Jon D. Koch

Absorption and fluorescence of toluene vapor at elevated temperatures

Absorption and fluorescence of the S0 → S1 (π,π*) transition in toluene are studied in the temperature range 300 K to 1130 K and 300 K to 930 K, respectively. Experiments are conducted in a shock-tube and in a heated flow-cell. Fluorescence spectra are investigated after excitation at 248 nm and 266 nm using a nitrogen diluent at a total pressure of 1 bar. Over the temperature range studied the fluorescence quantum yield decreases exponentially by three orders of magnitude for 266 nm excitation and double exponentially by three orders of magnitude for 248 nm excitation. The fluorescence spectrum shifts to the red with increasing temperature. The vibrational structure of the absorption spectrum found at ambient conditions vanishes above 600 K. The absorption feature broadens and the maximum shifts to the red. Taking advantage of the distinctive temperature dependence of the fluorescence, we suggest potential techniques using toluene as a sensitive tracer molecule for temperature imaging in both homogeneously and inhomogeneously mixed flow-fields.

Ultraviolet absorption spectra of shock-heated carbon dioxide and water between 900 and 3050 K

Abstract Spectrally resolved UV absorption cross-sections between 190 and 320 nm were measured in shock-heated CO 2 between 880 and 3050 K and H 2 O between 1230 and 2860 K. Absorption spectra were acquired with 10 μs time resolution using a unique kinetic spectrograph, thereby enabling comparisons with time-dependent chemical kinetic modeling of post-shock thermal decomposition and chemical reactions. Although room temperature CO 2 is transparent (σ −22 cm 2 ) at wavelengths longer than 200 nm, hot CO 2 has significant absorption (σ>10 −20 cm 2 ) extending to wavelengths longer than 300 nm. The temperature dependence of CO 2 absorption strongly suggests sharply increased transition probabilities from excited vibrational levels.

Impact of UV absorption by CO2 and H2O on no lif inhigh-pressure combustion applications

The influence of UV light absorption by hot CO2 and H2O is evaluated for laser-induced fluorescence(LIF) measurements of NO in high-pressure combustors. UV lasers are ubiquitously used to measure LIF from species like NO, OH, HCO, and O2, as well as Raman and Rayleigh scattering in combusting environments. However, attenuation of the laser probe and/or signal by optical absorption from major combustion species is seldom considered. In this paper, we show that neglecting UV attenuation by major product species like CO2 may lead to large errors in combustion measurements. Absorption cross sections between 190 and 320 nm are measured in shock-heated CO2 and H2O at temperatures ranging from 900 to 3050 K. The absorption cross section of CO2 has strong temperature dependence and increases by 4 orders of magnitude at 193 nm between 300 and 2000 K. The measured temperature-dependen tabsorption spectra for CO2 and H2O are fit to an empirical function to provide a tool for facile assessment of potential errors and quantitative corrections for UV combustion diagnostics. LIF measurements of NO in a high-pressure burner and an internal combustion engine are adjusted for CO2 and H2O absorption to demonstrate the importance of these corrections.

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 acetones 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.

Water temperature and concentration measurements within the expanding blast wave of a high explosive

We present an application of absorption spectroscopy to directly measure temperature and concentration histories of water vapor within the expansion of a high explosive detonation. While the approach of absorption spectroscopy is well established, the combination of a fast, near-infrared array, broadband light source, and rigid gauge allow the first application of time-resolved absorption measurements in an explosive environment. The instrument is demonstrated using pentaerythritol tetranitrate with a sampling rate of 20 kHz for 20 ms following detonation. Absorption by water vapor is measured between 1335 and 1380 nm. Water temperatures are determined by fitting experimental transmission spectra to a simulated database. Water mole fractions are deduced following the temperature assignment. The sources of uncertainty and their impact on the results are discussed. These measurements will aid the development of chemical-specific reaction models and the predictive capability in technical fields including combustion and detonation science.

