A fluid heat exchanger assembly having an upper wall and a lower wall extending between the inlet and the outlet for establishing a direction of flow to cool an electronic device. A plurality of projections extend linearly transversely across the direction of flow to define rows of projections with linear cavities between adjacent projections so that fluid flows into and out of the cavities as the fluid flows across the rows of projections for contraction and expansion of the coolant flow to maximize heat transfer. The projections may be rectangular, triangular or convex, as viewed in cross section.