The invention includes a heat transfer geometry having first and second flow channels in parallel with each other. The flow cross-sectional area of individual channels varies along the length of the flowpath, with one channel undergoing an expansion and the other undergoing a contraction. Different amounts of additional heat transfer surface are located within different regions. In at least some instances, contraction and expansion may occur as a result of a shift of both the left and right boundaries which principally define the channel, and may occur symmetrically with respect to a centerline of the individual channel. With a cell being a first channel and associated second channel, the overall exiting flow may be offset slightly from the overall entering flow. An array may be formed containing multiple cells, and cells at edges of the array may be atypical so that the overall array fits within a simple geometric envelope.