Laser Induced Fluorescence Of Acetone Research Articles

Quantification of Mixing Quality Effects on Flashback Limits for H2-Rich Fuel Gases in Lean Premixed Combustion Using Laser-Induced Fluorescence of Acetone

Abstract One of the main challenges arising from hydrogen-rich fuel mixtures is to prevent flame flashback. In typical gas turbines, the fuel is commonly injected through holes in the axial swirler vanes to achieve a high mixing quality. However, this injection method is not perfect and can cause nonhomogeneous mixing regions, so that locally rich fuel clusters can significantly increase flashback propensity. This work aims to establish a link between the local mixing quality of fuel and air and flashback limits obtained experimentally under elevated pressure conditions. The nonreacting experiments have been conducted at an atmospheric mockup test rig and acetone-planar laser-induced fluorescence (PLIF) has quantified the local fuel concentration. Zones of high equivalence ratio are evident close to the center body wall. The near-wall equivalence ratio fields reveal that the critical probability of the local equivalence ratio being greater than the one for perfect premixing is between 20% and 35% for all hydrogen concentrations at the flashback limits observed. A probability of 35% is selected as a critical threshold to derive a correlation between the local and the global equivalence ratio in technical premixing. Even though the correlation is specific to the investigated burner configuration, the presented methodology offers valuable insights into the impact of the local mixing quality on flashback propensity, which can improve flashback prediction models formulated for perfect premixing conditions.

Acetone Tracer Laser-Induced Fluorescence (LIF) at 282 nm Excitation as a Diagnostic Tool in Elevated Pressure and Temperature Systems.

This paper provides new data at constant pressure, variable temperature and constant temperature, variable pressure over the range of temperature (295-750 K) and pressure (0.5-40 atm), in air and nitrogen bath gases at both constant number density and constant mole fraction needed to enable acetone laser-induced fluorescence (LIF) as a diagnostic tool in an elevated temperature and pressure environment. Results clearly indicate that the effect of elevated pressure on the temperature sensitivity of acetone LIF is negligible when the excitation wavelength is chosen near the absorption maximum, making acetone LIF ideal for measuring temperature fields in high-pressure systems. The data also show that the temperature sensitivity of fluorescence is relatively insensitive to changes in pressure at 282 nm excitation. Furthermore, at 282 nm, fluorescence per mole fraction can be used to extract temperature while fluorescence per molecule is relatively insensitive to temperature, making it optimal for concentration or pressure measurements.

Acetone photophysics at 282 nm excitation at elevated pressure and temperature. II: Fluorescence modeling

This is the second in a series of two papers that presents an updated fluorescence model and compares with the new experimental data reported in the first paper, as well as the available literature data, to extend the range of acetone photophysics to elevated pressure and temperature conditions. This work elucidates the complete acetone photophysical model in terms of each and every competing radiative and non-radiative rate. The acetone fluorescence model is then thoroughly examined and optimized based on disparity with recently conducted elevated pressure and temperature photophysical calibration experiments. The current work offers insight into the competition between non-radiative and vibrational energy decay rates at elevated temperature and pressure and proposes a global optimization of model parameters from the photophysical model developed by Thurber (Acetone Laser-Induced Fluorescence for Temperature and Multiparameter Imaging in Gaseous Flows. PhD thesis, Stanford University Mechanical Engineering Department, 1999). The collisional constants of proportionality, which govern vibrational relaxation, are shown to be temperature dependent at elevated pressures. A new oxygen quenching rate is proposed which takes into account collisions with oxygen as well as the oxygen-assisted intersystem crossing component. Additionally, global trends in ketone photophysics are presented and discussed.

Experimental and Numerical Investigation on Spark Ignition of Linearly Arranged Non-Premixed Swirling Burners

