Dependence of mainstream angle of vortex generator arrays on heat transfer enhancement in boundary layer flow on flat plate based on direct numerical simulation

•Heat transfer characteristics of tetrahedral VG arrays with respect to the mainstream angle was investigated in a laminar boundary layer flow.•The heat transfer downstream the VG is enhanced by a pair of time-averaged vortices when the mainstream angle is zero degrees.•When the mainstream angle increases, turbulent heat transfer becomes the dominant factor.•In the case of double-row VGs the heat transfer downstream the second row further increases due to the intensification of turbulence when VGs are aligned in the mainstream direction.•VG pitch in the streamwise direction has less influence than that in the spanwise direction which indicates that the effect of VGs is maintained downstream.