Droplet supercritical transition and gas autoignition in n-dodecane sprays under elevated conditions

• Strong coupling found between droplet phase change and gas ignition. • Phase change regimes of droplets are mapped across size and ER conditions. • Three thermodynamic paths to critical point are identified in transition regime. • Shock intensity alters phase change regimes and ignition delay significantly. Detonative combustion using liquid fuels holds significant potential for enhancing the specific impulse in aerospace propulsion. The complex heat and mass transfer interactions between fuel sprays and shocked gas result in diverse phase transition and ignition dynamics. This fundamental study investigates the phase transition and autoignition characteristics of n -dodecane droplets under shock-relevant conditions, examining droplets with initial diameters d ranging from 5 to 70 μm and liquid equivalence ratios ϕ l0 from 0.2 to 3. An idealized isochoric reactor model is employed, integrating a simplified phase change formulation with a skeletal chemical mechanism. Under fixed initial gas conditions mimicking air compressed by shock at Mach 4, smaller droplets (≤20 μm dia.) at low ϕ l0 (≤0.3) undergo supercritical transitions from liquid to supercritical fluid, shifting to complete subcritical evaporation at higher ϕ l0 . In contrast, larger droplets extend the supercritical transition regime boundary toward higher ϕ l0 , with 70 μm droplets exhibiting this transition across all studied ϕ l0 . Three distinct evolutions of supercritical transitions are identified, governed by droplet thermodynamic response. Moreover, during these supercritical transitions, rapid mass transfer significantly impacts gas temperature, heat release, and alters chemical pathways. To assess the effect of initial gas conditions, simulations are conducted under post-shock conditions derived from Mach numbers of 3.5, 4.0, and 4.5. Results show that higher Mach numbers significantly reduce phase change times and promote supercritical transitions. The intensified evaporation accelerates fuel vapor accumulation and shortens ignition delay, demonstrating the strong coupling between droplet phase transition and early ignition chemistry. This study highlights the intricate coupling between droplet phase change and chemical kinetics under conditions behind a shock wave, providing fundamental insights into the initiation of spray detonation.