ABSTRACTThe ignition characteristics of a non-premixed multiple-burner linear combustion chamber was investigated experimentally and numerically, focusing on the determination of the mechanisms driving flame propagation from burner to burner. For different inter-burner spacings, overall equivalence ratios and bulk velocities, measurements of the velocity field and the mixture fraction distribution have been performed, respectively, with laser doppler anemometry and planar laser-induced fluorescence of acetone in the un-ignited flow. It was shown that in every individual burner, gas mixes with air within a central recirculation zone (CRZ) where the mixture is flammable except in the axial central rich gas jet and the annular air jet. Flammable mixture from the CRZ is extracted by the annular jet and this results in the existence of bridges of positive flammability factor in the inter-burner region. These bridges allow flame fragments to travel from the CRZ of the ignited burner to the CRZ of the adjacent unignited one, leading to burner-to-burner flame propagation. The ignition probability that sparking within a burner results in ignition of the adjacent one was obtained by performing many separate ignition trials with a laser spark. Ignition probability contours were also computed using a previously developed stochastic low-order ignition model and a large eddy simulation (LES) time-averaged solution of the cold flow. The quantification of the probability a flame kernel leads to burner ignition explained the differences existing between experimental results and the model. The results presented in this article extend our understanding of the mechanisms underlying the global ignition behavior of non-premixed annular combustion chambers.

Fuel–Air Mixing Experiments in a Directly Fueled Supersonic Cavity Flameholder

In the present study, planar laser-induced acetone fluorescence was used to quantify the streamwise concentration distribution of an ethylene fuel surrogate (acetone-seeded nitrogen) injected directly into the recirculation region of a cavity flameholder embedded in a nonreacting supersonic airflow. The effect of injection configuration and fueling rate were studied. Parallel injection from the cavity front wall resulted in an ultra-fuel-rich recirculation region (beyond the upper flammability limit of an ethylene–air mixture) with a mixing efficiency that degraded with increased fueling rate. Downstream angled injection from the cavity floor yielded a substantial improvement in terms of the spatial uniformity and flammability of the local mixture. Its mixing efficiency was observed to be independent of fueling rate. The superior mixing ability of the latter injection configuration has the potential to provide increased cavity flameholder operability through a broader range of ignition and blowout limits and more flexibility in terms of ignitor placement than conventional direct-fueling schemes. Further investigation of its performance over a wider range of reacting conditions is warranted.

Gas Phase Oxygen Quenching Studies of Ketone Tracers for Laser-Induced Fluorescence Applications in Nitrogen Bath Gas

ABSTRACT In this paper we report the quantitative oxygen quenching effect on laser-induced fluorescence of acetone, methyl ethyl ketone, and 3-pentanone at low pressures (∼700 torr) with oxygen partial pressures up to 450 torr. Nitrogen was used as a bath gas in which these molecular tracers were added in different quantities according to their vapor pressure at room temperature. These tracers were excited by using a frequency-quadrupled, Q-switched, Nd:YAG laser (266 nm). Stern–Volmer plots were found to be linear for all the tracers, suggesting that quenching is collisional in nature. Stern–Volmer coefficients (ksv) and quenching rate constants (kq) were calculated from Stern–Volmer plots. The effects of oxygen on the laser-induced fluorescence of acetone, methyl ethyl ketone, and 3-pentanone were compared with each other. Further, the Smoluchowski theory was used to calculate the quenching parameters and compared with the experimental results.

Lean blowoff behavior of asymmetrically-fueled bluff body-stabilized flames

Bluff-body stabilized flames were studied in an enclosed, asymmetrically-fueled duct with a two-dimensional triangular flame holder. Acetone laser-induced fluorescence was used to characterize the fuel distribution for both uniform and non-uniform fuel profiles. Flame dynamics were captured with high-speed chemiluminescence imaging during stable operation and near blow off conditions for three cases with varying fuel–air gradients across the flame holder. Particle imaging velocimetry was used to measure the velocity field. It was discovered that for a given velocity, increased fuel profile asymmetry caused an increase in the blowoff equivalence ratio, produced greater vortex shedding coherence, and for lower velocities resulted in dynamic coupling between the heat release and the duct acoustics. High-speed imaging of the acoustically uncoupled cases revealed the same flame blowoff process as previously observed in uniformly fueled cases. The blow off process in the acoustically coupled cases was dominated by acoustically influenced velocity straining the flame adjacent to the wake stagnation zone causing local extinction and rapid entrainment of reactants into the recirculation zone. From the Mie scattering images gathered for PIV, density transition contours were extracted and used as flame contours to calculate local aerodynamic strain rates and curvature. Statistics revealed conditional relationships between the local strain, wake geometry and fluid mechanics.

