LOW, INTERMEDIATE, AND HIGH TEMPERATURE REACTION ZONES IN MULTISTAGE ?-HEPTANE COUNTERFLOW DIFFUSION FLAMES

Abstract

Understanding multistage flames involving cool flame and low temperature chemistry (LTC) is important for both combustion applications and fire safety, since these processes can influence engine efficiency, second-stage ignition, detonation sensitivity, and potentially smouldering-to-flaming transition. n-Heptane counterflow diffusion flames with oxygen addition to the fuel side were numerically investigated to clarify how fuel-side oxygen addition and strain rate affect multistage flame structures and their chemistry in the counterflow diffusion flame module of Cantera. The fuel-side premixed oxygen mole fraction, X_O₂, was varied from 0.0 to 0.8, and the strain rate, σ, over the range (0.75, 280] s⁻¹. Based on the temperatures and spatial locations of local heat release rate (HRR) peaks, a regime map was constructed and 13 flame types were identified. Multistage flames with low temperature (LT) reaction zones were found mainly at relatively low strain rates (σ ≲ 27.5 s⁻¹) and fuel-side oxygen addition levels (X_O₂≈ 0.25–0.75). Generally, increasing strain rate could weaken or suppress the LT and intermediate temperature (IT) reaction zones while strengthening the high temperature (HT) zones, whereas increasing fuel-side oxygen addition can promote the formation and strengthening of additional reaction zones. Two four-stage oxidation flames were identified as the most novel flame structures: one consisting of one reaction zone at LT followed by three reaction zones at HT, and the other consisting of one reaction zone at LT, one reaction zone at IT, and two reaction zones at HT. These flames can be interpreted as the same four-stage oxidation process consisting of a LTC stage, a higher-temperature transition stage in which long-chain hydrocarbons break into smaller intermediates and heat release is mainly associated with reactions of C₁–C₅ small fragments, an HT oxidation stage characterized by further oxidation of C₁–C₂ fragments and CO's formation and conversion to CO₂, and a final HT oxidation stage dominated by H/O radical chemistry, H₂O-forming pathways, and further conversion of CO to CO₂. The first two stages may be initiated differently: the LTC stage could be initiated by sufficient O₂ addition through the main LTC pathway, whereas the higher-temperature transition stage could be initiated by H₂O₂ decomposition together with H-atom-driven chain-branching and chain-propagating reactions involving O₂. Further flame structure analysis showed that the effect of higher oxygen addition on HRR depends on reaction zones: in the LTC zone it mainly acts through species concentration changes and strengthens the leading LTC heat-releasing reactions, whereas in the transition zone it acts more broadly through temperature-related kinetic effects across the reaction network instead of leading heat-releasing reactions. Soot formation could be possible in the four-stage flames near the IT reaction zone.