Acetone Planar Laser-induced Fluorescence Research Articles

Flow Structure behind Spanwise Pin Array in Supersonic Flow

This work focused on the experimental characterization of a complex flow structure behind a cross-flow array of cylindrical pins installed on the wall of a supersonic duct. This geometry simulates several common gas dynamic configurations, such as a supersonic mixer, a turbulence-generating grid, or, to some extent, a grid fin. In this work, the instrumentation employed is essentially non-intrusive, including spanwise integrating techniques such as (1) fast schlieren visualization and (2) Shack–Hartmann wavefront sensors; and planar techniques, namely (3) acetone Mie scattering and (4) acetone planar laser-induced fluorescence. An analysis of the data acquired by these complementary methods allowed the reconstruction of a three-dimensional portrait of supersonic flow interactions with a discrete pin array, including the shock wave structure, forefront separation zone, shock-induced separation zone, shear layer, and the mixing zone behind the pins. The main objective of this activity was to use various visualization techniques to acquire essential details of a complex compressible flow in a wide range of temporal–spatial scales. Particularly, a fine structure in the supersonic shear layer generated by the pin tips was captured by a Mie scattering technique. Based on the available publications, such structures have not been previously identified or discussed. Another potential outcome of this work is that the details revealed could be utilized for adequate code validation in numerical simulations.

Flame development in prechamber assisted engine: High-speed PLIF

Prechamber-assisted combustion reduces emissions and improves engine performance through lean and knock limit enhancement. The spark plug ignition is replaced by multiple, high-temperature, radical-rich jets that entrain and ignite the main chamber charge, enabling the engine to operate at leaner air–fuel mixtures. Current work reports first cycle resolved planar laser-induced fluorescence (PLIF) measurements following the flame development process, starting from the mixture formation inside the prechamber to the post-combustion jets in the main chamber. The mixing and flame development inside the prechamber is visualized at 100 kHz with Acetone PLIF using an inventive engine-mounted optical prechamber (OPC) setup. Following the burning (/burned) prechamber jet interaction with the unburned main chamber, the mixture is imaged using the fuel and flame tracer (FFT) PLIF approach. The innovative 50 kHz FFT-PLIF approach is based on the fluorescence of acetone as fuel (unburned) and combustion-generated SO2 as a flame (burned) tracer with 266 nm laser excitation. The main chamber is fueled with premixed methanol seeded with 6.8 acetone and 2.6% (m/m) di-tert-butyl disulfide (DtBDS) while prechamber is injected with methane at 6 bar using solenoid and check valve assembly. The flow of the main chamber mixture into the prechamber creates a turbulent jet inside the prechamber. The inflow increases as the pressure ratio drops, generating significant recirculation inside the prechamber. Partially oxidized products remain near the top center of the prechamber even as the flame propagates through the throat. Combined flame and fuel images reveal the dynamics of the interaction layer between the prechamber jet and the main chamber fuel–air mixture.

Investigation of dynamic mixedness characteristics of a transverse acoustically excited turbulent jet by high-repetition-rate acetone tracer planar laser-induced fluorescence technique

The dynamic mixedness characteristics of a bluff-body stabilized turbulent jet under transverse acoustic excitations were investigated using high-repetition-rate acetone planar laser-induced fluorescence (PLIF) at 7 kHz and multipoint scanning hot-wire measurements. Acetone mixedness imaging was applied for the turbulent jet to assess the interaction between the turbulent jet and the imposed transverse acoustic excitations at a driving frequency of 50 Hz. The high-repetition-rate acetone PLIF images showed that the acetone mixedness distribution swung left and right frequently under the transverse acoustic excitation, and the deflection angle could reach about 6°. The mixedness area of a turbulent jet flow could also be increased by 13.3% when excited by a transverse acoustic wave. Meanwhile, the sequence of instantaneous acetone PLIF images illustrated how the wrinkled edges were generated when acoustic excitations were imposed. The curvature of the acetone PLIF interface showed that the portion of large curvatures increased by 60% after applying an acoustic wave of 123 dB. Multipoint hot-wire measurements further stressed that the turbulent intensity at the transverse acoustic excitation of 123 dB increased to be about 1.3 times the natural turbulence. The proper orthogonal decomposition results showed that the large and small scales of the jet wrinkles both increased with the sound pressure level. RANS transient simulation also implied that distorted velocity streamlines could be achieved inside the turbulent jet due to the transverse acoustic excitation. They could further lead to inhomogeneous convection and increased mixing between the turbulent jet and the surrounding air.

