An annular combustion system comprising: an inner liner and an outer liner disposed radially outward of the inner liner, the inner liner and the outer liner defining therebetween an annulus circumscribing a centerline of the annular combustion system, the annulus including a plurality of primary combustion zones at an upstream end thereof and a plurality of secondary combustion zones downstream of the primary combustion zones; a plurality of fuel nozzles, at least one fuel nozzle discharging a combustible mixture into each primary combustion zone of the plurality of primary combustion zones; a plurality of panel fuel injectors, each panel fuel injector being disposed between adjacent fuel nozzles and extending in an axially downstream direction to separate adjacent primary combustion zones, each panel fuel injector discharging a combustible mixture into at least one secondary combustion zone; wherein each panel fuel injector of the plurality of panel fuel injectors comprises: a first side wall; a second side wall opposite the first side wall; and an aft end connecting the first side wall and the second side wall; wherein the first side wall, the second side wall, and the aft end define therebetween a premix air plenum and a fuel distribution plenum; and wherein a plurality of premixing channels (230, 232) are disposed between the first side wall and the second side wall, each premixing channel of the plurality of premixing channels (230, 232) being in fluid communication with the premix air plenum and the fuel distribution plenum and having an injection aperture (234, 236) formed in one of the first side wall and the second side wall. an inner liner and an outer liner disposed radially outward of the inner liner, the inner liner and the outer liner defining therebetween an annulus circumscribing a centerline of the annular combustion system, the annulus including a plurality of primary combustion zones at an upstream end thereof and a plurality of secondary combustion zones downstream of the primary combustion zones; a plurality of fuel nozzles, at least one fuel nozzle discharging a combustible mixture into each primary combustion zone of the plurality of primary combustion zones; a plurality of panel fuel injectors, each panel fuel injector being disposed between adjacent fuel nozzles and extending in an axially downstream direction to separate adjacent primary combustion zones, each panel fuel injector discharging a combustible mixture into at least one secondary combustion zone; wherein each panel fuel injector of the plurality of panel fuel injectors comprises: a first side wall; a second side wall opposite the first side wall; and an aft end connecting the first side wall and the second side wall; wherein the first side wall, the second side wall, and the aft end define therebetween a premix air plenum and a fuel distribution plenum; and wherein a plurality of premixing channels (230, 232) are disposed between the first side wall and the second side wall, each premixing channel of the plurality of premixing channels (230, 232) being in fluid communication with the premix air plenum and the fuel distribution plenum and having an injection aperture (234, 236) formed in one of the first side wall and the second side wall. a first side wall; a second side wall opposite the first side wall; and an aft end connecting the first side wall and the second side wall; wherein the first side wall, the second side wall, and the aft end define therebetween a premix air plenum and a fuel distribution plenum; and wherein a plurality of premixing channels (230, 232) are disposed between the first side wall and the second side wall, each premixing channel of the plurality of premixing channels (230, 232) being in fluid communication with the premix air plenum and the fuel distribution plenum and having an injection aperture (234, 236) formed in one of the first side wall and the second side wall.