OH-PLIF Images Research Articles

On turbulent burning velocities of V-shaped turbulent premixed flames with circular fractal turbulence generators

The characteristics of turbulent burning velocities of V-shaped turbulent flames are experimentally investigated by varying the shape parameters of cross- and square-type circular fractal turbulence generators (FTG). For both type FTGs, the thickness reduction ratio, Rt, of 0.4, 0.6, 0.8, and 1.0, and the blockage ratios, σ, of 30 and 50% are used. The characteristics of non-reacting turbulent flows such as the mean flow velocity, U¯, velocity fluctuation, u′, and integral length scale, L, are measured using a hot-wire anemometer while the progress variable of reacting flows is evaluated using OH-PLIF images. The turbulent premixed flames are distributed in the Borghi-Peters diagram from the wrinkled flamelet regime to the thin reaction zone regime, depending on the shape parameters of the FTG. The local displacement speed, ST,LD, and the local consumption speed, ST,LC, are evaluated using the measured data of turbulent reacting and non-reacting flows. ST,LD and ST,LC are found to increase with increasing σ and decreasing Rt, and hence, the averaged ST,LD, and ST,LC can be expressed as a function of u′/SL and/or L/δL. Correlations between ST or u′ and the FTG shape parameters (Rt and σ) are also proposed, which can be used to control and predict ST and u′ for fundamental studies and industrial applications.

Effects of the secondary air on the combustion characteristics of turbulent premixed CH4/NH3/air flames in a two-stage swirl combustor

This study experimentally investigates the effects of secondary air injection on the flame structure and CO/NO emission characteristics of turbulent premixed CH4/NH3/air flames in a two-stage swirl combustor. The equivalence ratio (ϕp) and velocity (Up) of the primary fuel/air jet and the injection location (Ls) and velocity (Us) of the secondary air jet are varied. The location of the secondary air injection of 30 and 250 mm are adopted to investigate two interaction cases in two-stage combustion (TSC): 1) a strong interaction where the secondary air is directly injected into the primary premixed flame and strongly affects the flame, and 2) a weak interaction where the secondary air is injected after the primary reaction is completed. A single-stage combustion (SSC) is also tested for comparison. It is found from averaged OH-PLIF images that the strong interaction cases of TSC with ϕp > 1 can produce a significant amount of NO emissions due to the direct interaction between the primary flame and the secondary air, while the weak interaction cases can produce an appreciable amount of CO emissions due to the incomplete combustion of the secondary flames. This result is further confirmed by CO/NO measurements that the strong interaction cases produce higher NO emissions than the weak interaction cases, while the CO emissions of the weak interaction cases become significant and peak at ϕp = 1.1 ∼ 1.2. Based on the OH-PLIF images and CO/NO emission characteristics of TSC, the optimal conditions of the primary and secondary jets are proposed to ensure the reduction of NOx emissions with low level of CO emissions.

Visualization of post-detonation fireball flowfields and comparison to CFD modeling

Visualization of flow structures within post-detonation fireballs has been performed for benchmark validation of numerical simulations. Custom pressed PETN explosives with a 12-mm diameter hemispherical form factor were used to produce a spherically symmetric post-detonation flow with low soot yield. Hydroxyl-radical planar laser induce fluorescence (OH-PLIF) was employed to visualize the structure ranging from approximately 10μs to 35μs after shock breakout from the explosive pellet. Fireball simulations were performed using the HyBurn Computational Fluid Dynamics (CFD) package. Experimental OH-PLIF results were compared to synthetic OH-PLIF from post-processing of CFD simulations. From the comparison of experimental and synthetic OH-PLIF images, CFD is shown to replicate much of the flow structure observed in the experiments, revealing potential differences in turbulent length scales and OH kinetics. Results provide significant advancement in experimental resolution of these harsh turbulent combustion environments and validate physical models thereof.

Extraction of turbulent flame structures and dynamic modes with corrected OH-PLIF images for a hydrogen micromix burner

Diluted micromix combustion technology is an effective way to realize stable H2 flames with low NOx emission, and the turbulent flame structures of a micromix burner are experimentally revealed in this work with corrected OH-PLIF images.

