Study on laminar burning characteristics of premixed high-octane fuel-air mixtures at elevated pressures and temperatures

Laminar burning characteristics of high-octane fuel-air premixed mixtures (ETBE, TBA, and ethanol) were studied in a constant-volume bomb at various equivalence ratios, initial temperatures, and initial pressures by using outwardly propagating spherical flames with a high-speed schlieren imaging system. The flame propagation speed, the laminar burning velocity, the Markstein length, the adiabatic flame temperature, the flame thickness, and the density ratio were obtained and the influence of equivalence ratio, initial temperature, and initial pressure on these parameters was analyzed. The experimental results show that both the unstretched flame propagation speed and unstretched laminar burning velocity increase with the increase of initial temperature and decrease with the increase of initial pressure. Thermal-diffusive instabilities (Markstein length) decrease at higher initial temperature and/or at lower initial pressure. Meanwhile, the unstretched flame propagation speed and the unstretched laminar burning velocity give maximum values on the rich mixture side. The onset of cellular instability was evaluated in terms of the hydrodynamic and diffusional-thermal instabilities, and the reasons for the onset of the cellular structure were analyzed. The results indicate that the propensity for cellular structure is enhanced due to the increase in hydrodynamic and diffusional-thermal instabilities at high initial pressure. Moreover, the flame instability is more sensitive to initial pressure compared to initial temperature.