Studies of Turbulent Flame Propagation and Chemistry Interaction at Elevated Temperatures and High Reynolds Numbers

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1 Temperature profile Paper # 7LT-311 Topic: Laminar & Turbulent 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, 213 Studies of Turbulent Flame Propagation and Chemistry Interaction at Elevated Temperatures and High Reynolds Numbers Bret Windom 1, Sang Hee Won 1, Bo Jiang, 1,2 Yiguang Ju 1 1 Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China A new high temperature, high Reynolds number, Reactor Assisted Turbulent Slot (RATS) burner has been developed providing a well-defined flow geometry and fuel/oxidizer reactivity to explore the new flame regimes experienced in propulsion engines that result from turbulence-chemistry interaction at elevated temperature and large ignition Damköhler numbers. The RATS burner enables turbulent flame studies for both gaseous and liquid fuels up to 7 K. The present study intends to answer one question: how does low temperature reactivity of large hydrocarbon fuels affect turbulent combustion regimes? The turbulent flow characteristics of the new burner at room temperature have been quantified using hot wire anemometry revealing moderate levels of turbulence with u /U spanning 8 15 % depending on the turbulent generation geometry. Experimental methods were validated by measuring and comparing turbulent flame speeds of methane/air flames at bulk Reynolds numbers exceeding 2,. Turbulent flame speeds, structures, and regimes of methane-air and n-heptane-air mixtures at elevated temperatures are investigated. The results demonstrate two different turbulent combustion regimes, a conventional thin reaction zone turbulent flame regime, and a new low temperature ignition (LTI) turbulent flame regime. The results from the methane/air and low reactant temperature n-heptane/air flames indicate a conventional turbulent flamelet regime and agree with previously reported flame speed data in the literature. However, the results of n-heptane/air flames at elevated reactant temperatures demonstrate a LTI flame regime and a distinct flame speed dependence on turbulent intensity. The present results indicate that the low temperature reactivity of large hydrocarbons can lead a new turbulent flame regime as the ignition Damköhler number is increased. 1. Introduction In many practical high speed propulsion systems such as scramjet and gas turbine engines, turbulent premixed and partially premixed flames at high temperature, elevated pressure, and high Reynolds numbers dominate the processes of Thin Flame Low T Fuel/Air Preheated (or EGR) gas Low temperature ignition regime ignition ~ flow ~ flame High T Well-stirred reactor regime ignition < flow Turbulent mixing Figure 1. Schematic detailing flame regimes which result from turbulent/chemistry interactions.

2 ignition, fuel oxidation, flame stabilization, and emissions. At the near limit conditions, particularly for flame stabilization near a high temperature recirculation zone and in afterburner relight, turbulent transport and kinetic coupling between unburned reactants and partially reacted high temperature oxidizer or stabilizing flame product stream strongly affect the combustion regime, flame chemistry, ignition, and flame stabilization limits. This environment of heated reactants can lead to increased reactivity of the fuel/oxidizer, which can strongly influence the turbulent chemistry interaction and the overall combustion processes and regimes in the main flame. As illustrated in Figure 1, three flame regimes may exist depending on the reactivity of the fuel, temperature, and heated residence time of the reactants. At low reactant temperatures, the chemically frozen reactants have long ignition time scales compared to that of the flame, which results in a thin flame reaction zone regime. At high reactant temperatures, ignition time scales can exceed the flow or turbulent time scales resulting in a flameless well stirred reactor regime. At intermediate temperatures, depending on the residence time and reactivity of the fuel, low temperature ignition (LTI) may ig ut ' / Inverse of Ignition Damköhlernumber, l 1 Flame Thin flames Laminar Ignition-deflagration- DDT Transition Turbulence-chemistry interaction Homogeneous Ignition, Shock tube Damköhler Ka Ka, t t t cr mix c ig, u numbers Distributed reaction zone Turbulent Ignition-deflagration- DDT transition, Acoustic