Planar Laser-induced Fluorescence Experiments Research Articles

Comprehending flame development and misfire at advanced engine conditions: Detailed experimental characterizations and machine learning-assisted kinetic analyses

Through comprehensive experimental and modeling efforts, this work unravels the underlying mechanisms governing flame development and misfire at advanced engine conditions that are representative of low-load and lean blow-out operations. Toward this, preliminary heat release, autoignition, and flame developing patterns are characterized, via a case study of n-heptane, at ultra-lean conditions in a well-controlled optical engine under various combustion modes including homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and reactivity-controlled compression ignition (RCCI). Changes in preliminary heat release and flame developing patterns at three overall equivalence ratios (0.12, 0.18, and 0.24) are first characterized under the PPC mode. Flame development characteristics including flame areas and number of initial flame kernels at close-to-misfire conditions are further extracted and compared across the HCCI, RCCI, and three PPC modes, with two distinctive and one transition regimes identified. Further analyses indicate that sustainable flame development and misfire are largely controlled by the spatial distribution of local equivalence ratio (phi) and local temperature in the mixture, which dictate the initial flame kernel generation and the subsequent flame propagation through localized preliminary heat release and autoignition. Chemical kinetic modeling is also undertaken, using a recently updated gasoline chemistry model, in conjunction with a backpropagation neural network, where the predicted ignition delay map well captures the different regions of flame development. Further kinetic analysis and heat rate of production per reaction analysis corroborate the CH2O planar laser-induced fluorescence experiments and highlight the important chemical kinetics that govern the initial flame development patterns.

Liquid and aerated jets behind different pylon configurations in supersonic crossflow

Effects of different pylon configurations on the liquid and aerated jets injected in supersonic crossflow of Mach number 1.65 were investigated experimentally. Pylon configurations with different bottom curvatures at the trailing edge were explored. Two liquid jets with momentum flux ratio of 0.1 and 0.4, and two aerated jets with gas-to-liquid mass ratio of 2% and 4% were selected in the current study. Acetone and air were used as the secondary liquid jet and the aeration gas, respectively. For the atomization quality of the jet, the droplet velocity field was obtained using fluorescence-based particle image velocimetry. The high-speed shadowgraph and cross-sectional planar laser-induced fluorescence experiments were performed for the mixing characteristics, such as penetration height and spreading area. The high-speed images were also used for the dynamic mode decomposition, which revealed dominant structures and their associated frequencies. Curved pylons with 45°and 60°trailing edge curvatures produced higher droplet velocity distribution than the other cases for liquid and aerated jets, respectively. The penetration height of the liquid jet at low momentum flux ratio tends to traverse towards the wall downstream of the injection. Aeration substantially improves the penetration height for the same liquid flow rate due to the annular nature of the aerated jet with a gas-to-liquid mass ratio of 2%. Cross-sectional planar laser-induced fluorescence results revealed a considerable increase in the spreading of the jet for the aerated jets as compared with the low momentum flux ratio liquid jet. This was due to the higher shear layer interaction of the aerated jet than that of the liquid jet. The aerated jets produced 60% higher spreading area than the liquid jet with the same mass flow rate. The curved pylons exhibited at least 10% increment in the spreading area as compared to the standard pylon cases for all the liquid and aerated jets. Three distinct modes were observed in the DMD analysis. The convective mode was the typical first and second modes of all the cases. The shear layer flapping and shear-induced entrainment breakup modes have also been observed with frequencies around 5 kHz.

Visualization and Experimental Characterization of Wrapping Layer Using Planar Laser-Induced Fluorescence.

