A cartridge is provided for dialysis or other blood processing therapy. In the cartridge, fibers may be substantially uniformly distributed near a midplane, but near an end, in the inter fiber space, there may be void flow channels, which may cause fluid flow in the inter fiber space to transition within a short region to uniform flow with minimal stagnation zones. Void flow channels may be be radially oriented, introducing fluid from the outer circumference, or axially oriented, introducing fluid along the axial direction through passageways through the potting material. The fluid flow in the inter fiber space may be perpendicular to the fibers, or radial with respect to a cartridge longitudinal axis. The cartridge may have blood flow in the inter fiber space, and flow of dialysate or ultrafiltrate in the lumens of the fibers, or the opposite situation.