Experimental and numerical investigation on soot volume fractions and number densities in non-smoking laminar n-heptane/n-butanol coflow flames

Abstract

In this work, the soot volume fractions (SVF) and number densities in laminar coflow flames of n-heptane/n-butanol blends were experimentally and numerically investigated. Five blends, B00, B25, B50, B75, and B100, based on the volume fraction of n-butanol in the liquid fuel, were employed to explore the soot behavior when n-butanol is added to the fuel stream. Mass flow rates of the blends were adjusted to keep the mole flow of carbon constant. The fuel stream was heavily diluted with N2 to carry the fuel vapor. Measured visible heights are 5.7 cm, 5.6 cm, 5.3 cm, 5.1 cm and 4.8 cm for the five target flames, respectively. A thermophoretic probe was used to sample particles at different height of the flames. SVF, soot number density, average primary diameter, and number of primary particles per aggregate were measured and calculated through transmission electron microscopy (TEM) images. CoFlame code was used to calculate the soot formation in the coflow flames with a newly developed n-heptane/n-butanol/polycyclic aromatic hydrocarbon (PAH) skeletal model and a fixed sectional soot method. The qualitative characteristics of SVF and number densities obtained from TEM images were well captured by the calculation, although the visible heights were underpredicted. Calculated peaks of soot mass fraction (SMF) and number densities show a decrease as more n-butanol is added, and all the peaks are reached around the height of 3.2 cm. Inception by the dimerization of pyrene (A4) plays the most important role in the evolution of soot number densities, while hydrogen-abstraction-carbon-addition (HACA) process dominates the soot mass addition. Lower mole fractions of A4 and C2H2 are responsible for the suppression of SMF and number densities with the addition of n-butanol. Condensation of A4 and fragmentation play small roles in the increase of SMF and number densities, respectively. Finally, all the particles are completely oxidized, first mainly by OH, and then by O2.