The efficiency of heat transfer η, i.e., the ratio between heat transfer enhancement and drag increase from a smooth channel, is discussed for the turbulent and laminar flows in a channel with a wing-type vortex generator (VG) installed on the wall. In this report, the effect of Prandtl number Pr on ηis mainly discussed. η decreases slightly in the turbulent flow with Pr while it increases noticeably in the laminar flow. In the laminar flow, since there is no background turbulence in the smooth channel, the fluctuation generated by VG directly enhances the heat transfer, which is greater in high Pr region where the thermal diffusion effect is weakened. In addition, the increase of convective heat transfer near the wall has an important effect on the good efficiency for the laminar flow. The effect of down-flow around VG on η is discussed from the comparison with the numerical results for the channel without the notch hole just under the wing.
