Planar Laser-induced Fluorescence Data Research Articles

Flame state recognition method of a scramjet based on PLIF image fusion features and an artificial neural network

In this study, a pattern recognition model is proposed to differentiate the flame state of a scramjet using an artificial neural network. The flame images are obtained from a scramjet ground test utilizing planar laser-induced fluorescence (PLIF). By extracting basic features, Hu moments and Zernike moments, the preprocessed flame images are compressed to mine valuable information. In order to reduce redundant input features and improve the efficiency of model operation, the partial least squares (PLS) method is introduced for feature screening and fusion. Then, a back propagation neural network (BPNN) model for multi-flame classification is established and analyzed. Finally, the flame states are determined by comparing the probabilities of different states. In order to optimize the recognition performance, the fusion features are studied and discussed. Experimental results show that when the filtered 11-dimensional features are used as input, the average recognition rate for the four states can reach 97.4%. These results demonstrate the significant potential of integrating PLIF and advanced data analysis methods, thereby broadening their application to intricate combustion fields.

Fluid flow and mixing in a novel intermittently rotating bioreactor for CAR-T cell therapy manufacturing

This work explores the mixing and fluid dynamics in a novel bioreactor currently used for the automated manufacturing of CAR-T cell therapy, which offers a single-dose cure for several forms of advanced blood cancer. The cylindrical bioreactor has a low aspect ratio and a free surface. Agitation is achieved by intermittent rotation of the entire vessel around its central axis. No engineering characterisation has been conducted to date for this system in a bioprocessing context. The study examines the fluid dynamics problems of spin-up from rest and spin-down to rest. Novel Particle Image Velocimetry and Planar Laser-Induced Fluorescence data is presented alongside reflective flakes results, shedding light on the different transient flow regions inside the intermittently rotating bioreactor, and determining the timescales of macro- and micromixing. The results presented can be used to design a custom rotation pattern of the bioreactor for improved mixing performance during the cell expansion step.

Application of Planar Laser-Induced Fluorescence for Interfacial Transfer Phenomena

The present review describes the current achievements in the applications of a planar laser-induced fluorescence (PLIF) method for the diagnostics of liquid films, bubbles, individual droplets, and sprays. Such flows are related with strongly curved interphases, which often results in additional high errors during the PLIF data quantification because of laser light reflection, refraction, and absorption. The present review demonstrates that a two-color PLIF approach and a PLIF modification for regularly structured illumination resolves the reflection- and refraction-caused errors. The latter modification ensures proper phase separation in the measurement cross-section and visualization of the interface dynamics. The former approach provides the accurate evaluation of the local temperature and concentration both in liquid and gaseous phases even in the case of strong variations of the laser sheet intensity. With intensified cameras, the PLIF method is used for multi-parameter diagnostics of the two-phase combustion of sprays in combustion chambers with optical access. It visualizes and quantifies the liquid fuel evaporation and mixing, to measure temperature in the gas and liquid phases and to reveal the regions of pollutant formation. The PLIF technique can also be easily combined with a particle image (or tracking) velocimetry method, to evaluate local heat and mass transfer.

Liquid fuel refill dynamics in a rotating detonation combustor using megahertz planar laser-induced fluorescence

