Contemporary interest exists for understanding how reaction zones stabilize and counter-propagate against incoming reactants. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. Observations of the leading-edge flame behavior with respect to upstream propagation and recession downstream are made with sequential CH-PLIF imaging, and data have been revisited in light of the recent research of McCraw et al. (Flow Turbul Combust 70(1):83–97, 2007). It is found that in cases where a distinct branch of the outer (fuel-lean) edge of the reaction zone is present, the edge of the flame is either witnessed to propagate upstream or locally disappear. In cases where no distinct branch other than the main branch is observed, the flame is witnessed to either remain stationary or drop back downstream. These observations support the notion that structures in the low speed, outer edge of the reaction zone are involved in the upstream phase of the flame propagation.

Contemporary interest exists in understanding the roles of leading edge flow deflection, secondary jet instabilities and islands of ignited gases in permitting lifted flames to stabilize. To assess these issues, elements of the leading-edge of a lifted turbulent jet flame have been investigated using laser-imaging techniques. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. In particular, evidence of flame islands, or flame fragments, upstream of the bulk-flame leading edge are reported and discussed. This evidence is presented in the form of sequential CH-PLIF images and well as CH-PLIF/Rayleigh scattering images. Images showing thermal characteristics of the regions surrounding the edge flame are also described.

This work examines the dynamics of confined bluff body flames in the process of lean blowoff (LBO) using simultaneous stereo-PIV (particle image velocimetry), OH PLIF (planar laser induced fluorescence) and formaldehyde (CH2O) PLIF. Flames at high density ratios blow off in at least two distinct stages: stage 1, where intermittent extinction occurs along the flame front, but the flame and flow remain qualitatively similar to stable conditions, and stage 2, where there is permanent downstream flame extinction and large-scale changes in dynamic flow characteristics. This paper particularly focuses on stage 2 processes, with the goal of understanding what ultimately leads to irrecoverable flame blowoff. A test facility was developed with the flexibility to achieve two goals: (1) approach LBO by keeping the parameters that influence its hydrodynamic stability approximately constant, particularly flow velocity (ubulk) and gas expansion ratio (σ), and (2) compare near-LBO dynamics under conditions where, well away from blowoff, the flame is globally stable (high σ case) and globally unstable (low σ case, where the Bénard-Von Karman, BVK, instability of the flow is present). The latter case was of particular interest as most prior detailed diagnostic studies of LBO have been performed at high σ, BVK-suppressed conditions. We find that the transient blowoff process remains largely unchanged in the high and low σ cases, apparently due to the fact that the BVK instability reappears in either case under conditions very close to LBO. In all cases, blowoff is preceded by permanent downstream extinction that moves progressively closer to the bluff body as LBO is approached. We also find that near-LBO dynamics are intrinsically three dimensional, due to both secondary instabilities of the shear layer and large out-of-plane motions believed to be due to confinement effects associated with bluff body-wall interactions. These three-dimensional structures often manifest themselves as burning reactant fingers which are caught in the backflow of the recirculation zone; under very near LBO conditions they impinge on the back of the bluff body and extinguish as well. At the very edge of blowoff, the recirculation zone is no longer composed of hot products and is unable to autoignite the oncoming reactant flow, leading to global extinction. The characteristic time associated with this feedback between downstream extinction and wake structure alteration that leads to blowoff is about two orders of magnitude larger than the characteristic flow time, D/ubulk. We also discuss several implications of these results on computations of LBO - in particular, LBO's intrinsically three-dimensionality and the need for many flow through times to capture it.

