A heat exchanger includes a first fluid manifold extending along a first fluid axis from a first fluid inlet to a first fluid outlet. The first fluid manifold includes a first fluid inlet header, a first fluid outlet header, and a nested helical core section. The first fluid inlet header is disposed to fork the first fluid inlet into a plurality of first fluid branches distributed circumferentially and radially about the first fluid axis. The first fluid outlet header is disposed to combine the plurality of first fluid branches into the first fluid outlet. The nested helical core section fluidly connects the first fluid inlet header to the first fluid outlet header via a plurality of nested helical tubes, and includes radially inner and outer groups of circumferentially distributed helical tubes.