An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species

An experimental and numerical investigation of counterflow prevaporized partially premixed n-heptane flames is reported. The major objective is to provide well-resolved experimental data regarding the detailed structure and emission characteristics of these flames, including profiles of C1–C6, and aromatic species (benzene and toluene) that play an important role in soot formation. n-Heptane is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems. A counterflow geometry is employed, since it provides a nearly one-dimensional flat flame that facilitates both detailed measurements and simulations using comprehensive chemistry and transport models. The measurements are compared with predictions using a detailed n-heptane oxidation mechanism that includes the chemistry of NOx and PAH formation. The reaction mechanism was synergistically improved using pathway analysis and measured benzene profiles and then used to characterize the effects of partial premixing and strain rate on the flame structure and the production of NOx and soot precursors. Measurements and predictions exhibit excellent agreement for temperature and major species profiles (N2, O2, n-C7H16, CO2, CO, H2), and reasonably good agreement for intermediate (CH4, C2H4, C2H2, C3Hx) and higher hydrocarbon species (C4H8, C4H6, C4H4, C4H2, C5H10, C6H12) and aromatic species (toluene and benzene). Both the measurements and predictions also indicate the existence of two partially premixed regimes; a double flame regime for ϕ<5.0ϕ<5.0, characterized by spatially separated rich premixed and nonpremixed reaction zones, and a merged flame regime for ϕ>5.0ϕ>5.0. The NOx and soot precursor emissions exhibit strong dependence on partial premixing and strain rate in the first regime and relatively weak dependence in the second regime. At higher levels of partial premixing, NOx emission is increased due to increased residence time and higher peak temperature. In contrast, the emissions of acetylene and PAH species are reduced by partial premixing because their peak locations move away from the stagnation plane, resulting in lower residence time, and the increased amount of oxygen in the system drives the reactions to the oxidation pathways. The effects of partial premixing and strain rate on the production of PAH species become progressively stronger as the number of aromatic rings increases.