Experiments were performed to examine the stability of hydrocarbon-fueled flames in cavity flameholders in supersonic airflows. Methane and ethylene were burned in two different cavity configurations having aft walls ramped at 22.5° and 90°. Air stagnation temperatures were 590 K at Mach 2 and 640 K at Mach 3. Lean blowout limits showed dependence on the air mass flowrates, cavity geometry, fuel injection scheme, Mach number, and fuel type. Large differences were noted between cavity floor and cavity ramp injection schemes. Visual observations, planar laser-induced fluorescence of nitric oxide, and shadowgraph imaging were used to investigate these phenomena. Cavity ramp injection provided better performance near the lean blowout limit, whereas injection from the cavity floor resulted in more stable flames near the rich limit. Ethylene flames have a wider range of stable operations than methane in all conditions. Lean blowout limits were not significantly different between the Mach 2 and Mach 3 cases at the lean limit; however, variation in Mach number had a measurable effect near the rich limit. Fuel flowrates at ignition were much greater than the lean blowout limit, but showed similar dependence on air mass flowrate.

Several previously unreported properties of turbulent premixed flames were measured because they are especially useful for the future assessment of direct numerical simulations and models. These new properties include local stretch rates, a wrinkling parameter, the degree of flamelet extinction, and the reaction layer thickness, which were quantified using simultaneous CH planar laser-induced fluorescence/particle image velocimetry (CH PLIF-PIV) diagnostics. Other reported properties that are useful for model assessment are flame surface density (Σ) and global consumption speed, which is one type of turbulent burning velocity. Also measured was the Meneveau–Poinsot stretch efficiency function (ΓK), which plays a central role in the coherent flamelet model. Some images of the flame–eddy interactions show how eddies exert strain and how flamelets “merge.” A highly wrinkled (corrugated) flame with well-defined boundary conditions was stabilized on a large two-dimensional slot Bunsen burner. It was found that the turbulent burning velocity of Bunsen flames depends on the mean velocity U¯, which was varied independently of turbulence intensity. It is concluded that conventional relations for the turbulent burning velocity of Bunsen flames are inadequate because they should include two additional parameters: mean velocity U¯ and burner width W. These parameters affect the residence times of the flame–eddy interactions. A scaling analysis is presented to explain the observed trends. It indicates that if the burner width is sufficiently large, the long flame will experience significant flamelet merging, which is one factor leading to the “bending” (nonlinear behavior) of the burning velocity curve. Images of CH layers show that flame surface area is lost by flamelet merging, but is not lost due to local extinction, as no extinction was observed. The stretch efficiency function increases with increasing integral scale, indicating that large eddies are more efficient in exerting flame stretch than small eddies.

An experimental investigation of the mixing and combustion processes that occur in and around a cavity-based flameholder in a supersonic flow is reported. Cavity-based flameholders are commonly found in hydrocarbon-fueledscramjet combustors; however, detailed information concerning the behavior of these devices, their optimal shape and fueling strategies, combustion stability, and interactions with disturbances in the main airflow (i.e., shock trains or shock-boundary layer interactions) is largely unavailable in the existing literature. This work is part of an ongoing research program aimed at providing information to help fill these voids and improve the overall understanding of cavities for use as scramjet flameholders.

Measurements of the scalar dissipation rate in the region immediately upstream of a lifted jet flame are presented. The scalar dissipation is determined in this isothermal region from a planar measurement of a two-dimensional conserved scalar (jet fluid) using laser Rayleigh scattering. Fields of the scalar dissipation rate are presented in addition to tabulated values for three different liftoff heights (Re-d = 4800; 6400, and 8300). Scalar dissipation rates do not reach levels thought to cause extinction of the leading edge based on comparison with extinction data for counterflow diffusion flames. Additionally, results are presented on the axial flame propagation velocities relative to the jet flow. The data indicate that over the three flow conditions, the flame velocity relative to the flow is approximately constant during the case of a quasi-stationary lifted flame. In light of these findings, it is suggested that concepts involving partially premixed flame propagation, rather than those of critical scalar dissipation rate, are central to modern lifted flame stabilization models.

OH planar laser-induced fluorescence (PLIF) and particle image velocimetry have been used to study the frequency response of laminar C3H8-air counterflow diffusion flames to assess the adequacy of the steady-flamelet models. Particle image velocimetry was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the two-line PLIF technique, and the reaction-zone width. Both measurements demonstrate that the frequency response of flames subjected to a time-varying flow field is diffusion-limited. At the 30-Hz and 50-Hz forcing frequencies, the maximum reaction-zone temperature and width were found to respond quasi-steadily. However, at higher forcing frequencies-i.e., 100 and 200 Hz-transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths were well fit by an offset sine function. In all cases when the oscillation amplitude is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, the results collapse onto a single line having a steep negative slope.

