LIF Measurements Research Articles

Design and implementation of a new apparatus for astrochemistry: Kinetic measurements of the CH + OCS reaction and frequency comb spectroscopy in a cold uniform supersonic flow.

We present the development of a new astrochemical research tool, HILTRAC, the Highly Instrumented Low Temperature ReAction Chamber. The instrument is based on a pulsed form of the CRESU (Cinétique de Réaction en Écoulement Supersonique Uniforme, meaning reaction kinetics in a uniform supersonic flow) apparatus, with the aim of collecting kinetics and spectroscopic information on gas phase chemical reactions important in interstellar space or planetary atmospheres. We discuss the apparatus design and its flexibility, the implementation of pulsed laser photolysis followed by laser induced fluorescence, and the first implementation of direct infrared frequency comb spectroscopy (DFCS) coupled to the uniform supersonic flow. Achievable flow temperatures range from 32(3) to 111(9) K, characterizing a total of five Laval nozzles for use with N2 and Ar buffer gases by impact pressure measurements. These results were further validated using LIF and direct frequency comb spectroscopy measurements of the CH radical and OCS, respectively. Spectroscopic constants and linelists for OCS are reported for the 1001 band near 2890-2940cm-1 for both OC32S and OC34S, measured using DFCS. Additional peaks in the spectrum are tentatively assigned to the OCS-Ar complex. The first reaction rate coefficients for the CH + OCS reaction measured between 32(3) and 58(5) K are reported. The reaction rate coefficient at 32(3) K was measured to be 3.9(4) × 10-10cm3molecule-1s-1 and the reaction was found to exhibit no observable temperature dependence over this low temperature range.

On the difficulty of interpreting NO-LIF measurements around 226 nm in confined ammonia flames

This paper addresses how NH3 and NO absorption along the laser axis disrupts NO-LIF measurements made around 225 nm in NH3/O2/N2 premixed flames confined at low pressure (10 kPa). Measurements were performed in 3 flames (equivalence ratio ϕ = 0.87, 1.1 and 1.3) with 24 % N2 dilution. Together with the LIF measurements, single-pass absorption measurements were obtained in all flames as a function of the height above the burner (HAB) and of the wavelength. A significant NH3 absorption of the laser beam is found in the pre-heat zone and flame front of each flame, as well as in the burned gas in the rich flame. In the lean flame, whatever the HAB, an additional significant absorption is due to the NO present in the gas surrounding the flame. Whereas NH3 absorbance only occurs inside the flame under all conditions, NO absorbance is mainly due to the high concentration of NO produced in the lean flame and spread in the entire confined vessel around the flame. From this analysis, relative NO rotational population profiles were recorded by measuring the temporal peak of the LIF signals (prompt-LIF) which were corrected for absorption to account for the available energy at burner's center. NO-LIF thermometry was conducted to determine the temperature in the flames through collecting excitation LIF spectra and performing spectral fitting using a dedicated software (thermo NO-LIF, https://pc2a.univ-lille.fr/thermo-no-lif/). The procedure developed to collect the NO profiles could not be transposed to determine the temperature in the lean flame. Indeed, the attenuation of the laser beam depends on whether the laser is tuned on a “cold” transition (strongly impacted by absorption outside the flame) or on a “hot” transition of NO, little affected. Thus, two tubes (with N2 flushing) were installed inside the vessel along the laser axis and surrounding the flame to suppress the NO absorbance.

Non-intrusive temperature measurements for transient freezing in laminar internal flow using laser induced fluorescence

This work presents two color LIF temperature measurements for the transient freezing in a square channel under laminar flow conditions. This is the first time non-intrusive temperature measurements were performed within the thermal boundary layer during the transient growth of an ice layer in internal flow. A combination of a local outlier factor algorithm and a smoothing operation was used to remove the top to bottom striations and reduce the other measurement noise. The temperature uncertainty in our measurements was between σ=0.3∘C and σ=0.5∘C. For the largest temperature difference between the bulk and the melting point of 14.6 °C, good results were obtained. As such, the current campaign demonstrates the potential of LIF as a non-intrusive temperature measurement technique for solid–liquid phase change experiments. However, some artefacts were present within the vicinity of the ice-layer due to the scattering of the laser light, especially near the inlet of the channel where the ice-layer is curved instead of flat. LIF measurements taken within a short time span prior to the onset of ice freezing showed approximately 2 °C of subcooling, consistent with previous findings. In addition, an anomalous behavior within the thermal boundary layer was observed, with a much smaller temperature gradient within the first few mm above the cold plate and a point of inflection in the temperature profile. The anomalous temperature behavior is possibly attributed to enhanced natural convection as a result of the subcooling at the cold plate surface.