The Effect of Ignition Procedure on Flashback of Hydrogen-Enriched Flames

Abstract The impact of different ignition sequences on the ignition dynamics of CH4-H2 flames in a bluff body burner is investigated at atmospheric conditions. Experiments are performed over a wide range of operating conditions covering pure methane injection (PH0) to pure hydrogen injection (PH100). A perforated plate of total porosity σ=0.17 is positioned at the outlet of the combustion chamber to increase the chamber back pressure and trigger transient flashback during ignition. Time-series of pressure, OH* chemiluminescence and OH-PLIF images of the propagating flame branch are recorded simultaneously to characterize the ignition process. Two ignition procedures are investigated. For ignition procedure A, designated as an early ignition procedure, the spark is initiated before fuel injection. For ignition procedure B, designated as a late ignition, the spark is only activated after the fuel injection. The impact of the fuel air mixing on the final stabilization state is investigated by changing the fuel delivery time (dt) before the initial spark. Three different time delays are considered dt = 1, 3, and 5 s. The final state of the flame is found to be highly sensitive to the selected ignition procedure and increasing dt favors the occurrence of flashback. At constant power, the magnitude of the pressure peak is driven by a competition between the fuel mass flowrate at the moment of ignition and the high reactivity of hydrogen, which shifts the flammability limit toward lower equivalence ratios, hence generating a lower reaction rate. For procedure A, the peak of the chamber over pressure shows a nonmonotonic growth for increasing levels of H2 in the fuel blend, while it linearly increases for procedure B. Experiments are then conducted at a fixed injection flow velocity Ub = 5 m s–1 and fixed laminar burning velocity Sl0=0.25 m s–1 by varying the level of hydrogen enrichment. For procedure A, the over pressure amplitude decreases with increasing the hydrogen enrichment leading to a soft ignition for all CH4-H2 blends. Under ignition procedure B, the amplitude of the over pressure reached during ignition is found to be relatively unaffected by the hydrogen concentration, but the flame stabilization mode shows a strong dependence to both the level of H2-enrichment and fuel delivery time dt. OH* as well as OH-PLIF images reveal that the trajectory of the flame leading point changes as dt increases. The different dynamics of the flame leading points is likely to be the cause the different types of stabilization modes observed.

Effect of low fuel temperature on combustion deterioration of kerosene swirling spray flames using OH-PLIF

The low fuel inlet temperature poses challenges to the flame stability of aircraft engine combustors. In order to investigate the combustion deterioration of swirl-stabilized kerosene spray flames under conditions of low fuel inlet temperature (T < −16 °C), a comprehensive system was established, comprising a burner commonly used in aircraft engines and a cooling setup. Upon analyzing OH-PLIF images, it becomes evident that sub-zero Celsius temperatures significantly lead to a decrease in OH radical concentration, resulting in a diminished heat release rate. The strongest signals decrease by a minimum of 5 times. Additionally, through the analysis of LIF spectra and detuned images, the differences between OH-LIF and kerosene-LIF are beneficial for recognizing the kerosene particles from the OH-LIF signals at the same time. For atomization assessment, employing the LOG operator blob detection method, it is found that at sub-zero Celsius temperatures, the overall count of fuel LIF particles experiences a slight reduction, while the count of larger particles increases across various air flow rates. This observation highlights an inferior droplet distribution at lower fuel temperatures. The image processing of kerosene LIF particles opens up the possibility for a rapid evaluation of atomization, especially when assessing combustion deterioration in kerosene spray flames through the use of a simple OH-PLIF technique.

10-kHz-rate two-color dual-plane stereo-PIV using an optical parametric oscillator

10 kHz-rate dual-plane stereoscopic particle-image velocimetry (DSPIV) was applied in a swirling jet flame measurement simultaneously with hydroxyl planar laser-induced fluorescence (OH-PLIF). A high-speed burst-mode laser system with an optical parametric oscillator were used in double-pulse laser generation of 532 nm for one plane and 607 nm for the second plane. Temporally-resolved particle image velocimetry (PIV) image sequences, nine-component velocity gradient tensor, and simultaneous OH-PLIF images are presented. Flow vorticities and strain rates are also obtained with the measured velocity gradients. Compared with previous polarization-based dual-plane PIV technique, two-color based DSPIV measurement has distinctive advantages in elimination of signal interference from different PIV plane due to polarization leakage and hence improves measurement accuracy.

