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Abstract The growth of carbon particles was studied in heated flows of a burnt-gas flow reactor containing mixtures of N 2 /C 2 H 2 , and N 2 /C 2 H 2 with addition of H 2 or CH 4 surrounded by a rich C 2 H 4 /air flame. Soot particle sizes and volume fractions were measured by laser-induced incandescence (LII) between 50 and 130 mm above the nozzle exit. The measurements indicate a soot-inhibiting effect of adding H 2 to the C 2 H 2 /N 2 flow on both, particle sizes and soot volume fractions. The effect of CH 4 addition to the C 2 H 2 /N 2 flows was ambivalent, depending on the methane-to-acetylene ratio. At gas mixtures with N 2 :CH 4 :C 2 H 2 = 0.42:0.35:0.23 and 0.39:0.32:0.29 by volume at fixed total flow rates, the measured soot volume fractions were substantially increased in presence of CH 4 , while the mean diameters of the particles were slightly decreased. Gas temperatures were measured by a generalized line-reversal method with Abel transformation. Temperatures of the surrounding C 2 H 4 /air flame were around 1600 K, and temperatures of the inner flows, where soot formation was measured, were between 1550 and 1630 K. Plug-flow reactor calculations provided a qualitative understanding of the influence of CH 4 on the soot particle growth. [ABSTRACT FROM AUTHOR]

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 266 nm, 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. [ABSTRACT FROM AUTHOR]

Gas-phase microwave plasma-assisted synthesis of freestanding graphene is a promising route to produce high-quality graphene flakes. However, a comprehensive understanding of the gas-phase kinetics that is required to advance production rates and yields is still lacking. Here, we use line-of-sight Fourier-transform infrared (FTIR) absorption spectroscopy as an in situ diagnostic to measure gaseous species formed during graphene synthesis, thereby elucidating the chemical kinetics mechanism. Different carbonaceous precursors that lead to the formation of either few-layer graphene flakes or soot-like particles are examined, and the results are compared with numerical simulations and mass spectrometry measurements. Quantitative FTIR measurements show a correlation between concentration of atomic carbon and hydrogen in the post-plasma region and particle morphology. Lower carbon and higher hydrogen concentrations lead to graphene formation, while higher carbon and lower hydrogen concentrations shift the reaction toward soot-like particles. We also investigate how the precursor chemical composition, precursor flowrate, and delivered microwave power affect the carbon concentration in the post-plasma region, thus enabling to control particle morphology. [Display omitted] [ABSTRACT FROM AUTHOR]