Abstract: A combustor for a gas turbine including a burner defining an axial fluid flow path between upstream and downstream ends, a burner casing coaxially surrounding the burner and defining an annular conduit for inlet gas flow from downstream to upstream ends of the burner, ports for introducing part of the inlet gas into the upstream end of the burner, a fuel nozzle introducing fuel into the upstream end of the burner, slots circumferentially disposed around the burner casing upstream of the ports and a conduit and valve arrangement for introducing external air to the annular conduit through the slots to generate an annular vortex to restrict inlet gas flow to the ports when the turbine is operating at predetermined operating conditions.