tile structure through the circumferential ports are discharged through the orifice 25 the metal immediately surrounding the orifice 25 is arcuately protruded downstream as shown at 29 thus forming a smooth contoured passageway for air aspirated into the zone formed by the refractory sleeve flange shaped member 24 is preferably coated with zirconium oxide for preservation of the metal in the presence of high temperatures the depth x of the projection of the flange in the downstream direction should be no further than that depth which is beneficial to increase flame temperature a preferred projection depth has been discovered to be approximately 10 percent of the flange face diameter d less the thickness of the flange face as previously mentioned hot recirculating flame gases in the prior art merge forcibly with the induced air flow through the central orifice at right angles retarding the initial combustion reaction by diluting the oxygen supply it is observed however that the projection of the flange member 24 in a downstream direction in the present invention diverts the recirculating gases substantially downstream thus delaying the combustion retarding mixture of air and relatively inert recirculating gas which occurs in the prior design the configuration of the flange.