coupling Flow reactors, Jet stirred reactor l / u ' Figure 2. A new turbulent flame diagram correlating flame regimes with ignition delay time, flame combustion time, and turbulence fluctuation time. occur, resulting in a partially oxidized reactant stream with shortened ignition delays. With ignition delays on the order of the flame and turbulent flow time scales, this unique phenomena can result in a LTI turbulent flame regime. For turbulent premixed combustion, direct numerical simulations are still limited to simple combustion chemistry and low Reynolds numbers 2,3. Although extensive experimental studies 4 have been conducted to quantify turbulent flame speeds and flame structures 5-7, most of the studies have been limited to the thin reaction zone flamelet regimes 8 and few emphases were placed in distributed reaction zones (e.g. LTI turbulent flame regime) where turbulent chemistry interaction is strong. Moreover, few experimental studies have been carried out to understand how the increase in reactivity of the unburned mixture at high temperatures can alter the turbulent combustion regime. For real jet fuel combustion at high pressure, an increase of mixture temperature to 6-8 K will result in low temperature combustion within residence times of a few milliseconds 9. Unfortunately, few studies have investigated the effects of turbulence on the fuel oxidation and flame regimes at elevated temperatures, particularly when the ignition timescales are comparable with that of fuel consumption timescales by flames 1. As illustrated in Fig. 2, the limitation of the previous experiments to low temperature and the thin reaction zone flamelet regimes provide very limited understanding of the physics of the distributed reaction zone, turbulent ignition to flame transition, flame acoustic coupling, and turbulent chemistry interactions at high Reynolds numbers and may have overlooked new flame regimes at elevated reactant temperatures. Therefore, fundamental understanding of the propagation limit, speeds, and the chemistry/transport coupling of turbulent partially premixed flames at elevated temperatures is of great importance to develop advanced propulsion systems and validated computational fluid dynamics (CFD) tools for engine design. Given the above discussion, a new, reactor assisted turbulent slot (RATS) burner has been designed and constructed. The new burner provides an experimental foundation to investigate turbulent chemistry interaction in both thin and broadened reaction zones, to measure quantitatively the propagation limits, speeds, and flame structures of p remixed turbulent flames, and to advance fundamental understanding of new turbulent flame regimes at large ignition Damköhler numbers and low critical Karlovitz numbers relevant to combustion in practical propulsion systems. The remainder of this study will detail the design and the characterization of the RATS burner including the quantification of the turbulent flow field using hot wire anemometry and validation experiments comprised of CH 4 /air turbulent flame speed (S T ) measurements. Results from recent investigation of n-heptane/air flames at elevated temperatures are presented revealing a significant increase in S T when reactant temperatures and heated residence times are sufficient to induce first stage ignition prior to introduction into the flame. CH 2 O PLIF reveals a much different flame structure for the reactor assisted turbulent flame as compared to the chemically frozen reactant flame. 1, f Ka Ka, t t / cr -1 ig S L S L ext mix t ig, u t c Temperature increase 2

3 Turbulent Generator #2 Turbulent Generator #1 Liquid Fuel Radiation Panel Heaters (x2) Heated Air Heated Mixing Chamber Fuel & Air Figure 3. Cross-sectional schematic of the reactor assisted turbulent slot (RATS) burner. The reactant supply and heating system are also detailed. 