The highly dynamic response of a continuously injected liquid fuel jet to rotating detonation waves is a critical parameter in the design, performance optimization, and modeling of a rotating detonation engine (RDE). In this work, this response is spatio-temporally resolved from the fuel injection point to the detonation channel using planar laser-induced fluorescence (PLIF) at imaging rates up to 1 MHz. A rotating detonation combustor (RDC) is operated on hydrogen and air to sustain stable detonation waves that interact in a one-way coupled manner with a single liquid fuel jet that propagates into the combustion chamber with cycle periods of ∼250 µs. Diesel is utilized as a realistic fuel surrogate with higher aromatic compounds to enable fluorescence excitation using the 355 nm third-harmonic output of a burst-mode Nd:YAG laser. By optimizing the technique to accommodate orders of magnitude variations in the fuel density throughout the injection process, the PLIF data enable measurements of (i) the overall refill dynamics after the arrival of the detonation wave, (ii) changes in the liquid spray trajectory with microsecond temporal resolution, and (iii) the time required to reestablish the quasi-steady axial refilling process as a function of peak chamber pressure relative to the air- and liquid-injector pressure drops. As the passage of the detonation wave imparts significant changes in the momentum flux ratio, the qualitative liquid break-up process and spatial distribution of the spray also vary significantly in time. Only as the injection system recovers late in the cycle does the fuel spray eventually return to a quasi-steady position and allow comparisons with theoretical jet trajectories. These data, enabled by ultra-high-speed PLIF imaging, represent some of the first detailed measurements for quantifying the dynamic response and recovery of liquid jets exposed to periodic detonations in an operating RDC.

Multimodal imaging for intra-droplet gas-cavity observation during droplet fragmentation.

Herein, planar laser-induced fluorescence (PLIF) is used in combination with shadowgraphy to study water-droplet aerobreakup. The acquired shadowgraph data are in agreement with previous visualization studies but differ from the PLIF results, yielding new insights into the fragmentation process. In particular, the PLIF data reveal changes in droplet topology during fragmentation that result from the entrapment or formation of gas cavities inside the liquid phase. In some instances, topological modification can be observed to arise from the presence of these cavities. In addition, the cavities may act as weak spots, facilitating droplet split-off.

Simultaneous application of two laser-induced fluorescence approaches for film thickness measurements in annular gas-liquid flows

This paper is devoted to the simultaneous application of two spatiotemporally resolved optical techniques capable of liquid film thickness measurements, namely Planar Laser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF), to co-current downward annular gas-liquid flows. A single laser sheet is used to excite the liquid film, which has been seeded with a fluorescent dye, along a longitudinal/vertical plane normal to the pipe wall. Two cameras, one for each technique, are placed at different angles to the plane of the laser sheet in order to recover, independently by the two techniques, the shape of the gas-liquid interface along this section. The effect of the angle between the laser sheet and the PLIF camera axis is also investigated. In film regions where the gas-liquid interface is smooth and flat, the conventional approach used for interpreting PLIF data is affected by total internal reflection of the fluorescent light at the free surface, or “mirror effect”, which leads to an overestimation of the film thickness that increases as the angle between the laser sheet and the camera axis is decreased. Nonetheless, local features such as light intensity maxima or minima are often located within the fluorescent signals that correctly identify the true interface, which in these conditions also coincides well with the BBLIF film-thickness measurement. When a correction for the mirror effect based on simple flat-film optical calculations is applied, this leads to PLIF results that correspond well to the true film thickness. Interestingly, it is further found that interfacial three-dimensionality, and in particular azimuthal/circumferential non-uniformity, can lead to underestimation of film thickness by PLIF that in some cases counteracts the overestimation due to the mirror effect. Smaller angles between the laser sheet and camera axis make PLIF less susceptible to this error. In regions where the film surface is rough, including on the surface of disturbance waves, the mirror effect is suppressed. The BBLIF measurement, on the other hand, is affected by loss of signal sensitivity or saturation in thick film regions, as well as distortions at complex or multiple interfaces in agitated flow regions with significant wave activity. Direct comparisons between PLIF and BBLIF measurement data confirm that local overestimations of the film thickness by the latter occur in film regions with higher interfacial curvature, especially at the front slopes of waves and around gas bubbles entrained in the liquid even in smooth films, while underestimations occur in regions with multiple interfaces and inside bubbles although it is often the case that features such as bubbles can be identified and corrected for in simple flows with smooth interfaces. Correction procedures are developed to compensate distortions caused by both methods that make these techniques more accurate for standalone employment. In highly complex, three-dimensional flows, the simultaneous application of both techniques is highly recommended to attain the most reliable information.