The effects of steam dilution on flame structure and stability for a H2/air micromix burner

The present work is dedicated to understand the effects of steam dilution on the structure and stability of a micromix H2/air swirl flame. The flame structure and stability were obtained through numerical simulations using Large Eddy Simulation (LES) with the Eddy-Dissipation-Concept (EDC) combustion model. In addition, OH-PLIF images and temperature distribution were used to verity the computed results. The experimental and simulated OH distributions show that great part of OH distributes on midstream and downstream of the flame. From the numerical results, it can be seen that the thermal-dilution effect of steam has the biggest impact on flame temperature. Detailed analysis show that the thermal-dilution effect of steam lowers the OH mass fraction, which is the same as third-body effect, however, the OH mass fraction is increased by the direct chemical effect due to decrease the net reaction rate of R4 (H2 + OH = H2O + H). The stability of the micromix hydrogen flame could be reduced due to the thermal-dilution effect, but the direct chemical effect could slightly improve the stability of the micromix hydrogen flame which is the same as third-body effect.

Influence of the Pilot Flame on the Morphology and Exhaust Emissions of NH3-CH4-Air Swirl Flames Using a Reduced-Scale Burner at Atmospheric Pressure

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

Lean blow-off of premixed swirl-stabilised flames with vapourised kerosene

Lean blow-off (LBO) conditions, duration of the extinction transient, and flame structure were investigated for swirl-stabilised, fully-premixed flames operated with methane and vapourised kerosene. High-speed (5 kHz) OH* chemiluminescence, OH-, and Fuel-PLIF imaging were used. Low-speed (10Hz) CH2O-PLIF imaging was also implemented to visualize the structure of CH2O. OH* chemiluminescence imaging of the LBO transient suggested that the primary reaction regions were confined within the central recirculation zone (CRZ). Sharp temporal increases in OH* signal were observed in the CRZ for both fuels, possibly due to flame re-ignition events during LBO transients. Both fuels showed a decrease in flame length and an increase in fragmentation in downstream regions. In the OH-PLIF images, pockets void of OH entered the CRZ from downstream. The CH2O- and Fuel-PLIF images suggest that these pockets contain either partially burned or fresh reactants. Furthermore, the CH2O- and Fuel-PLIF images indicated that CH2O and Fuel exist in the outer recirculation zone (ORZ) during the early stages of the LBO transient. The blow-off duration was calculated by integrating the OH* signal and for kerosene was found to be 1.5 times longer than for methane. In addition, the equivalence ratio at which kerosene blew-off was consistently lower than that of methane for a range of bulk flow velocities. These results indicate that fuel chemistry effects are important for the LBO condition and duration.

Simulation of Plasma Assisted Supersonic Combustion over a Flat Wall

This work presents a simplified methodology to couple the physics of a nanosecond pulsed discharge to the process of supersonic combustion in a flat wall combustor configuration. Plasma and supersonic combustion are separately simulated and then coupled by seeding plasma-generated radicals on the combustion domain. The plasma model is built assuming spatial uniformity and considering only the kinetic effects of the nanosecond pulsed discharge. Therefore, a zero-dimensional kinetic scheme accounting for the generation of plasma species is utilized. For the combustion model, the complete set of Favre-averaged compressible Navier Stokes equations along with finite rate chemistry is solved through a control-volume based technique via the commercial software Ansys Fluent. The computational results are compared against experimental studies showing that the proposed methodology can capture the main kinetic effects of the nanosecond pulsed discharge on supersonic combustion. OH concentration contours reveal the presence of an enhanced flame when the plasma is applied following the trends from experimental OH PLIF images. In addition, time evolving temperature and OH concentration contours show that the ignition delay time is reduced with the application of the discharge.

