The enhancement of surface heat transfer rate via longitudinal vortices is demonstrated numerically. The vortices are generated by delta winglet-pairs of low aspect ratios and moderately high angles of attack. These winglet-pairs are attached to the channel walls. Strong distortion of the usual two dimensional boundary layer structure due to the vorticity induced swirling flow is observed. Thus the embedded vortices can enhance local heat transfer rate which culminates in a substantial increase in the combined-spanwise-average heat transfer. The study is also extended to turbulent flows through the k — ε closure model of turbulence.