2. Methods and Design 2.1 Reactor Assisted Turbulent Slot Burner ( RATS -Burner) The RATS burner was designed with the capability of generating large turbulent Reynolds numbers while incorporating reactant heating to induce a varying degree of pre-flame low temperature chemistry. A cross-sectional schematic of the RATS burner is provided in Fig. 3. In addition, a cross-sectional model along with detailed photographs of the burner exit and flame are illustrated in Fig. 4. The stainless steel burner is 5 cm tall providing sufficient residence time to control the pre-flame low temperature chemical reactions by careful heating of the burner and fine-tuning of the reactant flow rates. As seen in the cross sectional schematic of the burner in Fig. 3 and the photograph of the burner exit in Fig. 4, the main reactant flow channel is tapered providing a uniform top hat flow profile at the nozzle exit. The burner provides two locations for turbulence generation. The first location before the convergence of the flow channel provides a smaller degree of turbulence due to the damping of the cascading eddies along the converging section of the flow channel. The turbulent generator following the converging section produces a higher degree of turbulence and has been quantified by use of hot wire anemometry, which will be discussed in more detail later. To sustain a flame at large Reynolds numbers, a piloted flame system is applied. As seen in the image of the flame shown in Fig. 4, the pilot flame runs along the length of the nozzle on each side of the main reactant flow channel. The pilot flame consists of premixed CH 4 /air (φ=1) for all measurements reported in the following study. The main turbulent generator used in this study was a 1 x 1 mm plate with twenty-five 2 x 1 mm slots (see inset in Fig. 4) evenly distributed along the length of the plate. This resulted in a 5 % blockage ratio and generated turbulence that was uniform along the length of the burner. Other geometries with higher blockage ratios were considered and tested, and though they did produce greater turbulent intensities, they resulted in nonuniform jetting flames and ambient gas entrainment. For these reasons we selected the aforementioned small slotted plate for the measurements Industrial Air Heater TC Location #3 CH 4 /Air (pilot flame) TC Location #2 TC Location #1 Bulk Ai Flow Additional turbulent generator location Reactants Pilot flame Turbulent generator Figure 4. Cross-sectional schematic of the flow reactor coupled counterflow edge flame burner (left), cross-sectional view of burner exit (middle), and turbulent CH 4 /air flame, Re D = 1, (right). 3

4 presented in this study, though, the improvement of the turbulent generation does remain a concern and a goal for future research. The heating system consists of a 24 long heavy-duty in-line heater (HottWatt HA2-24) to heat the bulk airflow to the desired temperature (up to 7 K). The hot air stream is delivered into the bottom of the burner where it mixes with a vaporized fuel/air mixture. The hot premixed fuel/air mixture travels the length of the burner through the main reactant flow channel before exiting the burner nozzle at which it interacts with the pilot flame and is ignited. To maintain the temperature of the premixed fuel/air stream as it travels the length of the burner, two radiation panel heaters are used to heat the outer sides of the burner. The reactant temperature is measured at varying points along the burner to ensure uniform temperature profiles. 2.2 Hot Wire Anemometry Cold flow hot wire anemometry was performed to quantify the turbulence intensity and turbulent le ngth scales achievable with the RATS burner. The hot-wire probes used were single-wire, normal probes manufactured by etching Wollaston wire of 2.5 µm in diameter (9% Pt 1% Rh) to produce a sensing length of ~2 mm resulting in an l w /d w ~ 8. The probes were operated by Dantec Streamline constant temperature anemometry (CTA) system in 1:1 bridge mode, with a sampling frequency of 8 khz. The raw data was high pass filtered at f a = f s /2 = 4 khz to prevent aliasing. All measurements reported in this study were taken 3 mm above the burner exit along the centerline. Hot wire measurements were performed at multiple locations along the length of the burner and at different heights indicating uniform velocity profiles and isotropic turbulence. Calibration of the hot wire was performed by measuring a calibrated volumetric flow rate of air through the open burner (i.e. without turbulent generators). Measurements and velocities of the open burner were verified with measurements from a pitot probe calibrated wind tunnel and revealed close agreement. The hot wire provided a measurement of the turbulent intensity (u /U) and the integral (l) and Taylor (λ) turbulent E(f) (m 2 /s) 1.E+ 1.E-1 1.E-2 1.E-3 1.E-4 1.E-5 1.E-6 1.E-7 1.E-8 1.E+ 1.E+1 1.E+2 1.E+3 1.E+4 1.E+5 frequency (s -1 ) Figure 5. Power spectrum with the optimized fit (red line) of the given expressions to determine l and λ. E(k) (m 3 /s 2 ) length scales. The