**6. Conclusions**

This paper investigated the laminar convection heat transfer in a tube equipped with twisted tape inserts. A parametric study was conducted to evaluate the impact of key design variables, including the twisted tape truncation percentage, pitch value, position in the tube and Reynolds number on the fluid flow and heat transfer characteristics of such a configuration using symmetric heat flux around the tube. To determine the trade-off between the heat transfer enhancement and the pressure drop penalty, the PEC number was calculated. The obtained results indicated that using the twisted tape and reducing its pitch value increases the Nusselt number, friction coefficient and PEC number due to the generation of efficient secondary and mixing flow. The average Nusselt number increased by about 151.42 for a Reynolds number of 1000 in the case of fully fitted twisted tape at a pitch value of L/4. It was also found that increasing the twisted tape truncation percentage reduced both heat transfer and pressure drop in comparison with the fully fitted twisted tape case. Moreover, the best position for the truncated twisted tapes was at the tube entrance to reach the highest thermal performance. Ultimately, it was concluded that for P = L, L/2, L/3 and L/4, the optimum cases from the viewpoint of energy saving are twisted tapes with λ = 75, 50, 50 and 0%, for which the related PEC numbers at a Reynolds number of 1000 are almost equal to 1.08, 1.24, 1.4 and 1.76, respectively. The finding of this research provides a framework for researchers working in this area toward higher performance based on energy saving according to both heat transfer enhancement and pressure drop reduction.

**Author Contributions:** R.M. and P.T. performed and designed the numerical simulations; M.G., H.A., R.M., H.M.A. and W.Y. analyzed the data and performed the discussion; M.G., H.A., R.M., P.T., H.M.A. and W.Y. wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by Dr Wahiba Yaïci from CanmetENERGY Research Centre, Natural Resources Canada.

**Conflicts of Interest:** The authors declare no conflict of interest.
