Numerical investigations of flow structure and heat transfer in a channel with periodically mounted transverse vortex generators (bars) have been conducted in the Reynolds number range of steady laminar to oscillatory transitional flow. The unsteady Navier-Stokes equations and the energy equation have been solved by a finite volume code. Due to the periodic geometry, instability leads at relatively low Reynolds number to self-sustained oscillations and vortex shedding from the bars. At Reynolds number Re = 150, the flow structure is already time periodic. With increasing Reynolds number their amplitudes and number of frequencies increase. The data for heat transfer and flow losses are presented for different periodicity lengths in a Reynolds number range of 100-400.