A tube for a heat exchanger includes a tube outer body enclosing a tube inner volume, and a corrugated insert received within the tube inner volume. The tube outer body has a pair of broad planar walls joined by arcuate end walls. The corrugated insert defines flow channels through the tube, with opening in flanks of the insert allowing for flow communication between adjacent flow channels. Bypass channels adjacent the arcuate end walls are fluidly isolated from the adjacent flow channels by the absence of such openings in the end flanks. Flow through the bypass channels is obstructed by flow blocks at one or both ends of the bypass channels.