Structured planar laser-induced fluorescence (S-PLIF) for the accurate identification of interfaces in multiphase flows

Annular flows are employed in numerous engineering and industrial processes relating to the chemical, oil and gas, solar and nuclear energy industries. Yet, the reliable and detailed (i.e., time- and space-resolved) measurement of film thickness in these flows continues to elude us, as the moving and wavy interface renders the application of optical diagnostics, such as planar laser-induced fluorescence (PLIF), particularly challenging. In this research article, we present a novel adaptation of PLIF, which we refer to as structured PLIF (S-PLIF), which can suppress the errors in PLIF-derived film thickness measurements that arise due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on the periodic modulation of the laser-light intensity and angled illumination of the examined region of the flow, in order to generate fluorescence images with alternating bright and dark regions at an angle to the channel wall and film interface. An image-processing methodology capable of recovering the location of the true gas-liquid interface from such images is presented, and the application of S-PLIF is demonstrated in liquid films in a vertical pipe over the Reynolds number range ReL≈150−1500. The results from this technique are compared to simultaneously recovered, “conventional” (uncorrected) PLIF data, as well as data from other techniques over the same range of conditions, demonstrating the efficacy of S-PLIF. A comparison amongst S-PLIF data obtained with the observation angle between the laser-sheet plane and the camera’s observation axis set to β=70∘ and 90 ∘ is also performed, showing that the employment of β=70∘ is highly advantageous in avoiding distortions caused by optical interactions of the emitted fluorescence at the free surface of circumferentially non-uniform films. The instantaneous and average film-thickness uncertainties of S-PLIF are estimated to be below 10% and 5%, respectively, when measuring smooth films; an improvement over the other optical measurement techniques considered in this work. Finally, the application of S-PLIF is demonstrated in the presence of a gas-shear flow with gas entrainment in the liquid, and simultaneously with particle image velocimetry (PIV).

An adaptive sampling algorithm for Doppler-shift fluorescence velocimetry in high-speed flows

We present an approach to improving the efficiency of obtaining samples over a given domain for the peak location of Gaussian line-shapes. The method uses parameter estimates obtained from previous measurements to determine subsequent sampling locations. The method may be applied to determine the location of a spectral peak, where the monetary or time cost is too high to allow a less efficient search method, such as sampling at uniformly distributed domain locations, to be used. We demonstrate the algorithm using linear least-squares fitting of log-scaled planar laser-induced fluorescence data combined with Monte-Carlo simulation of measurements, to accurately determine the Doppler-shifted fluorescence peak frequency for each pixel of a fluorescence image. A simulated comparison between this approach and a uniformly spaced sampling approach is carried out using fits both for a single pixel and for a collection of pixels representing the fluorescence images that would be obtained in a hypersonic flow facility. In all cases, the peak location of Doppler-shifted line-shapes were determined to a similar precision with fewer samples than could be achieved using the more typical uniformly distributed sampling approach.

Mixture fraction, soot volume fraction, and velocity imaging in the soot-inception region of a turbulent non-premixed jet flame

An experimental study is performed to investigate the feasibility of conducting simultaneous mixture fraction, soot volume fraction and velocity imaging in sooting jet flames. The measurements are performed in the soot-inception region of ethylene jet flames, where the yellow luminous region first appears in the flame. Three-component velocity and soot volume fraction are measured by stereoscopic particle image velocimetry and laser-induced incandescence, respectively. The mixture fraction is inferred from laser-induced fluorescence of krypton gas seeded into the fuel stream. To obtain mixture fraction from the fluorescence signal, the signal must be corrected for density and fluorescence quenching effects. This correction is accomplished by invoking an assumed state relationship that is derived from a laminar strained-flame calculation. Once properly calibrated, the krypton planar laser-induced fluorescence data give the mixture fraction, temperature and major species near the regions of soot formation. The krypton is seeded into the fuel jet at a mole fraction of approximately 4%. The fluorescence of krypton is achieved by two-photon absorption at 214.7nm and the resulting fluorescence is collected at 760.2nm. The krypton fluorescence signal is rather weak, particularly near the reaction zones where density is lowest, and so adequate signal-to-noise ratios could be achieved in a region only about 1mm in height, which effectively limited this study to a line measurement of mixture fraction. The temperature field derived from the mixture fraction field was compared to temperatures obtained from thermocouple measurements. The mean radial temperature profiles using the different techniques show excellent agreement and this serves to validate the methodology used to map from fluorescence signal to mixture fraction and temperature. The resulting data are of high enough quality as to allow the investigation of the kinematics, thermo-chemical state and even the dissipation fields near regions of soot formation.

