Self-quenching in toluene LIF

• Published in: Proceedings of the Combustion Institute 2017, Vol.36 (3), p.4505-4514
• Main Authors: Fuhrmann, D., Benzler, T., Fernando, S., Endres, T., Dreier, T., Kaiser, S.A., Schulz, C.
• Format: Article
• Language: English
• Subjects: Laser-induced fluorescence; Self-quenching; Tracer-LIF
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/j.proci.2016.06.045

Toluene is frequently used as laser-induced fluorescence (LIF) tracer for visualizing mixing processes, for example, in internal combustion engines. The signal evaluation relies on a linear dependence of the LIF signal on tracer concentration – which is not present in many practically relevant cases. This paper presents an investigation of the dependence of the LIF signal intensities on the toluene concentration, revealing a non-linear signal response already at concentrations approximately ten times below those given by the room-temperature vapor pressure. Toluene was vaporized in a mass-flow controlled evaporator and investigated in a free jet. Nitrogen was used as bath gas with a variable addition of oxygen. After excitation at 266nm, an intensified CCD camera recorded the spectrally filtered fluorescence. In separate experiments, the effective fluorescence lifetime upon picosecond UV-laser excitation was determined. The results indicate that the fluorescence lifetime decreases with increasing tracer concentration due to self-quenching. Results from imaging and fluorescence lifetime measurements are consistent. The investigation reveals that the self-quenching of toluene is dominated by collisions with excited-state toluene molecules, which causes an additional dependence of the magnitude of self-quenching on the laser fluence.