Numerical study of mixed convection heat transfer for vertical annular finned tube heat exchanger with experimental data and different tube diameters

The commercial software along with the inverse method and the experimental data is used to obtain the mixed convection heat transfer and fluid flow characteristics of the single-tube vertical annular finned tube heat exchanger with different tube diameters. Various flow models along with near-wall treatments are introduced to investigate their effects on the obtained numerical results. The inverse method of the finite difference method along with the experimental data is first applied to determine the fin temperature and the heat transfer coefficient for a smaller tube. Afterwards, the air temperature and velocity profiles, the fin temperature distribution and the heat transfer coefficient on the fins are determined by the commercial software along with various flow models. More accurate results are obtained if the resulting heat transfer coefficient and the fin temperature at the measurement locations are respectively close to the inverse results and the experimental data. In addition, the comparison between the obtained air velocity pattern and the existing experimental pattern is desirable. An important finding is that the choice of appropriate flow models, near-wall treatments, friction factor, y⁺ value and the number of grid points varies with air velocity and tube diameter to obtain more accurate results. The heat transfer coefficient increases with increasing air velocity and fin spacing. However, the friction factor increases as the fin spacing increases and the air velocity decreases. The two vortices are symmetrical in the wake region behind the tube and rotate in the opposite direction. Increasing the number of grid points may not necessarily obtain more accurate results. The proposed correlation of the heat transfer coefficient is closer to the obtained inverse and numerical results than the existing correlation. To our knowledge, few researchers have used the similar method to investigate this problem in the open literature.