Effects of ambient pressures on cool flames in n-dodecane spray studied with laser diagnostics and large-eddy simulations

Cool flames, as a typical phenomenon in low-temperature combustion, have garnered wide interests owing to their crucial role in engine performance and emissions. In the present study, the auto-ignition process of n-dodecane spray injection into a constant volume combustion chamber was experimentally and numerically investigated. The focus is on providing insights into the underlying effects of ambient pressures on cool flames in n-dodecane spray combustion from the viewpoint of low-temperature chemistry. Laser diagnostics reveal the initial detection of formaldehyde (CH2O) at the spray periphery, followed by downstream progression toward the spray head. In addition, the production of CH2O is noted to occur earlier at higher ambient pressures. Subsequent large-eddy simulations illustrate that both CH2O and hydrogen peroxide (H2O2) shift from fuel-lean to fuel-rich regions, and this process is promoted at higher ambient pressures. Moreover, the primary distribution regions of CH2O exhibit confinement to a broader temperature span of 800–1400 K, in contrast to H2O2. Lastly, detailed chemical kinetic analyses show that the high sensitivity of R134 to the variation of ambient pressures explains for the observations in laser diagnostics. Furthermore, elevated ambient pressures are proved to accelerate low-temperature combustion and advance high-temperature combustion within the reaction system.