*4.2. Verification of Numerical Results*

Since the numerical model of the smooth channel proposed in this paper is similar to the physical models presented by Ma et al. [34] and Zhou et al. [14], the *Nu*, *f* of the numerical model and the experimental model are compared to verify the reliability of the numerical model. As is found in Figure 3a, the Nusselt numbers are in reasonable conformity with two correlations from Ma et al. and Zhou et al., with average deviations of 9.5% and 10.8%, respectively. Similarly, according to Figure 3b, the friction factors of the simulation results are in reasonable agreement with Ma and Zhou's results, with average differences of 26.9% and 16.9%, separately. Therefore, it is credible that the numerical model and solution method in this study are reliable.

**Figure 3.** Validation of numerical results (**a**) *Nu*, (**b**) *f* .
