A heat exchanger assembly comprising: a first annular duct; a first airflow pathway disposed within and formed by the first annular duct, wherein the first airflow pathway is configured to transport a first airflow; a second annular duct disposed radially outward from the first annular duct; a second airflow pathway disposed between and formed by the first and second annular ducts, wherein the second airflow pathway is configured to transport a second airflow, wherein the first annular duct forms a fluidic barrier between the first and second airflow pathways (36, 38); and a heat exchanger with a partially annular shape, wherein the heat exchanger comprises: an inner portion disposed radially inward of the first annular duct; and an outer portion disposed between the first and second annular ducts (36, 38), wherein the heat exchanger is configured to cool a third airflow by the first airflow, when the third airflow is first passing through inner portion, and then by the second airflow, when the third airflow is passing through the outer portion; wherein the inner portion and the outer portion form a core, characterized in that the first annular duct includes an axial cutout, wherein a portion of the heat exchanger is mounted within the axial cutout; and the heat exchanger assembly further comprises a flange extending axially from the heat exchanger, wherein the flange is mounted to the first annular duct. a first annular duct; a first airflow pathway disposed within and formed by the first annular duct, wherein the first airflow pathway is configured to transport a first airflow; a second annular duct disposed radially outward from the first annular duct; a second airflow pathway disposed between and formed by the first and second annular ducts, wherein the second airflow pathway is configured to transport a second airflow, wherein the first annular duct forms a fluidic barrier between the first and second airflow pathways (36, 38); and a heat exchanger with a partially annular shape, wherein the heat exchanger comprises: an inner portion disposed radially inward of the first annular duct; and an outer portion disposed between the first and second annular ducts (36, 38), wherein the heat exchanger is configured to cool a third airflow by the first airflow, when the third airflow is first passing through inner portion, and then by the second airflow, when the third airflow is passing through the outer portion; wherein the inner portion and the outer portion form a core, characterized in that the first annular duct includes an axial cutout, wherein a portion of the heat exchanger is mounted within the axial cutout; and the heat exchanger assembly further comprises a flange extending axially from the heat exchanger, wherein the flange is mounted to the first annular duct. an inner portion disposed radially inward of the first annular duct; and an outer portion disposed between the first and second annular ducts (36, 38), wherein the heat exchanger is configured to cool a third airflow by the first airflow, when the third airflow is first passing through inner portion, and then by the second airflow, when the third airflow is passing through the outer portion; wherein the inner portion and the outer portion form a core, characterized in that the first annular duct includes an axial cutout, wherein a portion of the heat exchanger is mounted within the axial cutout; and the heat exchanger assembly further comprises a flange extending axially from the heat exchanger, wherein the flange is mounted to the first annular duct.