Uncertainty assessment of species measurements in acetone counterflow diffusion flames

To quantitatively understand the uncertainty of intrusive species sampling measurements using a microprobe, velocity and speciation profiles of acetone counterflow diffusion flames have been experimentally investigated with cross validations using non-intrusive particle image velocimetry (PIV) and laser induced fluorescence (LIF) measurements. It is shown that the separation distance between the fuel and oxidizer nozzles needs to be sufficiently large to achieve uniform radial velocity profiles at the nozzle exit and accurate measurements of fuel concentration distributions in flames. The impacts of the diffusion flame location relative to the stagnation plane and the diffusion flame thickness on quantitative species sampling are investigated by varying the fuel to oxygen ratio as well as nitrogen and helium as fuel diluents. The results show that the diffusion flame needs to be located on the fuel side far from the stagnation plane in order to obtain reliable speciation measurements of fuel oxidation-related species. For helium dilution in the fuel side, a significant deviation from the model prediction is found due to the excessively fast diffusion velocity of helium. The impact of the intrusive probe on the flow field and the structure of the counterflow diffusion flame are identified by acetone and OH LIF measurements. The uncertainty in the speciation measurement associated with flow perturbations by the probe is quantified and found to be comparable to the outer diameter of the probe, ±0.3mm. A simple Reynolds number analysis shows that the flow near the probe is just on the outskirts of the Stokes regime. Finally, the structure of the acetone diffusion flame is measured quantitatively with species measurements of ethane, ethylene, and acetylene. The comparison between predictions and measurements indicate that the current C2 kinetic mechanism needs to be improved for quantitative prediction of the acetone flame structures.

Scalar dissipation rate statistics in turbulent flows using Planar Laser Induced Fluorescence measurements

A shearless mixing layer with isotropic decaying turbulence has been examined with a turbulent axisymmetric opposed-jet flow and a plume in pipe flow. The study has been carried out experimentally with Planar Laser Induced Fluorescence of acetone (used as a tracer to measure the passive scalar ξ) to study the conditional statistics of the scalar dissipation χ. For all flows examined, it has been found that: (i) the PDFs of the conditional χ show a consistent small deviation from log-normality at high values of ξ; (ii) the ratio F=〈χ′2|η〉1/2/〈χ|η〉 is independent of η the sample space variable for ξ), within statistical error; (iii) F increases from zero (at the injector in the plume experiment) to a value between 1 and 1.4 downstream and at later times; (iv) the rise of F to this value occurs over a few Kolmogorov timescales; and (v) the long-time value of F is close between the two experiments and seems to increase with the turbulent Reynolds number. The development of conditional scalar dissipation fluctuations is being discussed by comparing the “early” and “late” mixing regions in the plume and the counterflow. The agreement between the PLIF measurements, which can only resolve two-dimensional fields, suggests that the present conclusions are not limited by resolution or the lack of measuring the third dimension of the scalar gradient. The results are useful for turbulent reacting flow models such as advanced flamelet, CMC, and transported PDF closures.

Measurement of number densities in supersonic flows using a method based on laser-induced acetone fluorescence

A method that is based on laser-induced fluorescence (LIF) and that involves the use of acetone as a seed can be utilized for measurement of supersonic flows with high temporal and spatial resolutions. In the present study, the feasibility of using this method to measure the number density in supersonic flows at temperatures lower than the atmospheric temperature is investigated. An experiment in which low-temperature conditions are created by supersonic isentropic flows in a convergent–divergent nozzle is carried out. LIF is realized by exciting acetone with the fourth harmonic of a Nd:YAG laser, and the LIF signal is detected by a CCD camera with an image intensifier. In order to theoretically determine the LIF characteristics, a multistep decay model that can be used for temperatures of 300–900 K is extended so that it can be used in the low-temperature regime. The constant included in the model is redetermined by fitting the model to the present experimental data. The LIF calculated by using this extended model is found to agree very well with the experimental data. The model also indicates that the number density measured by the LIF method in the temperature range 170–290 K is insensitive to temperature.

Flow visualization of supersonic free jet utilizing acetone LIF

The flow field of a supersonic free jet is investigated both numerically and experimentally. Two-dimensional axi-symmetric numerical code is developed and applied to a nozzle geometry, which is equivalent to the nozzle used in the experiment. Experiments of flow visualization are also carried out with a simple nozzle geometry. Acetone laser-induced fluorescence (Acetone LIF) is used to visualize cross sections of the flow field induced by supersonic jets. A conventional optical visualization technique with a high-speed video camera is also used.