Time-resolved measurements of near infrared emission spectra from explosions: Pure pentaerythritol tetranitrate and its mixtures containing silver and aluminum particles

Measurements of chemical transients and thermodynamic conditions are difficult to obtain yet fundamentally important in understanding the behavior of explosives. We have constructed a fast near infrared (NIR) spectrometer and have made temporally and spectrally-resolved emission measurements during postdetonation combustion of pure pentaerythritol tetranitrate (PETN) charges and PETN charges doped with 10 wt % microparticles composed of silver (Ag) and aluminum (Al). We have observed postdetonation PETN emission spectra between 750 and 1500 nm at rates up to 46 992 spectra/s. The instrument captures the highly structured spectra immediately following breakout as well as the longer-lived broadband NIR emission signals from hot particles. The early spectra reveal spectral signatures related to PETN and the reacting constituents of the particles. The later spectra provide a means to infer the gray-body temperature history of the particles.

In Situ PLIF and Particle Image Velocimetry Measurements of the Primary Entrainment Fuel Jet in a Naturally Aspirated Water Heater

The time-averaged characteristics of a fuel jet have been measured via acetone planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) in the primary mixing region of an operating gas-fired water heater. These measurements allow for experimental characterization of the cross-sectional scalar and velocity fields as well as the estimation of the mass entrainment as the flow enters the burner in a practical system. In these experiments, reasonable results were obtained when only the fuel jet was seeded with acetone or PIV particles rather than the entire flow, thus demonstrating the potential for simplified experimental configurations in some applications where controlling or seeding the entire flow may be difficult. The entrainment characteristics of the fuel jet are compared with benchmarks from literature. The commercial device exhibits a larger mass entrainment rate than is found in typical free jets that have been studied in the literature. This may be a result of the jets low Reynolds number (9,600) in comparison with other literature studies, and a result of initial conditions.

Use of a fast near-infrared spectrometer for absorption and emission measurements within the expanding blast wave of a high explosive

We demonstrate the use of a fast InGaAs array and spectrometer to measure properties related to near-infrared absorption and emission (750 nm -1500 nm) following a high explosive detonation. Using a broadband light source and a rigid absorption gauge, gas temperatures are measured at a rate of 20 kHz for a period of several milliseconds behind the blast wave from a PETN, PBXN-5, and PBXN-113 detonations. The temperature and concentration of water vapor is determined by fitting experimental transmission spectra to a simulated database. Strong emission signatures obtained during the PETN breakout event (integrated over approximately the first 20 microseconds) indicate the presence of high energy nitrogen and oxygen atoms. Measurements from water absorption at a distance of 23 cm from the PETN charge indicate temperatures decaying from 1600 K to 600 K during the first few milliseconds, and measurements of non-ideal explosives with optically thick postdetonation environments are also demonstrated. These measu...

A FAST NIR EMISSION SPECTROMETER FOR EXAMINING EXPLOSIVE EVENTS: EMISSION SPECTRA OF PETN EXPLOSIONS CONTAINING SILVER AND ALUMINUM

A fast Near‐Infrared (NIR) spectrometer was constructed to make temporally and spectrally resolved emission measurements during post‐detonation combustion of pure pentaerythritol tetranitrate (PETN) charges and PETN charges doped with 10% (by mass) Ag and Al microparticles. The post‐detonation spectra are observed between 750 nm and 1500 nm at rates up to 46,992‐spectra/sec, and key features are identified. Immediately following break‐out of the detonation, all measured spectra are highly structured due to atomic and molecular emission. This emission decays within the first 40 μs following break‐out and is found to have lifetimes similar to that of emission from various species collected in the visible (390–600 nm) by a time‐resolved streak spectrometer. For the particle‐doped charges, broadband NIR emission signals can be used to determine time‐resolved gray‐body temperatures of the particles. At early time (0–40 μs after breakout) gray‐body temperatures in the range of 3000 to 4500 K are measured, betwe...

Temperature-dependent absorption by CO2: implications for UV diagnostics in high-temperature flames

Absorption spectra of hot (900 – 3050 K) CO2 have been measured at 190 – 320 nm. A parameter set allows the calculation of absorption cross sections relevant for laser diagnostic in combustion processes.