LIF Imaging Research Articles

Development of a temporal filtering technique for suppression of interferences in applied laser-induced fluorescence diagnostics

A temporal filtering technique, complementary to spectral filtering, has been developed for laser-induced fluorescence measurements. The filter is applicable in cases where the laser-induced interfering signals and the signal of interest have different temporal characteristics. For the interfering-signal discrimination a picosecond laser system along with a fast time-gated intensified CCD camera were used. In order to demonstrate and evaluate the temporal filtering concept two measurement situations were investigated; one where toluene fluorescence was discriminated from interfering luminescence of an aluminum surface, and in the other one Mie scattering signals from a water aerosol were filtered out from acetone fluorescence images. A mathematical model was developed to simulate and evaluate the temporal filter for a general measurement situation based on pulsed-laser excitation together with time-gated detection. Using system parameters measured with a streak camera, the model was validated for LIF imaging of acetone vapor inside a water aerosol. The results show that the temporal filter is capable of efficient suppression of interfering signal contributions. The photophysical properties of several species commonly studied by LIF in combustion research have been listed and discussed to provide guidelines for optimum use of the technique.

B212 標準添加LIF法によるジェット燃料火災中の―酸化窒素濃度・温度計測

An LIF imaging spectroscopy was applied to the quantitative measurements of NO concentration and temperature profiles in a jet fuel flame. The NO mole fractions were determined comparing the LIF signals of NO-seeded flame and non-seeded flame. A two-line thermometry technique was used for obtaining the temperature distributions. A narrow-band pass filter was used to sufficiently remove noise lights, and two-dimensional distributions of concentration and temperature of NO were obtained. The flame was stabilized inserting a honeycomb ceramic tube on the burner exit of preheated kerosene-air mixture. The result shows that NO is produced via the thermal NO mechanisms and that the NO concentration and temperature were higher in comparison with a methane-air premixed flame in a standard Bunsen burner.

LIF imaging of OH radicals in DC positive streamer coronas

In this study, the LIF detection of the hydroxyl (OH) radicals was performed in a nozzle-to-plane electrode system having an electrode gap of 30 mm during the steady-state positive streamer corona discharge at atmospheric pressure. For monitoring the ground-state OH radicals, OH transition [ A 2 Σ +( v′ = 1) ← X 2Π( v″ = 0)] at 282 nm was used. The time relationship between the regular streamer coronas, laser pulse, OH fluorescence and laser-induced streamer was measured. The time dependence of OH radicals between the successive streamers was measured for the evaluation of OH dynamics when the discharge was in a steady-state condition. The two-dimensional OH distribution in the DC streamer corona discharge was observed. The obtained results showed that the ground-state OH radicals were generated mainly in the filamentary part of the streamers. It was found that LIF detectable amount of ground-state OH radicals stayed in the region where streamers propagate during the steady-state DC positive streamer corona in open air.

Measurements of Temperature and Concentration of Nitrogen Monoxide in Flames by LIF Imaging Spectroscopy with Standard Addition Method

An NO-LIF imaging spectroscopy of A2Σ+→X2Π (0, 0) excitation (225-226 nm) and A2Σ+→X2Π (0, 2) emission (246-248 nm) was applied to methane-air flames with equivalence ratio of 0.8, 1.0, 1.2, and a methane diffusion flame for measuring mole fractions of nitrogen monoxide (NO) and temperature. The mole fractions were determined comparing the NO-LIF intensities of non NO-seeded and NO-seeded cases. Temperatures in flames were also obtained using two-line method. Nitrogen monoxide was doped in both burner nozzle flow and surrounding air co-flow to enable two-dimensional measurements of mole fraction in the flanges. The results of NO mole fraction images which should be closely correlated to the obtained temperature distributions showed good agreement with the well known NO formation mechanisms. Narrow-band filters for removing non-resonant lights were not enough to remove the strong Mie scattering of laser light by soot in the diffusion flame. Introduction of an imaging spectrometer provided high signal-to-noise ratio of NO-LIF imaging. The NO-LIF acquisition technique for the diffusion flame also revealed that NO A2Σ+←X2Π (0, 0) excitation was a good choice to capture strong NO-LIF while avoiding non-resonant lights such as LIF of soot precursors and Laser Induced Incandescence (LII) of soot.

