In a confined combusting flow, the solid wall is heated by the flame, which in turn affects both the hydrodynamic and chemical characteristics of the flow, and hence the behavior of the flame. In this study, the transition behavior of a methane air tribrachial flame in a confined flow is numerically studied on the basis of fluid solid coupling and multi-step mechanism. The effects of the channel and nozzle wall on both the cold and reacting flows are investigated. The obtained results show that the boundary layer of the nozzle wall causes a velocity variation. This variation accounts for the jump in the final axial location of the triple point between the attached and lifted states. In a sufficiently narrow channel, the following may be concluded. (1) The influences of the incoming velocity and premixing of the center jet on the stoichiometric contour become remarkable. (2) The thermal expansion of the unburned mixture caused by heat transfer from the hot channel wall and the nozzle's flow boundary layer results in extra flow redirection upstream from the flame front. (3) The channel wall heated by the flame accelerates the unburned mixture, which causes the flame to propagate downstream, and intensifies the combustion in the nearby region to help the flame propagate upstream. The final location of the flame front depends on the tradeoff between these two factors. (4) The production of H is remarkably boosted in every major route, resulting in the intensification of elementary reactions that consume fuel and oxygen, including the key chain branching reaction R35 (H + O-2 -> O + OH). (C) 2015 Published by Elsevier Ltd.