Joint two-shot CH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) measurements near the stabilization region of lifted methane/air diffusion flames stabilized under different flow conditions are presented. The simultaneous technique allows for a determination of the propagation rate of the CH zone relative to the fuel flow. Simultaneous single-shot CH-PLIF and PIV techniques have been used in the past to examine lifted jet flames; however, the double-shot technique of the current study is desirable because it yields information on flame dynamics-as indicated by sequential CH-PLIF-relative to the unburned mixture. Three flow conditions were examined corresponding to three different liftoff heights. While the average velocity at the stabilization point varies between 0.83 m/s for the lowest flow condition (Re-d = 4800) and 1.28 m/s for the highest (Re-d = 8300), the velocity at the stabilization point during instances of zero CH movement (during the time interval of the CH pulses) is constant for all three flow conditions (1.14 +/- 0.4 m/s). Furthermore, the flame is able to stabilize itself against the incoming unburned mixture only when the gas velocity is below a certain limit, above which the flame is convected downstream with the flow.

Simultaneous planar laser-induced fluorescence of the CH and OH radicals and two-dimensional particle image velocimetry were used to investigate the structure of turbulent non-premixed methane/nitrogen jet flames (Re-jet = 18,600) in an oxygen coflow. The motivation for this study is to investigate the relationship among regions of high CH/OH concentration and kinematic quantities such as vorticity, strain rate, and dilatation. The results show that in the lower part of the flame, the direction of the two-dimensional principal compressive strain axis exhibits a preferred orientation of about 45degrees with respect to the flow direction, whereas near the flame tip, the strain exhibits a more random orientation. Furthermore, CH structures are more likely to align orthogonal to the principal compressive strain axis in the downstream half of the flame. Probability density functions (PDFs) show that the most probable value of vorticity on CH structures is about DeltaU/delta (where DeltaU is the difference between the jet centerline and coflow velocities and delta is the full width at half-maximum of velocity profile), but near zero on the OH structures. Furthermore, joint PDFs of strain and dilatation show that CH structures are more likely to be associated with positive dilatation than are OH structures. These results are consistent with previous studies that have shown that jet flame kinematics are substantially affected by heat release and further show that these effects are more closely correlated with zones of high CH concentration than with zones of high OH concentration.

The thin, wrinkled CH reaction layers within moderate- (Re = 9,100) and high- (18,600) Reynolds-number turbulent non-premixed jet flames were identified by using planar laser-induced fluorescence, and the in-plane strain rates on these reaction layers were measured using simultaneous Particle Imaging Velocimetry (PIV). The PIV diagnostics resolved the Taylor scale; the strain-limited diffusion length scale was fully resolved for half the cases studied and nearly resolved for the others. In the high-Reynolds-number jet, instantaneous strain rates on the flame surface are highly intermittent, with peak values exceeding 10,000 s−1. Mean strain rates, conditioned on the CH-peak contour, are relatively constant (150 s−1) in the Re = 9100 flame and increase (650–1700 s−1) with axial location in the Re = 18,600 flame, resulting from the flame wrinkling process. The CH-layer thickness does not appear to respond in amplitude or in phase with the strain field, indicating that quasi-steady conditions do not occur. The strain field apparently oscillates at frequencies as high as 5–10 kHz—which is the inverse of the crossing time of integral-scale eddies—perhaps because only the low-frequency component of strain effectively acts on the flame. Mean axial velocities, conditioned on the CH-peak contour, were found to remain constant from the flame base to tip and to approximately equal the product of the stoichiometric mixture fraction and the fuel-exit velocity, in agreement with prediction.

