James T. Wiswall a, Margaret S. Wooldridge b & Hong G. Im a a Department of Mechanical Engineering, University of Michigan, Ann

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1 This article was downloaded by: [University of Michigan] On: 06 September 2011, At: 12:03 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Combustion Science and Technology Publication details, including instructions for authors and subscription information: The Effects of Platinum on Propane/Air Stagnation Flow Flames James T. Wiswall a, Margaret S. Wooldridge b & Hong G. Im a a Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA b Departments of Mechanical and Aerospace Engineering, University of Michigan, Ann Arbor, MI, USA Available online: 19 Feb 2011 To cite this article: James T. Wiswall, Margaret S. Wooldridge & Hong G. Im (2011): The Effects of Platinum on Propane/Air Stagnation Flow Flames, Combustion Science and Technology, 183:6, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan, sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Combust. Sci. and Tech., 183: , 2011 Copyright # Taylor & Francis Group, LLC ISSN: print= x online DOI: / THE EFFECTS OF PLATINUM ON PROPANE/AIR STAGNATION FLOW FLAMES James T. Wiswall, 1 Margaret S. Wooldridge, 2 and Hong G. Im 1 1 Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA 2 Departments of Mechanical and Aerospace Engineering, University of Michigan, Ann Arbor, MI, USA Platinum and quartz stagnation surfaces were used to quantify the effects of heterogeneous reaction on premixed atmospheric pressure propane/air flames. The flame extinction limits were determined for a range of flame stretch conditions (30 90 cm 1 ) and exhaust gas composition (CO 2, CO, O 2, and NO) was determined for a range of equivalence ratios between the fuel lean and fuel rich extinction limits and dilution levels (N 2 to O 2 ratios from 3.76 to 5). The stagnation surface was maintained at high temperature, isothermal conditions of 900 C for all experiments. The results show that the extinction limits and the products of combustion are insensitive to the presence of the platinum catalyst under the high surface temperature conditions considered in this work. The results are discussed in the context of the physical and chemical mechanisms important during extinction. Keywords: Catalytic combustion; Platinum catalyst; Propane; Stagnation point flow INTRODUCTION Modern combustion systems are moving toward lower temperatures and more fuel-lean operation to achieve lower emissions and higher efficiencies. However, premixed fuel-lean systems can become constrained by lean extinction limits. Catalysts have been proposed as a means to extend lean limits, thereby achieving stable operation at very lean conditions (Li and Im, 2006, 2007), and catalysts have been shown to extend combustion operating conditions (Ahn et al., 2005). Yet, there are fewer studies isolating the fundamental mechanisms important in flame=catalyst interactions, as opposed to catalyst studies of fuel oxidation and exhaust gas remediation. Moreover, the experimental data can be complicated by system dependent behavior (e.g., heat losses) that obfuscates the effects of the catalyst. Fundamental studies of flame=catalyst interactions are vital to assessing the potential of catalysts toward augmenting premixed flame systems. Received 8 June 2010; revised 30 August 2010; accepted 30 August Address correspondence to Margaret S. Wooldridge, Department of Mechanical Engineering, University of Michigan, 2350 Hayward St, Ann Arbor, MI , USA. mswool@ umich.edu 540