Distribution characteristics of a supercritical hydrocarbon fuel jet injected into a high-speed crossflow

The mixing process and distribution characteristics of a supercritical endothermic hydrocarbon fuel (EHF) jet injected into a high-speed crossflow are investigated experimentally by the acetone Planar Laser-induced Fluorescence (PLIF) technique. The instantaneous and averaged distribution of supercritical fuel in the central plane and the cross-sectional planes with various Mach numbers, injection pressure, and temperature are measured and analyzed. A three-species surrogate is employed to calculate the flow parameters of the EHF jet. The boundary and penetration depth of the jet plume are identified by the probability density method. The mixing process of the supercritical fuel is analyzed using the fluorescence concentration distribution. Results show that the momentum flux of the EHF jet increases with the increase of injection temperature when the injection pressure is close to the critical pressure, while it decreases firstly and then increases as the injection pressure is larger than the critical pressure. The penetration depth is dominated by the jet-to-air momentum flux ratio (q), and the EHF jet plume boundary is enlarged with the increase of the q. In particular, the penetration depth formula is given in: y/d=0.67q0.97x/d0.29 for supersonic crossflow and y/d=0.15q0.57x/d0.60 for subsonic crossflow, respectively. The EHF jet plume with lower q presents a semi-circle shape in the cross-sectional plane and mainly expands in the width direction. While it mainly expands in height direction downstream as a semi-Omega shape in the larger q condition. The results indicate that the increase in injection temperature and pressure can enlarge the EHF jet plume distribution and enhance the mixing effect. In comparison, increasing the injection pressure obtain more significant benefits, providing a reference for the fuel supply system and injector design.

Simultaneous visualization of instantaneous unburnt and preheating zones in turbulent premixed flames under transverse acoustic excitations

Instantaneous unburnt and preheating zones of bluff-body stabilized turbulent premixed flames under transverse acoustic excitations were investigated using simultaneous planar laser-induced fluorescence (PLIF) of acetone and CH2O, as well as multi-point hot-wire measurements. The PLIF images show that the unburnt zone marked by acetone images, the preheating zone marked by CH2O images, and the pixel-by-pixel product of acetone/CH2O have an increasing distribution area when slowly enlarging the sound pressure level (SPL). Wrinkled and bent edges of the unburnt and preheating zone can be seen at conditions away from the flame blow-off in the presence of the transverse acoustic excitations, and their sizes and areas increase as the flame blow-off is approached. At conditions near the flame blow-off with enlarging SPL to 123 dB, the flame turns from side to side over time and a large scale of the acetone/CH2O regions can be observed to deflect inside the center product zone, implying that the cold reactants can enter the product zone from the unburnt/preheating zones. The unburnt/preheating mainstream presents strong wrinkles and partial fractures. Such a phenomenon indicates that the local extinction of the shear layer flame can also be facilitated due to the turbulent fluctuation enhanced by the transverse acoustic wave. For a low flow velocity, increasing variations of the unburnt and preheating zones in the presence of the transverse acoustic wave can be revealed. The curvature of the acetone PLIF shows that the unburnt zones are more likely to be wrinkled with an increasing SPL and flow velocity. The root-mean-squared velocity measurements stress that the transverse acoustic wave mainly affects the turbulent premixed flame by enhancing the turbulent fluctuations.

Asymmetric forcing of convectively unstable transverse jets

In gas phase experiments we explore the effect of asymmetric and helical excitation of the flow about the exit plane of a jet injected perpendicularly into crossflow. Simultaneous acetone planar laser-induced fluorescence imaging and stereo particle image velocimetry quantify the effects of such excitation on jet structure, vorticity, and scalar mixing, while proper orthogonal decomposition of the images enables determination of characteristic dynamical signatures related to important jet features.