Droplets on nanotextured oil-impregnated surfaces have high mobility due to record-low contact angle hysteresis (∼1-3°), attributed to the absence of solid-liquid contact. Past studies have utilized the ultralow droplet adhesion on these surfaces to improve condensation, reduce hydrodynamic drag, and inhibit biofouling. Despite their promising utility, oil-impregnated surfaces are not fully embraced by industry because of the concern for lubricant depletion, the source of which has not been adequately studied. Here, we use planar laser-induced fluorescence (PLIF) to not only visualize the oil layer encapsulating the droplet (aka wrapping layer) but also measure its thickness since the wrapping layer contributes to lubricant depletion. Our PLIF visualization and experiments show that (a) due to the imbalance of interfacial forces at the three-phase contact line, silicone oil forms a wrapping layer on the outer surface of water droplets, (b) the thickness of the wrapping layer is nonuniform both in space and time, and (c) the time-average thickness of the wrapping layer is ∼50 ± 10 nm, a result that compares favorably with our scaling analysis (∼50 nm), which balances the curvature-induced capillary force with the intermolecular van der Waals forces. Our experiments show that, unlike silicone oil, mineral oil does not form a wrapping layer, an observation that can be exploited to mitigate oil depletion of nanotextured oil-impregnated surfaces. Besides advancing our mechanistic understanding of the wrapping oil layer dynamics, the insights gained from this work can be used to quantify the lubricant depletion rate by pendant droplets in dropwise condensation and water harvesting.

Mixing enhancement of ethylene secondary jet injected into supersonic cross-flow using curved pylon

Mixing of ethylene secondary jet injected behind a curved pylon in supersonic cross-flow at a Mach number of 1.65 has been studied numerically with experimental validation. For the validation of numerical results, Schlieren, wall pressure measurements, particle image velocimetry, and planar laser-induced fluorescence experiments were performed. Ethylene fuel was used for the simulation, and air was used as a substitute fuel in the experiments. In this study, the curved pylon with different curvatures has been studied at 3.5 bar and 7 bar secondary injection pressures. The results are compared with the standard pylon and conventional transverse injection cases. The results showed improvements in mixing parameters when curved pylons were used. Results indicated that complete mixing was achieved in pylon cases at a much shorter distance than in the transverse injection. The complete mixing distance for the pylon cases was observed around z/dj = 50 and z/dj = 80 for 3.5 bar and 7 bar injection cases, respectively. Even though the mixing efficiencies were similar for all the pylon cases, the flammable plume area and total pressure loss differed. Changes in the bottom curvature angle at the trailing edge of the pylon caused different streamwise vortex structures, which affected the flammable plume area. A considerable difference was found in the penetration height among pylons at 3.5 bar injection pressure, and the difference became minimal at 7 bar injection pressure. Penetration height was greatly affected by the interaction of the secondary jet with the low-pressure region and the momentum flux ratio of the secondary jet. At least a 1 % reduction in total pressure loss was found for curved pylon cases when compared with the standard pylon.

Treating NOx emission of hydrogen fueled combustion engines by NOx storage and reduction catalysts: A transient kinetic study including PLIF measurements

NOx storage and reduction (NSR) catalysts are a well-known and broadly used technology to reduce NOx emissions from combustion engines, which may also be applied for hydrogen fueled engines in the future. In this study, Pt- and Pd-based NSR-catalysts were investigated in the absence and presence of water to understand how NO oxidation as well as the storage and reduction phases are influenced by the gaseous environment with H2 as a reductant. A planar channel configuration was chosen for conducting planar laser-induced fluorescence experiments during the storage phase in addition to steady-state oxidation measurements and transient lean/rich cycles in a packed bed reactor. The presence of steam significantly decreases the NO oxidation activity of both noble metal catalysts. The Pt/BaO/Al2O3 catalyst is more active during transient lean/rich cycles, however, it suffers an activity loss during repeated cycles, whereas the activity of the Pd/BaO/Al2O3 sample is slightly more stable in the wet gas feed over time. All experiments showed a strong correlation between the NO2 formation over the catalyst and its storage capability. The influence of water in the exhaust gas on the NSR-catalysts shows a strong temperature dependency on storage and reduction of NO for both catalysts containing Pt and Pd. The storage behavior is also strongly influenced by both the experimental configurations chosen revealing the significance of the interaction of intrinsic catalytic kinetics and mass transfer in the surrounding flow field.

