Planar Laser-induced Fluorescence Measurements Research Articles

Flame- and flow-conditioned vorticity transport in premixed swirl combustion

This paper presents an experimental analysis of flame-induced enstrophy transport in premixed swirl combustion at Karlovitz numbers between 20 and 50. Such flames possess a large-scale pressure field that – in addition to the pressure fields associated with small-scale turbulent vortices – can interact with density gradients to produce baroclinic torque. Simultaneous tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain high-resolution velocity and progress-variable fields. This allows statistical evaluation of the various terms in the enstrophy transport equation. The impact of small- and large-scale pressure gradients is assessed by conditioning the baroclinic torque on the position of the fluid within the instantaneous flame front, within the flame brush, and axially within the combustor. At all conditions studied, the baroclinic torque was a significant contributor to enstrophy transport, with a comparable magnitude to vortex stretching and viscous diffusion. Enstrophy attenuation and production by baroclinic torque tended to occur towards the reactant and product sides of the instantaneous flame surface, respectively. However, the value of the baroclinic torque also depended equally strongly on the position in the combustor. Hence, both small- and large-scale pressure fields can result in significant enstrophy changes through baroclinic torque. A scaling is proposed to quantify the significance of swirl-induced pressure field on baroclinic torque enstrophy production; it is shown to increase with increasing swirl number and decreasing Damköhler numbers. This is evidence both that flame-induced vorticity dynamics are significant in swirl combustion, and that large-scale geometry-dependent flow fields can impact flame-generated turbulence.

Utilization of the Planar Laser-Induced Fluorescence Technique (PLIF) to Measure Temperature Fields in a Gas-Stirred Ladle

A 1/17th water physical model of a 200-ton steel ladle furnace with a single gas injection was used to simulate bath heating using a single burner to mimic the heat flux due to electric arcs in the industrial steel ladle. Two phases were considered, using water to simulate the molten steel and air to simulate the argon injection at a flow rate of 1.54 NL min−1. The planar Laser-Induced Fluorescence (PLIF) technique was for the first time experimentally implemented to measure temperature fields in a longitudinal plane of the gas-stirred ladle model. PLIF employs a laser source of 532-nm wavelength to light water seeded with rhodamine B, which emits fluorescence depending on its temperature, after a complex calibration is made. Next, the fluorescence is captured by a camera with a 550-nm wavelength filter. The PLIF measurements were validated by local thermocouple measurements at five different locations in the measurement plane. Temperature fields measured by PLIF are in good agreement with those obtained locally by thermocouples, so the PLIF technique can be used to measure temperature fields with the advantage of getting a complete temperature contour field, in contrast to point values of temperatures with thermocouples. Experiments were carried out to study the thermal mixing for two common tuyere positions, i.e., axisymmetric and eccentric (mid-radius) positions. Results on the injection mode show that axisymmetric gas injection is a more efficient heat transfer configuration between the burner and the liquid phase than is the symmetric injection mode for the particular heating configuration studied in this work.

Near blowout dynamics of a premixed, swirl stabilized flame

This paper analyzes the flame dynamics of near LBO (lean blowoff) swirl stabilized flames, using simultaneous OH and CH2O (formaldehyde) PLIF (planar laser induced fluorescence) measurements. Prior studies have shown that recirculation stabilized flames approach blowoff through two distinct stages – “stage 1” characterized by local extinction, where the overall flame and flow field remain largely unchanged, and “stage 2”, characterized by a fundamental change in the flow field, accompanied by violent flame flapping and wake disruption. This paper quantifies extinction spots along the flame edge, and entrained reactants within the combustion product region to analyze these stages in greater detail. Extinction spots were quantified by the overlapping regions of OH and CH2O – numerous such spots were found near blowoff. The entraining of unburnt reactants into the recirculation zone was quantified by detecting low intensity OH pockets that were not surrounded by CH2O. As expected, the flame near blowoff displayed significantly more entrained reactant pockets relative to a stable flame. Unexpectedly however, the volume of these pockets is tiny compared to the products, even on the edge of blowoff. Once they enter the wake, they are short-lived, suggesting that they are diluted and/or quickly react. This was surprising given the non-trivial baseflow and flame position disruption at these conditions, suggesting a striking similarity between the average composition of the wake, to that of a stable flame.

