Lean premixed swirling flames stabilized at the sudden expansion of a combustion chamber generally take a V or a M shape, depending on the burner geometry and flow conditions. It is not rare to observe transitions between these shapes as operating conditions of the combustor are modified, but the governing mechanisms of these transitions are not well understood. An experimental study is conducted to analyze the transition from an initially V-shaped flame to a M-shaped flame for swirling flames fed by CH4/H2/air mixtures anchored on a central bluff body when flow conditions and geometrical elements of the combustor are modified. Two situations are identified depending whether strong flame wall interactions take place at the quartz tube confining the flame. When the V-flame front is impinging and quenched at the combustion chamber wall, the transition to a M-shape is triggered by a flashback mechanism along the wall followed by the propagation of the flame tip along the outer shear-layer of the swirling jet in the direction of the external rim of the burner. This mechanism is controlled by the mixture Lewis number, a Karlovitz number based on the velocity gradient at the combustor wall, the swirl imparted to the flow and the cross section area ratio between the injection unit and combustion chamber defining a confinement ratio. Experiments conducted at a given mixture Lewis number indicate that the onset of flashback is determined by decreasing the Karlovitz number under a critical value. It is shown that this critical value decreases when the Lewis number increases, but it also depends on the confinement ratio and swirl number. In the absence of direct interaction between the V-flame tip and the chamber wall, the situation differs and this flashback mechanism along the combustion chamber wall ceases. Attempts are made in this case to identify the governing parameters triggering the V- to M-shape transition for small confinement ratios.

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