [Fig. 1b] and 120 ccm [not shown]) without burner heating in g, a large portion of the base of the hemispherical blue flame partially lifted. The extinguished area expanded to the top portion of the flame (Fig. 1b). For higher burner temperatures (75 C [not shown] or 200 C [Fig. 1c]), the flame was stable. The luminosity of the blue flame zone was generally highest at the flame base and decreased toward the central region around the axis. A stable ethylene flame in quiescent g was formed at lower fuel flow rates (23.3 ccm [Fig. 1d] and 37.7 ccm [Fig. 1e]). At the lowest flow rate (13.5 ccm [not shown]), a stable flame was obtained only once in four drops. As the ethylene flow rate was increased, the size (height) of the flame increased, and the soot formed at ignition tended to remain inside the blue flame until the impact at t = 2.2 s (Fig. 1f). Figure 2 shows the temporal variations in the radiometer output signals from methane and ethylene laminar diffusion flames in a quiescent g environment. The signal generally varies with the hot radiating gas temperature and volume in the flame. The background signal from the heated burner is subtracted from the measured values to determine the thermal radiation from the flame (and the heated igniter during the initial period t < 0.5 s, which is not used). For methane flames (Fig. 2a), the radiometer output shows distinct behaviors between no burner heating and heating (75 C and 200 C) cases. If the burner was cold, the radiometer signal continued to decrease due to the heat loss from the flame to the burner. For higher fuel flow rates (100 ccm and 120 ccm) with no active heating, the cooling by the burner led to partial detachment of the flame base. The radiometer captured this partial extinguishment event revealed by the video observation (Fig. 1b). The radiometer responses for 75 C and 200 C heater settings are quite similar. For 50 ccm 3

4 ( a ) ( b ) ( c ) ( d ) ( e ) ( f ) Figure 1. Video images (enhanced intensity) of laminar diffusion flames in a quiescent g environment (t = 1.9 s). Fuel: (a c) methane, (d f) ethylene. Fuel flow rate (at 21.1 C) and the burner heater setting: (a) 50 ccm, heater off, (b) 120 ccm, off, (c) 120 ccm, 200 C, (d) 23.3 ccm, off, (e) 37.7 ccm, off, (f) 60 ccm, 200 C. ( a ) ( b ) Figure 2. Temporal variations in the radiometer output signals from laminar diffusion flames in a quiescent g environment. (a) Methane, (b) ethylene. fuel flow rate, the signals maintained at a nearly constant level (steady state), while for 100 ccm and 120 ccm, it continued to increase as the flame (or a hot gas volume) grew larger. The preheating of the burner sped up the initial transient processes, in which the flame responded to the heat transfer (loss) to the burner. The results for ethylene flames (Fig. 2b) exhibited essentially the same trend, except that no partial extinction was observed. 4

5 Figure 3 shows a correlation for the radiometer output levels from methane and ethylene flames in a quiescent g environment at t = 1.9 s (except for methane flow rates of 100 ccm [t = 1.2 s] and 120 ccm [t = 1.0 s]). For the abscissa, the fuel flow rate was converted to the rate of oxygen required to accomplish stoichiometric combustion by multiplying the fuel flow rate and the stoichiometric coefficient (2 for CH 4 and 3 for C 2 H 4 ). All data points lie on a single line, indicating that the radiometer detected the radiant heat flux from the flame (hot gasses), which is proportional to the oxygen consumption rate. If the diffusion flame zone can consume the incoming fuel and oxygen completely at a fixed location, a stationary flame can be obtained. A threshold below which the flame becomes stationary (with burner heating) seems to be quite small (~70 ccm expressed in the stoichiometric O 2 requirement). 