Predicting LIF Signal Strength for Toluene and 3-Pentanone under Engine-Related Temperature and Pressure Conditions

Abstract

Laser-induced fluorescence (LIF) imaging of mixing processes frequently employs 3-pentanone or toluene as a fluorescence tracer. The analysis of measured LIF signals typically requires corrections for the influence of temperature, pressure, and gas composition on the signal strength in cases where these variables are not constant for the process under study, e.g., in internal combustion engines. However, fluorescence quantum yield data at simultaneous high temperature and high pressure are not well characterized. Therefore, the ability of two fluorescence models to predict the signal strength for 3-pentanone and toluene, respectively, under those conditions has been evaluated through comparison to LIF measurements using 248 nm excitation in a motored optical engine. The temperature-pressure manifold that was covered ranges from 0.45 bar, 328 K to 8 bar, 600 K. A semi-empirical, photophysical model for 3-pentanone combines the effects of temperature, pressure, and excitation wavelength on fluorescence quantum yield. The qualitative influences of p and T reflect an increasing non-radiative decay rate with the excited electronic state's vibrational energy level and the tendencies of collisions to remove the excess vibrational energy. The model for toluene seeks to quantify the fluorescence quantum yield via the effects of intra-molecular deactivation as well as collisional deactivation dominated by molecular oxygen. Model-predicted LIF signal strengths for 3-pentanone did not capture the signal modulations measured under the engine conditions, but agreement was much better using predictions based directly on the measured temperature and pressure dependencies in cell experiments. The toluene LIF model is able to reproduce the observed LIF signal strength in the engine with good accuracy. It is shown that quantitative analysis of toluene LIF requires knowledge of temperature and oxygen partial pressure. Therefore, the frequently applied assumption that the toluene-LIF signal is proportional to the equivalence ratio is not correct for the range of pressures and temperatures typical for the compression stroke in internal combustion engines.