A thermal management system for a heat engine, the system including an forming at least in part a core flowpath and a cavity, wherein the core flowpath and the cavity are separated by a double wall structure formed by at least a portion of inner wall, and wherein the double wall structure includes a plenum. A first opening provides fluid communication between the cavity and the plenum, and a second opening provides fluid communication between the plenum and the core flowpath. The inner wall is configured to receive a first flow of fluid. An outer wall forms a passage extended at least partially around the core flowpath. The outer wall is configured to receive a second flow of fluid fluidly separated from the core flowpath.