4.2 Computation Figure 4 shows the calculated temperature (left half) and the OH mole fraction (right half) in methane (1.35 mm/s) and ethylene (1.02 mm/s) flames in a quiescent 0g environment. The elapsed time after ignition is ~0.5 s. Each fuel (at 294 K) was pre-flowed for 0.3 s and ignited near the burner rim (maintained at 294 K). The incipient Figure 3. Radiometer output from CH 4 and C 2 H 4 flames at t = 1.9 s (except for methane, 100 ccm [t = 1.2 s] and 120 ccm [t = 1.0 s]) as a function of the stoichiometric O 2 requirement for the fuel flow rate. flame propagated toward the centerline and formed a button-shaped flame, which then expanded downstream to form a nearly-stationary hemi-ellipsoidal flame (Fig. 4). For larger fuel flow rates, the flame continued to grow. Unlike the experimentally observed dome-shaped or hemispherical flames (Fig. 1), the flame base extended outside and below the burner rim. Figure 5 shows the calculated velocity, temperature, and heat-release rate fields in methane and ethylene flames. The ( a ) ( b ) Figure 4. Calculated flame structures of laminar diffusion flames in a quiescent 0g environment (left half: temperature, right half: OH mole fraction). Elapse time after ignition: ~0.5 s. (a) Methane, 1.35 mm/s; (b) ethylene, 1.02 mm/s. 5

6 temperature and the OH mole fraction (Fig. 4) were higher in the central region. By contrast, the heat-release rate contours show a peak reactivity spot (the reaction kernel [Takahashi and Katta, 2000]) in the flame base, and the value decreased toward the central region. This result describes the experimental observation that the luminosity of the blue flame zone was highest in the base region and decreased toward the centerline (Fig. 1). The peak heat-release rate values (CH 4 : 11.4 W/cm 3, C 2 H 4 : 13.4 W/cm 3 ) in quiescent 0g flames were an order-of-magnitude smaller than corresponding 1g flames. It is notable that the peak value previously calculated (CH 4 : 10.1 W/cm 3 [Takahashi and Katta, 2002]) for a spherical jet diffusion flame in quiescent 0g using a much smaller reaction mechanism (24 species and 81 elementary steps) was comparable to the present full-chemistry computation. Although the initial fuel injection velocity was very small, the flow (of the initial fuel, fuel fragments, and products) accelerated rapidly by thermal gas expansion passed through the flame zone, particularly in the central region. If the flame temperature decreases and the leakage of the reactants increase, extinction may occur. However, the heat-release rate contours and isotherms showed a continuous high-temperature reaction zone and thus the flame tip was not open. The combined diffusion-convection fuel flux must be counterbalanced by the oxygen flux by diffusion in the opposite direction. If a subtle balance between the fluxes of the fuel (fragments) and the oxygen at the stoichiometric proportion is obtained at a fixed location, the flame becomes stationary. Otherwise, the diffusion flame zone continues to move passively. Even if the flame becomes stationary, the diffusion processes would continue indefinitely in a pure quiescent 0g environment. 5. Summary ( a ) ( b ) Figure 5. Calculated velocity, temperature, and heat-release rate of laminar diffusion flames in a quiescent 0g environment. Elapse time after ignition: ~0.5 s. (a) Methane, 1.35 mm/s; (b) ethylene, 1.02 mm/s. Hemispherical (or hemi-ellipsoidal) laminar diffusion flames over a porous cup burner in a quiescent g environment were formed experimentally and computationally. The flame zone generally expanded outwardly, except for very low fuel injection velocities, for which the flame zone approached a stationary condition in a relatively short period (seconds). The heat transfer (loss) to the burner affected the behavior of the flames studied. Thus, preheating the burner is important for flame stability and helps speeding up the initial transient processes. Although the calculated temperature and the radical mole fractions were highest at the centerline, the heat-release rate peaked at the flame base and decayed toward the central region, thereby explaining the observed blue flame luminosity variation. The peak reactivity spot (reaction kernel) in the flame base supports the trailing diffusion flame even under the low-speed 0g conditions. Differences in the flame shape between the observation and computation need to be investigated further in the future. 6