turbulent length scales were determined similar to an approach demonstrated by Coppola and Gomez 11 in which the power spectra in terms of frequency and wavenumber were modeled using correlative expressions. The power density spectra along with the correlative fits can be seen in Fig. 5. In brief, the one-dimensional power spectrum in terms of frequency was estimated using the Welch method 12. The integral length scale, l, was determined by optimally fitting (through the optimization of l) the integral region of the turbulent power spectrum in frequency space, E(f), with the Von Karman s turbulent spectrum 13 1.E+ 1.E-1 1.E-2 1.E-3 1.E-4 1.E-5 1.E-6 1.E-7 1.E-8 1.E+ 1.E+1 1.E+2 1.E+3 1.E+4 1.E+5 k (m -1 ) [ ( ) ] (1) where f is the frequency, u 2 is the velocity variance, and U is the mean velocity. A similar approach was applied to determine the Taylor length scale. Invoking Taylor s frozen hypothesis 13, the power spectrum was determined in wavenumber, k, space using. The mean energy dissipation rate, ε, was determined by optimally fitting (through the optimization of ε) the power spectrum to the following equation 14 (2) 4

5 integral length (l, mm) Taylor length (λ, mm) u'/u where A = 1.62 is a constant. The Taylor length scale was then estimated from the mean energy dissipation using 14 (3) ( ) (4) where ν is the kinematic viscosity. The turbulent intensity (u /U) as measured using hot wire anemometry with and without the addition of the turbulent generating plates can be seen in Fig. 6 as a function of the bulk Reynolds number, Re D (Re D = UD H /ν, D H = hydrodynamic diameter of the burner exit). The measured turbulent intensities with the addition of the turbulent generator ranged between % and slowly rise as the bulk velocity (U) is increased. Without the turbulent generator, turbulent intensities equaled ~8 %. The calculated integral and Taylor turbulent Reynolds numbers (Re l = u l/ν and Re λ = u λ/ν) along with the corresponding integral and Taylor turbulent length scales are provided in Fig. 7. Both Re l and Re λ increase as Re D increases. This is due to the rise in the fluctuating velocity component. The integral length scales maintain a value of ~ 2.75 mm as Re D is increased which is expected as the integral length scale is mainly determined by the geometry of the turbulent generating device. In this case, the turbulent 5% Re D Figure 6. Measured turbulence intensities for the RATS burner vs. bulk Reynolds number. generator was comprised of slots with hydrodynamic diameters equal to ~ 3. mm, which corresponds to the measured integral length scale. The Taylor length scale fell as Re D was increased from a value of ~1 mm to.5 mm. Rel Re integral integral length % 15% 13% 11% 9% 7% with turbulent generator w/o turbulent generator Re D Re D Figure 7. Measured turbulent integral Reynolds numbers, Re l, and length scales, l, (left) and turbulent Taylor Reynolds numbers, Re λ, and length scales (λ) versus bulk Reynolds number, Re D. Reλ Re Taylor Taylor length OH and CH 2 O PLIF To quantify the turbulent flame edge length and surface density parameters of the turbulent flame, planar laser induced fluorescence (PLIF) of the OH radical was performed. The optical system can be seen in Fig. 8. A frequency doubled Nd:YAG laser (Spectra-physics, lab-17-1, 532 nm, 5 mj/pulse) was used to pump a Rhodamine 59 dye laser (Syrah, CSTR-G-3) to generate the excitation frequency for the Q 1 (6) OH transition. The resulting beam (~283 nm, 11 mj/pulse) was spread and focused into a sheet ~ 1 μm thick and ~6 mm tall and directed across the burner to 5

6 image the traverse cross section of the turbulent flame. The resulting fluorescence emission was imaged at a 9 angle using a gated ICCD camera (Princeton Instruments, PI-Max). A band pass filter centered about 35 nm consisting of the combined effects of UG-11 and WG-35 optical filters was positioned immediately in front of the ICCD to isolate the OH fluorescence signal from other possible sources of stray light. As seen in Fig. 8, the system has been arranged to perform simultaneous formaldehyde (CH 2 O) PLIF. To excite the CH 2 O molecule, the third harmonic of the fundamental Nd:YAG at 355 nm is used. A band pass filter centered at 45 nm positioned in front of the camera provides an isolated CH 2 O fluorescence signal. The CH 2 O PLIF measurements were performed on the n-heptane/air experiments as detailed in the next section. 