LIF Images Research Articles

Imaging Aluminum Particles in Solid-Propellant Flames Using 5 kHz LIF of Al Atoms.

Laser-induced fluorescence imaging of aluminum atoms (Al-PLIF) is used to analyze the spatio-temporal behavior of aluminized solid propellant combustion. Using alternating LIF and chemiluminescence emission images of the particles in the gaseous and liquid phase evolving close to and far above the dynamically varying propellant surface, sequences of images were recorded and analyzed. The good sensitivity achieved enabled us to track the dynamics of the flame in the vicinity of particles detected all along the flame extension and up to 1.5 MPa. Analysis of wide-field images enabled droplet velocity measurements due to the high LIF sampling rate (5 kHz). The observed typical plume structures were in good agreement with alumina-formation prediction and previous shadowgraphy visualization. High-resolution sequences of images showed gaseous distribution behavior around the molten particles. The Al vapor phase was thus found to extend between 3 and 6.5 radii around the particles. Particle detachment dynamics were captured just above the propellant surface.

Reliable LIF/Mie droplet sizing in sprays using structured laser illumination planar imaging

In this article, Structured Laser Illumination Planar Imaging (SLIPI) is used in combination with the LIF/Mie ratio technique for extracting a reliable two-dimensional mapping of the droplets Sauter Mean Diameter (SMD). We show that even for the case of a fairly dilute spray, where single scattering events are in majority, the conventional LIF/Mie technique still remains largely affected by errors introduced by multiple light scattering. To remove this unwanted light intensity on both the LIF and Mie images SLIPI is used prior to apply the image ratio. For the first time, the SLIPI LIF/Mie results are calibrated and compared with measurement data from Phase Doppler Interferometry (PDI).

A new burner for studying auto-ignition in turbulent dilute sprays

This paper introduces a new burner to study the auto-ignition characteristics of dilute turbulent spray flames. The spray is formed upstream of the exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The fluid dynamics is kept intentionally simple and the boundary conditions are well-defined such that the burner lends itself easily to computations. The stability characteristics are presented here for a range of liquid fuels as a function of the controlling parameters. For each fuel, three types of flames are identified based on their visual appearance. These flames are all lifted but differ in the shape of the leading edge and heat release zones. These characteristics are similar for all fuels and three flames, one from each type, are selected for further study. Measurements of mean temperature, velocity and droplet fields as well as high-speed LIF images of OH, Mie scattering from droplets and chemiluminescence of CH ∗ are made in the selected flames of ethanol. It is found that the leading edge of the flame, which is a hardly visible light-blue zone close to the jet exit plane, is marked by the presence of OH (but not CH ∗), and slight increases in peak temperature above the levels of the co-flow. Further downstream, there exists an intense blue region that is characterized by higher OH levels, significant emission of CH ∗, and temperatures that are much higher than those of the co-flow. This is the region where the bulk of the heat release is taking place. Pockets of CH ∗ are seen to appear in this region and grow as they are advected further downstream.

Fluorescence lifetime imaging in a flame

A novel method for two-dimensional fluorescence lifetime imaging is presented. The technique is demonstrated on averaged planar laser-induced fluorescence (PLIF) signals recorded in a flame. Although demonstrated on averaged images, the concept applies equally well to single-shot images. Formaldehyde was probed through pico-second excitation at 355nm in a rich methane/oxygen flame (Φ=2.6). Images were recorded with a dual ICCD camera detection setup with different gate characteristics of the two cameras. The recorded images were analyzed using simulated values of LIF signal detection to generate two-dimensional images of effective lifetimes. Measured lifetimes range from roughly 1 to 4.5ns. The lifetime image data were used for quenching correction of the LIF images, which, after correction, showed better qualitative agreement compared to a formaldehyde concentration profile simulated with the GRI 3.0 mechanism.