The velocity and thermal fields of two parallel plane jets using simultaneous PIV and two-color LIF measurements

This paper presents the results of simultaneous measurements of the velocity and thermal fields of two parallel plane jets. Jets with two different distance ratios having Reynolds numbers of 850 and 910 are reported, with distance ratios selected based on whether or not they have a periodic vortex shedding phenomenon, and were experimented using simultaneous particle imaging velocimetry and two-color laser induced fluorescence. Results showed that jets with a distance ratio of 1.4 have a periodic vortex shedding phenomenon, which are absent in jets with a distance ratio of 2.0. The overall structure of the temperature fields are dissimilar to the velocity fields and major differences in Reynolds shear stress and turbulent heat flux were also apparent, showing that transfer processes of momentum and heat flux differ greatly. The spreading of turbulence statistics showed the mixing capabilities of the periodic vortex shedding phenomenon.

Flow regime transitions for Aiding flow around a heated cylinder: A numerical and experimental investigation

Experiments and numerical simulations were performed to examine the interplay between buoyancy and momentum effects for mixed convection flow around a 13.4 mm diameter cylinder in water. PIV and LIF measurements were made to obtain experimental velocity and temperatures over a Ri range from 0 to 9. LES models were validated against the measured experimental conditions and then used to numerically study the parametric behaviour over a wider range of conditions. The CFD model was found to perform well, with a small tendency to over predict temperature and velocity measurements for higher buoyancy conditions, Ri > 1.9. Using the experimental LIF results, three flow regimes around the cylinder were identified, grouped, and made into a flow regime map. From this, two sets of transition criteria were developed and proposed to predict the collapse of the recirculation zone and the total suppression of vortex formation. This was done via observations from the experimental data and additional CFD simulations to examine the separate effects of fluid properties and incident turbulence levels, both of which were shown to influence vortex suppression significantly. The criteria proposed was found to accurately capture the visually observed flow regimes experimentally in water.

Experimental and modeling study of NO formation in methyl acetate + air flames

Premixed laminar flat non-stretched flames of methyl acetate + air have been studied at atmospheric pressure. The heat flux method was employed to stabilize the flames over equivalence ratios ɸ = 0.7–1.5 at initial gas temperatures from 298 to 348 K. The laminar burning velocities of methyl acetate + air mixtures have been measured at these conditions and compared with the literature data. Laser-induced fluorescence was used to measure NO concentrations in the post-combustion region of the flames at 298 K. The detailed kinetic mechanism of the authors was extended by the reactions of methyl acetate, mostly relying on the model developed by Ahmed et al. (2019). The mechanism was tested against selected literature data from measurements in a shock tube and jet-stirred reactor. Moreover, it was compared with new measurements of the laminar burning velocity and NO concentrations in methyl acetate flames. In all cases, good agreement between the experimental and modeling results was observed. A comparison with LIF measurements obtained for methane + air flames at the same conditions shows that NO concentrations in methyl acetate flames are lower than those in methane + air flames both in stoichiometric and rich mixtures. These differences were interpreted using sensitivity and rate-of-production analyses of NO formation via different routes.

Laser induced fluorescence detection of R6G dye adsorbed on Fe3O4 nanomaterials

This study investigates the comparison between magnetite nanoparticles loaded tea waste (MNLTW) and magnetite nanoparticles loaded reduced graphene oxide nanosheets (MNPs@rGO), as effective adsorbents for the removal of R6G dye from effluents. XRD, TEM, and SEM techniques were used to characterize the proposed adsorbents. The adsorption process was optimized under the parameters of initial concentration, contact time, pH, and the quantity of adsorbents. An amount of 0.15 g of both adsorbents removed about 87% of R6G by MNLTW and 65% by (MNPs@rGO) from 100 ml of a 23.95 mg/l concentrated solution in 40 and 60 min for MNLTW and (MNPs@rGO), respectively at pH 7. LIF measurements have developed better results than those of spectrophotometer. The isothermal analysis according to Langmuir isotherm was (R2 = 0.98) for (MNPs@rGO) and Temkin isotherm (R2 = 0.89) for MNLTW. The kinetic study is well described with a pseudo-second order model.

