A helicoidal structure promotes cooling of a liner applied in an annular space of a double-shell structure formed of the liner and a flow sleeve to cool a duct assembly, by increasing the residence time and cooling area of cooling compressed air on the surface of the liner. The helicoidal structure includes a helicoidal rib protruding from a surface of the liner to guide the cooling compressed air at a predetermined angle with respect to an axial direction of the liner, the helicoidal rib having cooling holes and forming a first cooling passage along which main-cooling compressed air flows and a second cooling passage along which auxiliary-cooling compressed air flows. The second cooling passage induces a flow of the auxiliary-cooling compressed air through the cooling holes, and the flow of the auxiliary-cooling compressed air is derived from a flow of the main-cooling compressed air in the first cooling passage.