| Abstract:This paper researched the effects of positive electric fields on the outwardly expanding premixed methane-air flames in a constant chamber. The input voltages were 0 kV、5 kV and 10 kV, the excess air ratios (λ) of mixtures were 0.8 and 1.4, representing the two mixed gas states of rich combustion and lean combustion, respectively.. The results showed the flame propagation in the opposite direction of electric field was improved, and the flame develop velocity and the flame stretch rate were increased as the electric field become more intense and these behaviors were more pronounced for lean mixtures. For the mixtures of λ=0.8 and 1.4 at 10 kV, the mean flame speed was increased by 36.4% and 49.5%, respectively. The improvement of flame speed enhanced the combustion process, and this led to reducation of the initial period of combustion and advancement of the timing of peak pressure. Meanwhile, the stretch effect of the flame propagation was increased. The joint functions of the two respects made the peak pressure for lean mixture increase, while the value kept almost the same for rich mixtures.Two alternative mechanisms to explain the positive electric field enhanced effects were proposed: 1) ionic wind effect due to negative ion-neutral molecule collisions in the field direction; 2) direction action of positive electric field on chemical kinetics via excited nitrogen molecule due to collisions with electrons. |