3. Results and Discussion 355 & nm band-pass filter burner ICCD light sheet optics Nd:YAG laser 355 nm nm 532 nm dye laser Figure 8. Optical setup for the simultaneous OH and CH 2 O PLIF measurements. 3.1 CH 4 /Air Flames OH-PLIF images for the methane/air flames (φ = 1, p = 1 atm, T = 3 K) were taken at Re D = 6, 12, and 18 corresponding to U = 5, 1, and 15 m/s. Images were taken with and without the installation of turbulent generating plates resulting in 6 total sets of images. Sets of 5 individual images were taken at each condition along with a time averaged image consisting of 5 shot accumulations. The individual images were processed to capture the details regarding the flame structure and surface area, while the accumulated images were primarily used to measure the turbulent burning velocity (S T ). Typical flame structure as measured using OH- PLIF for each condition can be seen in Fig. 9. Overall, the flame is noticeably more corrugated with the addition of the turbulent generator, especially at lower Re D. As Re D is increased, the flame structure for the cases with and without the turbulent plates become more similar. This observation is most likely due to the bulk turbulence (i.e. turbulence generated by the bulk velocity of the flow through the slot burner) becoming more dominant on the flame structure than the smaller eddies produced with the turbulent generating plates. Re D Without turbulent generator With turbulent generator 6, 12, 18, Figure 9. OH-PLIF images of CH 4 /air (3 K) turbulent flames at varying Re D with and without the addition of the turbulent generators. The time-averaged images were used to measure the turbulent burning velocity (S T ). The large number of accumulations (5) resulted in a well-defined flame cone, as can be seen in Fig. 1. The raw images were processed to define the flame edge. This was accomplished by binarizing the image using a threshold value determined by the bimodal histogram of all pixel data and performing a canny edge detection to accurately map the boundary of the time averaged flame. This process can be seen in Fig. 1. S T was determined by first calculating the length of the time averaged flame, P avg. From mass continuity S T was then determined by the following geometrical relationship 57 mm 6

7 ( ) (5) where U is the bulk mean velocity of the mixture at the nozzle exit and w is the slot burner width (w = 1 mm in the RATS burner). The turbulent burning velocity normalized by the laminar flame speed (S T /S L ) for each set of CH 4 /air flames is plotted as a function of u /S L. Two separate plots designed to show two distinct trends are provided in Fig. 11. The first plot, Fig. 11a, compares the culmination of the data to a correlative fit from experimental measurements of S T using a cylindrical jet burner 15 and spherically expanding flames 5. As can be seen in Fig. 11a, the current data appears to follow the trends previously published despite having different burner geometries. In addition, Fig. 11b, displays the trend of S T when U is held constant and u is no longer dependent on U. This is done by varying the turbulence intensity by using different turbulent generators. In this case, the two data at each mean velocity represent cases with and without the turbulent generator or u /U equal to 14% and 8 %, respectively. This removes the dependence that the two factors can have on the turbulent flame wrinkling. In other words, typically one increases u by increasing U. This results in stronger eddies as marked by the increase in u, but reduces the eddy/flame interaction time as the more intense eddies are convected through the flame more quickly (due to the increase in U). Thus, the effect of increasing u may be negated. As previously demonstrated, this results in highly non-linear behavior marked by the correlative fits (the lines in Fig. 11b), which were derived using methane/air burning velocities also from a slot burner 16. Interesting to note, S T appears to bend and fall off as u is increased, and appears to reach higher values of u before fall-off as U is increased. Explanation of the bending behavior of S T has been attributed to possible merging of flamelets or to a geometric effect with Bunsen flames that become shorter and have less time to wrinkle as they propagate faster 16. Although not shown here, flame wrinkling/surface area measurements made on the CH 4 /air flames demonstrate a strong correlation to the bending behavior of S T shown in Fig. 11b to the degree of flame wrinkling and the change in surface area. 3.2 n-heptane/air Turbulent Flame at Elevated Temperatures Figure 1. Time averaged CH 4 /air flame, Re D = 12,, consisting of 5 individual flames (left), binarized image of flame (middle), and detected edge used for the determination of the turbulent burning velocity. 