Acetone laser-induced fluorescence behavior for the simultaneous quantification of temperature and residual gas distribution in fired spark-ignition engines

Although the fluorescence behavior of acetone has already been examined widely, the amount of data is still not sufficient for the quantification of signals over the parameter field relevant for combustion engines. This leads to large uncertainties when new excitation wavelengths are applied or in cases where temperature and pressure and bath gas composition dependences of the fluorescence yield must be extrapolated from models. This work presents calibration results of the fluorescence signal intensities in nitrogen, air, and an exhaust-gas-air mixture in the wide range from 298 to 748 K and from 0.2 bar (0.02 MPa) to 20 bars for the two important excitation wavelengths 308 and 248 nm. Based on this data, measurements of temperature and exhaust gas concentrations in a fired spark ignition engine were performed with high accuracy in single-shot images also.

Measurement of Fuel Concentration Profile at Leading Edge of Lifted Flame with Acetone Laser-Induced Fluorescence

This is a study of the leading-edge characteristics of a methane-air triple flame. Few experiment results are available for physical examination of such characteristics, so further experimental investigations are strongly needed to understand the stability mechanism in a mixture with a steep concentration gradient. To this end, we measured concentration profiles at the leading edge of a flame using acetone laser-induced fluorescence (acetone LIF). The results demonstrated that the lifted height of the flame changed when acetone was added to the mixture and correlated well with increased C2 radical behind the flame edge. However, the OH radical luminous intensity, measured with a spectroscope, did not change with addition of acetone. Moreover, the burning velocity obtained by the Bunsen-burner method remained constant when acetone was added to the mixture. Therefore, acetone had little influence on burning intensity. Acetone LIF can thus be employed to measure the local concentration gradient at the leading edge of a flame. The acetone LIF signals could be corrected to consider the thermal effect by using silicone oil vanishing-plane data. From the corrected acetone LIF data, the width between the lean and rich flammability limits (flammability limit width) in the flow upstream of the flame with a steep concentration gradient was clearly observed and could be quantitatively compared with the recent numerical results.

Numerical and Experimental Research on a Mixing Process of Film Cooling Air with Mainstream

To understand the mixing phenomena o film cooling air with mainstream hot gas is very important to attain higher film cooling effectiveness. But the mixing process of film cooling air with mainstream hot gas is extremely complicated. Detailed mixing of film cooling air injected through typical conventional and shaped film cooling holes with mainstream has been studied experimentally. Acetone Laser-induced Fluorescence (LIF) was used to measure film cooling effectiveness. The experimental results were compared with numerical analysis. For the process of numerical solution of flow equations, the standardκ-emodel with wall functions was implemented as the turbulence model. Numerical analysis could predict the distribution of film cooling effectiveness well, but in the near-wall region of downstream of film cooling holes it overpredicted the value of film cooling effectiveness. Acetone LIF was proved to be a powerful measurement method of mixing flow field and to be applicable for the verification of numerical analysis.

Measurement of Fuel Concentration Profile at Leading Edge of Lifted Flame with Acetone Laser-Induced Fluorescence

This research proposes a study of the leading edge of the methane-air triple flame characteristics. Few experiment results are available for physical examination, so an experimental investigation must be conducted to understand the stability mechanisms in a mixture with a steep concentration gradient. To this end, applying acetone-laser-induced fluorescence (acetone-LIF) is studied for measuring concentration profiles at the leading edge of the flame. The results demonstrate that the lifted height is changed when acetone is added to the mixture. It correlates closely with increasing C2 radical behind the flame edge. Meanwhile, the OH radical luminous intensity measured with a spectroscope is not changed. Moreover, the burning velocity of methane-acetone-air mixture obtained by the Bunsen burner method remains constant. Therefore, the influence of acetone on burning intensity is small. Acetone LIF can thus be advantageous for measuring the local concentration gradient at the leading edge. The acetone LIF signals can be revised considering the thermal effect using the silicone oil vanishing plane. After temperature correction, the transition of the flammability limit width upstream of the flame at a steep concentration gradient can be observed as clearly as in the recent numerical result.