3 EFFECTS OF PLATINUM ON FLOW FLAMES 541 Many studies have been conducted on the effects of catalysts on premixed fuel= air extinction and ignition limits, including those relevant to the present paper, using the stagnation flow configuration (Dupont et al., 2002; Giovangigli and Candel, 1986; Law et al., 1981; Law and Sivashinsky, 1982; Li and Im, 2006, 2007; Veser and Schmidt, 1996; Vlachos et al., 1994; Williams et al., 1991; Yamaoka and Tsuji, 1991). Law et al. (1981) discussed the advantages of the stagnation flow configuration for flame extinction studies, including the steady, two-dimensional, self-similar nature of the flow. In addition, stagnation flow allows relatively independent control of the characteristic times for surface and gas-phase reactions through stretch and dilution, making the stagnation flow system an excellent tool for characterizing the effects of catalysts on extinction and other combustion and oxidation properties. Last, the boundary conditions at the stagnation surface can be well defined in terms of the surface temperature and catalytic versus inert stagnation surface materials, further isolating the impact of the catalyst materials. The present study focuses on extending our understanding of platinum catalyst interactions with premixed propane=air flames. Law et al. (1981) quantified the lean extinction limits of premixed propane=air flames over a range of operating conditions, including surface temperatures, reactant flow rates, fuel=air equivalence ratios, reactant preheat, and the use of a platinum stagnation surface. They found that the platinum had no effect on the lean extinction limits of the flame system. Our earlier work (Wiswall et al., 2009) also reported that the presence of a platinum stagnation surface did not affect lean extinction limits for premixed propane=air and methane=air flames at moderate reactant flow rates (v avg ¼ m=s). However, the platinum stagnation surface was active toward propane oxidation on the catalyst surface for surface temperatures above 350 C (Wiswall et al., 2009; Wiswall, 2009). In this study, we extend the studies of the effects of platinum on premixed propane= air combustion to higher stagnation surface temperatures, higher reactant flow rates, and higher levels of dilutions. These conditions were selected to weaken the gas-phase flame system and enhance the catalytic activity. We further quantify the impact of the catalyst through emissions measurements, rich extinction limits, and nitrogen-diluted extinction limits. EXPERIMENTAL SETUP Figure 1 shows a photograph of the University of Michigan stagnation flow reactor (UM SFR) facility with the major components labeled in the figure. The key dimensions of the UM SFR are presented in Figure 2. This facility is a new SFR that replaces the reactor used in Wiswall et al. (2009), with significantly improved performance and capabilities. In particular, the flow uniformity is dramatically improved, the reactor is equipped with a higher temperature heater and a controller, and gas sampling has been integrated with the system. The previous system was limited to stagnation surface temperatures <750 K. Higher surface temperatures increase the surface reactivity relative to the gas-phase chemistry, and these studies in particular were conducted at temperatures where propane has been demonstrated to be catalytically active (Wiswall et al., 2009). The ability to conduct experiments at isothermal stagnation surface conditions allows surface reactivity to be precisely controlled, the exo- or endothermicity of the surface reaction to be quantified,

4 542 J. T. WISWALL ET AL. Figure 1 Major components of the University of Michigan stagnation flow reactor (SFR) facility. activation energies for global surface reactions to be determined (although activation energies were not studied in this work), and more uniform boundary conditions to be created enabling comparison with computational studies. The improved flow design yields a larger region of high-temperature uniform flow conditions, increasing the flame=surface interactions and also improving the validity of assuming one-dimensional flow. Specifically, results from the new SFR yield a visible flame region which typically varies by less than 1 mm in curvature in the central region of the disk flame (with a minimum diameter of 10 mm) based on digital imaging, compared with the previous SFR results where the flame curvature could exceed 2.5 mm in the central region of the flame (where flame diameters were 1 cm). The improved flow design also enabled the development of the exhaust gas sampling system, which allows more data to quantitatively characterize the flame=surface interactions. In total, the changes to the experimental setup dramatically extend the range of conditions that can be studied and the measured outcomes. For the experiments, the fuel=air flow is directed upward and impinges on the stagnation surface where the platinum catalyst is mounted. A mixing chamber filled with glass beads ensures complete mixing of the propane and air. An alumina monolith with 600 cells per square inch (Corning Inc.) is placed within the mixing chamber

5 EFFECTS OF PLATINUM ON FLOW FLAMES 543 Figure 2 Cross-sectional view of the UM SFR showing key dimensions. The insert shows the location of the thermocouple measurements on the heater assembly. immediately downstream of the glass beads to create a uniform velocity profile. After the monolith, the mixture flows through a nozzle with an 18.26:1 area contraction ratio (exit diameter ¼ 10 mm i.d.) and impinges on the heated stagnation plane. The nozzle exit plane is located 20 mm from the stagnation surface (separation distance=nozzle diameter ¼ L=D ¼ 2.0). The flow profile produced using the UM SFR configuration is quite uniform, as indicated by the photograph insert in Figure 1 and the images presented in Figure 3. A pyrolytic graphite heater (Momentive Ceramics Inc.) mounted in a stainless steel enclosure purged with nitrogen is used to maintain a constant stagnation surface temperature. The temperature of the stagnation plane is measured using a K-type thermocouple probe (Omega Engineering Inc.) placed in a groove machined into the stainless steel mounting surface. For this study, the stagnation surface was maintained at 900 C (with an uncertainty of <7 C) using an electric heater and PID temperature controller. Using the PID controller, the system equilibrates to the set-point temperature within 15 min.