6 5 U = 1 m/s U = 5 m/s w/ turbulent generator 2 2 w/o turbulent generator 1 Kobayashi (22) 1 Bradley (1992) u'/s L u'/s L Figure 11. Normalized turbulent burning velocity for CH 4 /air presented with all data combined (a) and displayed for constant U (b). Also included are fits by Konbayashi 15 and Bradley 5 to the combined data and Filatyev et. al. for the constant U data 16. S T /S L 8 7 (a) S T /S L 7 6 (b) U = 15 m/s 7

8 To study the effect of nonchemically frozen reactants on turbulent flame propagation and turbulent flame regimes, n-heptane was chosen for investigation due to its low temperature reactivity. n- Heptane undergoes a two-stage ignition process at temperatures exceeding 55 K as indicated by the modeled ignition delays shown in Fig. 12. (Kinetic mechanism by Mehl et. al. 17,18 ). As previously mentioned, the RATS burner has controllability over the reactant temperatures and the heated residence time so that the low temperature chemistry reactions can be carefully controlled to investigate the influence of pre-flame reactivity on the turbulent flame. The effect of high reactant Temperature (K) Reactant residence time at U mean = 1 m/s First stage ignition Second stage ignition Time (ms) Figure 12. Modeled n-heptane/air ignition delay results at varying temperatures. temperatures on the n-heptane/air flames can be easily observed in the overall color of the flame. Long exposure CCD images at varying temperatures and residence times were taken to observe this phenomenon. As illustrated in Fig. 13, significant differences in the flame emission, indicating differences in chemistry, were observed. First, it is noticed that the flame shortens as the temperature increases indicating a rising flame propagation. It is also obvious that the flames at higher temperatures result in a product stream, which becomes redder as temperature increases. This emission is a result of products that result from a difference in the flame chemistry, which is induced as reactant temperature (i.e. reactivity) is increased. 6 K 65 K 7 K 5 K 6 K 65 K 7 K Figure 13. Series of n-heptane/air turbulent flames (φ =.5, U mean = 1m/s) with increasing reactant temperatures. Similar to the methane/air flames described above, averaged OH-PLIF images were acquired for the n-heptane/air flames at elevated temperatures to quantify turbulent flame speeds. The normalized turbulent flame speeds plotted versus the normalized turbulent intensity for the n-heptane/air flames at elevated temperatures and varying equivalence ratios can be seen in Fig. 14. Also included are turbulent flame speed predictions by Bradley which are derived with the following expression 8

9 ( ) (6) where δ L is the laminar flame thickness (δ L = 2α/S L ) and Le is the mixture s Lewis number (Le = α/d). The data presented in Fig. 14 spans a wide range of temperatures and equivalence ratios, which strongly influence the laminar 8 Normalized turbulent flame speed, S T /S L Prediction by current understanding 4 K - 6 m/s 5 K - 6 m/s 55-6m/s 6 K - 6 m/s 65 K - 6 m/s 7 K - 6 m/s 4 K - 1 m/s 5 K - 1 m/s 55 K - 1 m/s 6 K - 1 m/s 65 K - 1 m/s 7 K - 1 m/s 4 K m/s 5 K - 15 m/s 55 K - 15 m/s 6 K - 15 m/s 65 K - 15 m/s 7 K - 15 m/s 55 K m/s 6 K m/s 65 K m/s Turbuluent intensity, u'/s L Figure 14. Culmination of normalized turbulent flame speed data taken spanning reactant temperatures from 4 7 K and mean velocities from m/s. Predictions provided by Bradley 5 and are represented by the dashed and solid lines. flame speed and, as a result, the laminar flame thickness. The lines representing the predicted S T /S L values by use of Eqn. 6 in Fig. 14 are computed with the minimum and maximum laminar flame thicknesses encountered in the entire set of experimental data. Thus, the predicted lines should encompass the entire data set. It should be noted that Le, l, and u are relatively constant for all data points and are thus not considered in computing the bounds in the S T /S L predictions using Eqn. 6. As expected, a majority of the experimental data is contained within the theoretical predictions. However, there are a number of data points that exhibit significantly higher normalized