An a Posteriori Procedure to Correct for the Error Due to Nonuniform and Unsteady Laser Power in Lif Measurements

An a Posteriori Procedure to Correct for the Error Due to Nonuniform and Unsteady Laser Power in Lif Measurements

Continuous-wave planar laser induced fluorescence with a fast camera.

We present planar, laser induced fluorescence (PLIF) measurements of the velocity-resolved distribution function of ions in a plasma using a modulated, narrow linewidth, continuous-wave laser. Plasma emission is acquired with a high frame rate camera, and the laser light is spread into a thin sheet so that an entire plane of the plasma is imaged at each interrogation wavelength. Fourier analysis is conducted on each pixel of the images to separate the modulated fluorescent emission from the background light. Argon ion temperatures and bulk flow maps are reported in a helicon plasma source, and standard single-point LIF measurements provide validation of the PLIF measurement.

Quantitative measurement of atomic hydrogen in low-pressure methane flames using two-photon LIF calibrated by krypton

In this work, the two-photon laser-induced fluorescence technique (TPLIF) was applied to measure the concentration profile of atomic hydrogen in low-pressure laminar premixed flames . Excitation of H-atoms was performed by two-photon absorption at 205 nm and collecting the fluorescence at 656.3 nm. For the first time in flames, the TPLIF signals from the H-atom have been calibrated using the TPLIF signal from krypton, directly seeded in the flame, excited at 204.13 nm and collecting the fluorescence at 826.5 nm. This method was previously demonstrated in plasma environments and recently applied in our group to calibrate O-atom TPLIF signals using xenon as a standard gas. The calibration requires the measurements of TPLIF signals of H and Kr atoms in a flame, where the quenching rates can be determined from time-resolved LIF measurements. Given the short fluorescence lifetime of H-atom, this last task was particularly challenging. A calibration flame was chosen to minimize collisions and the response time of the detection was determined using a deconvolution method. We found that the quenching rate is fairly constant around 4.6 × 10 8 s −1 at 5.3 kPa in a large portion of the flame. The calculated quenching rate overestimates the measured value from 35% to 500% depending on the chosen assumption on the dependence of the quenching coefficient with temperature. The quantitative measurement of H-atom mole fraction was carried out in three nitrogen-diluted low-pressure methane flames. The experimental profiles were compared with the calculated ones using chemical modeling. The variation in the experimental H-atom mole fraction in the range of equivalence ratios agrees well with the simulated values. Quantitatively, the calculated mole fractions agree within 30% with the experimental ones. The method is robust but its accuracy is limited by the uncertainty in the knowledge of the ratio of the two-photon cross-sections of Kr and H atoms. Application of this calibration method to atmospheric is discussed.

Solution to the perturbation of LIF measurements via photodissociation, OH measurement in atmospheric‐pressure multihollow DBD

AbstractThis study solves the problem of the invasivity of laser‐induced fluorescence (LIF) measurements caused by photodissociation. It is shown with the example of OH LIF that the excess signal caused by laser‐induced photodissociation can be distinguished from the true LIF signal of the naturally presented species by its different dependence on the energy of laser pulses. This fact can be further used for the detection of photodissociable species not accessible by the standard LIF. This method is applied to OH radicals in atmospheric‐pressure multihollow dielectric barrier discharge plasma ignited in a number of holes. OH radicals are concentrated at the effluents of these holes, but they are able to spread to a distance of 5 mm, meaning that the discharge is suitable for the treatment of distant and rough surfaces.

Facility pressure effects on a Hall thruster with an external cathode: I. Numerical simulations

We employ the 2D (r–z) code Hall2De to conduct numerical simulations of the SPT-140, a Hall thruster that operates with an external hollow cathode. The simulations are informed by direct measurements of the plasma conditions in the acceleration channel that were obtained using the laser-induced fluorescence technique. We validate our simulation results with additional plasma measurements, wear test erosion rates, and performance data as a function of background pressure. The comparisons of the simulation results with thrust measurements provide insight into the longstanding question of how background pressure affects Hall thruster performance. We find that in thrusters with an external cathode changes in the thrust with varying backpressure can be partially explained by changes in the plasma density near the cathode. We argue that such changes in the density affect the voltage coupling and, eventually, the thrust. However, accounting for this mechanism alone in the simulations over-predicts the thrust measured during ground tests for backpressures less than 10 μTorr. Also, at these backpressures, the measurements showed a higher rate of change of the thrust compared to the simulations. We propose that one explanation for this discrepancy is that the acceleration region may be axially shifting downstream with decreasing backpressure. Though possible, we also recognize that such shifts are not observed in LIF measurements at the lowest pressures for which such diagnostics are possible. We discuss alternative explanations in Part II of this article.

