thereto the velocity profile is shown in fig4 and is seen to have a low value at the outer extremity of the passage 14 which increases to a maximum at the inner surface thereof and which nearly equals the velocity of the main flow from the passage 12 fig5 depicts an alternative arrangement in which a rectangular primary nozzle 26 has a multi passaged structure 28 for the main flow and which is surrounded by a further passage 14a for the subsidiary flow in this example the wall 22 of the present invention is included and is provided with apertures 24 as is described hereinbefore so as to achieve similar benefits with the rectangular nozzle as stated hereinbefore present nozzles of circular cross section have been provided with fuel air swirlers in the passage corresponding to the passage 14 herein with resulting benefit rectangular nozzles however were not suitably shaped to have them the present invention is shown to have efficacy in both circular and rectangular cross section nozzles and moreover obviates the need for swirler devices in the circular cross section burners it is envisaged however that some nozzle designs may benefit by employing both appertures and swirlers it is envisaged that the wall 22 be modified to provide a further embodiment of the present invention thus there could.