6 544 J. T. WISWALL ET AL. Figure 3 Photographs of typical stagnation flow disk flame (top panel) and ring flame (lower panel) of premixed propane and air at atmospheric pressure for the quartz stagnation surface with no chamber wall present (disk flame conditions: v ave ¼ 1.0 m=s, / ¼ 0.66; ring flame conditions: v ave ¼ 1.0 m=s, / ¼ 0.61). The curved metal pieces in the foreground of the image are the mounting clips for the catalyst. The top panel shows the definition used to determine the flame separation distance, x sep. The platinum catalyst used in these studies was deposited by physical vapor deposition onto a square quartz wafer 2.54 cm 2.54 cm 1 mm thick. In order to form a stable bond of the platinum to the quartz, a layer of titanium (10 nm in thickness) was first deposited on the bare quartz. A 100 nm thick layer of platinum was then deposited onto the titanium layer. The wafer was mounted on the heated stainless steel surface using stainless steel clips. The catalyst was used without further treatment or conditioning. High purity propane (Cryogenics, 99.9%) mixed with synthetic air (O % and N 2 ) was used for all experiments. The reactant flow rates were controlled and measured using a mixing manifold and calibrated flow meters (Omega Engineering Inc.). Each flow meter was corrected for ambient variations in temperature and atmospheric pressure. The air and propane flow meters were calibrated yielding uncertainties of 2% and 3.5% of the respective volumetric flow rates. The corresponding uncertainties in the fuel=air equivalence ratio / and the average reactant velocity at the exit of the nozzle were 0.04 / and 0.02 v avg. The majority of the lean extinction limits in the present work were measured with the flow open to the atmosphere. Gas sampling measurements were taken only when the chamber wall was in place. The cylinder to contain the gases (chamber

7 EFFECTS OF PLATINUM ON FLOW FLAMES 545 wall) for gas sampling is shown in Figure 2. The configuration where the flow is open to the atmosphere is shown in Figure 1. The previous SFR facility used a coflow arrangement to suppress entrainment from room air and to minimize the shear flow losses from the premixed fuel=air reactant flow. A coflow arrangement was not necessary with the new SFR as highly uniform flame structures are a result of the reactant flow geometry and entrainment effects are outside the flow region of interest (i.e., outside the catalytically active region of the stagnation surface). The exhaust gases are sampled through two ports (to maintain flow symmetry) located downstream of the catalyst surface. A transparent (borosilicate glass) cylindrical enclosure (shown in Figure 1) is used to isolate the flow system from dilution by room air when gas-sampling measurements are made. The species O 2,CO 2, CO, NO, and unburned hydrocarbon (UHC) volume fractions are measured in the exhaust using nondispersive infrared gas sensing equipment (Horiba Inc.). The CO 2 sensor is subject to interference from propane, and the data are corrected for this effect. Specifically, known quantities of propane and air were sampled ( %, mole basis) and the UHC and CO 2 sensor response signals were recorded. Calibration curves were created for propane based on the UHC equivalent and for CO 2 interference (which ranged from 0.2 to 1% interference, mole basis). The calibration response curves were fit to polynomial expressions, which were used to correct the CO 2 measurements made in this study. The results are not corrected for enrichment or dilution effects due to nonisokinetic sampling or due to the location of the gas sampling in the SFR. The sample gases are cooled to room temperature, and a water separator is used to remove condensed water before entering the gas analyzer. Exhaust gas species are sampled at a flow rate of 5000 ml=min for approximately 5 min after the system reaches a steady state for the prescribed catalyst temperature. Flame structure and separation distance from the stagnation plane were recorded using digital images that were acquired for each operating condition. When a disk flame was stabilized (discussed subsequently), the flame separation distance, x sep, was defined as the distance from the center of the luminous region of the flame and the stagnation plane, as shown in Figure 3. RESULTS AND DISCUSSION Flame extinction limits were determined for fuel-lean, fuel-rich, and nitrogendiluted conditions. Exhaust gas species measurements were made over the range of stable operating conditions between the lean and rich extinction limits for a fixed reactant flow rate and fixed level of dilution. All exhaust gas sampling measurements were made with the chamber in place. Without the chamber wall, the flow is open to the atmosphere, and in this configuration there is no fuel rich extinction limit as a stable diffusion flame is formed. NO measurements were made at / ¼ 1.0 at air levels of dilution to the extinction limit by dilution. Several flame structures could be stabilized in the SFR, and the results are consistent with the flame structures described in the stagnation flame studies by Zhang and Bray (1999) and Fernandes and Leandro (2006). Specifically, as any of the three extinction limits were approached, a disk flame structure such as that shown in Figure 3 is stabilized. For the inert quartz surface at lower nozzle exit velocities (v avg < 120 cm=s) and the open chamber system, the disk flame transitions to a

