Measurements and modeling of acetone laser-induced fluorescence with implications for temperature-imaging diagnostics

Review of Optical Thermometry Techniques for Flows at the Microscale towards Their Applicability to Gas Microflows

Thermometry techniques have been widely developed during the last decades to analyze thermal properties of various fluid flows. Following the increasing interest for microfluidic applications, most of these techniques have been adapted to the microscale and some new experimental approaches have emerged. In the last years, the need for a detailed experimental analysis of gaseous microflows has drastically grown due to a variety of exciting new applications. Unfortunately, thermometry is not yet well developed for analyzing gas flows at the microscale. Thus, the present review aims at analyzing the main currently available thermometry techniques adapted to microflows. Following a rapid presentation and classification of these techniques, the review is focused on optical techniques, which are the most suited for application at microscale. Their presentation is followed by a discussion about their applicability to gas microflows, especially in confined conditions, and the current challenges to be overcome are presented. A special place is dedicated to Raman and molecular tagging thermometry techniques due to their high potential and low intrusiveness.

Monitoring Catalytic 2-Propanol Oxidation over Co3O4 Nanowires via In Situ Photoluminescence Spectroscopy

Spectroscopic methods enabling real-time monitoring of dynamic surface processes are a prerequisite for identifying how a catalyst triggers a chemical reaction. We present an in situ photoluminescence spectroscopy approach for probing the thermocatalytic 2-propanol oxidation over mesostructured Co₃O₄ nanowires. Under oxidative conditions, a distinct blue emission at ∼420 nm is detected that increases with temperature up to 280 °C, with an intermediate maximum at 150 °C. Catalytic data gained under comparable conditions show that this course of photoluminescence intensity precisely follows the conversion of 2-propanol and the production of acetone. The blue emission is assigned to the radiative recombination of unbound acetone molecules, the n ↔ π* transition of which is selectively excited by a wavelength of 270 nm. These findings open a pathway for studying thermocatalytic processes via in situ photoluminescence spectroscopy, thereby gaining information about the performance of the catalyst and the formation of intermediate products.

Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here we combine broadband rotational spectroscopy with detailed chemical modeling based on rate constants obtained from automated ab initio transition state theory-based master equation calculations and high-level thermochemical parametrizations. Broadband rotational spectroscopy offers quantitative and isomer-specific detection by which branching ratios of polar reaction products may be obtained. Using this technique, we observe and characterize products arising from H atom substitution reactions in the flash pyrolysis of acetone (CH₃C(O)CH₃) at a nominal temperature of 1800 K. The major product observed is ketene (CH₂CO). Minor products identified include acetaldehyde (CH₃CHO), propyne (CH₃CCH), propene (CH₂CHCH₃), and water (HDO). Literature mechanisms for the pyrolysis of acetone do not adequately describe the minor products. The inclusion of a variety of substitution reactions, with rate constants and thermochemistry obtained from automated ab initio kinetics predictions and Active Thermochemical Tables analyses, demonstrates an important role for such processes. The pathway to acetaldehyde is shown to be a direct result of substitution of acetone's methyl group by a free H atom, while propene formation arises from OH substitution in the enol form of acetone by a free H atom.

Excitation wavelength dependence of the fluorescence lifetime of anisole

Photo-physical models that describe the pressure- and temperature-dependent fluorescence quantum yield of organic fluorescence tracers rely on an accurate prediction of the initial excited-state population, collision-dependent relaxation processes, and state-dependent relaxation processes. In case the initial excited-state population distribution reached after the laser excitation equals on average the thermal distribution, the fluorescence quantum yield becomes pressure independent. This initial distribution critically depends on the temperature-dependent ground-state population before excitation as well as the excitation wavelength. The ability to predict this behavior is a critical check for the validity of the existing photophysical models. The dependence of the effective fluorescence lifetime of anisole on the excitation wavelength (256-270 nm) was investigated at temperatures between 325 and 525 K for pressures between 1 and 4 bar. For each temperature, a unique excitation wavelength was found where the fluorescence lifetime is pressure-independent. The comparison of the experimental results with the predictions based on the established photophysical step-ladder models revealed a systematic underestimation of the required excitation photon energies for direct excitation into the thermalized level. An improved modeling approach based on quantum chemistry calculations for implementing simulated excitation spectra and state-dependent transition probabilities overcomes these limitations. Our results show for the example of anisole that the fluorescence step-ladder models that exist for aromatic fluorescence tracers must be modified to correctly predict the effect of the excitation wavelength.