Acetone PLIF visualization of the fuel distribution at plasma-enhanced supersonic combustion

This study examines the mixing and flameholding characteristics of a plasma stabilized planewall supersonic combustor. Fuel injection, ignition, and flameholding are provided by Plasma Injection Modules (PIMs) installed in a M = 2, 76.2 × 76.2 mm duct. Two configurations were explored: the first involves a plasma filament collocated within a single transverse fuel jet, while the second involves combustion stabilization using a series of three PIMs. Unburned fuel distributions were assessed with acetone planar laser induced fluorescence (PLIF), where acetone vapor was seeded into the gaseous fuel. PLIF measurements were performed for both streamwise and spanwise laser sheet orientations. Supporting datasets were collected including schlieren imaging, chemiluminescent imaging, and pressure distributions. PIM actuation is found to increase jet penetration, crossflow expansion, and initiate fuel jet breakup. The average jet cross sectional area is shown to increase by >20% with the addition of plasma. PLIF imaging at PIM stabilized combustion revealed a significant unburned fuel concentration in a local separation region near the PIMs, and identified the minimum plasma pulse duration for flowfield transition from initial conditions to the combustion pattern.

Effects of controlled vortex generation and interactions in transverse jets

This experimental study examines the effects of controlled vortex generation and interactions on a round transverse jet for a range of jet-to-crossflow momentum flux ratios. Utilizing acetone planar laser-induced fluorescence (PLIF) imaging, alterations in jet structure, penetration, and molecular mixing created by such vortical interactions are quantified. Differences are found to depend on the state of the natural instabilities in the jet's upstream shear layer, with implications for jet control strategies.

Optical diagnostics on the pre-chamber jet and main chamber ignition in the active pre-chamber combustion (PCC)

We studied the relationship between pre-chamber jet and main chamber ignition in the pre-chamber combustion (PCC) of an optical engine, fueled with methane and equipped with an active pre-chamber with two rows of orifices. Acetone planar laser-induced fluorescence (PLIF) and OH* chemiluminescence imaging techniques were simultaneously applied to visualize the pre-chamber jet and the reaction zone in the main chamber, respectively. The pre-chamber fueling was constant and the main chamber fueling was increased to form an ultra-lean case and a lean case with global excess air ratios (λ) of 2.3 and 1.8, respectively. Results indicate that a higher pressure difference between pre-chamber and main chamber (▵P) produces larger pre-chamber jet penetration speed; the maximum pre-chamber jet penetration speed appears at timing around the peak ▵P. Over enrichment of the pre-chamber charge reduces the peak ▵P and thus does not favor a faster pre-chamber jet discharge. In addition to the main pre-chamber jet, a weaker post jet discharge process is visualized; the former is due to the pre-chamber combustion while the latter due to the ▵P fluctuation and the cylinder volume expansion. The post pre-chamber jet is accompanied by a post reaction zone in the ultra-lean case (λ=2.3) and there are two unburned regions in the main chamber: one is around the pre-chamber nozzle and the other between the adjacent reaction zones. These two unburned regions are consumed by flame propagation in the lean case (λ=1.8). The weak pre-chamber jet from the upper-row orifice does not produce any distinct reaction zone, indicating that the pre-chamber orifice location and arrangement on the nozzle also matters in the pre-chamber design. The pre-chamber jet penetration length is longer than that of the reaction zone during pre-chamber discharge; the penetration length difference between the pre-chamber jet and reaction zone decreases with increasing main chamber fueling.

Study on the Pre-Chamber Fueling Ratio Effect on the Main Chamber Combustion Using Simultaneous PLIF and OH* Chemiluminescence Imaging

