A thermal management system includes a housing, and a monolithic core structure disposed within the housing. An outer surface of the core structure defines at least part of a first passageway. An inner surface of the core structure defines at least part of a second passageway. The core structure includes a separator wall that isolates a first flow passing through the first passageway from a second flow passing through the second passageway. The first passageway is in thermal communication with the second passageway. The core structure includes one or more heat exchanger features, or fins, that are positioned within the first passageway, the second passageway, or both the first and second passageways. The core structure may have a compliant segment coupled to two or more walls.