W. Koban

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.

Novel strategies for imaging temperature distribution using Toluene LIF

The distinct temperature dependence of toluene fluorescence has enabled the application of toluene laser-induced fluorescence (LIF) for quantitative imaging of temperature. Two novel thermometry techniques based on toluene LIF are introduced and demonstrated: a single-color detection method, which can be applied for temperature measurements in homogeneously seeded flows and a two-color detection technique (i.e. the simultaneous detection of two different wavelength regions of the fluorescence spectrum) that can be applied in inhomogeneously seeded systems.

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.

Toluene Laser-Induced Fluorescence (LIF) Under Engine-Related Pressures, Temperatures and Oxygen Mole Fractions

Laser-induced fluorescence (LIF) is frequently used for the investigation of mixing processes in internal engine combustion. Toluene is one of the main fluorescing compounds of commercial gasoline. Understanding its fluorescence properties is therefore crucial for the correct interpretation of signal intensities observed under engine (i.e. high temperature and high pressure) conditions. Toluene LIF signal has been investigated as a function of temperature and oxygen concentration in order to enable quantitative fuel tracer imaging. Signal behavior and interpretation for engine-related conditions is demonstrated based on a semi-empirical fluorescence model. Toluene as well as gasoline-LIF is strongly quenched by oxygen. It has therefore been suggested for a direct measurement of fuel/air equivalence ratios. Contrary to this frequently used FARLIF assumption, oxygen quenching is not dominant at elevated temperatures and thus the toluene signal is not proportional to the fuel/air ratio under all the pressure and temperatures conditions in the compression stroke of IC engines. The correct signal interpretation is demonstrated for various practical cases.

Laser-based temperature imaging close to surfaces with toluene and NO-LIF

Two novel techniques based on Laser-Induced Fluorescence (LIF) were applied to measure gas-phase temperature distributions in boundary layers close to wall surfaces. Single- line toluene-LIF thermometry was used to image temperature in a nitrogen gas flow above a heated wall. The nitrogen gas flow was doped with evaporated toluene. When excited at 266 nm, the toluene LIF-signal shows an exponential dependence on temperature. This behavior was used to calculate absolute temperatures from LIF images after calibration at known conditions. The second technique, multi-line NO-LIF thermometry was applied to image temperature in the quenching boundary layer close to a metal wall located on a flat flame burner. A small amount of nitric oxide was mixed into the air/methane mixture. NO molecules were excited in the A-X (0,0)-band at 225 nm. NO-LIF excitation spectra were acquired by tuning the excimer laser wavelength and recording the NO LIF-signal with an ICCD camera. Absolute temperatures were calculated for every pixel by fitting simulated excitation spectra to the experimental data. Temperature distributions close to the wall surface were measured at two different flow-rate conditions. A high nominal spatial resolution of 0.016 mm/pixel in direction perpendicular to the wall was reached. Wall surface temperatures were recorded simultaneously by embedded thermocouples and compared with gas-phase temperature near the wall surface.