Kinetic interactions and nonlinear blending behavior in laminar flames of PMH/DTBP mixtures

Sustainable aviation fuels (SAFs) are essential for decarbonizing the aviation sector. However, the inherently low reactivity of highly branched alkanes, such as 2,2,4,6,6-pentamethylheptane ( iso -dodecane, PMH), poses challenges for ignition and flame stability. This study investigates the use of di‑tert‑butyl peroxide (DTBP) as a reactivity enhancer for PMH, focusing on its effect on laminar flame speed. Experiments were conducted in a constant-volume spherical reactor at 373 K and 1 bar over equivalence ratios from 0.7 to 1.4. A newly developed reduced kinetic mechanism for PMH/DTBP (228 species and 1349 reactions) was used to interpret the measurements. The results reveal a pronounced nonlinear blending behavior. Under lean conditions ( φ = 0.7), the 50% DTBP blend has no effect on the laminar flame speed relative to neat PMH within the experimental uncertainty. Conversely, under rich conditions ( φ = 1.4), DTBP addition clearly promotes flame propagation relative to neat PMH, although the 90% DTBP blend remains comparable to neat DTBP within the uncertainty range. Although the adiabatic flame temperature increases monotonically with DTBP fraction, this thermal effect alone cannot account for the observed flame-speed trends. Kinetic analyses show that DTBP fundamentally alters the radical pool through HO₂-related chemistry. Under lean conditions, enhanced HO₂ formation suppresses the concentrations of key chain-branching radicals (H and OH), slowing flame propagation. Under rich conditions, the HO₂ chemistry shifts to favor radical-regeneration pathways, increasing OH production and strengthening the main chain-branching reactions. The combination of elevated thermal energy and enhanced radical chemistry leads to the significant promotion of flame propagation. These findings provide new insights into the kinetic role of peroxide additives and pave the way for designing high-reactivity SAF blends. Novelty and significance statement This work presents the first systematic experimental and kinetic investigation of di‑tert‑butyl peroxide (DTBP) addition on the laminar flame speed of 2,2,4,6,6-pentamethylheptane (PMH), a representative highly branched alkane in sustainable aviation fuels (SAFs). The study reveals an equivalence-ratio-dependent blending behavior in which DTBP has little measurable influence on flame speed at φ = 0.7 for the 50% blend within experimental uncertainty, while it clearly promotes flame propagation under rich conditions relative to neat PMH. Through kinetic analysis, we identify a mechanistic transition within the HO₂/RO₂ radical network, shifting from radical trapping to radical regeneration. This finding reveals a new framework for understanding peroxide-assisted reactivity control in highly branched hydrocarbons.