Influence of viscoelasticity on mixing performance of primary and secondary circulation flows in stirred vessels

The aim of this study was to experimentally verify the mixing performance of primary and secondary circulation flows appearing in turbulence in stirred vessels of Newtonian and viscoelastic fluids. Impeller torque measurements, flow visualization, and particle image velocimetry and planar laser-induced fluorescence measurements were performed. In the case of the Newtonian fluid, a tornado-like flow that was a combination of primary and secondary circulation flows was observed with small-scale turbulent eddies. This flow required a moderate torque power and shortened the mixing time. Conversely, a large-scale primary circulation flow of a slow rigid vortex with no small-scale turbulent eddies was observed in the viscoelastic fluid. Although the discharge flow was enhanced or diminished dependently on the Reynolds number and surfactant concentration, it induced slow large-scale secondary circulation flows in the stirred vessel. As a result, the tornado-like flow disappeared, and these flows resulted in a long time constant of the mixing. Even with such flow characteristics, while the low-concentration case indicates that a low torque corresponding to the driving power is needed to drive the flow, the high-concentration case suggests that the high torque is due to the occurrence of additional viscoelastic stress.

PLIF measurements of instantaneous flame structures and curvature of an acoustically excited turbulent premixed flame

In this work, we experimentally measured the instantaneous flame structure and curvature of a methane fueled premixed turbulent acoustically-excited flame in a longitudinal combustor. For this, we implemented simultaneous formaldehyde (CH2O) and hydroxyl (OH) planar laser-induced fluorescence (PLIF) and high-speed schlieren imaging techniques to quantify the dynamic response of this acoustically-excited flame, which was fixed at 260 Hz. The Reynold number of the jet flow was varied from 1216 to 8108. Both imaging techniques revealed that the vortex appeared at the flame top in the presence of acoustic excitations. As the jet velocity increased, the jet flame was squeezed dramatically along the axial direction. The CH2O-PLIF is found to successfully capture the evolution of vortex shedding process. The vortex appears as a sharp angle, when the acoustically-excited flame is squeezed. The vortex size is then increased gradually over an acoustic fluctuation period. To measure the heat release rate from the flame, we used the pixel-by-pixel product of OH×CH2O as a quantitative index. Based on the experimentally measured CH2O, OH and their product characterizing the heat release, it is shown that the more the flame is squeezed by the acoustic waves, the more CH2O is produced and the more unsteady heat is released. Finally, the characteristics of the jet flame curvature were investigated. The variation of the probability density function of the flame curvature revealed that the acoustic excitation with a proper phase could smooth the flame front.

Gas density structure of supersonic flows impinged on by thin blades for laser–plasma accelerator targets

Density transition injection is an effective technique for controllably loading electrons into a trapped phase for laser plasma accelerators. One common technique to achieve the required fluid structure is to impinge a thin blade on the plume of a supersonic nozzle. Density transitions induced in this way are often assumed to be bow shocks and therefore sharp, but simulations and fluorescence measurements presented in this work show that in many cases of interest, the density transition accessible to a laser propagating transverse to the shock is an intercepting shock, and therefore, shock thickness and density vary with pressure, laser height, and blade position. The fluid dynamics of a supersonic nozzle impinged on by a thin, flat object are explored through simulations and relevant features are verified via planar laser-induced fluorescence measurements. The implications of the results for tuning electron beam injectors in laser plasma accelerators are discussed.

Experimental investigation into turbulent negatively buoyant jets using combined PIV and PLIF measurements

Turbulent negatively buoyant jets occur when the buoyancy of a jet opposes its source momentum. In these flows, the fluid will rise until it reaches a stagnation point and a return flow is established, forming a fountain (Hunt and Burridge, 2015). This study looks at both the initial negatively buoyant jet stage of this flow, before the return flow has established, and the fully developed fountain stage. Two-dimensional particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) are used to simultaneously measure the velocity and scalar concentration fields. An experimental and image processing procedure for the PLIF is introduced that accounts for pulse-to-pulse variations in laser power and beam profile for an Nd:YAG laser, which has been demonstrated to reduce the error in scalar concentration measurements. The flow is investigated experimentally using a 1m3 tank of salt-water ambient with freshwater + ethanol negatively buoyant jets, allowing for measurements to be taken at Fro=30 and Reo=5900. The entrainment coefficient for a negatively buoyant jet has been estimated as α ≅ 0.054, lower than a neutral jet at α ≅ 0.058. A finding consistent with existing literature (Bloomfield and Kerr, 2000; McDougall, 1981).