Experimental Investigation of Flow Separation Control Using an Array of Synthetic Jets

An experimental study of flow control using an array of three synthetic jets has been undertaken in a separated laminar flow over a flap in water. PIV was employed to obtain the information about the extent of flow separation delay at different synthetic jet operating conditions. Laser induced florescence (LI F) flow visualisation technique was used to reveal the characteristics of the vortical structures produced by the synthetic jets which result in a delay of separation. It was obser ved that the flow separation delay is typically associated with the presence of two strea ks or three streaks of high streamwise velocity in the separating flow. Based on the resul ts from the present experiment, a parameter map indicating the flow patterns observed at different synthetic jet operating conditions is produced. In addition, a contour map showing the separation control effectiveness at the corresponding conditions is al so obtained. It was found that the twostreak flow pattern is mostly produced at a jet-to- freestream velocity ratio (VR) between 0.3 and 0.5 whereas the three-streak pattern occurs at 0.5<VR<1.5 at medium dimensionless frequencies. The LIF images reveal th at the two-streak flow pattern is caused by the legs of the hairpin vortices produced by synthetic jets whereas the threestreak pattern is caused by the longitudinal struct ures induced by the trailing legs of tilted vortex ring type structures. For the experimental c onditions tested in this study, operating the synthetic jets at a dimensionless stroke length around 2 and a VR around 0.5, which generate vortical structures with a streamwise spac ing not larger than the local boundary layer thickness before separation, would deliver th e best flow control effect with the least energy consumption.

Visualization of gas–liquid mass transfer and wake structure of rising bubbles using pH-sensitive PLIF

A planar laser-induced fluorescence (PLIF) technique for visualizing gas–liquid mass transfer and wake structure of rising gas bubbles is described. The method uses an aqueous solution of the pH-sensitive dye Naphthofluorescein and CO2 as a tracer gas. It features a high spatial resolution and frame rates of up to 500 Hz, providing the ability to capture cinematographic image sequences. By steering the laser beam with a set of two programmable scanning mirrors, sequences of three-dimensional LIF images can be recorded. The technique is applied to freely rising bubbles with diameters between 0.5 and 5 mm, which perform rectilinear, oscillatory or irregular motions. The resulting PLIF image sequences reveal the evolution of characteristic patterns in the near and far wake of the bubbles and prove the potential of the technique to provide new and detailed insights into the spatio-temporal dynamics of mass transfer of rising gas bubbles. The image sequences further allow the estimation of bubble size and rise velocity. The analysis of bubble rise velocities in the Naphthofluorescein solution indicates that surfactant-contaminated conditions are encountered.

Development of a two-line OH-laser-induced fluorescence thermometry diagnostics strategy for gas-phase temperature measurements in engines

This study aims at optimizing two-line OH thermometry strategies for in-cylinder measurement in internal combustion engines. Various aspects are investigated experimentally, such as the selection of suitable OH lines and the possibility of using a single calibration coefficient for variable mixture composition, temperature, and pressure conditions. Two kinds of experimental systems have been investigated. First, a laminar methane-air burner flame at atmospheric pressure, whose stability allowed the determination of OH-laser-induced fluorescence (LIF) intensity ratios from nonsimultaneous imaging. The temperature distribution in the flame is presented for OH-transition pairs with various temperature sensitivities. The burner flame was studied for equivalence ratios from phi=0.93 to 1.30 in order to check for the stability of calibration over various flame conditions. Additionally, OH LIF images were acquired in an optical engine for the chosen OH transitions yielding data about the effect of pressure on OH LIF signals under realistic experimental conditions.

斜面上の波動場における組織渦構造と気体交換機構

The gas transfer of CO2 at the air-water interface when regular surface waves run up a sloping flat bed is investigated experimentally. The CO2absorption process is visualized by using a laser-induced fluorescence te chnique (LIF). We found out in the vicinity of the air-water interface, or ganized vortices whose rotation direction is opposite to that of the offshore vortex train, which was reported by Matsunaga et al. The counter-rotating vortices are found to promote efficiently the gas transfer at the wavy air-water interface in the offshore zone. The local gas transfer velocity is obtained from an image processing for LIF images. It is quantitatively confirmed that the transfer velocity increases due to the organized vortices.