<div class="section abstract"><div class="htmlview paragraph">Pre-chamber combustion (PCC) enables leaner air-fuel ratio operation by improving its ignitability and extending flammability limit, and consequently, offers better thermal efficiency than conventional spark ignition operation. The geometry and fuel concentration of the pre-chamber (PC) is one of the major parameters that affect overall performance. To understand the dynamics of the PCC in practical engine conditions, this study focused on (i) correlation of the events in the main chamber (MC) with the measured in-cylinder pressure traces and, (ii) the effect of fuel concentration on the MC combustion characteristics using laser diagnostics. We performed simultaneous acetone planar laser-induced fluorescence (PLIF) from the side, and OH* chemiluminescence imaging from the bottom in a heavy-duty optical engine. Two different PC Fueling Ratios (PCFR, the ratio of PC fuel to the total fuel), 7%, and 13%, were investigated. The “negative” regions of the PLIF fields were used to visualize PC jets and the MC combustion. The absence of acetone seeding in the PC, and its consumption during the MC combustion contributed to the loss of PLIF signal (negative regions). The instantaneous PLIF/OH* fields showed the appearance of PC jets near its pressure maximum. The PC jet interacting with the piston surface was accompanied by an increase in OH* chemiluminescence intensity and its area. Increasing PCFR resulted in a lag of ~0.6° crank angle in the PC and MC pressure and the pressure difference (Δ<i>P</i><sub>PC-MC</sub>) between the two, for the two PCFR cases. This led to a phase lag in the PC flame jet penetration distance, which in turn delayed the increase in the total OH* chemiluminescence intensity for the higher PCFR case. For similar Δ<i>P</i><sub>PC-MC</sub>, we observed wider OH* regions with increased PCFR. This result distinguished the radical concentration effect on the MC combustion from the fluid mechanical effects controlled by Δ<i>P</i><sub>PC-MC</sub>.</div></div>

Turbulent nonpremixed cool flames: Experimental measurements, Direct Numerical Simulation, and manifold-based combustion modeling

Turbulence, low-temperature chemistry, and their interactions in the form of turbulent cool flames are critical to understanding and improving advanced engines. Design of such engines requires tractable simulations which in turn necessitate turbulent combustion models that can account for cool flames. While manifold-based turbulent combustion models are an attractive option for hot flames, their applicability to cool flames is not yet fully understood. This is partially due to the lack of turbulent cool flame experiments, which has made model validation difficult. This work addresses these points with a combined experimental and computational investigation of turbulent nonpremixed dimethyl ether cool flames. First, a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner is developed and tested. Turbulent cool flames are studied using the formaldehyde planar laser-induced fluorescence (PLIF), acetone PLIF, and planar Rayleigh scattering techniques. The acetone PLIF signals are converted into mixture fraction values, and measurements of time-averaged temperature are derived from the Rayleigh scattering signals by taking advantage of the similarities between cool flames and unburned mixtures. Second, a Direct Numerical Simulation (DNS) of the CARAT burner is conducted. The flame is shown to be sensitive to thermal boundary conditions such that the stabilization method is dependent on a 10 K change in inlet temperature. Comparisons of first- and second-order statistics of temperature, mixture fraction, and formaldehyde between the DNS and experiment show good agreement. The validity of manifold-based turbulent combustion models in turbulent cool flames is then explored by first studying the effective Lewis numbers in the flame using a local differential diffusion parameter analysis. Turbulent cool flames are shown to have effective unity Lewis number transport even at a much lower Reynolds number than typically required for hot flames. Good agreement is shown for temperature and formaldehyde between DNS conditional means and one-dimensional nonpremixed flame solutions. Additionally, the motion of the cool flame in mixture fraction space is shown to be well described by one-dimensional nonpremixed flame solutions. These points indicate that the fundamental physics of turbulent cool flames are analogous to turbulent hot flames, implying that the same reduced-order manifold-based modeling approaches utilized for turbulent hot flames can be utilized for turbulent cool flames.

Effects of Axisymmetric Square-Wave Excitation on Transverse Jet Structure and Mixing

The influence of temporal square-wave excitation on structural and mixing characteristics of an equidensity, gaseous jet in crossflow (JICF) was explored in the present study. As in separate unforced and sinusoidally excited JICF experiments, acetone planar laser-induced fluorescence imaging enabled this detailed quantification for the JICF for mean jet-to-crossflow momentum flux ratios ranging from (with a convective unstable upstream shear layer, or USL, in the absence of forcing) to (with a globally unstable USL). Such square-wave excitation of the jet fluid required adaptive feedforward control, not only to create more accurate temporal square waveforms but to enable more accurate comparison among alternative forcing conditions. Square-wave excitation of the JICF demonstrated a significant influence on the naturally globally unstable JICF, where specific nondimensional stroke ratios within -dependent ranges could produce deeply penetrating, periodic vortices with improved jet penetration and spread. Enhanced jet penetration did not always correlate with better molecular mixing, however; there was a stronger correlation of improved mixing at higher values with creation of a more symmetric jet cross section via square-wave excitation, especially one with a clear counter-rotating vortex pair structure.