Simultaneous Rayleigh scattering and CH planar laser-induced fluorescence (PLIF) measurements near the stabilization region of a lifted methane–air diffusion flame are presented. The goals of this investigation are to establish flow patterns responsible for complete breaks in the CH profile that indicate local flame extinction and evaluate the stabilization mechanisms over a range of flow conditions. Considerable attention has been given to vortex–flame interactions as a primary extinction mechanism of turbulent diffusion flames. The existence of holes in the flame zone is thought to result from the radial penetration of the flame by vortices from the internal fuel jet. In this investigation, Rayleigh scattering is used as a qualitative indication of gas temperature, thereby providing valuable information about the fluid near regions of local extinction, as indicated by well-defined breaks in the CH layer. The extent of premixedness in the region upstream from the CH structure is also assessed from the Rayleigh signal level. Furthermore, the roles of premixedness in flame stabilization, the nature of the leading edge, and lift-off height oscillation are discussed.

Acetone fluorescence provides a useful way to visualize the fluid mixing process within supersonic wind tunnels, some of which operate in the low temperature (240–300 K) and low pressure range (0.1–1 atm). Measurements are presented to quantify the dependence of the acetone laser induced fluorescence (LIF) signal on temperature and pressure in this range. The temperature and pressure sensitivity of the acetone LIF signal resulted in less than an 8% variation over the experimental conditions for a laser excitation wavelength of 266 nm. Condensation of the acetone vapor was identified as a potential problem for this diagnostic technique. Methods to prevent and check for condensation are discussed.

The stabilization of lifted jet diffusion flames has long been a topic of interest to combustion researchers. The flame and flow morphology, the role of partial premixing, and the effects of large scale structures on the flame can be visualized through advanced optical imaging techniques. Many of the current explanations for flame stabilization can benefit from the flow and flame information provided by laser diagnostics. Additionally, the images acquired from laser diagnostic experiments reveal features invisible to the eye and line-of-sight techniques, thereby allowing a deeper insight into flame stabilization. This paper reports visualizations of flame and flow structures from Particle Image Velocimetry (PIV), Planar Laser-Induced Fluorescence (PLIF) and Rayleigh scattering. The techniques are surveyed and the success of visualization techniques in clarifying and furthering the understanding of lifted-jet flame stabilization is discussed.

It is shown that simultaneous images of the CH and OH concentration fields can be obtained throughout a high-Reynolds-number (18,600) turbulent nonpremixed, nonsooting jet flame, and that the CH-OH boundary is a useful marker of the instantaneous stoichiometric contour. Previous CH-OH imaging was confined to the flame base. The structure of the fuel-decomposition zone—identified by the CH images—includes the following regions: those with high-curvature cusps; those with low CH concentration; and those where the flame "pinches" due to oxidizer being entrained to the centerline. It is found that the reaction zone that is associated with fuel decomposition (i.e., the CH layer) remains thin and rarely exceeds 1 mm, even near the tip of the high-Reynolds-number flame. CH layers in the turbulent flame are not thicker than the CH layers in the laminar jet flame at the same x/d location. In fact, CH layer thickness is relatively insensitive to Reynolds number and the level of turbulence. This implies that turbulence does not broaden the CH reaction zone, and that flamelet concepts are justified in modeling the reaction zone associated with fuel decomposition. The CH layers become thicker in the streamwise (x) direction, which is expected because scalar gradients and the dissipation rate are expected to decrease in the streamwise direction. Imaging the CH layer makes it possible to measure the flame surface density (Σ), which has a typical value of 0.2 mm−1. Surface density is shown to be related to the turbulent brush thickness and the degree of wrinkling.

Planar laser-induced fluorescence (PLIF) imaging of OH has been completed through the piloting section of a hydrocarbon-fueled scramjet. This pilot consists of flush-wall fuel injection followed by a recess, or cavity, in one wall. Images were obtained for both gaseous (ethylene) and liquid (JP-7) fuel combustion. For the gaseous-fuel tests, ethylene as introduced through four flush-wall, low-angle injectors placed upstream of the cavity. For the liquid-fuel tests, injection was normal to the crossflow through seven injectors (four in the bottom wall, three in the top wall). Introducing a small amount of aas into the liquid in the bottom wall injectors enhanced atomization of the liquid column. Flight conditions between Mach 4 and 5 anti dynamic pressures between 23.9 and 71.7 kPa are simulated. Instantaneous images show the dynamics of the combustion process, suggest the process is premixed in nature, and reveal the presence of large-scale structures. Average images at different axial locations show the effects of total temperature and dynamic pressure on the combustion process. Increasing temperature broadens the time-averaged flame zone, while increasing dynamic pressure tends to force the flame against the combustor sidewall. At a given axial location, the time-averaged reaction zone for ethylene is larger than thai for JP-7.