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.

Carbonyl-based blue autofluorescence of proteins and amino acids

Intrinsic protein fluorescence is inextricably linked to the near-UV autofluorescence of aromatic amino acids. Here we show that a novel deep-blue autofluorescence (dbAF), previously thought to emerge as a result of protein aggregation, is present at the level of monomeric proteins and even poly- and single amino acids. Just as its aggregation-related counterpart, this autofluorescence does not depend on aromatic residues, can be excited at the long wavelength edge of the UV and emits in the deep blue. Differences in dbAF excitation and emission peaks and intensities from proteins and single amino acids upon changes in solution conditions suggest dbAF’s sensitivity to both the chemical identity and solution environment of amino acids. Autofluorescence comparable to dbAF is emitted by carbonyl-containing organic solvents, but not those lacking the carbonyl group. This implicates the carbonyl double bonds as the likely source for the autofluorescence in all these compounds. Using beta-lactoglobulin and proline, we have measured the molar extinction coefficients and quantum yields for dbAF in the monomeric state. To establish its potential utility in monitoring protein biophysics, we show that dbAF emission undergoes a red-shift comparable in magnitude to tryptophan upon thermal denaturation of lysozyme, and that it is sensitive to quenching by acrylamide. Carbonyl dbAF therefore provides a previously neglected intrinsic optical probe for investigating the structure and dynamics of amino acids, proteins and, by extension, DNA and RNA.

Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.

Naphthalene laser-induced fluorescence measurements at low temperature and pressure

Few studies on naphthalene vapor fluorescence have been conducted at low temperature and pressure conditions. The current study focuses on conducting measurements of naphthalene quenching and absorption cross section in a temperature- and pressure-regulated test cell with 266 nm laser excitation. The test-cell measurements were of the naphthalene-fluorescence lifetime and integrated fluorescence signal over the temperature range of 100 to 525 K and pressure range of 1 to 40 kPa in air. These data enabled the calculation of naphthalene-fluorescence quantum yield and absorption cross section over the range of temperatures and pressures tested, which were then fit to simple functional forms for future use in the calibration of naphthalene laser-induced fluorescence (LIF) measurements. Furthermore, the variation of naphthalene-fluorescence signal with respect to temperature was investigated for four different excitation wavelengths, demonstrating that a two-line naphthalene LIF thermometry technique may be feasible.

Three-dimensional reconstruction method for flame chemiluminescence distribution with complicated structure

In this study, reconstruction of flame chemiluminescence distribution with complicated structure was numerically investigated and experimentally validated. The ill-conditioned equations were constructed using the quasi-Monte Carlo method and solved by an algebraic reconstruction technique, where the convergence criterion was the Euclidean norm of the dimensionless displacement vector. Results of a phantom study revealed that the number of camera angles is the main restriction on reconstruction accuracy, and increase of the flame's nonhomogeneity improves the sensibility of reconstruction accuracy to image resolution. Results of experimental reconstruction showed the CH* distribution in a Meker burner flame. This work provides a better understanding in how to establish experimental systems for complicated flame reconstruction.

