A device for mixing two gas streams, the device includes: an inner pipe, wherein: the inner pipe is arranged substantially concentrically within an outer pipe and forms an annulus between an outer diameter of the inner pipe and an inner diameter of the outside pipe; the inner pipe is closed at a downstream end; and the inner pipe comprises a plurality of perforations; and the outer pipe, wherein: a downstream end of the outer pipe extends into a reactor; the outer pipe is closed at the downstream end; and the downstream end of the outer pipe comprises a plurality of perforations. The mixed gas stream can enter the reactor. The reactor can be an Oxidative Coupling of Methane (OCM) reactor.