Mixing by internal waves quantified using combined PIV/PLIF technique

We present a novel characterization of mixing events associated with the propagation and overturning of internal waves, studied thanks to the simultaneous use of Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) techniques. This combination of techniques had been developed earlier to provide an access to simultaneous velocity and density fields in two-layer stratified flows with interfacial gravity waves. Here, for the first time, we show how it is possible to implement it quantitatively in the case of a continuously stratified fluid where internal waves propagate in the bulk. We explain in details how the calibration of the PLIF data is performed by an iterative procedure, and we describe the precise spatial and temporal synchronizations of the PIV and PLIF measurements. We then validate the whole procedure by characterizing the Triadic Resonance Instability (TRI) of an internal wave mode. Very interestingly, the combined technique is then applied to a {precise} measurement of the turbulent diffusivity K_t associated with mixing events induced by an internal wave mode. Values up to K_t=15 mm^2\cdot s^{-1} are reached when TRI is present (well above the noise of our measurement, typically 1 mm^2\cdot s^{-1}), unambiguously confirming that TRI is a potential pathway to turbulent mixing in stratified flows. This work therefore provides a step on the path to new measurements for internal waves.

Evaluation of Two Measurement Techniques to Quantify Fuel–Air Mixing of a Gas Turbine Premixer at Atmospheric Conditions

In the present study, two measurement techniques are adopted to evaluate the fuel–air mixing under atmospheric conditions using an industrial fuel–air premixer. These techniques are CO2 mixing and planar laser induced fluorescence (PLIF) in water. In these techniques, CO2 and fluorescent dye are injected as fuel simulants. CO2 measurements are used to validate PLIF in water. In the CO2 technique, CO2 concentrations are converted to fuel mass fractions, whereas in the PLIF technique, a modified post processing method is used to convert the LIF signal into fuel mass fraction. The experiments are conducted at the same Reynolds number and momentum flux ratio for two injection strategies. To study the effect of the flow aerodynamics on the mixing results, high-speed particle image velocimetry (PIV) measurements are conducted in water at the same Reynolds number. A comparison of fuel concentrations measured with the CO2 and PLIF techniques shows good quantitative agreement at all momentum flux ratios. However, deviations between the two techniques are observed at locations of high fuel concentration gradients. The unsteady mixing is evaluated using the PLIF technique with high temporal resolution. Analysis of PIV and PLIF data shows that unsteady mixing is lower at regions of high fluctuations in velocity. Moreover, it is found that there is high unsteady mixing at locations of high concentration gradient.

Evaluating temperature and fuel stratification for heat-release rate control in a reactivity-controlled compression-ignition engine using optical diagnostics and chemical kinetics modeling