7 Acknowledgements This research was supported by the NASA Space Life and Physical Sciences Research and Applications Division (SLPSRA). The authors thank Drs. J.G. Quintiere and P.B. Sunderland (University of Maryland) for fruitful discussion. Assistance by J. Owens, J.E. Rymut, A.L. Ogorzaly, and E.S. Neumann in conducting the drop tower experiment is acknowledged. References Anon. (1998). Flammability, odor, offgassing, and compatibility requirements and test procedures for materials in environments that support combustion, NASA-STD-6001, February. Brahmi, L, Vietoris, T., Rouvreau, S., Joulan, P., David, L., and Torero, J.L. (2005). Microgravity laminar diffusion flame in a perpendicular fuel and oxidizer streams configuration. AIAA Journal 43 (8) Chen, C.H., and T ien, J.S. (1986). Diffusion flame stabilization at the leading edge of a fuel plate. Combustion Science and Technology 50, de Ris, J.N. (1969). Spread of a laminar diffusion flame. Proceedings of the Combustion Institute, Vol. 12, 241. Dietrich, D.L., Ross, H.D., Shu, Y., Chang, P., and T ien, J.S. (2000). Candle flames in non-buoyant atmospheres. Combustion Science and Technology 156, 1-24 Egorov, S.D., Belayev, A.Yu., Klimin, LP., Voiteshonok, V.S., Ivanov, A.V., Semenov, A.V., Zaitsev, E.N., Balashov, E.V., and Andreeva, T.V. (1995). Fire safety experiments on Mir orbital station, Third International Microgravity Combustion Workshop, NASA CP-10174, pp Emmons, H. (1956). The film combustion of liquid fuel. Z. Angew Math Mech. 36, Fernandez-Pello, A.C., and Williams, F.A. (1977). A theory of laminar flame spread over flat surfaces of solid combustibles. Combustion and Flame 28, 251. Hirano, T., Iwai, K., and Kanno (1974). Measurement of the velocity distribution in the boundary layer over a flat plate with a diffusion flame. Astronautica Acta 17, Ivanov, A.V., Balashov, Ye.V., Andreeva, T.V., and Meilikhov, A.S. (1999). Experimental flammability in space. NASA CP Katta, V.R., Goss, L.P. and Roquemore, W.M. (1994). Numerical investigations of transitional H 2 /N 2 jet diffusion flames. AIAA Journal 32, 84. Ohki, Y., and Tsuge, S. (1974). On flame spreading over a polymer surface. Combustion Science and Technology 9, 1. Olson, S. L. (1991). Fuel thickness effects on flame spread and extinction limits in low gravity as compared to normal gravity. Eastern States Section/The Combustion Institute Meeting. Quintiere, J.G. (1981). A simplified theory for generalizing results from a radiant panel rate of flame spread apparatus. Fire and Materials 5, 52. Ramachandra, A., and Raghunandan, B.N. (1984). Investigation on the stability and extinction of a laminar diffusion flame over a porous flat plate. Combustion Science and Technology 36, Roquemore, W.M., and Katta, V.R., (2000). Role of flow visualization in the development of UNICORN. Journal of Visualization 2, Ross, H.D. (2001). Microgravity combustion: fire in free fall. Academic Press, San Diego, CA. Sheen, D.A, You, X.Q., Wang, H., and Lovas, T. (2009). Spectral uncertainty quantification, propagation and optimization of a detailed kinetic model for ethylene combustion. Proceedings of the Combustion Institute 32, Takahashi, F., and Katta, V.R. (2000). A Reaction kernel hypothesis for the stability limit of methane jet diffusion flames. Proceedings of the Combustion Institute 28, Takahashi, F., and Katta, V.R. (2002). Reaction kernel structure and stabilizing mechanisms of jet diffusion flames in microgravity, Proceedings of The Combustion Institute 29, Takahashi, F., Linteris, G.T., and Katta, V.R. (2008). Extinguishment of methane diffusion flames by carbon dioxide in coflow air and oxygen-enriched microgravity environments. Combustion and Flame 155, Wang, H., You, X., Jucks, K.W., Davis, S.G., Laskin, A., Egolfopoulos, F., and Law, C.K. (2007). USC Mech Version II. High temperature combustion reaction model of H 2 /CO/C 1 C 4 compounds, available at < University of Southern California, Los Angeles, CA. Wichman, I.S., and Williams, F.A. (1983). A simplified model of flame spread in an opposed flow along a flat surface of a semi-infinite solids. Combustion Science and Technology 32, 91. Zhang, Y., Bustamante, M.J., Sunderland, P.B., Quintiere, J.G., and Ferkul, P. (2013). A burning rate emulator for study in micro-gravity. Proceedings of the 8 th U.S. National Combustion Meeting, Park City, UT, May. 7