Abstract: One embodiment is a heat exchanger comprising a shell surrounding a first fluid plenum, a plurality of flow chamber walls positioned inside the shell, and a diffuser positioned inside the shell. The plurality of flow chamber walls: at least partially define a plurality of first fluid radial flow channels in flow communication with a first fluid inlet and the first fluid plenum, at least partially define a plurality of second fluid axial flow channels in flow communication with a second fluid inlet, and comprise an arcuate shape and arranged in an array to at least partially define arcuate shapes of the first fluid radial flow channels and the second fluid axial flow channels. The diffuser includes a diffuser surface at least partially defining a diffusion flow path from the first fluid inlet and the plurality of first fluid radial flow channels.