The Effect of Mixture Fraction on Edge Flame Propagation Speed

Transcription

1 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, 213 The Effect of Mixture Fraction on Edge Flame Propagation Speed Hatsachai Prahanphap 1, Jesse Piotrowicz 1, Richard S. Boles 1, David Matinyan 1, Wenyu Li 1, Paul D. Ronney 1 1Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA , USA Abstract The propagation speeds (U edge ) of non-premixed edge flames were measured as a function of stoichiometric mixture fraction (Z st ) and global strain rate (σ) for several fuel/oxidant/diluent combinations using counter-flow slot-jet burner. It was found that For Le fuel 1 and Le O2 1 (e.g. CH 4 -N 2 vs. O 2 -N 2 mixtures), for fixed Z st, U edge is positive for 15/s <σ< 12/s and negative outside this range, with a maximum U edge at σ /s. For fixed σ, U edge increases monotonically with increasing Z st, probably because as Z st increases, the flame location moves from the oxidizer side to the fuel side of the stagnation plane, thereby facilitating production of radicals; when the flame is on the oxidizer side of the stagnation plane, fuel must diffuse across the stagnation plane and upstream to reach the flame front for radical production For Le fuel > 1 and Le O2 1 (e.g. C 3 H 8 -N 2 vs. O 2 -N 2 mixtures), for fixed σ, U edge exhibits a minimum at Z st.2. This is proposed to be due to a shift of the more fully consumed reactant from fuel to oxygen as Z st decreases, leading to a smaller effective Le at low Z st which outweighs the chemical effect mentioned above For Le fuel > 1 and Le O2 < 1 (e.g. C 4 H 1 -CO 2 vs. O 2 -CO 2 mixtures), the same trends are observed as for the Le fuel > 1 and Le O2 1 cases but increase in U edge at low Z st is not as dramatic as expected. For less strong mixtures (e.g. CH 4 /O 2 /N 2 = 1/2/9.5) with Le fuel 1 and Le O2 1, the trend U edge is monotonically increasing with increasing Z st is still valid for negative propagation speed edge flame. These results indicate that varying Z st may have both chemical and Lewis number effect on the non-premixed edge flame speed. For Le fuel 1, the chemical effect dominates through the whole range of Z st whereas for Le fuel > 1, Lewis number effects become important at low Z st. Consequently, the near-extinction behavior of highly turbulent non-premixed flames may depend critically on (1) the fuel type (i.e. Lewis number), (2) degree of dilution of both fuel and air by combustion products, and (3) the local strain rate at the flame front. 1

