Characterization of Nile Red as a Tracer for Laser-Induced Fluorescence Spectroscopy of Gasoline and Kerosene and Their Mixture with Biofuels

Abstract

Suitable fluorescence tracers (“dyes”) are needed for the planar measurement of droplet sizes by using a combination of laser-induced fluorescence (LIF) and Mie scattering. Currently, no suitable tracers have been characterized for application in planar droplet sizing in gasoline and kerosene fuels, as well as biofuel blends. One promising tracer is nile red, which belongs to the fluorophore group. For its utilization for droplet size measurements, preliminary characterization of the fluorescence of the respective fuel tracer mixtures are mandatory. For this purpose, the fluorescence and absorption behavior of nile red dissolved in the surrogate fuels Toliso and Jet A-1 as well as in biofuel blends was investigated. The fluorescence signal for nile red that was dissolved in the two base fuels Toliso and Jet A-1 showed a linear behavior as a function of dye concentration. The temperature effect on spectral absorption and emission of nile red was investigated in a specially designed test cell. An ethanol admixture to Toliso led to a spectral shift towards higher wavelengths. The absorption and emission bands were shifted towards lower wavelengths with increasing temperature for all fuels. Both absorption and fluorescence decreased with increasing temperature for all fuels, except for E20, which showed an increased fluorescence signal with increasing temperature. Jet A-1 and its blends with hydroprocessed esters and fatty acids (HEFA) and farnesane did not exhibit explicit variations in spectral absorption or emission, but these blends showed a more distinct temperature dependence compared to the Toliso-ethanol-blends. The effect of photo-dissociation of the LIF signal of the fuel tracer mixtures was studied, and all fuel mixtures besides Toliso showed a more or less distinct decay in the fluorescence signal with time. In summary, all investigated fuel-tracer mixtures are suitable for LIF/Mie ratio droplet sizing in combination with nile red at moderate temperatures and low evaporation cooling rates.

Investigation of Five Organic Dyes in Ethanol and Butanol for Two-Color Laser-Induced Fluorescence Ratio Thermometry

“…All measurements were performed in the linear regime of fluorescence and the dye concentrations (see Table 1) were chosen to have sufficient fluorescence intensity required for future 2c-LIF (two-color laser-induced fluorescence) thermometry of droplets in ethanol and butanol sprays [41,42]. Note that compared to dye concentration for the thermometry of liquid solutions in a test cell, atomizing sprays or single droplet studies require a relatively higher dye concentration (for better signal to noise ratio) [43,44], especially in two-dye approaches using FL [19]. Figure 3 shows the absorbance spectra of FL in (a), EY in (b), RhB in (c), Rh6G in (d), and SRh101 in (e) mixed with ethanol and butanol.…”

Analysis of the LIF/Mie Ratio from Individual Droplets for Planar Droplet Sizing: Application to Gasoline Fuels and Their Mixtures with Ethanol

“…Its properties lead to a high sensitivity to the chemical and physical environment of surrounding solvent molecules [39]. For the preliminary characterization of the spectral emission of the fuel-dye mixture in a dye cell, the selected concentration of nile red was 9.38 mg/L [28]. This relatively low concentration prevents measurement uncertainties caused by excessive extinction effects due to the long probe path in the dye cell (in comparison to the droplet diameter).…”

“…The LIF spectra were recorded under a detection angle of 90 • using a photo spectrometer (model USB 4000, Ocean Optics, Largo, USA, wavelength range 495.9-831.8 nm, 3648 pixels, slit size 10 µm, integration time 100 ms, average over 50 subsequent spectra for each measurement). Further details of the setup can be found in Reference [28]. The temperature-dependent fluorescence signals for the investigated fuel-dye mixtures are shown in Figure 2.…”

“…Nile red (C 20 H 18 N 2 O 2 ) is another LIF-tracer which has already been employed in a previous study in commercial gasoline and diesel fuels [24] for the investigation of the primary breakup of sprays in the near-nozzle region. Its spectral absorption and fluorescence characteristics in gasoline-ethanol blends as well as aviation fuels (also blended with biofuels such as farnesane and HEFA (Hydroprocessed Esters and Fatty Acids)) was recently studied by our group [28]. To ensure that aromatic fuel components of a realistic gasoline fuel do not become excited by the laser, a visible wavelength is preferred.…”

“…% isooctane, 35 vol. % toluene) was applied to model gasoline (see also References [1,28,32,33] and information in Section 2.2.). Furthermore, the relevant ethanol-gasoline blend E20 (80 vol.% Toliso, 20 vol.…”