Miniaturization is a method of allowing defined reaction conditions in modern process engineering. It also allows a continuous operation mode, short diffusion paths and control of shear stress. An unfavorable consequence of miniaturization is the use of small flow rates and ineffective laminar mixing. Regarding processes that are well known in macro scale, problems arise when they are transferred to micro scale. In micro scale, transport mechanisms in boundary layers play a major role, but are widely unknown. Miniature hollow-fiber membrane reactors (mini HFMR) are developed to aim a defined adjustment of process parameters. Their conceptual characteristics are indirect aeration, integrated educt feed and product removal and controlled shear rate. Because secondary flows are appropriate to enhance mixing [1], their principles will be applied to the mini HFMR. Dean vortices on one side and the von Karman vortex street on the other are applied to enhance the transport between membrane and liquid flow. To work straightforward, first numerical simulations are performed. Later on the results of the simulations will be validated by experiments. Based on two different reactor types it will be shown that secondary flows enhance the mass transfer significantly while keeping the shear stress low. This way miniaturization turns out to be an all appropriate method for various fields of bio process engineering.