System for injecting a mixture of air and fuel into a turbomachine combustion chamber

System for injecting a mixture of air and fuel into a turbomachine combustion chamber, comprising a fuel injector and a Venturi positioned downstream of the injector, coaxial therewith, the Venturi comprising an interior surface delimiting a premixing chamber in which fuel is mixed with a stream of air taken from an external space and passing through a primary swirler positioned upstream of the Venturi, wherein the Venturi comprises an internal annular air-flow cavity, this cavity being connected by air inlet ducts to the external space and by air outlet ducts to the premixing chamber, the air outlet ducts opening onto the interior surface of the Venturi so as to prevent the deposition of soot and the formation of coke on this surface.