An annular duct including a modular annular heat exchanger for a gas turbine engine is provided, where the modular annular heat exchanger includes a plurality of radial modules in circumferentially adjacent arrangement. Each radial module includes a cooled fluid inlet plenum segment, a plurality of blades, and a cooled fluid outlet plenum segment. The plurality of blades is configured in circumferentially adjacent arrangement and defines an angular space that is conformal between each circumferentially adjacent blade. The cooled fluid inlet plenum segment, the plurality of blades, and the cooled fluid outlet plenum segment are in serial axial flow arrangement and define an internal cooled fluid flowpath and an external cooling fluid flowpath parallel to the internal cooled fluid flowpath. Each radial module further includes an inner annular ring segment and an outer annular ring segment. The inner annular ring segment and the outer annular ring segment define a plurality of blade retainers. The blade retainers define an axial, radial, and circumferential position of the blades, the cooled fluid inlet plenum segment, and the cooled fluid outlet plenum segment.