8 546 J. T. WISWALL ET AL. diffuse ring flame as the equivalence ratio approaches the lean extinction limit. Figure 3 also shows a photograph of the typical ring flame. The ring flames were diffuse with no distinct flame front. The ring flame was not observed for any of the platinum studies or for conditions when the chamber wall was present. As the flames approached any of the extinction limits, the disk flame stabilized closer to the stagnation surface, and the separation distance from the stagnation plane to the disk flame (x sep ) decreased. Thus, the influence of the catalyst at the stagnation surface is maximized near extinction. Lean Flame Extinction Limits Table 1 presents the SFR conditions studied and the measured results for the lean extinction limits and the corresponding flame locations at extinction for the open flame system. Table 2 presents a summary of the conditions where the transitions from disk flames to ring flames were observed. As noted in the tables, at the lowest flow rates considered in the study, the heater could not maintain the stagnation surface at the set point temperature of 900 C. For these conditions, the heater was power limited; instead asetpointoft s ¼ 875 C was used. As seen in Table 1 and Figure 4, at these low flow rate conditions, the flames stabilize almost 3 times farther from the stagnation surface compared to the higher flow rate conditions. At low flow rates, the stagnation surface does not benefit from the higher radiative and convective heating rates that the higher flow rate flames create, and the heater cannot offset the losses. Figure 4 presents a comparison of the lean extinction limits and the separation distance at extinction for the platinum and quartz surfaces for the open flame system Table 1 Lean extinction limits for the open propane=air flame system with T s ¼ 900 C v ave (cm=s) / min x sep (mm) Pt Quartz RF RF RF RF Note. The flame distance x sep is measured relative to the stagnation surface, where RF designates conditions where ring flame extinction occurred. For the lowest velocities studied, the heater could not maintain the stagnation surface at 900 C. A steady surface temperature of T s ¼ 875 C was used for these conditions.

9 EFFECTS OF PLATINUM ON FLOW FLAMES 547 Table 2 Equivalence ratio where disk flame to ring flame transitions were observed for the open chamber system v ave (cm=s) / r x sep (mm) Quartz Note. For the lowest velocities studied, the heater could not maintain the stagnation surface at 900 C. A steady surface temperature of T s ¼ 875 C was used for these conditions. Figure 4 Experimental results for the flame location, lean extinction limits, and disk-to-ring transitions for the quartz and platinum surfaces in the open flame system.

10 548 J. T. WISWALL ET AL. as a function of the average nozzle exit velocity, v avg. Note that the Le > 1 flames extinguish at finite flame distances from the surface as expected based on idealized laminar flame theory, and as was demonstrated in the experiments of Law et al. (1981). However, as shown in Figure 4, the flames in this study were much closer to the stagnation surface compared to those of Law et al. (1981) for all velocity conditions. The difference is attributed to different heat transfer properties between the two systems. The close proximity of the flames to the stagnation surface led us to anticipate more significant catalytic effects on the flame extinction behavior in this study. The conditions for disk-to-ring flame transitions are included in Figure 4 and show the ring flame structure exists for a very limited range of equivalence ratios (between / min and / r ) before total extinction. The ring flame behavior may be attributed to entrainment of room air in these open flow experiments. A shroud flow of inert gas can eliminate the development of the ring flame, and we conducted experiments with and without a shroud flow of inert gases. We found the outcome of the experiments was not affected by the presence of the shroud flow=ring flame. This is consistent with expectations because the center portion of the flame remains uninfluenced by entrainment, which is confined to the flow edge. Further, when approaching the lean extinction limit, the effect of atmospheric and inert entrainment is to dilute and therefore weaken the flame near the edges. Therefore, the dominant portion of the flame controlling the extinction behavior is the center of the flame that has not been influenced by entrainment. The lean extinction limit is seen to decrease as the impinging velocity decreases in Figure 4. At higher velocities the flames stabilize closer to the stagnation plane, thereby weakening the flames due to increased heat transfer to the stagnation surface. The flame distance for the quartz surface was found to be slightly larger for most velocity conditions. However, the lean extinction limits for the platinum and quartz surfaces are almost indistinguishable and fall within the measurement uncertainties. The data are compared with previous results for propane=air flames in different stagnation burner configurations (Law et al., 1981; Wiswall et al., 2009). To compare the different flow geometries in the three systems, the nominal stretch of the flow in Figure 5 is defined as the ratio of the nozzle exit velocity to the nozzle exit diameter, D. Without detailed flow field measurements, this is a reasonable estimate for a consistent measure of stretch for the three different systems. In particular, the present system and the one used in Wiswall et al. (2009) have L=D (nozzle to surface distance to nozzle diameter) ratios greater than 1. Thus, we defined the nominal stretch based on the nozzle diameter considering our previous experience that the axial velocity field exhibits plug-flow behavior until it reaches within one diameter of the stagnation surface (Wiswall et al., 2010). As seen in Figure 5, the lean extinction limits in the present results are systematically lower than the results in Law et al. (1981). This is attributed to the higher surface temperature (by C) in the present results, such that the flames are subjected to much lower heat losses. This is further evidenced by the fact that at extinction the flames are closer to the stagnation surface (x sep ¼ 1 4 mm) than was reported in Law et al. (1981) (x sep ¼ 3 6 mm). Hence, the thermal and mass transport between the flame and the stagnation surface are more closely coupled in the present configuration. Law et al. (1981) attributed the insignificant catalytic effect on flame extinction to the fact that the flames were quenched at a distance from the stagnation plane.

