Structure and propagation speed of autoignition-assisted flames of jet fuels

Abstract

The flame structure and propagation speed of autoignition-assisted fuel-lean and fuel-rich flames of jet fuels are numerically and analytically investigated at elevated pressures and temperatures. The results show that while the propagation speed increases dramatically for large residence time, for fuels with strong low temperature chemistry (LTC), the propagation speed may first increase slightly at relatively small residence time. The species evolution in time shows that with the increase of propagation speed, the trajectories for autoignition-assisted flames first rapidly and then gradually converge to the autoignition trajectory. Furthermore, transport budget analysis shows that as the propagation speed increases, the balance in the high temperature reaction zone evolves from reaction-diffusion balance to reaction-convection balance, demonstrating the transition from flame propagation to autoignition. An analytical model, which only needs to calculate the corresponding 0D autoignition process, is proposed to predict the autoignition-assisted flame speed. The good qualitative agreement with the 1D simulation result indicates that the autoignition assistance on the propagation speed mainly results from the temperature rise ahead of the preheat zone. Finally, different scaling correlations for the normalized flame propagation speed Sl/Sl0 are compared, showing a universal scaling for n-heptane, n-dodecane, and Jet-A over a wide range of pressures (5–35 atm), temperatures (600–1015 K), and equivalence ratios (0.5–1.2) with the normalized maximum mole fractions of CO.