A heat exchanger comprising a flow manifold and a heat exchanger core, the heat exchanger core comprising alternating first circuit layers and second circuit layers, the flow manifold comprising: a plurality of fractal flow splitters arranged in a grid pattern that comprises a plurality of vertical layers fluidly connected to a corresponding first circuit layer, wherein two or more of the plurality of fractal flow splitters are in each vertical layer; a plurality of flow channels , each flow channel being fluidly connected to an associated fractal flow splitter; a plurality of flow dividing vanes each disposed within a flow channel and configured to divide the associated flow channel into two or more sub-channels; and an outer manifold surrounding the plurality of fractal flow splitters and configured to direct a first circuit flow into, or out of, the flow manifold; wherein: each of the fractal flow splitters defines an open end and a plenum end , and is configured to provide a transition from the open end to the flow manifold; each of the fractal flow splitters is configured to direct a portion of the first circuit flow toward, or away from, one or more of the flow dividing vanes , thereby directing flow into, or away from, the two or more sub-channels; characterized in that each of the open ends has a circular, oval, oblong, or elliptical shape; two adjacent first circuit layers define a vertical spacing; and the vertical spacing ranges from 0.075 - 0.5 inch (1.9 - 13 mm). a plurality of fractal flow splitters arranged in a grid pattern that comprises a plurality of vertical layers fluidly connected to a corresponding first circuit layer, wherein two or more of the plurality of fractal flow splitters are in each vertical layer; a plurality of flow channels , each flow channel being fluidly connected to an associated fractal flow splitter; a plurality of flow dividing vanes each disposed within a flow channel and configured to divide the associated flow channel into two or more sub-channels; and an outer manifold surrounding the plurality of fractal flow splitters and configured to direct a first circuit flow into, or out of, the flow manifold; wherein: each of the fractal flow splitters defines an open end and a plenum end , and is configured to provide a transition from the open end to the flow manifold; each of the fractal flow splitters is configured to direct a portion of the first circuit flow toward, or away from, one or more of the flow dividing vanes , thereby directing flow into, or away from, the two or more sub-channels; characterized in that each of the open ends has a circular, oval, oblong, or elliptical shape; two adjacent first circuit layers define a vertical spacing; and the vertical spacing ranges from 0.075 - 0.5 inch (1.9 - 13 mm). each of the fractal flow splitters defines an open end and a plenum end , and is configured to provide a transition from the open end to the flow manifold; each of the fractal flow splitters is configured to direct a portion of the first circuit flow toward, or away from, one or more of the flow dividing vanes , thereby directing flow into, or away from, the two or more sub-channels; characterized in that each of the open ends has a circular, oval, oblong, or elliptical shape; two adjacent first circuit layers define a vertical spacing; and the vertical spacing ranges from 0.075 - 0.5 inch (1.9 - 13 mm). <EOS>