reference will now be made in detail to present embodiments of the invention , one or more examples of which are illustrated in the accompanying drawings . the detailed description uses numerical and letter designations to refer to features in the drawings . like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention . each example is provided by way of explanation of the invention , not limitation of the invention . in fact , it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof . for instance , features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment . thus , it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents . embodiments of the present invention may be machined and assembled to create a premixed direct injection ( pdi ) fuel nozzle design . in general , the fuel nozzle design comprises two components which may be separately machined or fabricated for subsequent assembly . one piece may be referred to as the tip or outer body , and the other piece may be referred to as the fuel cartridge or fuel plenum . the fuel plenum directs fuel downstream against a front wall of the outer body to provide impingement cooling to the front wall . after impinging against the front wall , the fuel then flows through passages to bore holes in the outer body where the fuel mixes with a fluid flowing through the bore holes before exiting the fuel nozzle and flowing into the combustion chamber . the fuel plenum and outer body , with their various bore holes and other passages , may be readily manufactured by machining instead of requiring more costly processes such as direct metal laser sintering . as a result , fuel nozzles according to various embodiments of the present invention may be less expensive to manufacture , while still providing improved cooling to the fuel nozzle and premixing the fuel prior to combustion . fig2 shows a cross - section of a perspective view of a fuel nozzle 30 according to one embodiment of the present invention . as will be explained , the fuel nozzle 30 generally includes two modular components , namely a fuel cartridge or fuel plenum 32 and an outer body 34 , which may be separately machined or fabricated for subsequent assembly . the fuel plenum 32 provides a chamber or conduit for fuel flow to and through the fuel nozzle 30 . for example , the fuel plenum 32 may comprise a longitudinal passage 36 centrally located in the fuel nozzle 30 , as shown in fig2 . an inlet 38 to the fuel plenum 32 may be connected to a fuel supply ( not shown ). possible fuels supplied to and used by commercial combustion engines include , for example , blast furnace gas , coke oven gas , natural gas , vaporized liquefied natural gas ( lng ), propane , and hydrogen . the fuel plenum 32 may further include a plurality of apertures 40 . the apertures 40 may be located , for example , at the downstream portion of the fuel plenum 32 , as shown in fig2 . the plurality of apertures 40 allow the fuel to flow through and out of the fuel plenum 32 . the outer body 34 includes a front wall 42 downstream of the fuel plenum 32 and proximate to the plurality of apertures 40 in the fuel plenum 32 . the front wall 42 is generally the closest portion of the fuel nozzle 30 to the combustion flame and therefore is subjected to higher temperatures than the remainder of the fuel nozzle 30 . fuel flowing through the plurality of apertures 40 exits the fuel plenum 32 and impinges on the front wall 42 to provide impingement cooling to the front wall 42 . the outer body 34 generally surrounds the fuel plenum 32 , creating a space or annular plenum 44 between the fuel plenum 32 and the outer body 34 . the outer body 34 further includes a plurality of bore holes 46 that extend longitudinally through the outer body 34 . the bore holes 46 may be arranged in any desired pattern . for example , as shown in fig2 , the bore holes 46 may be arranged in substantially concentric circles around the fuel plenum 32 . the bore holes 46 are generally cylindrical in shape , although the present invention is not limited to any particular shape of bore holes 46 , unless specifically recited in the claims . each bore hole 46 generally includes an inlet 48 , which may be beveled , as shown in fig2 , to facilitate an even distribution of fluid flow into and through the bore holes 46 . the outer body 34 further includes a plurality of passages 50 between at least some of the bore holes 46 and the fuel plenum 32 . the plurality of passages 50 provide fluid communication between the fuel plenum 32 and at least some of the plurality of bore holes 46 . specifically , fuel exiting the fuel plenum 32 through the plurality of apertures 40 impinges on the front wall 42 to provide impingement cooling to the front wall 42 . the fuel then flows through the annular plenum 44 until it reaches one of the plurality of passages 50 where it flows into the associated bore hole 46 . in this manner , the fuel mixes with the fluid ( e . g ., compressed working fluid from a compressor ) flowing through the bore hole 46 before exiting the bore hole 46 and entering the combustion chamber . the fuel plenum 32 and outer body 34 may be separately machined and manufactured for subsequent assembly . for example , the fuel plenum 32 and / or outer body 34 may be cast from a molten metal . the various bore holes 46 and passages 50 in the outer body 34 may then be drilled to accurately and inexpensively position , size , and orient the various elements in the outer body 34 . if desired , the inlet 48 to various bore holes 46 may be further machined to include a beveled surface or otherwise increase the surface area of the inlet 48 for specific boreholes 46 , depending on particular design considerations . the fuel plenum 32 may then be inserted into the annular plenum 44 defined by the outer body 34 and attached to the outer body 34 . various methods and means are known in the art for attaching or connecting the fuel plenum 32 to the outer body 34 . for example , brazing , welding , complementary threads , seal rings , and other equivalent techniques and connections are known in the art for attaching or connecting the fuel plenum 32 to the outer body 34 . depending on the particular design needs , the connection between the fuel plenum 32 and the outer body 34 may be permanent or temporary to allow for removal of the fuel plenum 32 during maintenance or repair . the particular embodiment shown in fig2 includes a continuous weld bead 52 between the fuel plenum 32 and the outer body 34 . in addition , this particular embodiment also includes a threaded connection 54 between the fuel plenum 32 and the outer body 34 . alternate embodiments within the scope of the present invention may include only one of these means for attaching or connecting the fuel plenum 32 to the outer body 34 , and / or other welding techniques , such as tack welding , and / or other mechanical fittings or connections between the fuel plenum 32 and the outer body 34 . fig3 shows a cross - section of a fuel nozzle 56 according to an alternate embodiment of the present invention . the fuel plenum 32 and outer body 34 in this embodiment are substantially similar to the embodiment previously described and illustrated in fig2 , and the same reference numbers are therefore used . in this particular embodiment , the means for attaching or connecting the fuel plenum 32 to the outer body 34 again includes a continuous weld bead 52 around the perimeter of the fuel plenum 32 . in addition , the cross - section of this particular embodiment illustrates the plurality of passages 50 between bore holes 46 located at different distances from the fuel plenum 32 . in this manner , the fuel may be more evenly distributed and mixed in specifically selected bore holes 46 . this written description uses examples to disclose the invention , including the best mode , and also to enable any person skilled in the art to practice the invention , including making and using any devices or systems and performing any incorporated methods . the patentable scope of the invention is defined by the claims , and may include other examples that occur to those skilled in the art . such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims , or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims .