AbstractA numerical study was performed to investigate improvement in mass transfer rates obtainable by introduction of secondary convection into a proposed membrane blood oxygenator. The three‐dimensional Navier‐Stokes equations were solved numerically for the velocity field in channel and the results used in the numerical solution for the localPo2in the oxygenator. The convective diffusion equation solved is nonlinear, in addition to being three‐dimensional, because of the presence of red cell oxygen sinks.The convection introduced was generated by moving one wall of the blood flow channel laterally to the main flow. Decreases in blood side film resistance up to 5 × 104were predicted. In the presence of moderate to strong convection, the limiting factor on oxygenation rate is imposed by the membrane situated between blood and gas phases. It appears that a considerable decrease in required blood priming volume would be possible if the method discussed here were imple