A fluid reactor for facilitating mixing and/or chemical reaction comprising: an elongated cylindrical flow chamber having a first flow inlet at one end, and a flow outlet at another end, the first flow inlet being operable to admit a flow of a bulk fluid into the flow chamber, a cone-pi element disposed in a position, centered within the chamber, and having an upstream conical portion with a leading apex generally facing the first flow inlet, an intervening cylindrical portion, having a uniform diameter and cooperating with the wall of said chamber to provide an annular passage around said cone-pi element, and a downstream conical portion with a trailing apex generally facing the flow outlet, the cone-pi element being attached to a wall of the flow chamber by at least one strut or pylon, a second flow inlet through a wall of the flow chamber at a position downstream of the first flow inlet and upstream of the cone pi element, the second flow inlet being operable to admit a relatively low mass flow of a second fluid into the bulk fluid flow upstream of the cone-pi element; wherein the at least one strut or pylon is positioned in the intervening cylindrical portion of the cone-pi element and comprises a third inlet in fluidic communication with a passage connecting through the cone-pi element to the apex of the upstream conical portion or to the apex of the downstream conical portion, wherein the third inlet is operable to feed a small mass flow rate of a third fluid into the reactor from the apex of the upstream conical portion or the apex of the downstream conical portion; and wherein the cone-pi element is operable to produce cavitation and/or vortical flow within the flow stream by causing the bulk fluid flow to move over the upstream conical portion of the cone-pi element and be accelerated and compressed as it enters the annular passage between the cone-pi element and the wall of the flow chamber, thereby effectuating the mixing and/or chemical interaction of the bulk fluid, the second fluid and the third fluid, and to deliver a mixed and/or reacted fluid from the flow outlet. an elongated cylindrical flow chamber having a first flow inlet at one end, and a flow outlet at another end, the first flow inlet being operable to admit a flow of a bulk fluid into the flow chamber, a cone-pi element disposed in a position, centered within the chamber, and having an upstream conical portion with a leading apex generally facing the first flow inlet, an intervening cylindrical portion, having a uniform diameter and cooperating with the wall of said chamber to provide an annular passage around said cone-pi element, and a downstream conical portion with a trailing apex generally facing the flow outlet, the cone-pi element being attached to a wall of the flow chamber by at least one strut or pylon, a second flow inlet through a wall of the flow chamber at a position downstream of the first flow inlet and upstream of the cone pi element, the second flow inlet being operable to admit a relatively low mass flow of a second fluid into the bulk fluid flow upstream of the cone-pi element; wherein the at least one strut or pylon is positioned in the intervening cylindrical portion of the cone-pi element and comprises a third inlet in fluidic communication with a passage connecting through the cone-pi element to the apex of the upstream conical portion or to the apex of the downstream conical portion, wherein the third inlet is operable to feed a small mass flow rate of a third fluid into the reactor from the apex of the upstream conical portion or the apex of the downstream conical portion; and wherein the cone-pi element is operable to produce cavitation and/or vortical flow within the flow stream by causing the bulk fluid flow to move over the upstream conical portion of the cone-pi element and be accelerated and compressed as it enters the annular passage between the cone-pi element and the wall of the flow chamber, thereby effectuating the mixing and/or chemical interaction of the bulk fluid, the second fluid and the third fluid, and to deliver a mixed and/or reacted fluid from the flow outlet.