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A premixed flame is a flame in which the oxidizer has been mixed with the fuel before it reaches the flame front. This creates a thin flame front as all of the reactants are readily available. If the mixture is rich, a diffusion flame will generally be found farther downstream.
If the flow of the fuel–oxidizer mixture is laminar, the flame speed of premixed flames is dominated by the chemistry. If the flow rate is below the flame speed, the flame will move upstream until the fuel is consumed or until it encounters a flame holder. If the flow rate is equal to the flame speed, we would expect a stationary flat flame front normal to the flow direction. If the flow rate is above the flame speed, the flame front will become conical such that the component of the velocity vector normal to the flame front is equal to the flame speed. As a result, the flame front of most premixed flames in daily life are roughly conical.