An experimental investigation of the breakup of an aerated-liquid jet in subsonic crossflow is described. The present test conditions were similar to those encountered in fuel injection in ramjet engines. Previous studies of spray structures of aerated-liquid jet in crossflow have been limited to the dilute spray area (downstream distance of greater than 100 jet diameters) using phase Doppler interferometry and along the liquid surface using wet holographic plates. The objective of the present study was to extend these earlier measurements to investigate the dense-spray near-injector region immediately downstream of the injector (0-50 jet diameters), in which secondary breakup may occur to bridge the gap between drop-size distributions along the jet surface and those obtained using phase Doppler interferometry in the far field of the injector. Three-dimensional microscopic digital holography was used to record and measure droplets sizes and locations within the three-dimensional volume of the spray. Earlier results of the primary breakup of aerated-liquid jets in crossflow show that the gas jet along the axis of the annular flow leaving the injector passage forces the annular liquid sheet into a conical shape that extends from the injector exit. Primary breakup occurs in a similar manner along both the upstream and downstream sides of the liquid jet (relative to the crossflow), which suggests relatively weak aerodynamic effects on the primary breakup. In the present study, the aerodynamic effects on the drop sizes in the wake region of the fuel injector were considered. The test conditions include different gas-to-liquid mass flow rate ratios and jet-to-freestream momentum flux ratios. The present measurements of the spray structure of aerated-liquid jets in crossflows shows a reduction in drop sizes with downstream distance that may be attributed to the drop secondary breakup.

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.

Advanced spray diagnostics are needed for studying the formation of drops in a variety of natural and technological spray processes, e.g. water falls, bow waves of ships, and many types of commercial spray atomizers, among others. Of interest is the dense-spray near-injector region which is typically opaque for spray diagnostics such as phase Doppler particle analyzers (PDPA). This is unfortunate because primary breakup processes that control spray size and velocity distributions occur in this optically challenging region.The present setup; digital holographic spray analyzer, allows the probing of dense spray regions and provides the user with droplet sizes and velocities measurements in three dimensions. The setup is based on typical in-line holography except that the holographic film is replaced with a CCD sensor. The actual process of capturing the hologram is a relatively simple process only requiring a laser, optics to form a collimated beam, and a digital camera. The hologram is then stored digitally and reconstructed numerically with a reconstruction program. After reconstructing the hologram in many different planes, the droplet size distribution is measured. In addition droplet velocities are measured by means of double pulsed exposure configuration and PIV program. All these processes can be automated which is the strength of this technique. The output is a three dimensional map of droplets locations, sizes, and velocities.This digital holographic spray analyzer was tested by measuring droplet sizes inside the dense spray created by an aerated injector subjected to a subsonic crossflow typical of test condition encountered in ramjet engine.

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.

High-temporal-resolution measurements of scalars and velocity are used to study vortex-induced annular (off-centerline) flame extinction during the interaction of a propagating vortex with an initially stationary counterflow hydrogen-air diffusion flame. Such an extinction process differs from classical one-dimensional descriptions of strained flamelets in that it captures the effects of flame curvature as well as dynamic strain. Planar laser-induced fluorescence (PLIF) measurements of the hydroxyl radical (OH) are used to track flame development, and simultaneous particle-image velocimetry (PIV) is used to characterize the two-dimensional flowfield. Measurements reveal differences in local normal strain rate profiles along and across the reaction zone and indicate that vortex-induced curvature in the annular region may initiate the extinction process. In addition, the effect of local flame extinction on vortex evolution and dissipation is determined from measured vorticity data.

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.

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.

A planar Doppler velocimetry (PDV) system was developed and demonstrated in a small-scale facility (Mach 1.36 freejet) and then applied in a large-scale subsonic wind tunnel, where measurements were made over a delta wing at a 23-deg angle of attack. This PDV system utilized a pulsed, injection-seeded, frequency-doubled Nd:YAG laser to interrogate the flow. Back-illuminated charge-coupled device (CCD) cameras in conjunction with an iodine filter were used to record images produced by the scattered laser light, permitting the determination of the velocity at each CCD pixel, The PDV instrument also included custom software and a frequency-monitoring system composed of photodiodes, gated integrators, and a second iodine cell. With this setup, we recorded the shot-to-shot iodine-filtered and reference images and the associated laser frequency. In the freejet, mean velocities in the core were measured by PDV to within 6.4 m/s (out of similar to 260 m/s) of the value obtained by laser Doppler velocimetry. In the wind tunnel, freestream empty-tunnel measurements indicated bias and random errors of less than 2 and 4 m/s, respectively. The dominant source of random error arose from laser speckle, and the dominant source of bias error came from the characterization of the iodine filters. Measurements over the delta wing showed similar velocity ranges but smaller vortex cores when compared to the velocity field predicted by a computational fluid dynamics model.

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.

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.

Scalar and velocity measurements are reported for two turbulent jet flames of CO/H2/N2 (40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters (4.58 mm and 7.72 mm). Simultaneous measurements of temperature, the major species, OH, and NO are obtained using the combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence. Three-component laser-Doppler velocimetry measurements on the same flames were performed at ETH Zurich and are reported separately. This paper focuses on the scalar results but includes some limited velocity data. Axial profiles of mixture fraction, major species mole fractions, and velocity in these two flames are in close agreement when streamwise distance is scaled by nozzle diameter. However, OH mole fractions are lower and NO mole fractions are higher near the stoichiometric flame length in the larger flame due to the lower scalar dissipation rates and longer residence times. Turbulent flame measurements are compared with steady strained laminar flame calculations. Laminar calculations show remarkably close agreement with measured conditional means of the major species when all diffusivities are set equal to the thermal diffusivity. In contrast, laminar flame calculations that include the normal Chemkin treatment of molecular transport are clearly inconsistent with the measurements. These results suggest that turbulent stirring has a greater influence than molecular diffusion in determining major species concentrations at the flow conditions and locations considered in the present experiments, which begin at an axial distance of 20 nozzle diameters. Analysis of the conditional statistics of the differential diffusion parameter supports this conclusion, though some evidence of differential diffusion is observed. With regard to validation of turbulent combustion models, this data set provides a target that retains the geometric simplicity of the unpiloted jet flame in coflow, while including a chemical kinetic system of intermediate complexity between hydrogen flames and the simplest hydrocarbon flames. Aspects of the measurements, including Favre-averaged profiles, conditional statistics, mixture fraction pdf’s, and departures from partial equilibrium, are presented and discussed in terms or their relevance to the testing of turbulent combustion submodels. The complete data are available on the World Wide Web for use in model validation studies.

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).