Numerical analysis of diameter influence on droplet fluorescence

Laser-induced fluorescence (LIF) is used in planar droplet sizing, assuming that the signal integrated over the droplet is proportional to its volume. Nevertheless, this assumption is rigorously valid in nonabsorbing mixtures. We performed an examination of the LIF signal with a fluorescence model, based on the Lorenz-Mie theory and on ray-tracing methods, for n-heptane droplets seeded by 3-pentanone. A parametrical study quantifies the bias caused not only by the absorption of the laser, but also by shadow zones in the droplets, which do not contribute to the fluorescence signal. Moreover, the effect of the first- and higher-order internal reflections is examined. The results of this study have immediately implications for the design of measurement techniques.

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.

Simultaneous laser-induced fluorescence and Raman imaging inside a hydrogen engine

We report on the simultaneous and two-dimensional measurement of laser-induced fluorescence (LIF) and Raman scattering (Ramanography) applied inside a hydrogen internal combustion (IC) engine. Two different LIF tracer molecules, triethylamine (TEA) and trimethylamine (TMA), were used for the LIF experiments. The LIF and Raman results were found to be in very good agreement. The simultaneous application of Ramanography and LIF imaging indicated that TMA is the more suitable LIF tracer molecule, compared to TEA.

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.

Two-dimensional Raman mole-fraction and temperature measurements for hydrogen-nitrogen mixture analysis

A two-dimensional laser Raman technique was developed and applied to directly probe the population number of selected rotational and vibrational energy levels of hydrogen and nitrogen. Using three cameras simultaneously, temperature and mole fraction images could be detected. Three different combinations of rotational and vibrational Raman signals of hydrogen and nitrogen were analyzed to identify the combination that is most suitable for future mixture analysis in hydrogen internal combustion engines. Here the experiments were conducted in an injection chamber where hot hydrogen was injected into room temperature nitrogen at 1.1 MPa.

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.

Rayleigh-calibrated fluorescence quantum yield measurements of acetone and 3-pentanone

We measured fluorescence quantum yields of acetone and 3-pentanone as a pure gas and with nitrogen diluent at room temperature at 20, 507, and 1013 mbar using 248, 266, and 308 nm excitation by calibrating the optical collection system with Rayleigh scattering from nitrogen. At 20 mbar with 308-nm excitation, the fluorescence quantum yields for acetone and 3-pentanone are 7 +/- 1 x 10(-4) and 1.1 +/- 0.2 x 10(-3), respectively, and each decreases with decreasing excitation wavelength. These directly measured values are significantly lower than earlier ones that were based on a chain of relative measurements. The observed pressure and excitation wavelength dependence is in qualitative agreement with a previously developed fluorescence quantum yield model, but the absolute numbers disagree. Changing acetone's fluorescence rate constant to 3 x 10(5) s(-1) from its previous value of 8 x 10(5) s(-1) resulted in good agreement between our measurements and the model.

Measurements of cavity ringdown spectroscopy of acetone in the ultraviolet and near-infrared spectral regions: potential for development of a breath analyzer

We report a study on the cavity ringdown spectroscopy of acetone in both the ultraviolet (UV) and the near-infrared (NIR) spectral regions to explore the potential for development of a breath analyzer for disease diagnostics. The ringdown spectrum of acetone in the UV (282.4-285.0 nm) region is recorded and the spectrum is in good agreement with those obtained by other spectral techniques reported in the literature. The absorption cross-section of the C-H stretching overtone of acetone in the NIR (1632.7-1672.2 nm) is reported for the first time and the maximum absorption cross-section located at 1666.7 nm is 1.2 x 10(-21) cm(2). A novel, compact, atmospheric cavity with a cavity length of 10 cm has been constructed and implemented to investigate the technical feasibility of the potential instrument size, optical configuration, and detection sensitivity. The detection limit of such a mini cavity employing ringdown mirrors of reflectivity of 99.85% at 266 nm, where acetone has the strongest absorption, is approximately 1.5 ppmv based on the standard 3 criteria. No real breath gas samples are used in the present study. Discussions on the detection sensitivity and background spectral interferences for the instrument development are presented. This study demonstrates the potential of developing a portable, sensitive breath analyzer for medical applications using the cavity ringdown spectral technique.