On the physical nature of the turbulent/turbulent interface

The existence of a turbulent/turbulent interface (TTI) has recently been verified in the far wake of a circular cylinder exposed to free-stream turbulence (Kankanwadi & Buxton, J. Fluid Mech., vol. 905, 2020, p. A35). This study aims to understand the physics within the TTI. The wake boundary, approximately 40 diameters downstream of a circular cylinder subjected to grid-generated turbulence, was investigated through simultaneous cinematographic, stereoscopic particle image velocimetry and planar laser induced fluorescence experiments. With no grid placed upstream of the cylinder, the behaviour of the resultant interface, our closest approximation to a turbulent/non-turbulent interface, exactly matched what is observed in existing literature. When background turbulence is present, viscous action is no longer the only method by which enstrophy is transported to the background fluid, unlike for turbulent/non-turbulent interfaces. The presence of rotational fluid on both sides of the TTI allows the vorticity stretching term of the enstrophy budget equation to be the dominant actor in this process. The role of viscosity within a TTI is greatly diminished as the vorticity stretching term takes over responsibilities for enstrophy production. The turbulent strain rate normal to the TTI was found to be enhanced in the interfacial region. Decomposing the vorticity stretching term into components aligned with the three principal strain-rate directions, it was found that the term most aligned with the interface-normal direction contributed to the largest share of enstrophy production. This indicates that better ‘organised’ vorticity on the wake side of the interface yields the enstrophy amplification leading to the previously discovered enstrophy jump across the TTI by Kankanwadi & Buxton (J. Fluid Mech., vol. 905, 2020, p. A35).

Flame structure and burning velocity of ammonia/air turbulent premixed flames at high Karlovitz number conditions

This paper presents experimental studies of the structures and burning velocities of premixed ammonia/air jet flames at high Karlovitz (Ka) number conditions. Simultaneous planar laser-induced fluorescence (PLIF) imaging of imidogen (NH) and hydroxyl (OH) radicals was performed to investigate the local flame structure and Laser Doppler Anemometry (LDA) measurements were employed for extracting complement relevant turbulent quantities from the flow field. All the selected cases are located in the regime of distributed reaction zones (DRZ) in the Borghi-Peters diagram, with a maximum Karlovitz (Ka) number and turbulent intensity (u′/SL) up to 1008 and 240, respectively. The OH- and NH-PLIF data were used to determine the flame surface density, flame-surface area ratio, and turbulent burning velocity (ST). The main findings include: (a) The NH layer remains thin and continuous over the investigated range of turbulent intensity and Karlovitz number, and the thickness keeps constant statistically without any significant broadening by turbulent eddies; (b) Spatial correlations of the NH and OH radicals show that overlap of NH and OH layers always exists in a thin region where OH has a weaker signal intensity; (c) The ratio of turbulent to laminar burning velocity (ST/SL) shows a nearly linear increase with turbulent intensity, while the ratio of wrinkled flame surface area to that of ensemble-averaged flame surface area increases only slightly with turbulent intensities. The slower increase of wrinkled flame surface area with turbulent intensity can be attributed to under-resolution in the current state-of-the-art PLIF experiments, the neglection of 3D flame wrinkles in 2D experiments, and the increase in flame stretch factor at high turbulent intensities.

Experimental study of a free-surface circular liquid sheet using planar laser-induced fluorescence

