The effects of adding long-chain ethers in n-heptane counterflow diffusion flames on the formation characteristics of soot precursors

Biomass-derived long-chain oxygenated fuels have good soot emission reduction properties and are promising diesel additives. In this work, the effects of 25% tripropylene glycol methyl ether (TPGME) and 25% polyoxymethylene dimethyl ether (PODE) additions on the soot precursor formation characteristics in n-heptane counterflow diffusion flames were experimentally and numerically analysed. The mole fractions of C1—C4 hydrocarbons in counterflow diffusion flames were measured by gas chromatography (GC). The experiments showed that the additions of TPGME and PODE suppressed the formation of ethylene and acetylene. The numerical simulations were performed through a reaction kinetic mechanism combined with the decoupling strategy, which could well capture the experimental observations. The simulations showed that the temperature differences in the key combustion zone were not more than 50 K, indicating that the oxygenated additives had little effects on the flame temperature. The dilution effect and thermal effect reduced the concentration of acetylene and ethylene, while chemical effect was beneficial to the formation of acetylene and ethylene. The rate of production (ROP) and reaction pathway analyses illustrated that unsaturated hydrocarbons were mainly generated via hydrogen abstraction and β-scission of n-heptane, and neither of the two additives had a remarkable impact on the pathways. Due to the presence of oxygen atoms, the carbon atoms on TPGME and PODE molecules trended to be converted into aldehydes and carbon monoxide (CO) instead of unsaturated hydrocarbons. Ultimately, the dilution effect and thermal effect played dominant roles in reducing the soot precursor emissions of n-heptane flame. © 2023 Materials China. All rights reserved.