2 1 Introduction The term edge flame refers to the transition from a burning to a non-burning condition along the length of a flame sheet. Edge flame behavior may illuminate factors that are important in many types of non-uniform flame phenomena, e.g., flames in highly turbulent flow fields such as in a reciprocating piston internal combustion chamber where holes in the flame sheet may open or heal, flames stabilized near a cold wall or splitter plate or the leading edge of a flame spreading across a condensed-phase fuel surface [1]. Edge flames may propagate in such a way that the non-burning region is overtaken by the burning region forming an advancing or ignition front having a positive edge speed (U edge ), or the burning region may retreat resulting in a negative U edge. The flow properties and chemical conditions affecting U edge include global strain rate (σ), effective Lewis number (Le eff ) of the mixture, heat losses, and for non-premixed flames, stoichiometric mixture fraction (Z st, which is traditionally defined as 1/(1+υX f /X o ) where υ is the stoichiometric oxygen to fuel mass ratio and X f and X o represent the initial mass fraction of fuel and oxygen for each stream.) By adjusting σ, Z st, X f and X o, values of U edge that are positive, negative, and zero (standing edge flames) can result. The study of the effects of Z st on non-premixed flames is relevant to many new combustion technologies. While traditional non-premixed combustion consists of a pure hydrocarbon fuel mixing with highly diluted oxidant (i.e. air) for which Z st is very low (typically.6), with recent interest in many new fuels and modes of combustion (e.g. biofuels, oxyfuel combustion, massive Exhaust Gas Recirculation), low-z st combustion is no longer the only relevant model; Z st can be as high as.8 for pure O 2 / diluted fuel conditions. An increase in Z st moves the reaction zone from the oxidizer to the fuel side of the stagnation plane in strained or turbulent flows, which in turn drastically affects the temperature/composition/time history of the reactants. These effects cause substantial and as yet not well understood changes in flame properties. Besides widely varying Z ST, new technology combustors may employ fuels with very high diffusivity (e.g. low molecular weight fuels such as hydrogen and methane) or very low diffusivity (e.g. heavy fuel oil, C 1 C 2 ). This in turn affects the fuel Le which is known has drastic effects on burning rates and extinction conditions. Non-premixed edge flames have been widely studied both analytically [2-4] and experimentally [1]. Prior work has concentrated on the prediction and measurement of the extinction limits and propagation rates of edge flames. Recently, Cha and Ronney [1] studied many of the parameters affecting nonpremixed edge flame behavior experimentally using a counter-flow slot-jet configuration to control global strain rate in a flow environment that is both spatially and temporally steady to allow a more direct comparison with analytical and computational results than would be possible using previous experimental approaches such as the traditional opposed round-jet configuration. A wide range of mixtures of methane or propane against oxygen diluted by N 2 or CO 2 were tested to study and the effects of mixture strength, Le and Z st. The present work is intended to extend portions of the Cha and Ronney study to a wider range of Z st and a range of fuel/o 2 /inert combinations having varying Le of fuel and O 2. Z st =.5 indicates that the concentration of diluent gas in the fuel and oxygen streams is 2

3 chosen so that the flame is centered at the stagnation plane. While select values of Z st above and below.5 were investigated in [1] for a wide range of strain rates, the present work seeks to treat Z st as the independent variable with strain rate as a secondary parameter. The same counter-flow slot-jet configuration is used, and combinations of methane, propane, or iso-butane against oxygen with N 2 and CO 2 as diluents are used to form mixtures with both Le fuel and Le oxidizer above, below and near unity. 2 Experimental Method The counter-flow slot-jet burner consists of two opposed rectangular jets, 13 cm long and.5 cm wide, and shrouded by inert N 2 gas jets also.5 cm wide to protect against diffusion of laboratory atmosphere into the reactive mixtures. The jets are positioned vertically with the fuel/inert stream flowing upward from below and the oxygen/inert stream downward from above. Stainless steel honeycomb inserts at each jet exit promote uniform flow across the width and depth of the jets. The fuel, oxidant and shroud streams have equal velocities and temperatures. Mass flow controllers are used to feed the gases to the jets at the flow rates needed to obtain the desired mixture fraction and strain rate. This apparatus produces a stable flame sheet parallel to the jet exits that is nearly centered at the stagnation plane for the case of Z st =.5. Mixture fractions above/below.5 result in the flame moving closer to the fuel/oxygen side of the stagnation plane. To measure positive edge speeds, a small round jet of inert gas was used to extinguish the flame at one end of the slot jets. The inert jet was then retracted and released to allow the flame to advance. To measure negative edge speeds, first a mixture having a positive edge speed was introduced into the jets, then a set of electrically heated wires prepositioned at both ends of the jets was switched on, then the mixture strength was slowly reduced to the desired value, then the small round jet of inert gas was used to extinguish the flame at one end. In all cases a high-speed (25 frames/sec or greater) video camera was used to record the flame propagation. The video was analyzed to measure the location of the flame edge within each frame and U edge was determined from the progression of this location. Values of U edge were measured across the range of Z st for several fixed strain rates for the following mixtures: CH 4 /O 2 /N 2 = 1/2/7.5 (essentially equivalent to a stoichiometric CH 4 -air mixture, though in addition to pure fuel vs. air, N 2 was shifted to the fuel side to assess the effect of Z st ), C 3 H 8 /O 2 /N 2 = 1/5/18.8 (essentially stoichiometric C 3 H 8 -air), iso-c 4 H 1 /O 2 /N 2 = 1/6.5/24.45 (essentially stoichiometric C 4 H 1 - air) and C 4 H 1 /O 2 /CO 2 = 1/6.5/15. The amount of dilution is chosen to obtain comparable adiabatic flame temperatures in each case. Additionally, CH 4 /O 2 /N 2 = 1/2/9.5 was tested to examine the behavior of a weaker mixture having a lower adiabatic flame temperature than the CH 4 /O 2 /N 2 = 1/2/7.5 mixture. 3

