A counter-flow heat exchanger comprising a heat exchanger core including an inner wall and an outer wall radially outward and spaced apart from the inner wall. A first flow path is defined within the inner wall and a second flow path is defined between the inner wall and the outer wall. The heat exchanger core includes a primary flow inlet, a primary flow outlet and a middle portion therebetween. The inner and outer walls are concentric at the primary flow inlet of the heat exchanger core. The inner wall defines a first set of channels extending axially from the primary flow inlet to the middle portion of the heat exchanger core diverging away from a radial center of the heat exchanger core. The inner wall and the outer wall define a second set of channels extending axially from the primary flow inlet to the middle portion of the heat exchanger core converging toward the radial center of the heat exchanger core.