This work presents an experimental study of various spatiotemporal events leading to the primary atomization of a radially expanding circular liquid sheet. A cylindrical liquid (water) jet orthogonally made to impinge on the horizontally placed cone-disc deflector to produce a thin free-surface circular liquid sheet from the peripheral edge of the deflector disc. High-speed planar laser-induced fluorescence (PLIF) at 6 kHz framing rate was carried out to capture the atomizing liquid sheet in the side and front views at various jet Weber numbers ($${\text{We}}_{{{\text{jet}}}}$$) over a range of 994 < $${\text{We}}_{{{\text{jet}}}}$$ < 8029. The side view PLIF comprising of three fields of view to capture the complete atomization process of expanding liquid sheet in a time-resolved manner starting from capturing radial wave features on liquid sheet down to the sheet breakup, ligament formation, and droplet production. The azimuthal wave features were captured in the front view PLIF experiments. The quantitative information of different spatiotemporal features was extracted from experimental data using rigorous image processing methods. Advanced feature extraction and transient analysis methods like proper orthogonal decomposition (POD) was applied to obtain the dominant flow features associated with sheet dynamics. The POD analysis showed that the most dominant phenomenon in the flow field for $${\text{We}}_{{{\text{jet}}}}$$ < 1500 is found to be the oscillation of the liquid sheet, the radial wave propagation in the radial direction for 1500 ≤ $${\text{We}}_{{{\text{jet}}}}$$ < 2000, and sheet breaks up at the outer edge for $${\text{We}}_{{{\text{jet}}}}$$ ≥ 2000. It is shown that the dominancy of each of these phenomena changes with $${\text{We}}_{{{\text{jet}}}}$$. Statistical analysis-based ligament length and thickness ($$L_{{{\text{lig}}}}$$ and $$t_{{{\text{lig}}}}$$) follow $${\text{We}}_{{{\text{jet}}}}^{ - 0.6}$$ and droplet diameter ($$D_{32}$$ and $$D_{10}$$) follow $${\text{We}}_{{{\text{jet}}}}^{ - 0.33}$$ power law. Further, the direct correlations obtained showed that the droplet diameter is a stronger function of ligament thickness compared to ligament length. Schematic of optics, laser light and camera arrangements for side view PLIF experiments of a circular liquid sheet. Three FOVs adopted for side view PLIF experiments at different radial locations; RA= 280 mm for A = 100 mm × 100 mm, RB = 360 mm for B = 80 mm × 80 mm, and RC = 420 mm for C = 40 mm × 40 mm Direct imaging of azimuthal waves of liquid sheet using front view PLIF experimental technique

Hydroxyl radical planar imaging in flames using femtosecond laser pulses

Hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) imaging is one of the most widely used laser diagnostic techniques to investigate reacting flows such as flames and plasmas. In conventional PLIF experiments, 10-Hz, commercial Nd:YAG/dye-laser-based systems are often used for OH excitation. In recent years, significant developments are also reported in using diode-pumped solid-state and pulse-burst laser systems for high-repetition-rate (kHz–MHz) measurements. In general, all these laser sources generate nanosecond-duration, narrowband laser pulses which are to be tuned to a specific ro-vibrational excitation transition line. In the present work, we investigate the use of broadband, ultrashort femtosecond-duration (fs) laser pulses for OH-PLIF imaging in flames. The fs excitation of the OH A2Ʃ+ ← X2∏ (1, 0) transition is followed by fluorescence detection from the (0, 0) and (1, 1) vibrational bands. Because of the broad bandwidth, the excitation laser is coupled to a large number of OH ro-vibrational transitions at the same time; hence, species selectivity is obtained by detecting fluorescence emission in the 310–325 nm spectral window. This scheme is shown to be free from fluorescence from other flame species as confirmed by high-resolution fluorescence spectra recorded under a variety of flame conditions. Measured OH number density profiles in CH4, C2H4 and H2 calibration flames are in good agreement with model predictions. Two-dimensional imaging of OH at 1-kHz repetition rate is also demonstrated in a turbulent diffusion flame. The present fs OH-PLIF scheme can find novel applications in fundamental chemical physics research, as well as in practical engine combustion and flame diagnostics.

