Apparatus includes a first and second conduits configured to form an annulus between them. An adjustable structure includes a body having an upper surface, a lower surface, and a circumferential surface abutting a portion of the internal surface of the second conduit. The structure is adjustable axially in relation to and removably attached to the first conduit via a hub. The hub defines a central passage for fuel or oxidant. The body has one or more non-central through passages configured such that flow of an oxidant or fuel therethrough causes the fuel or oxidant to intersect flow of fuel or oxidant exiting from the central passage in a region above the upper surface of the body.