The combustion process in a dual-fuel, reactivity-controlled compression-ignition (RCCI) engine is investigated using a combination of optical diagnostics and chemical kinetics modeling to explain the role of equivalence ratio, temperature, and fuel reactivity stratification for heat-release rate control. An optically accessible engine is operated in the RCCI combustion mode using gasoline primary reference fuels (PRF). A well-mixed charge of iso-octane (PRF=100) is created by injecting fuel into the engine cylinder during the intake stroke using a gasoline-type direct injector. Later in the cycle, n-heptane (PRF=0) is delivered through a centrally mounted diesel-type common-rail injector. This injection strategy generates stratification in equivalence ratio, fuel blend, and temperature. The first part of this study uses a high-speed camera to image the injection events and record high-temperature combustion chemiluminescence. The chemiluminescence imaging showed that, at the operating condition studied in the present work, mixtures in the squish region ignite first, and the reaction zone proceeds inward toward the center of the combustion chamber. The second part of this study investigates the charge preparation of the RCCI strategy using planar laser-induced fluorescence (PLIF) of a fuel tracer under non-reacting conditions to quantify fuel concentration distributions prior to ignition. The fuel-tracer PLIF data show that the combustion event proceeds down gradients in the n-heptane distribution. The third part of the study uses chemical kinetics modeling over a range of mixtures spanning the distributions observed from the fuel-tracer fluorescence imaging to isolate the roles of temperature, equivalence ratio, and PRF number stratification. The simulations predict that PRF number stratification is the dominant factor controlling the ignition location and growth rate of the reaction zone. Equivalence ratio has a smaller, but still significant, influence. Temperature stratification had a negligible influence due to the NTC behavior of the PRF mixtures.

Mars Science Laboratory Reaction Control System Jet Computations with Visualization and Velocimetry

Numerical predictions of the Mars Science Laboratory reaction control system jets interacting with a Mach 10 hypersonic flow are compared to experimental nitric oxide planar laser-induced fluorescence data. The steady Reynolds-averaged Navier–Stokes equations using the Baldwin–Barth one-equation turbulence model were solved using the OVERFLOW code. The experimental fluorescence data used for comparison consist of qualitative two-dimensional visualization images, qualitative reconstructed three-dimensional flow structures, and quantitative two-dimensional distributions of streamwise velocity. Through modeling of the fluorescence signal equation, computational flow images were produced and directly compared to the qualitative fluorescence data. Post processing of the experimental and simulation data enabled the jet trajectory to be extracted for a more quantitative comparison. The two-dimensional velocity fields were reconstructed through interpolation of a series of single-component velocity profiles. Each distribution of single-component velocity was obtained using molecular tagging velocimetry. After validating the numerical model, the numerical solution was further examined to gain insight into hypersonic jet-in-crossflow interaction. Novel nitric oxide planar laser induced fluorescence experiments are proposed based on this analysis.

Visualization and analysis of the characteristics of transitional underexpanded jets

Underexpanded jets can be formed when high-pressure gaseous fuel is injected directly into an engine cylinder. In such conditions, shock waves are formed immediately near the nozzle exit. In the present study, the flow structure and turbulent mixing of pulsed jets issuing from a circular nozzle is investigated using acetone planar laser-induced fluorescence (PLIF). By monitoring axial and various radial cross-sections under different injection pressure conditions, different features of gaseous jets are visualized and interpreted. The temporal development of the axial cross-sections reveals three typical jet flow patterns (subsonic, moderately underexpanded, and highly underexpanded) during the injection. These stages are (1) well described with the observed shock structures and (2) noted to take a considerably long portion of the full injection process. The visualizations from the radial cross sections show how the nozzle inflow conditions may influence the primary and the azimuthal (secondary) instability of the jet which influences the turbulence transition process and the mixing process. The results indicate the importance of inner nozzle flow on the flow behavior. For example, systematic asymmetries in the mean concentration fields are observed. In addition to PLIF data, numerical simulations can be used to support the experimental picture of the jet behavior. We give examples of large-eddy simulations (LESs) in order to further explore the behavior of the underexpanded jets. Results show that LES is able to reproduce the basic physics of underexpanded jets. LES and PLIF compare favorably in terms of the barrel shock structures and the description of the normal shocks. LES also provides detailed flow field information including temperature, Mach number, concentration and scalar dissipation rate (SDR).