11 EFFECTS OF PLATINUM ON FLOW FLAMES 549 Figure 5 Comparison between the present results for propane=air lean extinction limits (.,, D) as a function of the flow stretch defined by v ave =D and the results of (,^) Wiswall et al. (2009) and (&) Law et al. (1981) at 200 C. In the present work, the flame is closer to the surface and the surface temperature is higher, yet the extinction limit was still insensitive to the catalytic surface. The results are surprising, as there was significant catalytic activity for the propane=platinum oxidation system for surface temperatures above 600 C (Wiswall et al., 2009). The results indicate the gas-phase reaction rates are still sufficiently fast relative to the surface reaction rates to negate the effects of the catalyst at these experimental conditions. A possible explanation is that the products of combustion inhibit catalytic activity. For example, Reinke et al. (2006) showed that high concentrations of H 2 O in exhaust gas dilution of methane inhibited platinum activity due to surface coverage by OH (at pressures of 5 14 bar). In the present work, the flame-catalytic surface interactions for propane and platinum appear primarily thermal (although surface measurements would be needed to confirm this theory), and the presence of surface reactions does not affect the lean combustion limits of the reactant gases. Further increase in the stagnation plane temperature may eventually allow the flame to stabilize even closer to the stagnation surface, in which case the catalyst may directly interact with the reaction zone of the flame. Such conditions are expected at temperatures above 1200 C for the propane=air system, but such temperatures are too high to be practically useful. Alternatively, the gas-phase flames could be weakened by diluting the reactant gases with inert gases; or recent work by Reinke et al. (2004) demonstrated that enhanced catalytic activity can be achieved at operating pressures above 1 atm. Exhaust Gas Species Measurements Figure 6 presents the measurements for CO, CO 2, and O 2 mole fractions for the range of stable disk flames from the lean to the rich extinction limits. Recall the gas sampling experiments were conducted using the closed system with the chamber wall

12 550 J. T. WISWALL ET AL. Figure 6 Experimental and calculated results for CO, CO 2, and O 2 exhaust emissions as a function of equivalence ratio for a fixed reactant flow rate (v avg ¼ 90 m=s) for the closed flame system. The solid symbols are the experimental data for the platinum surface, the open symbols are for the quartz surface, and the lines are the results of adiabatic equilibrium calculations corrected for water condensation. The uncertainties of the species measurements are within the size limit of the symbols. The vertical lines represent the measured lean and rich extinction limits. in place. Identical to the open system, the lean extinction limits for the closed system were unaffected by the presence of the Pt catalyst. Consequently, only the gas sampling results are highlighted here. As observed for the lean extinction limits, the gas sampling results for the inert and the catalyst surfaces are within the experimental uncertainties, indicating the catalyst is not affecting the major products of combustion or the rich extinction limit. Values for CO, CO 2, and O 2 mole fractions based on adiabatic equilibrium calculations (corrected for water condensation) are included in the figure and are in excellent agreement with the experimental data, indicating complete combustion is occurring throughout the range between the extinction limits. Hence, even if fuel is penetrating past the flame zone and reaching the stagnation surface, particularly for the fuel rich conditions, the propane oxidation is not impacted by the Pt catalyst beyond the control case of the inert surface. Because platinum is active toward hydrocarbon oxidation at the stagnation temperatures used in this work and in particular for fuel rich conditions (Veser and Schmidt, 1996; Williams et al., 1991; Wiswall et al., 2009; Wiswall 2009), the results indicate the relative strength of the gas phase reactions is still well above that of the catalytic reactions. The results for NO for the conditions corresponding to the results of Figure 6 are presented in Figure 7. The measured NO values for both stagnation surface materials are an order of magnitude less than the values predicted by chemical equilibrium. This is an indication that although complete combustion is achieved, the flame temperatures are significantly lower than the adiabatic flame temperature for these conditions. Although Pt has demonstrated catalytic activity toward NO at lower temperatures (T < 300 C; Burch and Millington, 1995), no differences were observed between the NO emissions using the platinum or quartz wafer for this study

