Flame speed scaling in autoignition-assisted freely propagating n-heptane/air flames

Abstract

Autoignition and flame propagation embody different controlling mechanisms for flame stabilization in various modern combustion devices such as the internal combustion engines with hot air and elevated pressure environments. In this study, the flame structure and propagation speed scaling for n-heptane/air mixtures are numerically investigated over a wide range of unburnt temperatures, i.e., 600–1000 K. Under these autoignitive conditions, the flame structure and propagation speed depend not only on the unburnt mixture properties but also on the residence time, which is further complicated by the two-stage ignition processes associated with the negative temperature coefficient (NTC) behavior. The budget analysis of different physical processes for species evolution, together with manifold examination, demonstrates the diminishing effect of molecular diffusion and the increasing contribution of autoignition when the flame is in transition from flame propagation to autoignition. It is shown that, for complex hydrocarbon fuels with NTC, the enhancement of flame propagation due to autoignition assistance cannot be fully characterized by the scaling which shows the flame speed (normalized with the reference flame speed, i.e., Sl/Sl0) only depends on the residence time (normalized with the corresponding ignition delay time, i.e., τres/τign). Sensitivity analysis is conducted and results illustrate the crucial roles of the important radicals H, OH and CO in n-heptane/air flames. Two distinct correlations are revealed in the transition from flame propagation to autoignition: a near linearity and an exponential growth between the propagation speed and the maximum mole fraction of the (H+OH) radicals, respectively. Finally for complex hydrocarbons with NTC, a general scaling is for the first time identified between the normalized flame speed and the maximum mole fraction of (H+OH)/CO radicals (normalized with the maximum (H+OH)/CO mole fraction from corresponding autoignition process, i.e., Xmax(H+OH)/Xmax(H+OH),ign and Xmax(CO)/Xmax(CO),ign), with one branch representing flame propagation and the other representing autoignition-assisted flame propagation.