A flame formulated in a mixing layer has a typical structure of a tribrachial (or triple) flame. The propagation velocity of a tribrachial flame is much higher than the laminar burning velocity of a stoichiometric premixed flame, and the propagation velocity usually decreases as the gradient of fuel concentration increases. A separate experimental study that used a jet flow in an open space reported that there is a maximum propagation velocity at a critical fuel concentration gradient coupled with the enhancement of a diffusion flame branch and that the critical concentration gradient can be varied even by the difference in velocity variation near the flame. This study investigates how a confined flow field affects the structure of a tribrachial flame. The mean velocity and the concentration gradient of fuel were controlled by a multislot bumer. Laser diagnostic methods were used to measure the velocity variation, the OH radical, and the temperature variation. Even in a confined geometrical space, the existence of the maximum propagation velocity was confirmed. Moreover, the critical concentration gradients in a confined channel were larger than those in an open jet case; that is, the role of the diffusion flame at the maximum propagation velocity becomes more significant in a confined structure due to the enhanced convective diffusion. This result shows the importance of the diffusion branch in a confined (or squeezed) stream tube. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All fights reserved.