Influence of the velocity field on scalar transport in gaseous transverse jets

The present experiments explored the dynamical character of the gaseous jet injected flush into cross-flow for variable jet-to-cross-flow momentum flux ratios$J$(5, 12 and 41) and density ratios$S$(0.35 and 1.0). Contoured nozzle and straight pipe injectors were studied here, with the jet Reynolds number fixed at 1900 as other flow parameters were varied. Simultaneous acetone planar laser-induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV) were used to study the relationships between scalar and velocity/vorticity fields, with a special focus on comparing PLIF-based extraction of scalar dissipation rates and local strain rates with PIV-based local strain rates in the upstream and downstream shear layers of the jet. There was remarkable similarity between the scalar and vorticity fields for the jet in cross-flow, spanning conditions for absolutely unstable upstream jet shear layers at low$J$or$S$values to conditions for convectively unstable shear layers for larger$J$, equidensity conditions (Megerianet al.,J. Fluid Mech., vol. 593, 2007, pp. 93–129; Getsingeret al.,Exp. Fluids, vol. 53, 2012, pp. 783–801). Proper orthogonal decomposition applied to both scalar and velocity fields revealed strengthening instabilities in both the upstream shear layer and in the jet’s wake as$J$was reduced. The simultaneous measurements allowed PLIF-extracted scalar dissipation rates and strain rates to be determined via a flamelet-like model and compared with PIV-extracted strain rates, each in the diffusion layer-normal direction. There was generally very good qualitative and quantitative agreement for these metrics in both the jet upstream and downstream shear layers for most flow conditions, with excellent correspondence to locations of shear layer vorticity roll up, although downstream shear layer strain rates in some cases showed lesser correspondence between PLIF- and PIV-based data. Such differences are shown to potentially result from diffusion and resolution effects as well as the influence of three-dimensional and transient effects which can be more significant in the lee side of the jet. Nevertheless, the present results reveal interesting dynamics and demonstrate the importance of strain fields in enhanced diffusion and transport phenomena.

Passive scalar mixing studies to identify the mixing length in a supersonic confined jet

Supersonic jet with a co-flow, closely bounded by walls is known as supersonic confined jet. Supersonic confined jet is encountered in practical devices like the supersonic ejector. Mixing of the primary and the secondary fluid inside the confined passage is complex. From a design perspective, it is necessary to have an accurate knowledge of the mixing length (L-MIX). Tracers that do not actively participate in the flow behavior but rather mark the fluids such that they faithfully follow the fluid motion are known as passive scalars. Passive scalars help in the understanding the progression of mixing amidst interacting flows. In this work, we have performed passive scalar mixing studies in a supersonic confined jet for different operating conditions using an existing low area ratio (AR = 3.7) rectangular supersonic gaseous ejector. Air is used as the working fluid in both the primary and the secondary flow. The design Mach number of the primary flow nozzle (M-PD = 1.5-3.0) and the total pressure of the primary flow (P-OP = 4.89-9.89 bar) are varied during the experiments. Using the planar laser-induced fluorescence (PLIF) technique and acetone as the passive scalar, L-MIX is determined. A 266 nm Nd-YAG laser with a repetition rate of 8 Hz is used to excite the acetone molecules in the flow field, and the emitted fluorescence is captured by an ICCD camera. A new method is proposed to study the passive scalar distribution from the acetone PLIF images through digital image processing. Spatial Scalar Fluctuations Intensity (SSFI or psi ) is a parameter defined at every transverse section along the flow direction. Based on the variation of psi along the jet, the location of L-MIX can be identified. L-MIX is defined as the length from the supersonic nozzle exit where psi first attains a value of 0.05. For the first time, L-MIX is quantified in a supersonic confined jet. L-MIX values are observed to be in the range of 3H to 6H for the cases under study, where H is the height of the confined passage. The behavior of L-MIX is closely dependent on the nozzle operating conditions. The values of L-MIX are found to be reduced by 17.67% for the over-expanded flows and increased by 15.76% for the under-expanded flows from the perfectly expanded condition. This study also provides other supersonic confined jet characteristics like the potential core length (L-PC) and the shock cell spacing (S-x) of the primary supersonic jet. Compared to the supersonic free jet, values of L-PC and S-x are found to be different in the supersonic confined jet.

