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 ; 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 , wherein the heat exchanger is configured to cool a third airflow (AF3) by the first airflow (AF1), when the third airflow (AF3) is first passing through inner portion , and then by the second airflow (AF2), when the third airflow (AF3) 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 ; 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 , wherein the heat exchanger is configured to cool a third airflow (AF3) by the first airflow (AF1), when the third airflow (AF3) is first passing through inner portion , and then by the second airflow (AF2), when the third airflow (AF3) 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 , wherein the heat exchanger is configured to cool a third airflow (AF3) by the first airflow (AF1), when the third airflow (AF3) is first passing through inner portion , and then by the second airflow (AF2), when the third airflow (AF3) 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 . <EOS>