Effect of velocity gradient on propagation speed of tribrachial flames in laminar coflow jets

The effect of velocity gradient on the propagation speed of tribrachial flame edge has been investigated experimentally in laminar coflow jets for propane fuel. It was observed that the propagation speed of tribrachial flame showed appreciable deviations at various jet velocities in high mixture fraction gradient regime. From the similarity solutions, it was demonstrated that the velocity gradient varied significantly during the flame propagation. To examine the effect of velocity gradient, detail structures of tribrachial flames were investigated from OH LIF images and Abel transformed images of flame luminosity. It was revealed that the tribrachial point was located on the slanted surface of the premixed wing, and this slanted angle was correlated with the velocity gradient along the stoichiometric contour. The temperature field was visualized qualitatively by the Rayleigh scattering image. The propagation speed of tribrachial flame was corrected by considering the direction of flame propagation with the slanted angle and effective heat conduction to upstream. The corrected propagation speed of tribrachial flame was correlated well. Thus, the mixture fraction gradient together with the velocity gradient affected the propagation speed.

Flame stabilization in high pressure LO x/GH 2 and GCH 4 combustion

Planar laser induced fluorescence (PLIF) of OH is used to examine flame stabilization in high pressure cryogenic flames formed by injecting a central jet of low speed liquid oxygen surrounded by a high speed gaseous stream of hydrogen or methane. In the LO x /GH 2 experiments injection conditions are transcritical as the chamber pressure is above critical but the temperature is below critical ( p LO x = 6.3 MPa > p c ( O 2 ) = 5.04 MPa, T LO x = 80 K < T c ( O 2 ) = 150 K ) . In the LO x /GCH 4 experiments the chamber pressure and LO x injection temperature are below critical p LO x = 2 MPa , T LO x = 80 K . Hydrogen or methane are injected at room temperature T GH 2 = T GCH 4 = 288 K. LIF images delineate the flame edge in the injector nearfield. The two flames are stabilized in the vicinity of the liquid oxygen injector lip but the anchor point is found to lie closer to the lip in the LO x /GH 2 case and its displacement from shot to shot is of a smaller amplitude than that corresponding to the LO x /GCH 4 flame. Interpretation of these data is based on a previous analysis which indicates that stabilization is essentially controlled by a dimensionless group formed by comparing the lip thickness to the flame edge thickness Ψ = h s/ δ f. It is found that Ψ slightly exceeds unity in the LO x /GH 2 case essentially fulfilling the stability condition while Ψ < 1 in the LO x /GCH 4 case. In this last situation the flame is thicker than the characteristic thickness h s and it is therefore sensitive to the high speed methane stream. Anchoring is imperfect and the flame edge moves with the turbulent eddies shed from the lip. Global stabilization is achieved dynamically but the reactive layer is not well established and the large amplitude motion of the edge is a symptom of a possible lift-off. Theoretical estimates indicate that LO x /GCH 4 flame stabilization requires a thicker lip size than the LO x /GH 2 propellant couple.

A Study of Spray Behavior for the Multi-Component Fuel (3rd Report, Analysis of Spray Vapor Distribution of Each Component)

The objective of this study is to analyze effect of fuel properties on vaporization behavior and each component's vapor distribution in multi-component fuel. In this study vaporization characteristics of multi-component fuel were investigated by LIF methods. A pulsed Nd-YAG laser was used as a light source. The experiment is conducted in a constant-volume vessel with optical access. Fuel is injected into the vessel with electronically controlled common rail injector. Tested fuel contains i-octane (C8H18), n-dodecane (C12H26) and n-hexadecane (C16H34) that are selected as low-, middle-and high-boiling point fuel, respectively. Experiment conditions are 42 MPa, 72 MPa and 112 MPa in injection pressure, 15 kg/m3 in ambient gas density, 700 K in ambient gas temperature, and different fuel mass fraction. Spatial distribution of each component's vapor and fuel mixing process were assessed on the LIF images. And homogeneity of fuel vapor is discussed using the entropy analysis. Experimental results show that fuel vapors are distributed to stratification, and diffusion comes active with increase of mass fraction of low boiling point component.

A301 物質拡散を伴う鉛直上向き噴流

The effect of chemical reaction on the jet diffusion has been investigated. The flow visualization of the jet issuing upward from the vertical pipe into uniform flow has been carried out using the LIF method. Moreover, the velocity distribution and the jet width have been measured by the PIV method in cases of various Reynolds number and combination of fluids. The wavelet components with the various scales were extracted from the LIF images by the wavelet multi-resolution analysis. As the result, it has been found that the jet width is relatively suppressed due to the chemical reaction at the interface between the jet and the ambient flow. Furthermore, it has been clarified that the difference of the flow structure is recognized between the jet with the chemical reaction and that without chemical one.