Experimental and Numerical Investigation on Spark Ignition of Linearly Arranged Non-Premixed Swirling Burners

ABSTRACTThe ignition characteristics of a non-premixed multiple-burner linear combustion chamber was investigated experimentally and numerically, focusing on the determination of the mechanisms driving flame propagation from burner to burner. For different inter-burner spacings, overall equivalence ratios and bulk velocities, measurements of the velocity field and the mixture fraction distribution have been performed, respectively, with laser doppler anemometry and planar laser-induced fluorescence of acetone in the un-ignited flow. It was shown that in every individual burner, gas mixes with air within a central recirculation zone (CRZ) where the mixture is flammable except in the axial central rich gas jet and the annular air jet. Flammable mixture from the CRZ is extracted by the annular jet and this results in the existence of bridges of positive flammability factor in the inter-burner region. These bridges allow flame fragments to travel from the CRZ of the ignited burner to the CRZ of the adjacent unignited one, leading to burner-to-burner flame propagation. The ignition probability that sparking within a burner results in ignition of the adjacent one was obtained by performing many separate ignition trials with a laser spark. Ignition probability contours were also computed using a previously developed stochastic low-order ignition model and a large eddy simulation (LES) time-averaged solution of the cold flow. The quantification of the probability a flame kernel leads to burner ignition explained the differences existing between experimental results and the model. The results presented in this article extend our understanding of the mechanisms underlying the global ignition behavior of non-premixed annular combustion chambers.

Transverse jet mixing characteristics

This experimental study explores and quantifies mixing characteristics associated with a gaseous round jet injected perpendicularly into cross-flow for a range of flow and injection conditions. The study utilizes acetone planar laser-induced fluorescence imaging to determine mixing metrics in both centreplane and cross-sectional planes of the jet, for a range of jet-to-cross-flow momentum flux ratios ($2\leqslant J\leqslant 41$), density ratios ($0.35\leqslant S\leqslant 1.0$) and injector configurations (flush nozzle, flush pipe and elevated nozzle), all at a fixed jet Reynolds number of 1900. For the majority of conditions explored, there is a direct correspondence between the nature of the jet’s upstream shear layer instabilities and structure, as documented in detail in Getsingeret al.(J. Fluid Mech., vol. 760, 2014, pp. 342–367), and the jet’s mixing characteristics, consistent with diffusion-dominated processes, but with a few notable exceptions. When quantified as a function of distance along the jet trajectory, mixing metrics for jets in cross-flow with an absolutely unstable upstream shear layer and relatively symmetric counter-rotating vortex pair cross-sectional structure tend to show better local molecular mixing than for jets with convectively unstable upstream shear layers and generally asymmetric cross-sectional structures. Yet the spatial evolution of mixing with downstream distance can be greater for a few specific convectively unstable conditions, apparently associated with the initiation and nature of shear layer rollup as a trigger for improved mixing. A notable exception to these trends concerns conditions where the equidensity jet in cross-flow has an upstream shear layer that is already absolutely unstable, and the jet density is then reduced in comparison with that of the cross-flow. Here, density ratios below unity tend to mix less well than for equidensity conditions, demonstrated to result from differences in the nature of higher-density cross-flow entrainment into lower-density shear layer vortices.

