Numerical investigation on heat transfer performance and flow characteristics in circular tubes with dimpled twisted tapes using Al_2O_3-water nanofluid

摘要

The present study numerically examines heat transfer and flow in circular tubes fitted with dimpled twisted tape inserts, and Al_2O_3-water nanofluid is employed. Considering the effects of dimples, protrusions, nanofluid volume fraction α and nanoparticle diameter d_p, the discussion and analysis on heat transfer, flow characteristics, turbulence kinetic energy TKE, thermal property, entropy generation and maximum local wall temperature T_max are detailedly provided. The results show that dimple side and protrusion side both realize great heat transfer enhancement, and dimple side behaves better compared with protrusion side. Utilization of dimples leads to an increase by 25.53% in convective heat transfer coefficient h at most compared with smooth tape. Heat transfer performance is greatly improved on both tape wall and tube wall owing to disturbance to flow structures. The overall TKE level significantly rises especially close to the core flow region when dimples are adopted, and overall swirl flow intensity greatly increase, further resulting in enhancement of turbulent mixing and heat transfer. Remarkable decline in average heat transfer entropy generation rate S_ah is identified, accompanied by slight rise in average friction entropy generation rate S_af, where average total entropy generation rate S_a inclines by 29.10% at most in comparison with smooth tape. Besides, the employment of nanofluids results in great improvement of heat transfer, with further enhanced effect when α increases, accompanied by slight growth in resistance. A maximum increase by 58.96% in h is identified compared with basefluid case, and it is quite helpful that in this case the maximum rise in friction factor is only 5.05%. The wall temperature distribution is greatly improved using nanofluid, and nanofluid case provides markedly improved thermal conductivity distribution in spite of slight growth in dynamic viscosity. The heat transfer in recirculation-caused low heat transfer regions is further promoted through nanofluids when dimple technique is adopted. Utilization of nanofluid brings about a significant reduction in S_ah, the effect of which is further promoted by a rising a, with a slight increase in S_af, where S_a is reduced by 28.89% at most. The T_max greatly declines using nanofluids with further enhanced effect when α ascends. Furthermore, utilization of smaller d_p gives rise to significant heat transfer enhancement compared with the larger one, with extremely mild resistance increase. Lower S_a and lower T_max are both realized when smaller d_p is used, and the effect of d_p reaches an extremely low level when it exceeds 40 nm.