An apparatus for mixing fluids within a pipe is provided. In one embodiment, the apparatus includes a fluid mixing device with a pipe having a pipe wall and an axial bore for conveying fluids through the pipe. The fluid mixing device also includes a sleeve disposed about the pipe and a cavity provided between an exterior surface of the pipe and an interior surface of the sleeve. The cavity and the axial bore of the pipe are in fluid communication with one another via an opening through the pipe wall. Additional systems, devices, and methods are also disclosed.