A double wall venturi chamber having a converging section, a diverging section and a cylindrical section wherein said chamber defines a venturi zone in which compressed air, fuel and combustion products flow downstream through converging section, diverging section and cylindrical section, and has a cooling gas passage between the walls of the venturi chamber, a least one cooling gas inlet in an outlet wall of the venturi chamber, and at least one cooling gas outlet in an inner wall of the venturi chamber, wherein said cooling gas outlet is in at least one of the diverging and the cylindrical section, and the outlet is downstream of the at least one cooling gas inlet and upstream of an axial end of the chamber.