High velocity flames in half‐open tubes produced by placing a grid augmenter in the path of an advancing flame are described. High speed schlieren motion pictures and instantaneous pressure measurements are used to study flame structure and accompanying flow in the tube. It is found that the grid augmenter causes a mixing of burning and unburned gases, resulting in an increase in reaction surface. As a result the total amount of reaction per unit time per unit volume is increased, thus leading to the observed high velocities.