Stability and characteristics of NH3/CH4/air flames in a combustor fired by a double swirl stabilized burner

Compared to hydrocarbons, ammonia's low reactivity and higher NOx emissions limit its practical application. Consequently, its implementation in combustion systems requires a different combustor geometry, by adapting existing systems or developing new ones. This study investigates the flame stability, NO emissions, and flame structure of NH3/CH4/air premixed flames in a novel combustor comprising a double swirl burner. A lean premixed CH4/air mixture of equivalence ratio, Φout, was supplied to the outer swirl, while a NH3/CH4/Air mixture fed the inner swirl. The molar fraction of NH3 in the inner fuel blend, xNH3, was varied from 0 (pure CH4) to 1 (pure NH3) over far-lean to far-rich inner stream equivalence ratio, Φin. This new burner's stability map was established in terms of Φin versus xNH3 for different Φout. Then, NO emissions were measured versus Φin for various xNH3 and Φout. Finally, based on the NO emissions, eight flames were down-selected for in-flame measurements, which included temperature and OH-PLIF. The stability measurements revealed that increasing xNH3 modifies the stability map by increasing the lean blowout limits and narrowing the flashback region. At Φout ≥ 0.6, a stable flame was achieved for a pure inner NH3/air mixture. Low NO emissions were achieved in this burner configuration at xNH3=1 by either enriching or far-leaning Φin. Enriching Φin led to a steep decrease in NO concentrations. However, to achieve low NO concentrations, precise control of Φout was needed. At Φin=1.4, 220 ppm NO at Φout=0.7 versus 690 at Φout=0.6 was measured. Moreover, substantially enriching Φin>1.2 led to a slight decrease in measured NO. Generally, the OH-PLIF images revealed a conical OH-layer at the burner exit. Certain flame conditions created OH-pockets inside the conical structure or formed a V-shaped OH-layer far downstream. This change in flame structure was found to impact NO emissions strongly.

Instantaneous flame front identification by Mie scattering vs. OH PLIF in low turbulence Bunsen flame

Simultaneous OH-PLIF and Mie scatter imaging were used to investigate turbulent premixed flame edge detection under a range of turbulence characteristics on a stabilised piloted Bunsen burner. A 527 nm wavelength laser beam is used to generate a Mie scattering sheet at 500 Hz, and a 283 nm wavelength laser sheet is created using 355 nm wavelength laser to pump an optical parametric oscillator to induce florescence from OH radicals at 5 Hz. A phase-locking technique is applied to synchronize and lock the two laser systems. The number density method has been used to detect flame edges in Mie scattering images, and three algorithms were applied to OH-PLIF images as a reference. A comparison of the methods and different parameter setting is made by using the metrics of location difference, flame surface density and curvature of flame edges. The processed data show that once a well-tuned window size is determined by applying the number density method, averaged spatial differences between Mie scattering images and OH-PLIF images are of the order of or smaller than the laminar flame thickness, demonstrating that under these conditions, high frequency Mie scatter measurements can be used as well as OH-PLIF images to define the flame edge at that spatial resolution. The positive result confirms that double-frame Mie scattering allows the measurement of high frequency conditional velocity distributions and flame properties simultaneously using solely Mie scattering, provided that the particle density is suitably designed to be around 16 px^2 per particle.Graphic

On the impact of H[formula omitted]-enrichment on flame structure and combustion dynamics of a lean partially-premixed turbulent swirling flame

Large Eddy Simulation (LES) with Conjugate Heat Transfer (CHT) is used to analyze the impact of H2-enrichment on the flame structure and combustion dynamics of a lean partially-premixed turbulent CH4/Air swirling flame. Experimentally, the combustor is operated at atmospheric pressure with H2 fuel fractions of up to 50%, by volume. LES-CHT results are compared and validated against time-resolved stereo PIV, OH* chemiluminescence, OH-PLIF imaging and acoustic pressure measurements. In terms of dynamics, for the pure CH4 and 20% of H2 enrichment cases, no thermoacoustic oscillation is observed in either the experimental or numerical data. As the fuel fraction of hydrogen is increased, the flame length reduces due to the increase in laminar flame speed and the heat release rate distribution becomes more compact. CHT simulations reveal that H2-enrichment leads to higher temperatures at the centerbody tip. At 50% H2, in agreement with experiments, LES predicts a bi-modal thermoacoustic oscillation, with two main frequencies corresponding to the quarter and chamber modes of the system. Dynamic Mode Decomposition is performed on the measured OH-PLIF images and LES 3D fields to extract each mode contribution to the overall flame dynamics. It is observed that both modes are characterized by local variations of equivalence ratio, while only the higher frequency (chamber) mode is characterized by vortices periodically detaching from the backplane and the centerbody walls causing a strong periodic wrinkling of the flame front during the thermoacoustic oscillation.

