Elsevier Editorial System(tm) for Combustion and Flame Manuscript Draft

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1 Elsevier Editorial System(tm) for Combustion and Flame Manuscript Draft Manuscript Number: Title: Extremely weak hydrogen flames Article Type: Short Communication Keywords: microcombustors; microflames; quenching limits Corresponding Author: Prof. Peter B Sunderland, Ph.D. Corresponding Author's Institution: University of Maryland First Author: Vivien R Lecoustre, Ph.D. Order of Authors: Vivien R Lecoustre, Ph.D.; Peter B Sunderland, Ph.D.; Beei H Chao, Ph.D.; Richard L Axelbaum, Ph.D. Abstract: Hydrogen jet diffusion flames were observed near their quenching limits. These involved downward laminar flow of hydrogen from a stainless steel hypodermic tube with an inside diameter of 0.15 mm. Near their quenching limits these flames had heat release rates in air and in oxygen of just 0.46 and 0.25 W, respectively (i.e., hydrogen flow rates of 3.9 and 2.1 μg/s). To the authors' knowledge, these are the weakest self-sustaining steady flames ever observed. Suggested Reviewers: Yuji Nakamura Ph.D. Associate Professor, Hokkaido University yuji-mg@eng.hokudai.ac.jp Paul Ronney Ph.D. Professor, Aerospace and Mechanical Engineering, University of Southern California ronney@usc.edu Kozo Saito Ph.D. Professor, Mechanical Engineering, Univ. of Kentucky saito@engr.uky.edu Carlos Fernandez-Pello Ph.D. Professor, Mechanical Engineering, Univ. of California, Berkeley ferpello@me.berkeley.edu

2 Manuscript Click here to view linked References Extremely weak hydrogen flames V.R. Lecoustre a, P.B. Sunderland a, *, B.H. Chao b, R.L. Axelbaum c a Department of Fire Protection Engineering, University of Maryland, College Park, MD 20742, USA b Department of Mechanical Engineering, University of Hawaii, Honolulu, HI 96822, USA c Department of Energy, Environmental and Chemical Engineering, Washington University, St. Louis, MO 63130, USA Submitted to: Combustion and Flame Article Type: Brief Communication Submission Date: April 7, 2010 Word count: 1402 (limit is 1500) Body: 1002 Table 1: 200 Figure 1: 200 *Corresponding Author: Peter B. Sunderland Dept. of Fire Protection Engineering University of Maryland 3104 J.M. Patterson Building College Park MD USA tel (301) fax (301) pbs@umd.edu

3 Abstract Hydrogen jet diffusion flames were observed near their quenching limits. These involved downward laminar flow of hydrogen from a stainless steel hypodermic tube with an inside diameter of 0.15 mm. Near their quenching limits these flames had heat release rates in air and in oxygen of just 0.46 and 0.25 W, respectively (i.e., hydrogen flow rates of 3.9 and 2.1 µg/s). To the authors knowledge, these are the weakest self-sustaining steady flames ever observed. 1. Introduction This study is motivated by a concern of fire hazards associated with small leaks in hydrogen systems and the use of microcombustors for power generation. Butler et al. [1] examined the fire hazards of small hydrogen leaks. They observed quenching limits of diffusion flames on hypodermic tube burners and found the quenching mass flow rates for hydrogen to be about an order of magnitude lower than those for methane and propane. At the quenching limits the flame heights were comparable to the quenching distance for premixed hydrogen flames, in agreement with other studies [2,3]. Significantly smaller quenching limits for hydrogen were also observed for leaking compression fittings [1]. These hazards are now acknowledged in an SAE recommended practice [4], which requires hydrogen vehicles not to have localized leaks in excess of the measured quenching limits of Ref. [1]. One hydrogen flame observed by Butler et al. [1] had a heat release rate (HRR) of 0.46 W. This flame, and its counterpart burning in pure oxygen, are further considered below. Microcombustors have potential advantages over batteries in terms of power generation per unit volume and energy storage per unit mass [5]. Recent developments in micro-electromechanical systems (MEMS) have enabled microcombustors with dimensions on the order of 1 mm [5]. The ability to burn weak but stable flames [6,7] is important in the design of 1