Experimental and Numerical Investigation of the Thermal Transition Area at the Heated Bottom-Wall of a Horizontal Rectangular Channel

This article represents a part of an ongoing work on the preparation of an experimental rectangular channel for the PIV and LIF measurements of vortex structures and temperature field in a non-isothermal water flow. The main aim of the current study is to develop a sufficiently accurate simplified numerical model of the real problem. The basic requirements for thermal properties of heated bottom-wall are specified. In the computational model, there are several simplifications such as 2D case and a constant temperature of a heated surface along the longitudinal coordinate. Results of the numerical simulation of the fluid flow and heat transfer are verified on the experimental data obtained in a laboratory channel with the same geometry and similar flow conditions. The presented results helped to define additional requirements on the design of a new experimental channel intended for investigation of the flow instabilities in a non-isothermal liquid flow.

LIF Measurements of Fluorescence Intensity Corresponding to Pressure-Induced Temperature Oscillations in a Rectangular Tube

LIF Measurements of Fluorescence Intensity Corresponding to Pressure-Induced Temperature Oscillations in a Rectangular Tube

Evaluation of heat release indicators in lean premixed H2/Air cellular tubular flames

Heat release rate in combustion systems must be understood in order to control thermoacoustic instabilities, flame extinction, and heat losses. Traditionally OH chemiluminescence (OH*) is used to trace heat release rate (HRR) in H2/air flames, but its accuracy as a tracer has not been assessed. Lean premixed H2/air cellular tubular flames are a good test case to evaluate HRR tracers due to the presence of highly reactive flame cells surrounded by regions of near extinction. Comparing the calculated heat release rate to OH* concentration, one finds that [OH*] profiles correlate with the regions of high reactivity (flame cells) but the correlation fails in the low reactivity regions where the HRR is much higher than the [OH*] value indicates. Alternate HRR tracers including [H] and pixel-by-pixel products of [O2]x[H], [OH]x[H2], and [O]x[H2] are analyzed with detailed numerical simulations. The chosen products derive from the main chain reaction steps that contribute to overall HRR in lean, premixed H2/air flames. Findings suggest that [H] is an accurate yet simple way of tracking HRR. Planar measurements of HRR are possible if LIF measurements of [H] are improved.

LIF Measurements of Temperature Amplitude Generated by Gas Column Oscillation

LIF Measurements of Temperature Amplitude Generated by Gas Column Oscillation

Pyrolysis behavior of thermally thick wood particles: Time-resolved characterization with laser based in-situ diagnostics

Infrared laser absorption spectroscopy (IRLAS) and laser-induced fluorescence spectroscopy (LIF) are applied in the present study for online in-situ characterization of volatiles products in the close vicinity of a single pyrolysing wood sphere. These advanced analytical techniques are combined with online measurements of temperature and mass evolutions in order to gain further insight into the complex processes occurring during slow pyrolysis of thermally thick wood particles. Three different heating rates (5, 10, 20°C/min) and two wood types (beech, pine) were used. Time-resolved absolute concentrations of the main permanent gases CO2, CO, CH4 and H2O were measured applying IRLAS. From the LIF measurements the qualitative temporal release of polycyclic aromatic hydrocarbons (PAHs) could be deduced. Different behaviors regarding the release of volatiles, temperature evolution and conversion rates were observed for both wood species, which could not be ascribed only to the different content of cellulose, hemicellulose and lignin. According to the measured volatiles evolution an enhancement of heterogeneous catalytic tar cracking seemed to be present with higher heating rates for both wood types. However, the evolution of exothermicity and PAH release differed for both wood types indicating different reaction pathways. An analysis of possible reaction pathways including secondary reactions is performed taking into account all experimental findings about the in-situ composition of the volatiles and the evolution of the PAHs as well as the exothermicity. The differences in the pyrolytic behavior of both species can be related to both the chemical and physical properties of the raw materials.