Using Planar Laser Induced Fluorescence to determine temperature fields of drops, films, and aerosols

By using thermocouples and Planar Laser-Induced Fluorescence (PLIF) method, the temperature fields of droplets, films, and aerosols of water have been measured at different heating schemes. Among them were the droplet immersion into a high-temperature gaseous area with the help of holders, lowering of the droplets on a heated substrate as well as their heating at free fall through air. The environment temperature varied from 20 °C to 800 °C. The Rhodamine B dye has been applied as a fluorophore. The effect of the bubbles nucleation in a droplet and the position of a light sheet on the PLIF measurements has been found out. The study described conditions for the emergence of “dead zones” in the temperature field, in which temperature was not able to be measured. At a fluorophore concentration of less than 2000 μg/l, a difference between luminosities of droplets, films, and aerosols did not exceed 3%.

Analysis of eigenmodes in a swirling jet and flame: 3D PIV and PLIF study

The existence of spiral structures was directly confirmed experimentally on the basis of 3D Particle Image Velocimetry (PIV) and planar laser-induced fluorescence (PLIF) measurements both in a strongly swirling non-reacting jet and in a fuel-lean premixed methane-air flame. Flows were characterized by the presence of vortex breakdown and precession of the vortex core. The differences in magnitude and spatial distribution of flow eigenmodes with distance from the nozzle are analyzed by using Proper Orthogonal Decomposition (POD) based on Spatial Fourier Transform over azimuthal coordinate. The analysis of flow eigenmodes reveal that for a high-swirl isothermal jet, the vortex core co-existed with the pair of counter-rotating helical vortices, which were located in outer shear layer and inside the recirculation zone. This double-vortex helical structure was also detected in a swirling flame, but its magnitude was suppressed compared with isothermal flow. It was determined that the change in the shape of the chemical reaction area was associated with two types of large-scale coherent structures: nearly axisymmetric mode m = 0 of the flame front deformation, presumably due to the effect of buoyancy forces on the combustion products, and the quasi-solid rotation of the asymmetric global mode |m| = 1 in the form of double-vortex structure due to precession of the swirling flow. It was shown that the energy of the axisymmetric mode m = 0 in a reacting jet increased downstream by almost 10 %, unlike other modes, whose energy was only diminished.

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.

Effects of sloshing gas–liquid interface on dynamics of meandering bubble plumes and mixing in a shallow vessel: PIV and PLIF measurements

Gas–liquid shallow vessels are widely used in steel-making and refining processes in the steel industry. The efficiency of these processes or the quality of steel depends on the extent of gas–induced liquid–phase mixing achieved through top–blowing of oxygen and bottom–blowing of inert gases. In the present work, the effects of combined top and bottom gas injection and sloshing interface on the dynamics of gas–liquid flow, and their role in the liquid–phase mixing in a shallow vessel, that corresponds 1:6 scaled–down model of a ladle used in steel refining, are investigated. The effects of interface sloshing and top blowing on the motion of multiple meandering bubble plumes on the liquid–phase velocity distribution, recirculatory flow structures and turbulent kinetic energy are analyzed using the time–resolved Particle Image Velocimetry (PIV) measurements. Further, the role of the aforementioned parameters on the liquid–phase mixing is analyzed using the Planar Laser Induced Fluorescence (PLIF) measurements. The effects of coaxial and eccentric top blowing configuration on the liquid flow structures and mixing are also investigated. In the case of combined top and bottom gas injection, in which gas was injected very close to the interface, the liquid–phase mixing was found to be significantly faster than that of the other configurations considered in the present work, due to the formation of strong downward flow regions and higher turbulent kinetic energy provided by the top blowing. The present investigations help to understand and to quantify the effects of the different top gas injection configurations on the meandering motion of bubble plumes, liquid flow structures, turbulence and their contributions to liquid–phase mixing.

Temperature characterization of an operando flow reactor for heterogeneous catalysis

In this work, we present several methods to determine the temperature of a catalyst sample, as well as the gas surrounding it, in a typical flow reactor used for operando research on heterogeneous catalysis. To determine the sample temperature, we present an approach using calibrated IR-camera imagery, as well as thermographic phosphors. For the gas temperature, we present methods to extract temperature information from planar laser induced fluorescence measurements, one of which can be used during operando studies with an active catalyst in place.

Simultaneous high-speed imaging of temperature, heat-release rate, and multi-species concentrations in turbulent jet flames.