Large-eddy simulation of highly underexpanded transient gas jets

Large-eddy simulations (LES) based on scale-selective implicit filtering are carried out in order to study the effect of nozzle pressure ratios on the characteristics of highly underexpanded jets. Pressure ratios ranging from 4.5 to 8.5 with Reynolds numbers of the order 75 000–140 000 are considered. The studied configuration agrees well with the classical picture of the structure of highly underexpanded jets. Similarities and differences between simulation and experiments are discussed by comparing the concentration field structures from LES and planar laser induced fluorescence data. The transient stages, leading eventually to the highly underexpanded state, are visualized and investigated in terms of a phase diagram revealing the shock speeds and duration of the transient stages. For the studied nozzle pressure ratio range, the Mach disk dimensions are found to be in good agreement with literature data and experimental observations. It is observed how the nozzle pressure ratio influences the Mach disk width, and thereby the slip line separation, which leads to co-annular jets with inner and outer shear layers at higher pressure ratios. The improved mixing with increasing pressure ratio is demonstrated by the probability density functions of the concentration. The coherent structures downstream of the Mach disk are identified using proper orthogonal decomposition (POD). The structures indicate a helical mode originating from the shear layers of the jet. Despite the relatively low energy content of the dominant POD modes, the frequencies of the POD time coefficients explain the dominant frequencies in the pressure fluctuation spectra.

Statistics of the scalar dissipation rate using direct numerical simulations and planar laser-induced fluorescence data

The statistics of the scalar dissipation rate (SDR) in gaseous flows with Schmidt numbers close to unity were examined in a joint numerical and experimental effort, in a shearless mixing layer in the presence of decaying turbulence using three-dimensional Direct Numerical Simulations (DNS), and in an axisymmetric plume formed by the continuous low-momentum release of an acetone-laden stream (used as a tracer to measure the passive scalar) along the centreline of a turbulent pipe flow of air downstream of a turbulence generating grid using Planar Laser-Induced Fluorescence (PLIF). For the flows examined good agreement was found between the DNS and the experiment, both of which indicate that: (i) the probability density functions of the unconditional and conditional SDR show small departures from lognormality; (ii) the ratio of the standard deviation of the unconditional SDR to its respective Reynolds-averaged mean, as well as the ratio of the standard deviation of the conditional SDR to its conditional mean (these ratios do not vary strongly with the value of the mixture fraction at which it is evaluated), both increase over a few Kolmogorov time scales from zero (at the injector nozzle in the experiment and initially in the DNS) to some value downstream and at later times; (iii) the long-time values of the ratios of the standard deviation to the mean of the conditional and unconditional SDR increase with the turbulent Reynolds number; (iv) for the same turbulent Reynolds number, the DNS and the experiment showed that the ratio related to the unconditional SDR increases to a long-time value of approximately 2.3 (±20%), while the ratio related to the conditional SDR increases quickly to a value that stays within the range 1.0–1.4 (or, 1.2±0.2) and reaches a maximum value of 1.3–1.4 by the end of the DNS run and at the downstream edge of the experimental domain. The development of the conditional SDR fluctuations is discussed by comparing the early and late stages of mixing. The agreement between the PLIF measurements, which can only resolve two-dimensional fields, and the DNS, which provides access to the fully resolved three-dimensional field, suggests that the present conclusions are not limited by resolution or the lack of measurement in the third dimension. The results are useful for the development and validation of turbulent reacting flow models such as advanced flamelet, Conditional Moment Closure (CMC), and transported Probability Density Function (PDF) closures.

Characteristics of horizontal liquid–liquid flows in a circular pipe using simultaneous high-speed laser-induced fluorescence and particle velocimetry

