portion 32 of the combustor 30 are a plurality of liner portions 42 of circular cross section and in the example shown the liner portions are cylindrical the portions 42 are of stepped construction i e each of the portions has a circular section of greater circumference or diameter than the preceding portion from the upstream to the intermediate portion to permit telescopic insertion of the portions the most upstream portion 42 has an annular array of apertures 44 for admitting primary air from within the plenum chamber 18 into the primary portion 32 of the combustor to support diffusion combustion of the fuel injected therein by the fuel injector 37 in accordance wih this invention the intermediate axial section 33 of the combustion chamber comprises a ceramic cylindrical shell 38 concentric with and attached to the upstream cylindrical section 32 and the discharge section which in turn exhausts into the transition duct 34 the ceramic wall 38 defines a plurality of axially spaced rows of apertures 40 42 in the embodiment shown in fig1 there are two such rows a first mixing chamber of duct 45 is defined by an annulus having a downstream facing open end 46 for receiving compressed air from the plenum chamber with the upstream end 48 in closed flow communication with the upstream row of apertures 40 in the ceramic cylinder 38 a second mixing chamber or annular duct 50 is defined by another annulus also having a downstream facing open end 52 for receiving compressed air from.