We report the high-speed imaging of multi-species and multi-parameter combustion diagnostics for turbulent non-premixed jet flames using a three-legged burst-mode laser system. Simultaneous OH/CH2O planar laser-induced fluorescence and Rayleigh-scattering imaging measurements at a 10-kHz rate are obtained. OH and CH2O concentrations, flame temperatures, and heat-release rates are simultaneously acquired in two-dimensions at 10 kHz.

Kinematics of local entrainment and detrainment in a turbulent jet

In this paper we investigate the continuous, local exchange of fluid elements as they are entrained and detrained across the turbulent/non-turbulent interface (TNTI) in a high Reynolds number axisymmetric jet. To elucidate characteristic kinematic features of local entrainment and detrainment processes, simultaneous high-speed particle image velocimetry and planar laser-induced fluorescence measurements were undertaken. Using an interface-tracking technique, we evaluate and analyse the conditional dependence of local entrainment velocity in a frame of reference moving with the TNTI in terms of the interface geometry and the local flow field. We find that the local entrainment velocity is intermittent with a characteristic length scale of the order of the Taylor micro-scale and that the contribution to the net entrainment rate arises from the imbalance between local entrainment and detrainment rates that occurs with a ratio of two parts of entrainment to one part detrainment. On average, an increase in local entrainment is correlated with excursions of the TNTI towards jet centreline into regions of higher streamwise momentum, convex surface curvature facing the turbulent side of the jet and along the leading edges of the interface. In contrast, detrainment is correlated with excursions of the TNTI away from the jet centreline into regions of lower streamwise momentum, concave surface curvature and along the trailing edge. We find that strong entrainment is characterised by a local counterflow velocity field in the frame of reference moving with the TNTI which enhances the transport of rotational and irrotational fluid elements. On the other hand, detrainment is characterised by locally uniform flow fields with the local fluid velocity on either side of the TNTI advecting in the same direction. These local flow patterns and the strength of entrainment or detrainment rates are also observed to be strongly influenced by the presence and relative strength of vortical structures which are of the order of the Taylor micro-scale that populate the turbulent region along the jet boundary.

Experiment study on the combustion performance of hydrogen-enriched natural gas in a DLE burner

Abstract The effect of hydrogen enrichment to natural gas swirling flame was experimentally investigated at atmospheric pressure conditions using a radially-staged DLE (Dry Low Emission) burner. The hydrogen volume content was varied as 0%, 5%, 11%, 21% and 26% in the fuel blend to assess that whether it is beneficial or detrimental to combustion characteristics. OH-PLIF (Planar Laser Induced Fluorescence) measurement was performed to examine macro flame topology at both stable combustion conditions. NOx and CO emission was also analyzed. In terms of unstable combustion, pressure fluctuation during combustion instability was monitored by dynamic pressure transducers to characterize the dominant frequency as well as pressure oscillation amplitude. Based on the real time pressure signal, phase-locked PLIF method was utilized to give phase averaged OH images during one thermo-acoustic cycle. The experimental results indicate that the flame shrinks distinctly when the addition of hydrogen is increased to certain content. To be specific, the flame structure changes little as the hydrogen content is lower than 5%, and the flame expanding angle also keeps well. Once the hydrogen concentration is boosted to be over 11%, the flame expands greatly towards the confinement wall. Meanwhile NOx emission increases gradually, but CO emission keeps stable with slightly decrease. The amplitude of the dynamic pressure pulsation reduces with hydrogen addition, but the dominant frequency presents to be irrelevant with hydrogen enrichment during combustion oscillation. It can be concluded that hydrogen addition in the natural gas at this realistic radially staged burner shows potential ability to inhibit combustion instability somehow. The 2D Rayleigh Index distribution then indicates proves that the main circulation zone and flame-wall interaction zone are two regions where local heat release and pressure fluctuation coupled to induce combustion thermo-acoustic problems.

Flame Structure and Dynamics in a Premixed Dual-Mode Scramjet Combustor from Fluorescence Imaging

Formaldehyde () planar laser-induced fluorescence (PLIF) imaging was performed in a premixed, ethylene-fueled dual-mode scramjet combustor to study the flame structure and dynamic flame behavior. PLIF is used as a marker to provide information on the structure and position of the lower-temperature portion of the reaction zone. Low-repetition-rate (10 Hz) measurements were acquired at several cross-flow planes along the axial flow direction. Statistical analysis of the topology indicates sparse concentrations of formaldehyde that are intermittently detected by the PLIF measurement. Comparisons between previous large-eddy simulations, laboratory-scale flames, and the current experimental measurements indicate discrepancies between the expected and measured formaldehyde concentrations and structure. High-speed formaldehyde PLIF also was conducted at 50 kHz to further investigate the intermittency of the formaldehyde layers in the flow.

