Characterization of preferential evaporation in binary ethanol-isooctane sprays: 2D droplet size and volume fraction measurements

Abstract Understanding preferential evaporation in multi-component fuel sprays is critical for optimizing combustion efficiency and reducing emissions in internal combustion engines. This study focuses on the development of a novel approach that allows for the simultanous detailed characterization of the Sauter Mean Diameter (SMD) and relative ethanol/isooctane volume fraction to elucidate the mechanisms governing preferential evaporation in binary fuel mixtures. This is achieved by combining Planar Droplet Sizing (PDS), a technique based on the ratio between the laser-induced fluorescence (LIF) signal of Nile red-doped fuel and the elastic scattering signal, with a two-color-LIF approach. As Nile red is a solvatochromic dye, i.e., it exhibits a shift in its fluorescence signal with changes in solvent polarity and temperature, the mole fraction of ethanol and isooctane in the spray can be determined, if the spray temperature is known. We performed extensive calibration on various Nile red-doped fuel mixtures in a heated cuvette, as well as in a droplet generator. Further, we minimized morphology-dependent resonance (MDR) effects in the LIF signal of the spray by the selection of spectral filters designed to cover all measured temperatures and concentrations. We found that in the ethanol spray temperatures decrease toward the spray edge. For the fuel-mixed samples, this coincides with a smaller overall SMD and a shift in the ethanol volume fraction in this region.