Abstract
In this paper, the heat transfer and flow characteristics of a helically coiled tube with twisted elliptical in a low Reynolds number (Re = 500−3000) flow were investigated numerically. The working fluid flowed in a laminar regime. Numerical results were compared with empirical correlations in the existing literature, demonstrating the accuracy of the analysis in this study. Firstly, we investigated the effects of geometric parameters and Reynolds number on the heat transfer and flow characteristics. The results showed that lower twist pitch length and semi-major axis length could induce sufficient fluid mixing and a larger temperature gradient near the tube wall, enhancing the heat exchange and producing larger friction resistance. Overall, the heat transfer performances were improved by about 1.04−1.21 times and 1.02−1.23 times for different semi-major axis lengths and different twist pitch lengths, respectively, compared to the smooth helical coil. Secondly, by changing the working fluid, it was found that the Nu when oil was chosen as the working fluid was all 6−6.8 times higher than that of water. Moreover, the effect of using Al2O3 nanofluids with a concentration of 0.10 wt.%, 0.25 wt.% and 0.50 wt.% was discussed. It can be seen that the heat transfer capacity of nanofluids increased by approximately 2−18% compared to the reference model with water as the working fluid, and the nanofluid solution with higher concentration has better heat transfer performance. Finally, Nu and f correlations are given to predict the heat transfer and pressure drop in practical applications.