turbulent flame speeds than Eqn. 6 would suggest. These data, which are circled in Fig. 14, represent points in which the temperature and the heated residence time of the reactants exceed the necessary condition for first stage ignition to occur. In other words, these data represent flames in which the reactants have undergone first stage ignition inside the RATS burner prior to entering the turbulent flame. This results in a reactant stream with increased reactivity and an increased ignition Damköhler number resulting in a new low temperature ignition (LTI) turbulent flame regime, which has the effect of accelerating the turbulent flame speed. It should be noted that as expected, the chemically frozen n-heptane/air normalized turbulent flame speeds (i.e. those that are encompassed by the predictions) are lower than those measured for methane/air flames at 3 K (Fig. 11a). This can be attributed to the differences in transport properties between the two fuels as indicated by the significant differences in Lewis number, which are ~1 for methane/air and ~3 for n-heptane/air. However, the normalized turbulent flame speeds for the reactor assisted n-heptane flames are very similar to the methane/air values. Thus, the effect of lowtemperature chemistry on the transport properties of the reactants and the subsequent influence on the turbulent flame needs to be better understood and is motivation of future experiments. 9

10 Based on the ignition delay simulations presented in Fig. 12, the first stage ignition of n-heptane produces a substantial concentration of CH 2 O. To determine the extent of low temperature chemistry as a result of varying temperature and residence times, CH 2 O was monitored at the nozzle exit without a flame using PLIF. The PLIF intensity was measured and is plotted versus heated residence time in Fig. 15. As expected, a decreasing heated residence time results in a decrease in the CH 2 O signal (i.e. CH 2 O concentration). In addition, reducing the temperature from 7 K to 6 K also reduces the CH 2 O concentration. Therefore, it is not a coincidence that the turbulent flame speed data, which exceeds the predicted values, are from flames that were generated at elevated temperatures above 55 K and occurred for reactants experiencing long heated residence times. To further investigate the role of LTI on the turbulent flame, single shot CH 2 O PLIF images were taken of flames at cases, which demonstrate the accelerated flame speeds, and at cases, which are predicted by Eqn. 6. Images of the two flames can be seen in Fig. 16. Despite the low signal to noise, it is very clear to see the difference in flame structure. The flame at 55 K (1 m/s exit velocity) results in CH 2 O isolated within the thin reaction zone, while the flame at 65 K (1 m/s exit velocity) has a distributed CH 2 O concentration resulting in an indistinguishable thin reaction zone. 4. Conclusion Normalized Intensity A new Reactor Assisted Turbulent Slot (RATS) burner capable of generating highly turbulent reacting flows has been designed, constructed, and tested. The RATS burner is capable of studying turbulent combustion of both gaseous and liquid fuels at elevated temperature up to 7 K providing turbulent flame conditions that simulate the complex environment experienced in high-speed propulsion engines. Turbulent intensities and length scales for the RATS burner have been quantified using hot wire anemometry. Results indicate a moderate yet uniformly distributed level of turbulence, which can be enhanced through the use of new turbulent generating plate geometries with larger blockage ratios. Two different turbulent combustion regimes, a conventional high temperature turbulent flame regime and a new low temperature ignition turbulent flame regime were identified by measuring turbulent flame structures (OH and CH 2 O PLIF), flame surface density, and flame speeds, at different initial fuel mixture temperatures. The results of the CH 4 /air and low temperature/short residence time n-heptane/air turbulent flame speeds indicate the conventional thin reaction zone turbulent flame regime and are consistent with previously derived turbulent flame speed correlations. However, for n-heptane/air mixtures, as the Nozzle Exit CH2O PLIF Intensity 7 K 65 K 6 K Residence Time (ms) Figure 15. Nozzle exit CH 2 O PLIF intensity with varying temperatures and exit velocities (i.e. heated residence time). 55 K 65 K Figure 16. CH 2 O PLIF images at varying reactant temperature (U mean = 1 m/s, φ =.4). Distinct differences in flame structure are observed as a result of pre-flame low temperature chemistry. 1

