A counter-flow heat exchanger core includes a first wall defining a longitudinal axis. The first flow path is defined within the first wall. The first flow path includes a primary flow inlet and a primary flow outlet downstream from the primary flow inlet. The heat exchanger core includes at least two hollow vanes circumferentially spaced apart and extending in a radially inward direction from the first wall. Each of the at least two hollow vanes includes a first vane wall and a second vane wall. The heat exchanger core includes a second flow path defined within the at least two hollow vanes between the first vane wall and second vane wall of each of the at least two hollow vanes. The heat exchanger core includes at least one fin extending between two of the at least two circumferentially spaced apart vanes.