Effects of turbulent flow on the forced ignition of premixed cool flames in DME/air mixtures

Cool flame ignition, governed by low-temperature chemistry, has received increasing attention due to its strong influence on the ignition and subsequent propagation of hot flames. In engines, ignition occurs in turbulent flows, yet the effect of turbulence on cool flame ignition remains poorly understood. This study investigates forced ignition of premixed dimethyl ether (DME)/air cool flames in a homogeneous isotropic turbulent flow using two-dimensional transient simulations considering detailed chemistry and transport. Results show that turbulence suppresses the initial ignition stage but promotes subsequent flame kernel growth. A negative correlation exists between displacement speed and local curvature: high positive curvature leads to negative displacement speed and local extinction, whereas low positive or negative curvature yields positive displacement speed, favoring outward flame propagation and kernel growth. The local ignition behavior is well characterized by the local Damköhler number. Moreover, turbulence delays the cool-to-hot flame transition and significantly affects hot flame propagation. The flame kernel evolution is also closely linked to the turbulence integral length scale. When this scale is comparable to the kernel size, turbulence induces substantial wrinkling of the flame front. Conversely, when the integral length scale exceeds the kernel size, turbulence tends to transport the entire kernel without major wrinkling, resulting in a near-circular flame kernel development. These findings provide valuable insights into the role of turbulent flow in cool flame ignition. Novelty and significance statement Despite extensive quiescent cool flame studies, the effects of turbulence on forced ignition of premixed cool flames remain unexplored. This limits low-temperature chemistry utilization for engine ignition control. This first DNS of cool flame ignition in isotropic turbulence quantifies how turbulence intensity and length scale govern ignition success, kernel growth, and cool-to-hot flame transition. Key advancements include: (i) turbulence suppresses initial ignition via enhanced heat loss but promotes subsequent propagation through wrinkling; (ii) higher turbulence intensity delays cool-to-hot flame transition, allowing a larger burned region that influences hot flame propagation; (iii) distinction between “kernel wrinkling” and “kernel convection” regimes based on length scale ratio. These findings provide fundamental insights for leveraging cool flames to improve ignition reliability and efficiency in practical turbulent combustors.