Predicting LIF signal strength for toluene and 3-pentanone under engine-related temperature and pressure conditions
Laser-induced fluorescence (LIF) imaging of mixing processes frequently employs 3-pentanone or toluene as a fluorescence tracer. The anal. of measured LIF signals typically requires corrections for the influence of temp., pressure, and gas compn. on the signal strength in cases where these variables are not const. for the process under study, e.g., in internal combustion engines. However, fluorescence quantum yield data at simultaneous high temp. and high pressure are not well characterized. Therefore, the ability of two fluorescence models to predict the signal strength for 3-pentanone and toluene, resp., under those conditions was evaluated through comparison to LIF measurements using 248 nm excitation in a motored optical engine. The temp.-pressure manifold that was covered ranges from 0.45 bar, 328 K to 8 bar, 600 K. A semi-empirical, photophys. model for 3-pentanone combines the effects of temp., pressure, and excitation wavelength on fluorescence quantum yield. The qual. 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-mol. deactivation as well as collisional deactivation dominated by mol. 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 temp. and pressure dependencies in cell expts. The toluene LIF model is able to reproduce the obsd. LIF signal strength in the engine with good accuracy. Quant. anal. of toluene LIF requires knowledge of temp. 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 temps. typical for the compression stroke in internal combustion engines.
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