Simultaneous images of the CH and OH reaction zones are reported for "Intensely Wrinkled" premixed flames, to determine whether reaction zones retain their thin "laminar flamelet" structure or become "distributed reaction zones." Intensely Wrinkled Flames (IWFs) were achieved by using a special burner with large coflow air velocities to obtain a normalized turbulence intensity (u '/(U) over bar) of; 3.6. which is 10 times greater than time turbulence intensity within jet flames. The images were used to measure profiles of the flame surface density (Sigma) and the average CH layer thickness (<()over bar>(CH)); it is argued that these parameters are the ones that should be used to assess new large eddy simulations (LESs), rather than insensitive parameters such as mean concentrations.In the regime of IWFs. the CH reaction zones remained as thin as those measured in laminar jet flames (i.e.. less than 1 min thick) and had the appearance of flamelets. These thin reaction zones were extin guished before they became thickened by intense turbulence, which provides experimental evidence to support laminar flamelet modeling concepts, "Shredded flames" occurred, within which the reaction zones were short, discontinuous segments, and. the degree of flame wrinkling was significantly larger than in jet flames. Shredded flames have not been observed previously. There is no evidence of small-scale wrinkling of the reaction zones at scales less than half the integral scale. The images showed where the instantaneous stoichiometric contour is located. since it exists at the boundary between the CH and OH layers. Flame surface densities were typically 0.3 mm(-1).

Experiments have been performed to investigate the leading edge of a lifted jet diffusion flame. The first portion of this study is a simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) investigation of a lifted methane flame. The simultaneous technique is an approach for establishing the 2-D velocity field in conjunction with the flame front location indicated by laser-induced fluorescence from CH radicals within the reaction zone. The results show that the lifted flame stabilizes in a region of relatively low incoming gas velocity. Furthermore, the radial movement of large-scale vortices appears to play a crucial role in local flame extinction. The second set of experiments involves a simultaneous CH and OH PLIF investigation of the same lifted flame. The relative positions of the two radical fields have remarkable agreement. The CH profile is indicative of the fuel-rich region of the reaction zone and closely follows the inner edge of the OH profile. Furthermore, the OH zone is more than three times as thick as the CH zone, and the structures in both images support the radial motion of vortices established by the joint PIV/CH-PLIF measurements.

The objective of this paper is to report some of the first experimental evidence for the "leading edge" flame as the stabilization mechanism in lifted jet diffusion flames [1-5]. CH fluorescence has been used to indicate the flame front location (i.e., region of chemical reaction) and thereby characterize features of the stabilization region [5, 6]. The "leading edge" flame phenomenon reported within refers to the outward-extending branch of CH fluorescence at the base of the streamwise CH zones. Whether the "leading edge" flame is a special case of the more general triple flame is a question which remains unanswered. It is evident from previous computational studies [7, 8] that the triple flame, when interacting with a vortex or pair of vortices, can take on characteristics of the "leading edge" flames introduced in the present study. Veynante ct al. [8] illustrate the contortion of the premixed branches of the triple flame by the flowfield where the premixed branches are swept into the trailing diffusion flame. These simulated triple flame/vortex interactions are consistent with the results of this study which show a trailing diffusion flame and the leading edge reaction zone structure. (C) 1999 by The Combustion Institute.

=18600). Here, PLIF images reveal a CH layer of thickness typically <1 mm from flame base to tip. Furthermore, in these permanently blue flames, we observe instantaneous flamefront strain rates – derived from the PIV data – in excess of ±104 s-1 without flame extinction.