Planar Laser-Induced Fluorescence Experiments and Modeling Study of Jets in Crossflow

Planar laser-induced fluorescence (PLIF) with acetone seeding was applied to measure the scalar fields of an axisymmetric freejet and an inclined jet in crossflow as applicable to film cooling. From the scalar fields, jet-mixing and trajectory characteristics were obtained. In order to validate the technique, the canonical example of a nonreacting freejet of Reynolds numbers 900–9000 was investigated. Desired structural characteristics were observed and showed strong agreement with computational modeling. After validating the technique with the axisymmetric jet, the jet in crossflow was tested with various velocity ratios and jet injection angles. Results indicated the degree of wall separation for different injection angles and demonstrated both the time-averaged trajectories as well as select near-wall concentration results for varying jet momentum fluxes. Consistent with literature findings, the orthogonal jet trajectory for varying blowing ratios collapsed when scaled by the jet-to-freestream velocity ratio and hole diameter, rd. Similar collapsing was demonstrated in the cases of a nonorthogonal jets. Computational fluid dynamic (CFD) simulations using the openfoam software were used to compare predictions with select experimental cases and yielded reasonable agreement. Insight into the importance and structure of the counter-rotating vortex pair (CVP) and general flow field turbulence was highlighted by cross validation between CFD and experimental results.

Experimental study of enhanced mixing induced by particles in Taylor–Couette flows

Local mixing dynamics was recently investigated experimentally in Taylor–Couette single-phase flow, thanks to simultaneous Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) techniques. The results highlighted the influence of the successive flow bifurcations and the role of azimuthal wave states on the dispersion of dye injected in Taylor–Couette flows.The present work extends this study to two-phase configurations with spherical solid particles. The respective effect of particle size and concentration on the vortices size and transition thresholds between the various flow regimes has been examined thanks to flow visualizations and PIV measurements. These hydrodynamic features have been complemented with PLIF experiments, that revealed a drastic enhancement of mixing due to the presence of particles regardless of the flow regime, highlighting the existence of significant particle-induced mixing in Taylor–Couette flows.

Refractive index and solubility control of para-cymene solutions for index-matched fluid–structure interaction studies

To deploy optical diagnostics such as particle image velocimetry or planar laser-induced fluorescence (PLIF) in complex geometries, it is beneficial to use index-matched facilities. A binary mixture of para-cymene and cinnamaldehyde provides a viable option for matching the refractive index of acrylic, a common material for scaled models and test sections. This fluid is particularly appropriate for large-scale facilities and when a low-density and low-viscosity fluid is sought, such as in fluid–structure interaction studies. This binary solution has relatively low kinematic viscosity and density; its use enables the experimentalist to select operating temperature and to increase fluorescence signal in PLIF experiments. Measurements of spectral and temperature dependence of refractive index, density, and kinematic viscosity are reported. The effect of the binary mixture on solubility control of Rhodamine 6G is also characterized.

In situ plasma activated low temperature chemistry and the S-curve transition in DME/oxygen/helium mixture

The effect of non-equilibrium plasma activated low temperature chemistry (PA-LTC) on the ignition and extinction of Dimethyl Ether (DME)/O2/He diffusion flames has been investigated experimentally in a counterflow burner with in situ nanosecond pulsed discharge at 72Torr. A uniform discharge is generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of DME/O2/He are studied by employing OH and CH2O Planar Laser Induced Fluorescence (PLIF) techniques at constant strain rates and O2 mole fractions on the oxidizer side with varying the DME mole fractions. Contrary to the conventional understanding, strong low temperature reactivity during ignition process is observed for DME with non-equilibrium plasma activation even at 72Torr and flow residence time of a few milliseconds. The OH PLIF shows strong OH signal at and after ignition, whereas extremely low OH signal before ignition. However, the CH2O PLIF experiments demonstrate that, with the increase of DME mole fraction on the fuel side, the CH2O PLIF signal intensity increases significantly before ignition and decreased rapidly after ignition. The low OH number density and high CH2O number density before DME ignition clearly demonstrates the existence of PA-LTC at low pressure. Moreover, at higher O2 mole fraction and discharge repetition frequency, the in situ discharge significantly modifies the characteristics of ignition and extinction, thus creating a new monotonically and fully stretched ignition S-curve without an extinction limit. Compared to our previous study of methane, the existence of strong low temperature reactivity in DME oxidation makes ignitions occur at much lower fuel mole fractions, thus accelerating the transition of ignition curve from conventional S-curve to the fully stretched S-curve. The transition from the conventional S-curve to the new stretched ignition curve at high plasma repetition rate indicates that the plasma could dramatically change the chemical kinetic pathways of DME oxidation by activating the low temperature chemistry even at low pressure. The chemical kinetic model for the plasma–flame interaction has been also developed based on the assumption of constant electric field strength in the bulk plasma region. Both experiments and modeling results reveal that the PA-LTC has a much shorter timescale comparing with that of thermally activated low temperature chemistry owing to the rapid radical production by plasma. The reaction pathways analysis shows that atomic O generated by the discharge is critical to controlling the population of radical pool.