Quantitative LIF measurements and kinetics assessment of NO formation in H2/CO syngas–air counterflow diffusion flames

Quantitative non-intrusive measurements of NO species using the Laser Induced Fluorescence technique are reported in laminar counterflow syngas–air diffusion flames for a wide range of strain rates and as a function of fuel composition (i.e. H2/CO ratio). The local strain rate of the flames was varied from 35s−1 to 750s−1 for three compositions of syngas (1:4, 1:1 and 4:1 H2/CO ratio). Numerical simulations corresponding to the experimental conditions were conducted using OPPDIF with six H2/CO chemical kinetic mechanisms from the literature. The peak [NO] predictions by mechanism M3 (GRIMech. 3.0) were found to be closest to measurements at low strain rates, while predictions using M1 (partial C1 mechanism by Li et al.) and M4 (full C1 mechanism by Li et al.) mechanisms improved at higher strain rates. A Quantitative Reaction Path Diagram (QRPD) analysis showed that mechanism M3 (GRIMech 3.0) predicted a shift from the NNH (dinitrogen monohydride) and prompt NO pathways towards NNH pathway contributing to NO formation with increasing strain rates. Mechanism M6 (Konnov 0.6) predicted significant prompt-NO contributions via an alternate CNN pathway in addition to the NNH pathway at low as well as high strain rates. Unlike M3 and M6, the M1 mechanism consistently predicted the NNH pathway to be the sole major route for NO formation across all strain rates. The N2O intermediate pathway was found to be insignificant for most of the flames. The sensitivity analysis concerning the effect of CH4 on NO formation routes indicated the need to include CH chemistry in the mechanisms, so as to model the NO formation accurately for H2/CO mixtures containing even trace amounts of hydrocarbons. Lastly, the NO emission indices (EINO) are reported as a function of strain rates for the three compositions of syngas showing 32%H2:8%CO case resulting in the lowest NO emissions and fuel consumption per unit power output.

Impact of heat release on strain rate field in turbulent premixed Bunsen flames

The effects of combustion on the strain rate field are investigated in turbulent premixed CH4/air Bunsen flames using simultaneous tomographic PIV and OH LIF measurements. Tomographic PIV provides three-dimensional velocity measurements, from which the complete strain rate tensor is determined. The OH LIF measurements are used to determine the position of the flame surface and the flame-normal orientation within the imaging plane. This combination of diagnostic techniques enables quantification of divergence as well as flame-normal and tangential strain rates, which are otherwise biased using only planar measurements. Measurements are compared in three lean-to-stoichiometric flames that have different amounts of heat release and Damköhler numbers greater than unity. The effects of heat release on the principal strain rates and their alignment relative to the local flame normal are analyzed. The extensive strain rate preferentially aligns with the flame normal in the reaction zone, which has been indicated by previous studies. The strength of this alignment increases with increasing heat release and, as a result, the flame-normal strain rate becomes highly extensive. These effects are associated with the gas expansion normal to the flame surface, which is largest for the stoichiometric flame. In the preheat zone, the compressive strain rate has a tendency to align with the flame normal. Away from the flame front, the flame – strain rate alignment is arbitrary in both the reactants and products. The flame-tangential strain rate is on average positive across the flame front, and therefore the turbulent strain rate field contributes to the enhancement of scalar gradients as in passive scalar turbulence. Although increases in heat release result in larger positive values of the divergence as well as flame-normal and tangential strain rates, the tangential strain rate has a weaker dependence on heat release than the flame-normal strain rate and the divergence.

The saturation of the fluorescence and its consequences for laser-induced fluorescence thermometry in liquid flows

The temperature dependence of the fluorescence emission of certain organic dyes such as rhodamine B has been widely utilized for measuring the temperature in liquid flows. Measurements are generally based on two assumptions: The fluorescence signal is proportional to the intensity of the laser excitation, and the temperature sensitivity of the dye is not affected by the laser irradiance. In the ratiometric methods, these assumptions allow justifying that the influence of the laser intensity can be totally eliminated in the intensity ratio of two spectral bands of the fluorescence emission and thus that measurements can be taken with no biases under experimental conditions, where the laser propagation is disturbed by the flow. However, when pulsed lasers are used (mainly in planar LIF measurements), the peak irradiance usually compares or exceeds the saturation intensity of the dyes. The present study assesses the consequences of a saturation of the dye emission on temperature measurements. Tests among fluoresceins and rhodamines reveal that the saturation can be accompanied by a significant loss of temperature sensitivity. The dyes, for which this loss of sensitivity is observed, mainly owe their temperature dependence to the fluorescence quantum yield and have a fluorescence signal decreasing with the temperature. The couple fluorescein/sulforhodamine 640 is finally proposed for an implementation of the ratiometric method, since its relatively high temperature dependence (+3 %/∘C) is not altered at high laser irradiances. The possibility of measuring instantaneous temperature fields with this pair of dyes using a single laser shot is finally demonstrated on a turbulent heated jet injected into quiescent water.