Premixed ’ames arise from the combustion of gaseous reactants that are well mixed prior to combustion. Diffusion ’ames arise from the combustion of separate gaseous fuel and oxidizer streams that combust rapidly as they mix. When the gas ¢ow is laminar, the ¢ame is laminar and has a smooth ¢ame front. When the gas ¢ow is turbulent, the ¢ame is turbulent and the ¢ame front is wrinkled or broken up. Laminar premixed ¢ames have a unique ¢ame speed for a given fuel-oxidizer mixture that is less than a meter per second for hydrocarbon-air mixtures at ambient pressure and temperature. Turbulence increases the ¢ame speed. For diffusion ¢ames, the rate of mixing of the reactants determines the ¢ame speed, and the reaction takes place at the interface between the fuel and the oxidizer. Stationary ¢ames are stabilized by ¢ow in a burner or by ¢ow into a stagnation region. In a quiescent combustible mixture, an ignition source initiates a ¢ame that propagates outward from the source at the laminar ¢ame speed. Given suf–cient volume, a ¢ame will transition into a detonation, which propagates at more than 1000 m/s and creates a pressure rise (see Chapter 8).