Embodiments of a heat transfer apparatus, and related methods, involve at least one boundary wall defining a first flow path through a neck portion, a first heat source external to and in thermal communication with the boundary wall, and a working fluid (e.g., a first fluid component with a second fluid component entrained therein). The neck portion may be shaped such that at least a portion of the second fluid component impinges upon at least a portion of the boundary wall as the working fluid flows therethrough, whereby heat is transferred from the first heat source to the working fluid through the boundary wall.