4 microcombustors and it also may allow flames to serve as permanent pilots, thereby replacing electric ignitors. Several studies have examined flames that are among the weakest self-sustaining flames ever observed. Ban et al. [8] observed and predicted the shapes of small hydrocarbon jet flames. Ronney et al. [9] observed the burning of microgravity premixed flame balls aboard a space shuttle with heat release rates as low as W. Matta et al. [2] observed weak propane jet flames with heat release rates as low as 1 W. Cheng et al. [10] observed and modeled hydrogen jet flames and found lean conditions to exist in the reaction zones. Nakamura et al. [3] numerically studied quenching limits of methane jet flames and predicted the existence of flames as weak as 0.5 W. 2. Experimental Methods and Results The experiments involved two hydrogen diffusion flames, one burning in quiescent air and the other in nearly quiescent oxygen. The burner was a stainless steel hypodermic tube with an inside diameter of 0.15 mm and an outside diameter of 0.30 mm. Tests with platinum tubes of similar dimensions had nearly identical quenching limits, suggesting that any surface reactions are insignificant. The hydrogen jets issued downward. Horizontal and upward-flowing orientations had quenching limits that were only slightly higher [1]. A pure oxygen ambient was obtained by placing the burner tip 40 mm above a 100 mm diameter supply of O 2 flowing upward at 20 mm/s through a plenum and a ceramic honeycomb flow straightener. There was no measurable change in the quenching limit with changes in oxygen velocity. Hydrogen flow rates were measured with a rotameter. Uncertainties in the quenching limit flow rates are estimated at ±10%. 2

5 The flames (and any glowing of the burner tip) were not visible even in a darkened laboratory and hence were detected with a thermocouple placed 10 mm above the burner tip. After ignition, the hydrogen flow rate was reduced slowly until each flame was extinguished at its quenching limit. Color images of the two hydrogen flames slightly above their quenching limits are shown in Fig. 1. These were recorded with a Nikon D100 camera at ISO 200, f/1.4, and a shutter time of 30 s. The test conditions for these flames are given in Table 1. The word WE on a U.S. dime is included at flame scale to show that the flames are smaller than the smallest letters on U.S. coins. The flames are hazy, suggesting distributed reaction zones rather than thin flame sheets. The flames diameters are seen to be smaller than 0.5 mm and the tube is seen to glow dimly for the hydrogen/air flame. The quenching limits, i.e., the lowest hydrogen mass flow rates that can sustain steady flames in air and in oxygen, are provided in Table 1. These limits are 3.9 and 2.1 µg/s, respectively, indicating heat release rates of 0.46 and 0.25 W (assuming complete combustion and a lower heating value of 120 kj/g). The quenching limit is lower in oxygen because the associated adiabatic flame temperature is higher. Also included in Table 1 are the Reynolds, Froude, and Peclet numbers: Re = u d / ν ; Fr = u 2 / g d ; and Pe = Re D ( l D / d ) Sc, (1) where u is the mean hydrogen velocity in the burner, d is the burner inside diameter, ν is kinematic viscosity, g is the acceleration of gravity, l D is a characteristic diffusion length scale, and Sc is the Schmidt number (the ratio of viscosity divided by mass diffusivity). Here l D is taken as 1 mm, and Sc is taken as and 0.22 for H 2 /air and H 2 /O 2 flames, respectively [11]. The viscosity and mean velocity in Eq. (1) pertains to hydrogen at laboratory pressure and 3

6 temperature. The low Peclet numbers of these flames (defined as flow velocity divided by diffusion velocity) indicate that diffusion dominates over momentum [3]. Their high Froude numbers indicate that momentum dominates over buoyancy [3]. Acknowledgments This work was funded by NIST (J. Yang, grant monitor) and by NASA (D.P. Stocker, grant monitor). The authors thank C.W. Moran and M.S. Butler for their assistance with the experiments. References [1] M.S. Butler, C.W. Moran, P.B. Sunderland, R.L. Axelbaum, Int. J. Hydrogen Energy 34 (2009) [2] L.M. Matta, Y. Neumeier, B. Lemon, B.T. Zinn, Proc. Combust. Inst. 29 (2002) [3] Y. Nakamura, H. Yamashita, K. Saito, Combust. Theo. Model. 10 (2006) [4] SAEJ2579, Recommended practice for general fuel cell vehicle safety, a surface vehicle recommended practice. Detroit, MI: SAE International; January, [5] A.C. Fernandez-Pello, Proc. Combust. Inst. 29 (2002) [6] Y. Tsuboi, T. Yokomori, K. Maruta, Combust. Sci. and Tech. 180 (2008) [7] Y. Tsuboi, T. Yokomori, K. Maruta, Proc. Combust. Inst. 32 (2009) [8] H. Ban, S. Venkatesh, K. Saito, J. Heat Transfer 116 (1994) [9] P.D. Ronney, M.S. Wu, H.G. Pearlman, K.J. Weiland, AIAA J. 36 (1998) [10] T.S. Cheng, Y.C. Chao, C.Y. Wu, Y.H. Li, Y. Nakamura, K.Y. Lee, T. Yuan, T.S. Leu, Proc. Combust. Inst. 30 (2005) [11] B.J. Lee, S.H. Chung, Combust. Flame 109 (1997)

7 WE 0.5 mm Fig. 1. Images of hydrogen jet flames near their quenching limits in air (left) and in O 2 (right). Camera exposures are at ISO 200, f/1.4, and 30 s. The word WE from a U.S. dime is shown at the same scale as the flames. Original in color. Table 1 Measured quenching limits of hydrogen jet flames Oxidizer m& H2 μg/s HRR W u m/s Re Fr Pe Air O