13 EFFECTS OF PLATINUM ON FLOW FLAMES 551 Figure 7 Experimental results for NO exhaust emissions as a function of equivalence ratio for a fixed reactant flow rate (v avg ¼ 90 m=s) for the closed flame system. The solid symbols are the experimental data for the platinum surface and the open symbols are for the quartz surface. The vertical lines represent the measured lean and rich extinction limits. where T s ¼ 900 C. This is consistent with the study of NO=Pt interactions by Burch and Millington (1995), who found the propensity for platinum to reduce NO peaks at approximately 275 C, and decreases at higher temperatures to virtually 0% conversion at 400 C. Figure 8 Experimental results for NO emissions as a function of nitrogen dilution for a fixed equivalence ratio of / ¼ 1.0. The vertical lines represent the extinction limit by dilution. The solid symbols are the experimental data for the platinum surface and the open symbols are for the quartz surface.

14 552 J. T. WISWALL ET AL. The experimental results for the extinction limit, defined by nitrogen dilution, are presented in Figure 8, along with the measured NO mole fractions as dilution increased from air levels to the dilution limit. Note that as the N 2 dilution level increases, the flow rate also increases. Both effects serve to weaken the gas phase flame, potentially emphasizing the role of the catalyst. However, as seen in Figure 8, both the dilution extinction limit and the NO levels were insensitive to the presence of the Pt catalyst on the stagnation surface even at these weakened flame conditions. CONCLUSION The present study reports new data quantifying the extinction limits of propane=air stagnation flow flames and the effects of a platinum stagnation surface at high stagnation surface temperatures. The work significantly extends the previous understanding of the impact of Pt on weak flames (i.e., near lean, rich, and dilution extinction limits). The speciation data provide new insights on the potential impact of platinum on the major products of combustion and NO. Given the high activity of Pt to NO, CO, and propane, the extinction limit data and exhaust gas measurements provide multiple quantitative measures of the strength of stagnation flames at both stable and weak flame conditions relative to the surface activity. The lack of sensitivity to the platinum catalyst at the weakened flame conditions studied indicates that significant changes to the approach (e.g., flame=burner geometry, surface temperatures) are required to extend flame operating limits by simultaneously coupling catalytic reactions with gas-phase reactions. ACKNOWLEDGMENTS This work was in part supported by National Science Foundation Grant CTS REFERENCES Ahn, J., Eastwood, C., Sitzki, L., and Ronney, P.D Gas-phase and catalytic combustion in heat-recirculating burners. Proc. Combust. Instit., 30, Burch, R., and Millington, P.J Selective reduction of nitrogen oxides by hydrocarbons under lean-burn conditions using supported platinum group metal catalysts. Catal. Today, 26, 185. Dupont, V., Zhang, S.-H., and Williams, A High-temperature catalytic combustion and its inhibition of gas-phase ignition. Energy Fuels, 16, Fernandes, E.C., and Leandro, R.E Modeling and experimental validation of unsteady impinging flames. Combust. Flame, 146, 674. Giovangigli, V., and Candel, S Extinction limits of premixed catalyzed flames in stagnation point flows. Combust. Sci. Tech., 48, 1. Law, C.K., Ishizuka, S., and Mizomoto, M Lean-limit extinction of propane=air mixtures in the stagnation-point flow. Proc. Combust. Inst., 18, Law, C.K., and Sivashinsky, G.I Catalytic extension of extinction limits of stretched premixed flames. Combust. Sci. Tech., 29, 277.

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