Effects of turbulence and carrier fluid on simple, turbulent spray jet flames

This paper presents simultaneous LIF images of OH and the two-phase acetone fuel concentration as well as detailed single-point phase-Doppler measurements of velocity and droplet flux in three turbulent spray flames of acetone. This work forms part of a larger program to study spray jets and flames in a simple, well-defined geometry, aimed at providing a platform for developing and validating predictive tools for such flows. Spray flames that use nitrogen or air as droplet carrier are investigated and issues of flow field, droplet dispersion, size distribution, and evaporation are addressed. The joint OH/acetone concentration images reveal a substantial similarity to premixed flame behavior when the carrier stream is air. When the carrier is nitrogen, the reaction zone has a diffusion flame structure. There is no indication of individual droplet burning. The results show that evaporation occurs close to the jet centerline rather than in the outer shear layer. Turbulence does not have a significant impact on the evaporation rates. A small fraction of the droplets escapes the reaction zone unburned along the centerline and persists far downstream of the flame tip. The proportion of this droplet residue increases with shorter residence times as observed for the higher velocity flame.

Measurements of the Reaction between NO and O3 in a Jet Stream of NO Dilute Gas by LIF Methods.

An experimental study on an effect of jet exhaust gas in a simulated ozone layer was conducted. A stratospheric condition was simulated in use of a vacuum chamber, where its pressure was sustained 2.67 kPa, this being equal to the one at the altitude 25 km. NO gas, which was diluted by N2, was fired from a nozzle as an exhaust gas in the simulated ozone layer. NO and NO2 concentration in jet streams were measured by LIF methods, and then reaction amounts between NO and O3 were derived from these LIF images. The reaction rate derived from NO-LIF was much bigger than the actual one, because O3 absorbed laser beam near about 226.1 nm to simulate NO molecular and NO-LIF signal. It was not proper to use NO-LIF method to determine the reaction amount between NO and O3. However, it was useful to visualize the flow in this condition because of its strong intensity. The reaction amount derived from NO2-LIF was valid compared with a result of simple calculations. NO2-LIF should be used to measure the reaction rate qualitatively between NO and O3 in the simulated ozone layer.

Measurement of velocity and velocity derivatives based on pattern tracking in 3D LIF images

Pattern tracking in consecutive 3D LIF images based on least squares matching (LSM) of grey levels has been developed recently for velocity and velocity gradient measurements. The shortcomings of this method are clearly shown. The present article presents an improvement on this method by introducing a local multi-patch (LMP) technique through the LSM approach. The method is validated using the flow field of a turbulent channel flow obtained by direct numerical simulation (DNS) and a synthetic image with grey-level patterns. The results show that LMP matching allows the determination of the velocity and the velocity gradient fields with high accuracy including the second derivatives. Measurements of a round non-buoyant jet are presented which demonstrate the good performance of the method when applied under laboratory conditions. This method can also be applied on two-dimensional images provided that the flow is strictly two-dimensional.

An investigation of neutral and ion number densities within laser-produced titanium plasmas in vacuum and ambient environments

Optical absorption spectroscopy in combination with laser-induced fluorescence imaging is applied to determine spatially and temporally resolved number densities within laser-produced titanium plasmas, expanding into vacuum and low-pressure nitrogen. Contour mapping of species number density and subsequent volumetric integration to yield the total number of absorbing species in the plume are demonstrated for Ti I expanding into vacuum. The results obtained indicate that for an incident KrF energy density of ∼4 J cm-2 the total plume content is >1017 Ti neutrals and ions. The ground-state neutral and ground-state ion yields are both observed to increase linearly with laser fluence above thresholds of ∼2.2 J cm-2 and ∼3.7 J cm-2, respectively. Reduction in absorption linewidths and spatial widening of the corresponding LIF images, observed for plume expansion in the presence of low-pressure ambient gases, reflects the reduction in species velocities and randomisation of the velocity distributions of plume species with increasing ambient pressure.