4 3 Results and Discussion CH 4 /O 2 /N 2 = 1/2/7.5 (Le fuel =.96, Le O2 = 1.1) Figure 1 shows the effect of Z st on U edge for the baseline CH 4 /O 2 /N 2 = 1/2/7.5 mixture. It was observed in [1] that U edge increases monotonically with increasing Z st rather than being symmetric with respect to Z st =.5. This is also seen in the present work with U edge increasing monotonically from below 8 cm/s to above 12 cm/s as Z st increases from below.2 to above.7 as more of the inert N 2 is shifted from the oxygen to the fuel stream. Chen and Axelbaum [5] reported this trend also and proposed it is due to a shift in the flame location from the oxygen side to the fuel side of the stagnation plane, thereby increasing the residence time that radical-generating fuel molecules and their decomposition products spend in the high-temperature region of the flame structure. U edge (cm/s) (1/s) 5(1/s) 75(1/s) (1/s) 125(1/s) Z st Figure 1: Edge flame propagation speed as a function of stoichiometric mass fraction for CH 4 /O 2 /N 2 = 1/2/7.5 C 3 H 8 /O 2 /N 2 = 1/5/18.8 (Le fuel = 1.86, Le O2 = 1.5) Figure 2 shows that, as in the CH 4 /O 2 /N 2 = 1/2/7.5 case, U edge increases monotonically from below cm/s to above cm/s with increasing Z st, but only for Z st larger than about.2. For lower Z st, the value of U edge shows an increase to a value near 8 cm/s. A minimum U edge 5 cm/s occurs near Z st =.2. The proposed cause of this increase is a shift in the fully consumed reactant from fuel to oxygen as Zst decreases, leading to a smaller Le eff and therefore stronger flames. According to Linan [6], which reactant (fuel or O 2 ) is more completely consumed depends on a parameter α+2β, where α = Y O,- 4

5 /νy F,+ is the ratio of oxidizer mass fraction in the oxidizer stream to fuel mass fraction in the fuel stream normalized by the stoichiometric oxidizer-to-fuel mass ratio (ν) and β is the non-dimensional difference between temperatures of oxidizer and fuel streams. When 2β < 1, the oxidizer/fuel is more completely consumed if α+2β is less than/greater than unity. In the present work, β =, so the oxidizer/fuel is more completely consumed if α is less than/greater than unity. Linan also establishes the case that L eff of the mixture is equal to the Le of the reactant that is more completely consumed. From the definition of α and Z st, when α < 1, Z st <.5, and the scarce reactant is O 2 resulting in Le eff Le O2, and when α > 1, Z st >.5, and the scarce reactant is fuel resulting in Le eff Le fuel. It is therefore proposed that the cause of the upswing in U edge below Z st =.2 is due to a shift in the fully consumed reactant from fuel to oxygen as Z st decreases, resulting in a smaller Le eff and therefore stronger flames. This effect is not observed for methane because Le is near unity for both the fuel and oxygen. U edge (cm/s) (1/s) 5(1/s) 75(1/s) (1/s) Z st Figure 2: Edge flame propagation speed as a function of stoichiometric mass fraction for C 3 H 8 /O 2 /N 2 = 1/5/18.8 5

