Heat Transfer And Flow Characteristics Of Small Scale Circular Cylinder Vortex Generator

Fluid flow and heat transfer in the channel have always been one of the focus in the engineering fields. Many macroscopic properties of fluid flow, such as drag variation, heat transfer, energy transfer and dissipation, are associated with coherent structure near the wall boundary layer. Effectively controlling coherent structure of the boundary layer during the process of heat transfer enhancement and flow drag reduction is very significant for theoretical study and practical application. Meanwhile, the relevant results are helpful for some engineering practical applications such as in thermal engineering field.In present work, small scale circular cylinder vortex generator is located in the turbulent boundary layer. A rectangle channel model is used to probe the influence of bluff body periodic wake flow on the boundary layer coherent structure near the wall, then the impact of bluff body on fluid flow and heat transfer characteristics are further explored. The flow and heat transfer characteristics in the rectangle empty channel, rectangle channel with small scale reference cylinder and rectangle channel with small scale slit-cylinder are numerically investigated by using large eddy simulation(LES) for Re=3745.In order to analyze and compare the variation of flow and heat transfer characteristics in the rectangle channel by adding small scale cylinder vortex generator, the flow and heat transfer characteristics in the rectangle channel without small scale vortex generator are firstly analyzed. In the numerical simulation of the channel with the reference cylinder and the channel with slit-cylinder, the gap ratio(the gap between the circular cylinder and the channel bottom G to the diameter of the cylinder D) is G/D=0~6.0, the slit width ratio(the slit width S to the diameter of the cylinder D) is S/D=0.35. The numerical results indicate that the cylinder wake flow in the turbulent boundary layer can significantly improve heat transfer performance and effectively suppress the augment of flow drag except for G/D=0. When the gap ratio G/D<2.0, the impinging jet derived from the slit can strengthen the interaction between the wake flow and the wall boundary layer. Compared with heat transfer performance of the channel with the reference cylinder, the overall thermal performance of the channel with slit-cylinder is improved. In addition, the synthesis performance coefficient of the channel with the reference cylinder and the channel with slit-cylinder are gradually increased with the increase of gap ratio. When the gap ratio G/D?2.0, the wake flow stability of the slit-cylinder becomes worse, the impinging jet can prevent the sweep motion of the fluid from the outside of the cylinder wake to some degrees, which result in the decrease of overall thermal performance deterioration. The synthesis performance coefficient for both cases is decreased as the gap ratio increasing. Finally, the impact of different diameter cylinder(D/H=0.05, 0.1) on flow and heat transfer characteristics in the channel is investigated, the results indicate that heat transfer performance of the channel with large diameter cylinder can be improved.The present work shows that flow structures of the bluff body wake flow in the turbulent boundary layer have determined periodicity, the vortex shedding of wake flow is a periodic disturbance, which can be used to enhance the overall thermal performance of the channel bottom.