High-repetition-rate burst-mode-laser diagnostics of an unconfined lean premixed swirling flame under external acoustic excitation.

Lean premixed swirling flames are important in practical combustors, but a commonly encountered problem of practical swirl combustors is thermo-acoustic instability, which may cause internal structure damage to combustors. In this research, a high-repetition-rate burst-mode laser is used for simultaneous particle image velocimetry and planar laser-induced fluorescence measurement in an unconfined acoustically excited swirl burner. The time-resolved flow field and transient flame response to the acoustic perturbation are visualized at 20 kHz, offering insight into the heat release rate oscillation. The premixed mixture flow rate and acoustic modulation are varied to study the effects of Reynolds number, Strouhal number, and acoustic modulation amplitude on the swirling flame. The results suggest that the Strouhal number has a notable effect on the periodic movements of the inner recirculation zone and swirling flame configuration.

OH planar laser-induced fluorescence measurements with high spatio-temporal resolution for the study of auto-ignition.

For the detailed understanding of transient combustion processes, in particular, of auto-ignition, quantitative measurements with high spatio-temporal resolution are desirable. These can, for instance, serve as validation data for time-resolved numerical simulations and in particular for the combustion models used in those simulations. In the current study, a jet-in-hot-coflow (JHC) burner, developed at the German Aerospace Center (DLR), the DLR JHC, was used to inject a turbulent methane jet into the hot exhaust gas of a lean hydrogen/air flame, and a steady state jet flame was established. In addition, fuel could be injected in a transient manner. Here, an auto-igniting jet was observed. The flame stabilization of the steady state jet flame and the auto-ignition during transient fuel injection were studied using high-speed laser-based and optical measurements. A strategy for quantifying high-speed OH planar laser-induced fluorescence is presented, and the measurement uncertainties are evaluated. The flame stabilization mechanism in steady state jet flames was assessed using probability density functions of the OH concentration at different axial and radial locations. The formation of auto-ignition kernels during transient fuel injection is evaluated based on time series of the OH concentration. It is shown how the OH concentration levels and PDF shapes can be used to characterize the chemical state of the reacting flow and to distinguish between auto-ignition and flame propagation.

HCCI and SICI combustion modes analysis with simultaneous PLIF imaging of formaldehyde and high-speed chemiluminescence in a rapid compression machine

Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM). A radiofrequency igniter with a high energy deposit (305 mJ) is used to investigate the SICI combustion phenomena in lean conditions (Φ = 0.5), relatively close to the frontiers of the SICI regime. Fluorescence images enable to monitor both the development of the cool flame process and the topology and dynamics of reaction fronts during the second stage of ignition. The results are first analyzed from a phenomenological point of view, bringing insights into the understanding of the both HCCI and SICI combustion processes as they take place in the RCM. Additional data are gathered from double-pulse 355 nm PLIF imaging, with focus on the temporal evolution of the cool flame and on the reaction front propagation during hot ignition. From a more quantitative point of view, an analysis of apparent velocities of the reaction zones is then presented, and large variations of these values are observed depending on the experimental conditions. These local quantities are closely related to the global heat release rate which is a key parameter for practical applications of HCCI and SICI combustion modes. The proposed simultaneous diagnostics finally lead to a better understanding in the local reaction modes, namely deflagration, spontaneous ignition fronts and bulk auto-ignition – e.g. volumetric auto-ignition -, which are implied in the combustion processes. The results highlight the complex aerothermal interactions taking place in the RCM vessel, in particular through the pre-ignition thermal stratification. The results suggest the latter strongly affects the HCCI combustion process, but also drives the heat release rate during the second stage of the SICI combustion mode. Furthermore, deflagration fronts are found to be significantly affected by cool flame chemistry, as well as by the large and small scale structures of the fluid flow.

OH planar laser-induced fluorescence measurement for H<sub>2</sub>/O<sub>2</sub> jet diffusion flames in rocket combustion condition up to 7.0 MPa

OH planar laser-induced fluorescence measurement for H<sub>2</sub>/O<sub>2</sub> jet diffusion flames in rocket combustion condition up to 7.0 MPa