Experimental study of the inverse diffusion flame using high repetition rate OH/acetone PLIF and PIV

Most previous work on inverse diffusion flames (IDFs) has focused on laminar IDF emissions and the soot formation characteristics. Here, we investigate the characteristics and structure of methane IDFs using high speed planar laser-induced fluorescence (PLIF) images of OH, particle image velocimetry (PIV), and acetone PLIF imaging for non-reacting cases. First, the flame appearance was investigated with fixed methane loading (mass flux) but with varying airflow rates, yielding a central air jet Reynolds number (Re) of 1,000 to 6,000 (when blow-off occurs). Next, it was investigated a fixed central air jet Re of 4500, but with varied methane mass flux such that the global equivalence ratio spanned 0.5 to 4. It was observed that at Re smaller than 2000, the inner air jet promotes the establishment of an inverse diffusion flame surrounded by a normal diffusion flame. However, when the Re was increased to 2500, two distinct zones became apparent in the flame, a lower entrainment zone and an upper mixing and combustion zone. 10kHz OH-PLIF images, and 2D PIV allow the identification of the fate and spatial flame structure. Many flame features were identified and further analyzed using simple but effective image processing methods, where three types of structure in all the flames investigated here: flame holes or breaks; closures; and growing kernels. Insights about the rate of evolution of these features, the dynamics of local extinction, and the sequence of events that lead to re-ignition are reported here. In the lower entrainment zone, the occurrence of the flame break events is counterbalanced by closure events, and the edge propagation appears to control the rate at which the flame holes and closures propagate. The rate of propagation of holes was found to be statistically faster than the rate of closure. As the flames approach blow-off, flame kernels become the main mechanism for flame re-ignition further downstream. The simultaneous OH-PLIF/Stereo PIV measurements indicate that the individual breaks events could be correlated to local vortical flow structure and strain rates fields. The detailed measurements provide a more complete understanding of IDF flame characteristics and structure than was previously possible.

Acetone planar laser-induced fluorescence for supersonic flow visualization in air and nitrogen jet

Laser based flow visualization techniques are indispensable tools for flow visualization in fluid dynamics and combustion diagnostics. Among these, PLIF is very popular because of its capability to give quantitative information about the flow. This paper reports the acetone tracer-based PLIF imaging of supersonic jet with air and nitrogen as bath gases. The tracer was seeded in the flow by purging bath gas through the liquid acetone at ambient temperature. Planar laser sheet from frequency quadrupled, Q-switched, Nd:YAG laser (266 nm) was used as an excitation source. Emitted PLIF images of a jet flow field were recorded on ICCD camera. In this study, the dependence of PLIF images intensity on oxygen by comparing nitrogen jet with air in supersonic regime was presented. A lower temperature at the exit of the supersonic jet condenses the tracer which in turn forms droplets. There was a significant decrease in the PLIF image intensity in the case of air. This may be attributed to the oxygen present in the air. It is shown that image adding and Gaussian image processing of PLIF images for steady-state jet improve the quality of images.

Simultaneous concentration and velocity field measurements in a shock-accelerated mixing layer

A novel technique to obtain simultaneous velocity and concentration measurements is applied to the Richtmyer–Meshkov instability. After acceleration by a Mach 2.2 shock wave, the interface between the two gases develops into a turbulent mixing layer. A time-separated pair of acetone planar laser-induced fluorescence images are processed to yield concentration and, through application of the Advection-Corrected Correlation Image Velocimetry technique, velocity fields. This is the first application of this technique to shock-accelerated flows. We show that when applied to numerical simulations, this technique reproduces the velocity field to a similar quality as particle image velocimetry. When applied to the turbulent mixing layer of the experiments, information about the Reynolds number and anisotropy of the flow is obtained.

In Situ PLIF and Particle Image Velocimetry Measurements of the Primary Entrainment Fuel Jet in a Naturally Aspirated Water Heater

The time-averaged characteristics of a fuel jet have been measured via acetone planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) in the primary mixing region of an operating gas-fired water heater. These measurements allow for experimental characterization of the cross-sectional scalar and velocity fields as well as the estimation of the mass entrainment as the flow enters the burner in a practical system. In these experiments, reasonable results were obtained when only the fuel jet was seeded with acetone or PIV particles rather than the entire flow, thus demonstrating the potential for simplified experimental configurations in some applications where controlling or seeding the entire flow may be difficult. The entrainment characteristics of the fuel jet are compared with benchmarks from literature. The commercial device exhibits a larger mass entrainment rate than is found in typical free jets that have been studied in the literature. This may be a result of the jet's low Reynolds number (9,600) in comparison with other literature studies, and a result of initial conditions.