6 Iso-C 4 H 1 /O 2 /N 2 = 1/6.5/24.45 (Le fuel = 2.13, Le O2 = 1.4) Figure 3 shows that as with the C 3 H 8 /O 2 /N 2 = 1/5/18.8 case, both the monotonic increase above Z st.2 and the upswing below Z st.2 are observed; however, the upswing is more significant, rising to a value of nearly 7 cm/s from a low of 3 cm/s near Z st =.2. The upswing was consistently observed for strain rates of 25/s to /s. Uedge (cm/s) Zst Figure 3: Edge flame propagation speed as a function of stoichiometric mass fraction for C 4 H 1 /O 2 /N 2 = 1/6.5/24.45 Iso-C 4 H 1 /O 2 /CO 2 = 1/6.5/15 (Le fuel = 1.74, Le O2 =.78) Figure 4 shows that as with the iso-c 4 H 1 /O 2 /N 2 = 1/6.5/24.45 case, both the monotonic increase above Z st.2 and the upswing below Z st.2 are observed; however, the upswing is less significant, moving from cm/s to just above 5 cm/s. The upswing is not as significant as would be expected considering that Le eff above/below Z st =.5 should be greater/less than unity. In other words, the iso-c 4 H 1 /O 2 /CO 2 mixture has the largest difference in Le between the fuel and oxidizer, so an even more dramatic change in U edge is expected as L eff shifts from that of the fuel to that of oxygen. 6

7 U edge (cm/s) 8 2 Strain = 25 (1/s) Strain = 35 (1/s) Strain = 5 (1/s) Z st Figure 4: Edge flame propagation speed as a function of stoichiometric mass fraction for C 4 H 1 /O 2 /CO 2 = 1/6.5/15 CH 4 /O 2 /N 2 = 1/2/9.5 (Le fuel =.96, Le O2 = 1.1) Figure 5 shows the effect of dilution on U edge for CH 4 /O 2 /N 2 mixtures. The results are qualitatively similar to CH 4 /O 2 /N 2 = 1/2/7.5 but are shifted to lower values of U edge for a given Z st. As with the stronger mixture, U edge increases monotonically with increasing Z st. For high strain rates at low Z st the edge speed is negative. 7

8 U edge (cm/s) Z st Figure 5: Edge flame propagation speed as a function of stoichiometric mass fraction for CH 4 /O 2 /N 2 = 1/2/9.5 4 Conclusions The effects of stoichiometric mixture fraction (Z st ) on nonpremixed edge-flame speeds (U edge ) were measured for a variety of fuel/o 2 /inert mixtures having a range of Lewis numbers. It was found that U edge depended on Z st in two unusual ways. First, in every case U edge increased monotonically with increasing Z st for sufficiently large values (>.2) of Z st ; this was attributed to chemical effects discussed in prior works [1, 5] and is probably germane to all nonpremixed hydrocarbon-o 2 systems, not just where edgeflame structures exist. Second, only for high molecular weight (i.e. high Lewis number) fuels, a non-monotonic trend was observed in that as Z st was decreased below about.2, U edge increased; this was attributed to a change in the more completely consumed reactant from fuel to O 2 and thus a decrease in the effective Lewis number of the flame edge. 5 Acknowledgment This work was supported by the National Science Foundation, Directorate for Engineering, Division of Chemical, Bioengineering, Environmental, and Transport Systems, under Grant No

9 References [1] M.S. Cha, P.D. Ronney. Propagation rates of nonpremixed edge flames. Combustion and Flame, 146 (26) [2] J.D. Buckmaster. Edge-flames and their stability. Combustion Science and Technology, 115 (1996) [3] J. Daou, A. Linan. The role of unequal diffusivities in ignition and extinction fronts in strained mixing layers. Combustion Theory Modelling, 2 (1998) [4] R. Daou, J. Daou, J. Dold. The effect of heat loss on flame edges in a non-premixed counterflow within a thermo-diffusive model. Combustion Theory and Modelling, 8 (24) [5] R. Chen, R.L. Axelbaum. Scalar dissipation rate at extinction and the effects of oxygen-enriched combustion. Combustion and Flame, 142 (25) [6] A. Linan. The asymptotic structure of counterflow diffusion flames for large activation energies. Acta Astronautica, 1 (1974)