Laser-induced fluorescence of ketones at elevated temperatures for pressures up to 20 bars by using a 248 nm excitation laser wavelength: experiments and model improvements

Laser-induced fluorescence of acetone and 3-pentanone for a 248 nm excitation wavelength was investigated for conditions relevant for internal combustion engines regarding temperature, pressure, and gas composition. An optically accessible calibration chamber with continuous gas flow was operated by using CO2 and air as a bath gas. According to the varying pressure and temperature conditions during the compression stroke of a spark ignition engine, fluorescence experiments were performed under isothermal pressure variations from 1 to 20 bars for different temperatures between 293 and 700 K. The ketone fluorescence behavior predictions, based on a model previously developed by Thurber et al. [Appl. Opt. 37, 4963 (1998)], were found to overestimate the pressure-related fluorescence increase for high temperature and small wavelength excitation at 248 nm. The parameters influencing the model only in the large vibrational energy regime were newly adjusted, which resulted in an improved model with a better agreement with the experiment. The model's validity for excitation at larger wavelengths was not influenced. For the air bath gas an additional collision and vibrational energy sensitive quenching rate was implemented in the model for both tracers, acetone and 3-pentanone.

Measurements of scalar dissipation in a turbulent plume with planar laser-induced fluorescence of acetone

In order to gain a better understanding of the scalar dissipation rate χ in turbulent flows and to test available models for this quantity, high-resolution two-dimensional planar laser-induced fluorescence measurements were undertaken in the mixing field formed by the axisymmetric injection of a fluorescent tracer (acetone) into a confined turbulent co-flow of air, with emphasis on the less explored early region close to the nozzle and on the spatial resolution and level of image denoising necessary for the correct measurement of χ . In the mean, the resulting plumes had Gaussian profiles and axial decay as expected from previous investigations. It was found that, with Kolmogorov lengthscale resolution and careful image processing prior to the calculation of the scalar gradients, the measured χ satisfied global conservation of scalar energy across the plume to within 20%. The estimated mean three-dimensional scalar dissipation rate was used to calculate C D (twice the timescale ratio) that was found to decrease from values higher than 10 adjacent to the nozzle, to approximately 2 at an axial distance of 2–3 nozzle diameters (corresponding to residence times of 0.1–0.2 turbulent timescales) and retaining this value further downstream. The data can assist the validation of models for 〈 χ 〉 .

Single-shot imaging of OH radicals and simultaneous OH radical/acetone imaging with a tunable Nd : YAG laser

Single shot imaging capability for OH radical distributions in various atmospheric pressure methane flames upon excitation with a tunable frequency-quadrupled Nd : YAG laser is demonstrated. The laser wavelength can be tuned with an intra-cavity etalon to produce laser-induced fluorescence (LIF) signals from OH via absorption in the OH A–X (2,0) P 1(10) line. Simultaneous single-shot imaging of the burnt and unburnt zones in laminar nonpremixed, premixed and turbulent flames is presented. The unburnt areas are visualized with LIF of acetone that is seeded to the methane fuel. Acetone levels are set to match signal intensities to that of the OH signals to allow imaging on a single intensified CCD camera.

アセトンレーザー誘起蛍光法による不足膨張自由噴流の密度場計測(OS3-4高速流れと衝撃波現象(その4))

アセトンレーザー誘起蛍光法による不足膨張自由噴流の密度場計測(OS3-4高速流れと衝撃波現象(その4))

Proof-of-principle laser-induced fluorescence measurements of gas distributions from supersonic nozzles

We have applied the technique of acetone laser-induced fluorescence (LIF) to the measurement of gas distributions from axisymmetric supersonic nozzles used to produce loads for z-pinch plasma radiation sources. Typical peak particle densities are ∼1017 particles/cm3 for loads imploded on the Double-EAGLE facility. The experimental approach uses a pulsed laser (266 nm wavelength, 2.2 mJ per pulse, 5 ns pulse width, and 3×107 W/cm2 intensity) to obtain a snapshot along a chord through the center of the gas density distribution at an arbitrary axial distance, z, from the nozzle exit. We report measurements at 4.3 and 20.0 mm from the exit of the nozzle for comparison with previous measurements. We find acceptable agreement between LIF and laser interferometer measurements. Strengths of the LIF approach include simplicity of implementation and high radial spatial resolution.