Heat Transfer Enhancement and Vortex Flow Structure in the Spirally Fluted Tubes

The turbulence kinetic energy and heat transfer performance of air in spirally fluted tube were numerically studied at a constant wall temperature with Reynolds number (Re) between 5000 and 45 000. Furthermore, the flow dynamics and heat transfer performance of spirally fluted tubes with five different geometric parameters as well as the effects of separation vortex and swirling wake flow on heat transfer and flow resistance were analyzed. According to the results, heat transfer is enhanced mainly because the fluid hit the windward side of the flute, thus generating a strong turbulence kinetic energy to further reconstruct the boundary layer. The second reason is that the formation of the recirculation zone between the flutes disturbs the boundary layer caused by the flow separation. With the increase of flute depth ratio (L-d/D), the separation vortex will become stronger and larger on the leeward side of flute. The separation vortex will break the boundary layer and improve the heat transfer capacity which is accompanied with the increase of fluid resistance. As the flute pitch length ratio (L-p/D) decreases, the spiral flow is strengthened, and meanwhile more wake flow is generated. The spiral flow causes little impact on enhancing heat transfer but inhibits the development of the separation vortex and fluid pulsation; in addition, the fluid resistance is reduced at the same time. The maximum value of the average Nusselt number appears when Re=5000, L-d/D=0.25 and L-p/D=1.00, which is 2.53 times the value of smooth tube. In view of the whole range of Reynolds number, the thermal performance enhancement factor indicates that L-d/D=0.15 and L-p/D=1.00 are the optimal geometric design parameters.