Effect of hydrogen addition on flame stability and structure for low heating value coaxial nonpremixed flames

ABSTRACT An investigation of effect of hydrogen addition on flame stability and structure i.e. combustion characteristics of low heating value gases (LHVGs) that consist of methane, propane, nitrogen is carried out experimentally and numerically. For the experiment, a coaxial non-premixed jet-type combustor and Planar Laser Induced Fluorescence (PLIF) system are used to evaluate the flame stability limit and flame shapes and visualize OH radical distribution, respectively. For the numerical analysis of hydrogen added LHVGs, the calculation using the FLUENT software with decreased GRI 3.0 mechanism is performed to study the 2D flame temperature distribution and radial flame structure. As a result, the flame length of LHVGs reduces as hydrogen is added. CH* and OH* radicals visualized by chemiluminescence system are distributed closer to the nozzle as hydrogen is added to LHVGs. OH-PLIF images show that OH intensity of LHVG 6000 is higher with hydrogen addition. The flame stability of LHVGs is improved with hydrogen addition. As for the numerical result, the reduction of flame length and width of LHVG 6000 are predicted with adding the hydrogen.

Effect of pilot hydrogen on the formation of dynamic flame in an ethylene-fueled scramjet with a cavity

Pilot hydrogen is used for the ignition of gaseous ethylene to improve combustion efficiency at low flight Mach numbers. The process of dynamic flame stabilization has been investigated experimentally in a scramjet equipped with a cavity by using a variety of fuel injection strategies involving the injection of pure hydrogen, pure ethylene, and combinations of fuels to understand the effects of pilot hydrogen on supersonic combustion. This study reports experiments conducted in a direct-connected facility that simulated flight conditions at a Mach number of 4, a total temperature of 953 K, and a total pressure of 0.82 MPa. The data were collected by using hydroxyl planar laser-induced fluorescence (OH-PLIF), methylene (CH), luminosity images, and 10-kHz static pressure transducers. The results show that the internal flow oscillated at a dominant frequency of approximately 450 Hz without fuel injection. Oscillation in internal flow was suppressed when pilot hydrogen was injected into the cavity, whereas the influence of other injection strategies was minor. The OH-PLIF images indicate that the pilot flame was unstable, with different locations and occupying different areas. The normalized pressure was apparently higher when pilot hydrogen was injected ahead of the cavity step. The pure ethylene flame was difficult to stabilize without the application of pilot hydrogen. Once the pilot hydrogen had been turned off, the ethylene flame moved toward the cavity ramp and its area expanded. Thus, pilot hydrogen is a precondition for the stabilization of the ethylene flame.

Effect of DME addition on turbulent flame structure in lean premixed CH4/DME/air mixtures

An experimental study was conducted on DME addition to lean premixed CH4/air Bunsen flame in order to investigate differential diffusion and turbulence effects on flame structure. Three DME volume fractions of 0, 20% and 50% were adopted in the presence of carefully controlled laminar burning velocity at a near-constant level to highlight the differential diffusion effects. By extracting the flame front from the OH-PLIF images, the flame front curvature, turbulent burning velocity and mean flame surface density were obtained and discussed. Results show that DME addition and turbulence will eventually make the curvature distribution more symmetrical. In addition with the turbulence intensity increasing, the DME addition seems to have minor effect on curvature. The turbulent burning velocity is decreased with DME addition, which is in agreement with earlier research results for Le > 1 mixtures. In this study, we prove that the mean flame surface density at different height can qualitatively represent the local consumption speed (ST,LC). DME addition only changes the ST,LC distribution structure of the flame, while turbulence not only changes the structure, but also increases the maximum ST,LC value. It is also found that the decrease of turbulent burning velocity (ST,GC), caused by DME addition at low turbulence intensity, is mainly manifested by an increase of the flame height. Moreover at thin reaction zone, the interaction between turbulence and differential diffusion effect promotes the change of ST,LC distribution. Therefore, the effect of differential diffusion on ST,GC is enhanced by turbulence as the diffusion layer is broadened.