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.

Experimental observation of the flame structure of a bimodal ammonium perchlorate composite propellant using 5 kHz PLIF

The self deflagration of a bimodal ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) propellant was studied using high speed (5 kHz) planar laser induced fluorescence (PLIF) for the first time. The qualitative OH concentration was characterized near the surface. In addition to OH, it was found that the larger AP particles can be imaged as they fluoresce when exposed to laser radiation centered at 283.2 nm. Single AP particle ignition delay, lifetime, and flame height are determined as a function of particle diameter over a range from 100 to 500 μm at 1 atm within the burning sample. High speed visible imaging was also completed to confirm the trends seen during PLIF experiments, although the fluoresced particles have much improved contrast. Ignition delay times and single particle burn times were compared with a model proposed by Shannon and Peterson. The measured final diffusion flame height above a regressing AP crystal was compared with an expression used by the Beckstead, Derr, and Price (BDP) model. It was found that the AP/HTPB propellant flame structure varies significantly with particle size, even at 1 atm. The models are found to adequately predict the observed trends, but do not capture the interaction of adjacent particles. The interaction of particles close to each other appear to affect behavior significantly and more detailed modeling is needed. It is shown that 5 kHz OH PLIF can be a valuable tool to characterize AP composite propellant combustion.

A comprehensive model for the quantification of linear and nonlinear regime laser-induced fluorescence of OH under A2Σ+←X2Π(1,0) excitation

An analytic model termed the ‘integrated quasi-steady-state’ (IQSS) model for the comprehensive quantification of both linear and nonlinear regime laser-induced fluorescence (LIF) is presented. The IQSS model is optimized for the hydroxyl radical (OH), subject to nanosecond A2Σ+←X2Π(1,0) excitation at pressures close to atmospheric. The IQSS model is particularly relevant to experimental conditions where the LIF signal is both spectrally and temporally integrated, such as in planar laser-induced fluorescence experiments. The IQSS model is based around a quasi-steady-state solution to a four-level rate-equation approximation of the OH molecule; this quasi-steady-state solution is then integrated with a triangular functional form for both the spatial and temporal variations to produce an analytic solution. In order to accurately predict LIF in the nonlinear regime, it is shown that both the temporal and the spatial variations of the laser pulse—or ‘wings’ of the laser pulse—must be adequately accounted for in the LIF model formulation. The IQSS model is successfully verified against detailed numerical simulations for variations in the laser irradiance, quenching environment and temperature. Experimentally, the IQSS model is successfully validated by comparing the predicted and measured OH LIF vs. irradiance dependence in the product gases of a methane–air laminar flame.

Three-dimensional concentration field measurements in a mixing layer using magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to measure the three-dimensional, time-averaged concen- tration distribution in a turbulent two-stream mixing layer. Test fluids and MRI scanning parameters were chosen to give good signal linearity, and a calibration/normalization procedure was developed to reduce the concentration measurement uncertainty. Plain deionized water mixing with a solution of 0.8% gadopentetate dimeglumine in deionized water were selected as test fluids. The concen- tration of the marked water was measured on an array of 220,000 0.69 mm 3 voxels covering the entire flow appa- ratus. Planar laser-induced fluorescence experiments were performed on the flow centerplane to provide validation data. The uncertainty of a single voxel measurement was estimated to be less than 12% with the largest source of uncertainty being turbulent dephasing. Averaging two runs in which the marked water was switched between the two streams reduced the uncertainty to only 4%. The complete magnetic resonance concentration (MRC) procedure including the adjustment of scanning parameters, a back- ground run, two reference/calibration runs, and multiple concentration measurement runs can be completed in 2- 3 h. This work establishes MRC as a viable technique for studying the mixing in complex turbulent liquid flows.

