The flame structure of unsteady two-dimensional premixed flame is numerically simulated with elementary reaction mechanism. In order to investigate unsteady behavior, a sinusoidal fluctuation is added to the inlet velocity of methane-air premixed mixture as follows : u0= um {1+A·sin (2πky) · sin (2πft)}. The average velocity um is 1.25-2.00m/s, the amplitude A is fixed to 1.0, the frequency f is 100, 250, 500 Hz, and the wavenumber k is 0.1-0.4mm-1. The Smooke's skeletal mechanism is adopted as the elementary reaction mechanism. The equivalence ratio is fixed to 0.75. The unsteady behavior and flame structure of premixed flame are examined by using the progress variable defined with mass fraction of oxidizer Cq and its gradient (dC/dn) q at flame front. The obtained results are as follows : The total heat release rate, the temperature gradient, the maximum concentration of species are determined by Cq and (dC/dn) q. Consequently, they are the most important descriptors of premixed flame which dominate the flame structure of premixed flame. The unsteady behavior is classified by the flame curvature and the flame propagation direction, and the classified regions are related to Cq and (dC/dn) q.