This paper describes a set of experiments on liquid–liquid flows in a horizontal circular tube. The liquids used in the experiments were an aliphatic hydrocarbon oil (Exxsol D80) and an aqueous solution of glycerol. The concentration of glycerol in the solution was adjusted so that the two liquids had the same refractive index, and optical distortions due to the curvature of the (transparent) circular tube test section were corrected for with the use of a graticule technique. The test section was far downstream of an inlet section that established an initially stratified co-current flow of the two immiscible liquids, with the Exxsol D80 oil flowing over the glycerol/water solution. The flows were investigated at the test section with the application of laser-based optical diagnostic methods, which included high-speed simultaneous Planar Laser Induced Fluorescence (PLIF), Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). These techniques allowed the reliable evaluation of the nature of the investigated horizontal liquid–liquid flows (i.e., the flow patterns from phase distribution information), together with the detailed spatiotemporally resolved measurement of key flow characteristics such as phase and velocity distributions, and also of important parameters such as droplet size. The resulting PLIF images provide a clear indication of the distribution of the phases within a plane that passed through the channel centreline, and are used to obtain qualitative information about the arising flow patterns. The images were also used quantitatively to generate data on phase distribution, in situ phase fraction, interface level and droplet size distribution. Much of the PLIF data on in situ phase fraction and interface level agrees well with predictions from a simple stratified laminar–laminar flow model. The particle velocimetry methods (PIV and PTV) provide data on the velocity profiles in the investigated flows. Over the range of superficial velocity conditions investigated, the velocity profile in the lower (heavier and more viscous) glycerol/water solution phase was typically characteristic of laminar flow, whereas in the upper (lighter and less viscous) Exxsol D80 oil phase the profile often showed a shape characteristic of turbulent flow.

Time-resolved blowoff transition measurements for two-dimensional bluff body-stabilized flames in vitiated flow

Flame holding and blowoff characteristics of bluff-body stabilized, turbulent flames were measured in an enclosed rectangular duct with a triangular flame holder in vitiated, premixed flows. Blowoff stability margins were characterized with chemiluminescence measurements performed by high-speed imaging to capture flame dynamics during the approach to flame blow off. As the equivalence ratio was decreased, local extinctions along the flames interacting with shear layers surrounding the bluff body recirculation zone occurred with greater frequency and proximity to the wake stagnation zone. Decreased equivalence ratio resulted in extinction events at the trailing edge of the stagnation zone, which allowed reactants to be convected into the recirculation zone and burned behind the bluff body. Increasing reactant dilution of the recirculation zone eventually resulted in flame lift-off or extinction of the flame in the neighboring shear layer. These near field shear layer flames convected to the wake stagnation zone, and were eventually quenched. Simultaneous particle imaging velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements captured the flame edge location and aerodynamic behavior as blowoff was approached. Two-dimensional hydrodynamic stretch along the flame front and flow field vorticity maps were extracted from the combined PIV/OH PLIF data. The distribution of flame stretch shifted to greater values as the equivalence ratio decreased and is believed to be the cause of local flame extinction in the wake stagnation zone that starts the blowoff process.

Extended Quantitative Fluorescence Imaging for Multicomponent and Staged Injection into Supersonic Crossflows

The fluorescence ratio technique for processing planar laser-induced fluorescence data was extended for quantitative imaging of the injectant mole-fraction and density in multicomponent and multiinjection configurations in a nonreacting supersonic mixing flowfield. The method was experimentally validated in a mixing flowfield formed by single or staged sonic transverse injection into a Mach 2.0 supersonic air stream. Gaseous helium and air were used as injectants. Injectant mole-fraction distributions obtained by the present method were confirmed by comparison with gas-sampling data. Density distributions were validated with the oblique shock relation. The new method worked well for the multistaged injection configuration. Phase-locked measurement for the pulsed secondary injection was also tested. Results demonstrated the applicability of the new method to the pulse injection as well.

Planar laser-induced fluorescence (PLIF) measurements of liquid film thickness in annular flow. Part II: Analysis and comparison to models

Film thickness distributions in upward vertical air–water annular flow have been determined using planar laser-induced fluorescence (PLIF). Film thickness data are frequently used to estimate interfacial shear and pressure loss. This film roughness concept has been used in a number of models for annular flow of varying complexity. The PLIF data are presently applied to the single-zone interfacial shear correlation of Wallis; the more detailed model of Owen and Hewitt; and the two-zone (base film and waves) model of Hurlburt, Fore, and Bauer. For the present data, these models all under-predict the importance of increasing liquid flow on pressure loss and interfacial shear. Since high liquid flow rates in annular flow induce disturbance wave and entrainment activity, further modeling in these areas is advised.