Flame Structure Characterization in a Dual-Mode Scramjet Using Hydroxyl Planar Laser-Induced Fluorescence

The flame front of a premixed ethylene–air turbulent flame in a high-speed flowfield was imaged using hydroxyl radical (OH) planar laser-induced fluorescence (PLIF). An electrically heated continuous flow facility produced inflow conditions with a total temperature of 1200 K, corresponding to a flight Mach number of about five. A statistically stationary turbulent flame stabilized over a scaled-down cavity was used to generate experimental data for future comparisons with direct numerical simulations. A laser sheet with a wavelength of 283.55 nm was used to excite the (8) transition of OH. The sheet waist had a full width at half-maximum of . Two-dimensional OH-PLIF images were collected at high in-plane spatial resolution (approximately ) with a field of view of . The images spanned a region of along the spanwise centerplane. The local OH gradient was used to isolate the flame front in all images through an automated process. Each image was processed to obtain the appropriate statistics, time-averaged flame surface density, and flame front curvature values. The distribution of local flame front curvature values was approximately constant across the domain.

Effect of the jet fuel cetane number on combustion in a small-bore compression-ignition engine

Three custom-made jet fuels with cetane numbers 30, 40 and 51 are investigated in an optically accessible small-bore compression-ignition engine. The low-temperature and high-temperature reactions are visualised using planar laser-induced fluorescence imaging of formaldehyde (HCHO-PLIF) and hydroxyl radicals (OH-PLIF) and is complemented with high-speed, line-of-sight integrated signal imaging of cool-flame and OH* chemiluminescence. For soot measurements, planar laser-induced incandescence (soot-PLII) imaging is performed. The cool-flame chemiluminescence and HCHO-PLIF images show that the low-temperature reaction develops quicker and covers a larger area in the combustion chamber for higher cetane number fuels. Also, the OH* chemiluminescence and OH-PLIF images indicate the transition from low-temperature reaction to high-temperature reaction occurs faster in both spatial distribution/concentration and temporal evolution, indicating the predominant effect of higher fuel reactivity despite lower charge premixing. The results obtained for single injection conditions are extended to pilot-main injection conditions in an attempt to further reduce the charge premixing to the level that its impact becomes measurable. Indeed, the significantly reduced charge premixing condition induces lower HCHO-PLIF and OH-PLIF for CN50 than those of CN40 while the soot-PLII is most intense. The CN30 exhibits the lowest soot-PLII than the other two fuels due to the enhanced charge premixing but the OH-PLIF signals are weaker due to the lower fuel reactivity. This study successfully identifies the important trade-off characteristics between fuel reactivity and charge premixing among the tested fuels and given injection conditions, CN40 shows the most optimised performance with the strong high-temperature reaction and low remaining in-cylinder soot.

Experimental investigation of unconfined turbulent premixed bluff-body stabilized flames operated with vapourised liquid fuels

The structure of turbulent unconfined bluff-body flames of vapourised liquid fuels was investigated at conditions far from and close to blow-off with high-speed (5 kHz) OH-PLIF imaging and 10 Hz CH2O-PLIF imaging. Four different fuels were considered: ethanol, heptane, and two different kerosene blends (a conventional Jet-A and an alcohol-to-jet kerosene, respectively denoted as A2 and C1 following the USA National Jet Fuels Combustion Programme. OH-PLIF images of ethanol flames far from blow-off display a high intensity of OH-LIF signal along the shear layer. In contrast, the OH-LIF signal was evenly distributed throughout the recirculation zone (RZ) of the heptane and kerosene flames. Regardless of the fuel used, close to blow-off the flame becomes shorter with peak OH-LIF signal intensities lying inside the RZ. All four fuels showed a decrease in flame surface density (Σ2D) and broadening of the 2-D curvature PDFs as their blow-off limits were approached. An increase in local turbulent consumption speed was observed in the downstream region as the flames approached blow-off. No significant variation in Σ2D, curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH2O-layer thickness was larger than the computed laminar flame value by a factor of two and six for conditions far from and close to blow-off, respectively. Moreover, heptane and kerosene flames showed more pockets of CH2O-LIF signal within the RZ as compared to ethanol, suggesting that considerably more partially-combusted fluid enters the RZ of the former than the latter. High-speed particle image velocimetry was performed to measure the local velocity fields and place various regions of the flame on the turbulent premixed regime diagram. It was observed that, regardless of fuel type, conditions close to blow-off occupy the same region on the regime diagram. However, the fact that the fuel type results in differences in some structural features near blow-off suggests that flames produced with heavy hydrocarbon fuels involve chemistry effects at blow-off that are not fully characterized by laminar flame properties.