Effect of Ignition Energy on the Initial Propagation of Ethanol/Air Laminar Premixed Flames: An Experimental Study

摘要

The ignition energy has a significant effect on the initial unsteady evolution of the flame kernel, which will affect the performance and emissions of a spark-ignition engine and the uncertainty in determining laminar burning velocity and Markstein length as a result of the inappropriate selection of the lower limit of the flame radius. In the present study, the outwardly propagating spherical flame was employed to experimentally investigate the initial propagation characteristics of ethanol/air premixed flames at different ignition energies. The emphases were to provide new insight into the effect of ignition energy on the initial propagation of the premixed flame and to develop a new reference flame radius (the lower limit of the flame radius) for deriving the laminar burning velocity and Markstein length of ethanol/air mixtures. Results show that the initial flame speed of ethanol/air mixtures increases with the increase of the ignition energy and the spark ignition has only a marginal effect on the initial flame speed when the ignition energy increases to a critical value. Furthermore, there exists an exponential relationship between the minimum stretched flame propagation speed and the ignition energy under the experimental conditions for the ethanol/air mixtures whose Lewis numbers are larger than a unit. According to the exponential relationship, a new parameter is defined as the minimum reliable ignition energy (MRIE), and the empirical formula for calculating MRIE is developed. The corresponding maximum radius of the flame affected by ignition energy at MRIE is defined as the reference flame radius, which is recommended to choose the lower limit of the flame radius to reduce the uncertainty in determining the laminar burning velocity induced by the spark-ignition energy.