Fluorescence Imaging of Reactive Processes

Abstract Laser induced fluorescence spectroscopy has emerged as the most powerful analytical tool to image reactive processes in a variety of systems. The purpose of the article is to review state-of-the-art example applications from a variety of research fields, which make use of LIF imaging for the visualisation of chemical reactions. Examples range from gas phase diagnostics of reacting flows to biomedical applications. There is a growing trend in the spectroscopy community to “work across disciplines” and one aim of this article is to highlight common principles behind seemingly unconnected research applications and to provide an incentive for spectroscopists to find novel application areas for their techniques and to point them to relevant literature.

Two-photon LIF imaging of atomic oxygen in flames with picosecond excitation

Interference-free planar laser-induced fluorescence measurements of atomic oxygen are demonstrated in steady and unsteady laminar premixed methane flames. Two-dimensional LIF measurements of atomic oxygen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the LIF signal on laser intensity. Previous two-photon LIF measurements of atomic oxygen in hydrocarbon flames using nanosecond laser excitation were complicated by significant interference from photolytically produced oxygen atoms. Recent results from line-imaging studies in our laboratory indicate that CO 2 is the dominant photolytic precursor. Here, we use picosecond excitation to image atomic oxygen with negligible photolytic interference. We demonstrate this capability in premixed CH 4/O 2/N 2 ( ϕ = 1.08 , X N 2 = 0.63 ) flames with an adiabatic flame temperature of 2480 K and an estimated peak O-atom mole fraction of 0.5%. Peak single-shot signal-to-noise ratios of 9 are achieved, and the estimated detection limit is approximately 80 ppm. Quantitative O-atom LIF measurements also require knowledge of the temperature-dependent quenching cross-sections, which are not currently available. The effects of collisional quenching on the O-atom LIF are predicted using two models for the temperature dependence of quenching cross-sections, and the results indicate that O-atom LIF signals can provide accurate measurements of O-atom profiles in flames. Measurements in an acoustically forced Bunsen flame illustrate the feasibility of using O-atom LIF to investigate flow–flame interactions.

CT Measurement of Three Dimensional Unsteady Flow by Laser Absorption.

The paper describes the concentration measurement technique for unsteady flow fields by employing the laser absorption in conjunction with a Computer Tomography technique. The technique can measure the three dimensional concentration distribution and provide an information about the entire structure of the fluid, while conventional LIF imaging techniques give a cross section of the field. The method will contribute to the investigation of the transport phenomena and the dynamic interaction among them in combination with other measurement technique for experimental study.

LIF imaging and 2D temperature mapping in a model combustor at elevated pressure

Planar laser-induced fluorescence (PLIF) has been used to measure time-resolved spatial distributions of the fuel, the OH radical, and the temperature field in a jet engine model combustor segment. For temperature measurements, a two-line PLIF scheme was used: two different spectral lines of the OH radical, which served as indicator molecule, were excited successively within a short time delay using nanosecond pulses from two UV laser systems operating on different wavelengths. The ratio of the two fluorescence signals depends on the temperature; this forms the basis of the temperature measurement. To our knowledge, this temperature mapping technique has been applied for the first time in the high pressure combustion of kerosene. The fluorescence signal resulting from excitation by one of the two lasers is proportional to the OH density and provides thus information on the OH radical distribution and on flame structure. The same technique can be utilized to excite fluorescence from the fuel, thus providing qualitative information on kerosene distributions. These measurements yield information on flame structure, heat release and mixing properties, which can serve as design aids, as well as for CFD code validation purposes.