Measurement of the mixing quality in premix combustors

The problem of mixing fuel and air is the essential point of low emission combustion in gas turbines. In premixed combustors and fuel staged combustors the quality of the fuel–air mixture is the determinant parameter for the amount of emissions of nitric oxides (NOx). The nearly perfect preparation of the fuel–air mixture is also a condition for trouble-free operation in catalytical combustion. Prevaporization of liquid fuels hampers the process of mixing. So the investigative work at the Department of Steam and Gas Turbines at the University of Bochum concentrated on experiments with liquid fuels. The results show that there is a great potential of reducing NOx emission even with liquid fuels and reveal the key role of prevaporization and mixing. The experiments were carried out at a premix combustion test rig at moderate pressure. By using the technique of planar-laser induced fluorescence (LIF), highly time and spatially resolved measurements of fuel concentrations were yielded from the experiments. The optical measurements showed the structure of the mixture field of fuel and air in the zone downstream of the flameholder. The pollutant emissions were simultaneously monitored with conventional gas analysers. As the main result, the strong dependence of the pollutant emissions on the mixture could be clearly revealed. On one hand the homogeneity over the cross-section of the combustor was the main condition for low emission combustion. On the other hand the time-resolved two-dimensional LIF images of the turbulent mixture field showed that the instationary distribution also had a considerable influence on the rate of emissions. Even the mixture of static mixers contained spatial and temporal inhomogeneities, which could be observed by using the LIF-technique but not with conventional methods.

Detection of Vegetation Stress Via a New High Resolution Fluorescence Imaging System

The UV-laser ( λ 355 nm) induced fluorescence emission spectra of green leaves comprise the blue(F440) and green (F520) fluorescence bands as well as the red (F690) and far-red (F740) chlorophyll fluorescence emission bands. Based on the four UV-laser induced fluorescence bands blue, green, red and farred a high resolution fluorescence imaging system was established, which allows a fast and large scale screening of fluorescence gradients and local disturbances in fluorescence emission over the whole leaf surface. The new imaging method not only permits to screen leaves by means of images in four fluorescence bands (LIF images) but, in addition, via images of the fluorescence ratios blue/red (F440/F690), blue/farred (F440/F740), the chlorophyll fluorescence ratio red/far-red (F690/F740) and the ratio blue/green (F440/F520) (LIF ratio images). By fluorescence imaging we could prove that in aurea tobacco the major part of the leaves' blue and green fluorescence is emitted from the main and side leave veins, whereas the major part of the leaves' red and far-red chlorophyll fluorescence is emitted from the vein-free leaf regions, which also have the highest chlorophyll content. A smaller proportion of the aurea tobacco leaves' blue-green fluorescence emission is derived from the cell walls of epidermis cells. The fluorescence ratios blue/red and blue/far-red are very sensitive to environmental changes, and thus permit early stress and strain detection in plants, and the evaluation of damage to the photosynthetic apparatus. Via monitoring the increase in chlorophyll fluorescence LIF images allow to detect differences in the time-dependent uptake of diuron and the progressing inhibition of photosynthetic electron transport in the treated leaf part. The novel fluorescence imaging technique sets a new dimension for early stress detection in the photosynthetic apparatus and in plants. It has many advantages over the previously applied point-data measurements of selected leaf points using conventional spectrofluorometers. The new fluorescence imaging system proved to be very suitable for remote sensing of plants in the near distance, and can be further developed for far distance remote sensing of the state of health of terrestrial vegetation. Some examples in the many possible ways of computer-aided fluorescence data processing (formation of different fluorescence ratios, screening of fluorescence profiles, histogramme plotting) are indicated.

Joint planar CH and OH lif imaging in piloted turbulent jet diffusion flames near extinction

Simultaneous planar LIF images of the OH and CH radicals have been obtained in piloted turbulent diffusion flames, extending previous single-point Raman/Rayleight/OH LIF measurements in the same flames. The results give visual confirmation of inferences from the point measurements that at sufficiently high mixing rates, the flame is locally interrupted and extinguished some 10–30 diameters from the nozzle, and is re-lit further downstream. The images reveal thin, tendril-like structures of CH closely wrapping around the rich side of the OH profiles. Air dilution of the flame substantially thickens the OH reaction zone to the lean side of stoichiometric, and comparisons with laminar flame calculations indicate that whilst flamelet concepts may be applicable to the inner reaction zone where CH is produced and consumed, they are not valid for the thickened recombination zone.