Measurements of the Statistical Distribution of the Scalar Dissipation Rate in Turbulent Axisymmetric Plumes

In this paper we reconsider the high resolution Planar Laser-Induced Fluorescence (PLIF) mixing measurements that were presented in Markides and Mastorakos (Chem. Eng. Sci. 61:2835–2842, 2006). The PLIF experiments were performed in flows created by the continuous injection, with various velocity ratios, of a passive scalar (acetone) from a finite sized round nozzle, into uniform turbulent co-flows confined within a cylindrical tube with a range of turbulent Reynolds numbers. Here, we extend our study of these flows to include an investigation of the Probability Density Functions (PDFs) of the scalar dissipation rate (χ), as well as of the scalar dissipation rate conditional on mixture fraction (χ|ξ). To this effect, we have further processed the resulting data from the earlier work, including an additional correction for density variations in the flow. The results were then reprocessed for the accurate recovery of the spatial gradients of the normalized scalar concentration (ξ), which resulted in direct measurements of χ. All results that are presented in this paper involve the two-dimensional scalar dissipation rate (χ2D) evaluated in the plane of imaging, and furthermore, results are only shown and discussed along the centreline of our axisymmetric configuration. The PDFs of χ were calculated and found to deviate slightly from a lognormal distribution, consistent with other published work. Moreover, the PDFs of χ|ξ followed a similar distribution, and showed a similar deviation. This deviation was more pronounced closer to the source and decayed downstream. The ratio of the standard deviation over the mean of χ and χ|ξ increased with streamwise distance and reached values of 2.8 and 1.2 respectively.

A turbulent boundary layer over a two-dimensional rough wall

Particle image velocimetry (PIV) measurements and planar laser induced fluorescence (PLIF) visualizations have been made in a turbulent boundary layer over a rough wall. The wall roughness consisted of square bars placed transversely to the flow at a pitch to height ratio of λ/k = 11 for the PLIF experiments and λ/k = 8 and 16 for the PIV measurements. The ratio between the boundary layer thickness and the roughness height k/δ was about 20 for the PLIF and 38 for the PIV. Both the PLIF and PIV data showed that the near-wall region of the flow was populated by unstable quasi-coherent structures which could be associated to shear layers originating at the trailing edge of the roughness elements. The streamwise mean velocity profile presented a downward shift which varied marginally between the two cases of λ/k, in agreement with previous measurements and DNS results. The data indicated that the Reynolds stresses normalized by the wall units are higher for the case λ/k = 16 than those for λ/k = 8 in the outer region of the flow, suggesting that the roughness density effects could be felt well beyond the near-wall region of the flow. As expected the roughness disturbed dramatically the sublayer which in turn altered the turbulence production mechanism. The turbulence production is maximum at a distance of about 0.5k above the roughness elements. When normalized by the wall units, the turbulence production is found to be smaller than that of a smooth wall. It is argued that the production of turbulence is correlated with the form drag.

The effect of photobleaching on PLIF

Organic dyes commonly used for planar laser-induced fluorescence (PLIF) are susceptible to photobleaching under laser excitation. Previous work has shown that photobleaching can induce significant errors in laser-induced fluorescence measurements made with single-point techniques. This paper presents an analytical model and experimental results that quantify the extent of photobleaching in the more common planar configuration of the technique. Experimental results for two common dyes, fluorescein and Rhodamine 6G, agree well with analytical predictions for typical PLIF parameters. Results indicate that even dyes such as fluorescein that have large photobleaching constants can be used with little error in many PLIF experiments.