An experimental and numerical study of a laminar triple flame

A lifted laminar axisymmetric diffusion flame is stabilized in the downstream region of a diluted methane jet that is surrounded by a lean methane-air co-flow and an outer co-flow of air. The flame shows a distinct triple flame structure in the stabilization region. It is investigated experimentally by PIV for the velocity field, OH-LIPF imaging, C 2H x -LIF imaging, and a 1D-Raman technique for major species concentrations, combined with a Rayleigh technique for temperature. This is complemented by numerical simulations solving the two-dimensional axisymmetric Navier-Stokes equations in the zero Mach number limit on an adapted mesh, coupled with balance equations for temperature and species. A simplified model for molecular transport properties was used with constant, but non-unity, Lewis numbers for all species. Chemistry is represented by a ten-step reduced mechanism for methane oxidation, which was derived starting from a 61-step elementary mechanism that includes the C 1 and C 2 chains. The agreement between the measured and the predicted flow field is very satisfactory. Owing to gas expansion, the velocity decreases immediately ahead of the flame and increases strongly at the flame front. Further downstream acceleration due to buoyancy is dominant and is predicted accurately. There is a good agreement between measurements and computations for flame shape and flame length. The measured OH-LIPF image and the computed OH concentrations indicate that OH is concentrated in the vicinity of stoichiometric mixture. The results from a newly developed C 2H x -LIF method are also supported by calculations. While these measurements were only qualitative, the temperature and mole fractions of the major species could be measured quantitatively with the combined Raman-/Rayleigh technique along a line and were found to agree well with the numerical predictions. It is found that the structure is a triple flame and is influenced essentially by two external parameters: heat exchange between the branches and heat loss at the curved flame front near the triple point.

Application of spontaneous Raman and Rayleigh scattering and 2D LIF for the characterization of a turbulent CH 4 /H 2 /N 2 jet diffusion flame

/H2/N2 diffusion flame. Important aspects of the measuring technique, such as accuracy, cross talk between different Raman bands, and the correction procedure for background from laser-induced fluorescence are discussed. In addition, a 2D LIF and Rayleigh imaging system were used to study the structures of OH, CH, NO, and temperature distributions in the flame. A comparison between two different CH detection schemes is presented. A main goal of the investigations was a detailed and accurate characterization of the investigated flame as well as the study of experimental techniques. Joint pdfs of the temperature and major species concentrations were determined at nearly 100 measuring locations covering the complete flame. Parts of the results are presented in the paper in order to discuss effects of differential diffusion, flame extinction, and interaction between flow field and chemistry. The measured data sets which are available on the Internet are well suited for testing and validating mathematical flame models.

LIF imaging applied to the photodissociation of polyatomic molecules

The photodissociation of HNO 3 at 193 nm and NO 2 at 308 nm in a nozzle beam is used to test the potential of LIF imaging techniques for the state selective detection of angular and velocity distributions of the photofragments NO and OH. The dissociation laser and the probe laser are spatially delayed to probe the products away from the dissociation laser beam. For S/N reasons these first experiments are carried out downstream from the nozzle so close to the exit (15 mm) that collision-free conditions are only valid for a short time. Therefore the experiments yield correct state-resolved TOF distributions only for the fastest fragments. Some indications for correlated product formation are found in the case of HNO 3 photodissociation. Although the S/N ratio is poor, several possible improvements indicate that the method is promising for a detailed analysis of correlated product formation in the photodissociation of polyatomic molecules.

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.

The Influence of Ambient Air Entrainment on Partially Premixed Burner Flames: LIF Imaging of CO and OH

Abstract The influence of the presence and availability of ambient, “secondary” air on the oxidation of CO in partially premixed burner flames is considered, using laser-induced fluorescence. Composite two-dimensional images of the distributions of OH-radicals and CO-molecules in partially premixed bunsen flames, burning in the open air, arc compared with expectations based on the assumption of a closed system. The results indicate that many features of the structure of these flames arc intrinsically due to the interaction of the flame with the ambient environment. The imaging technique is further applied to elements from a burner system from a household appliance. In these experiments, the availability or secondary air is varied by changing the distance between two of the component burners. The images show a declining OH density between the burners as the distance decreases, with a concomitant increase in CO.

Quantitative two-photon LIF imaging of carbon monoxide in combustion gases

Two-dimensional imaging of CO concentration in combustion gases is demonstrated using two-photonexcited planar laser-induced fluorescence. A quantitative model is presented for the simultaneous twophoton excitation of several rotational transitions of the B(1)Sigma(+) ? X(1)Sigma(+) system and the subsequent visible fluorescence (B(1)Sigma(+) ? A(1)Pi). The model is verified by comparison of predicted and measured excitation spectra and of temperature-corrected relative fluorescence measurements to standard probe measurements of the center line CO distribution in a CO-air diffusion flame. In addition, CO imaging experiments in a premixed methane-air flame indicate the production of C(2) by laser photodissociation of acetylene.