Heat transfer and pressure drop characteristics of mini smooth and microfin tubes were studied numerically using water as working fluid at Reynolds number ranging from 7500 to 17500. Seven microfin tubes were used with the same inner diameters of 4.6 mm and 18° helix angle and with number of fins ranging from 30 to 50, fin apex angle ranging from 10° to 40°, and fin height ranging from 0.1 to 0.15 mm. The numerical results fit well with the empirical correlations for heat transfer coefficients and pressure drops. The results indicate that the j-factor of the microfin tubes is approximately 1.2∼1.4 times of that in smooth tubes at the same Re. The j-factor increases with increasing number of microfin and the microfin height and with decreasing fin apex angle. The f-factor of the microfin tubes is approximately 1.05∼1.25 times of that in the smooth tube at the same Re, and the difference between the factors increases with the Re rising. The performance evaluation criterions (PEC) of the seven microfin tubes ranges from 1.15 to 1.35, indicating that microfin tubes exhibit better comprehensive performance compared with smooth tubes. The fluid at the center has a strong tendency to move towards the heated wall along the radial direction due to the directing effect of the microfins. The distinctive flow pattern in the radial direction can sufficiently enhance the turbulent flow near the wall and strengthen the mixing between the cold fluid at the center and hot water at the wall, leading to the enhancement of heat transfer in the near-wall region.

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