11 initial mixture temperature increases it is found that a new low temperature ignition (LTI) turbulent flame regime occurred. The new turbulent flame regime was confirmed by the CH 2 O PLIF images. The results show that the turbulent flame speeds in the new turbulent flame regimes increase significantly and are not bounded by the conventional turbulent flame speed correlation. The present results suggest as the ignition Damköhler number increases, the low temperature chemical reactivity of large hydrocarbon fuels plays a strong role on the pre-flame fuel oxidation, transport, ignition, and turbulent flame speed. This new phenomenon needs to be addressed in turbulent combustion relevant to practical propulsion systems. Acknowledgements This research was funded by the Air Force Office of Scientific Research Energy Sciences and Combustion Research program Grant No. FA The first author thanks the CEFRC (Combustion Energy Frontier Research Center) of Princeton University for the Postdoctoral fellowship. YJ and SHW acknowledge discussions with Dr. Campbell and other members of the Air Force Research Laboratory. References 1 Torrez, S. M., Driscoll, J. F., Ihme, M., Fotia, M. L., Reduced order modeling of turbulent reacting flows with application to ramjets and scramjets, Journal of Propulsion and Power, Vol. 27, 211, Poludnenko, A.Y. and Oran, E.S., The interaction of high-speed turbulence with flames: Turbulent flame speed, Combustion and Flame, Vol. 158, 211, pp Bell, J. B., Day, M. S., Grcar, J. F., Lijewski, M. J., Driscoll, J. F., and Filatyev, S. A., Numerical simulation of a laboratory-scale turbulent slot flame, Proceedings of the Combustion Institute, Vol. 31, 26, pp Driscoll, J. F., Turbulent premixed combustion: Flamelet structure and its effect on turbulent burning velocities, Progress in Energy and Combustion Science, Vol. 34, 28, pp Bradley D., How fast can we burn?, Proceedings of the Combustion Institute, Vol. 24, 1992, pp Kobayashi, H., Kawahata, T., Seyama, K., Fujimari, T., and Kim, J-S., Relationship between the smallest scale of flame wrinkles and turbulence characteristics of high pressure, high temperature turbulent premixed flames, Proceedings of the Combustion Institute, Vol. 29, 22, pp Ayoola, B. O., Balachandran, R., Frank, J. H., Mastorakos, E., Kaminski, C. F., Spatially resolved heat release rate measurements in turbulent premixed flames, Combustion and Flame, Vol. 144, 26, pp Peters, N., Turbulent Combustion, Cambridge University Press, New York, 2, Chap Dooley, S., Won, S. H., Chaos, M., Heyne, J., Ju, Y., Dryer, F. L., Kumar, K., Sung, C-J., Wang, H., Oehlschlaeger, M. A., Santoro, R. J., and Litzinger, T. A., "A Jet Fuel Surrogate Formulated by Real Fuel Properties", Combustion and. Flame, Vol. 157, No. 12, 21, pp Coriton, B., Frank, J. H., Hsu, A. G., Smooke, M. D., Gomez, A., Effect of quenching of the oxidation layer in highly turbulent counterflow premixed flames, Proceedings of the Combustion Institute, Vol. 33, 211, pp Coppola, G., and Gomez, A., Experimental investigation on a turbulence generation system with high-blockage plates, Experimental Thermal and Fluid Science, Vol. 33, 29, pp Oppenheim, A.V. and Schafer, R.W., Discrete-Time Signal Processing, Prentice-Hall, Upper Saddle River, NJ, Barrett, M.J. and Hollingsworth D.K., On the calculation of length scales for turbulent heat transfer correlation, Journal of Heat Transfer Transactions of the ASME, Vol. 123, 21, pp Hinze, J.O., Turbulence, McGraw-Hill, New York,

12 15 Kobayashi, H., Experimental study of high-pressure turbulent premixed flames, Experimental Thermal and Fluid Sciences, Vol. 26, 22, pp Filatyev, S. A., Driscoll, J. F., Carter, C. D., and Donbar, J. M., Measured properties of turbulent premixed flames for model assessment, including burning velocities, stretch rates, and surface densities, Combustion and Flame, Vol. 141, 25, pp Mehl, M., Pitz, W.J., Westbrook, C.K., Curran, H.J., "Kinetic Modeling of Gasoline Surrogate Components and Mixtures Under Engine Conditions", Proceedings of the Combustion Institute, Vol. 33, 211, Mehl, M., Pitz, W.J., Sjöberg, M., and Dec, J.E., "Detailed kinetic modeling of low-temperature heat release for PRF fuels in an HCCI engine," SAE 29 International Powertrains, Fuels and Lubricants Meeting, SAE Paper No , Florence, Italy,