## 1 Introduction  
The industrial demand for concentric tube heat exchangers (CTHE) has increased in recent years due to their design simplicity, space consideration, lower cost, and efficiency [1]. However, the heat transfer area limitation of a CTHE can result in poor heat transfer in most applications. To improve the performance of CTHEs while maintaining their compact design, augmented heat transfer (AHT) methods were introduced and are continuously being developed. AHT demonstrates a modification of flow to achieve better temperature distribution inside the CTHE and, in doing so, promotes better heat transfer. AHT methods are classified as passive, active, and compound, and these methods were studied and used in different heat transfer applications in the industry [2], namely steaming and condensation [3, 4, 5], boiling and evaporation systems [6, 7], refrigeration and air-conditioning, and pharmaceutical and medical applications. Various research studies focus on different passive AHT since it is relatively easier to install. Table 1 summarizes various types of inserts used and their impact on heat transfer rate. Despite significant improvements in heat transfer rates, the shape complexity of developed inserts often leads to high manufacturing costs and fabrication constraints. On the other hand, active and compound AHT methods are less explored compared to passive AHT [14; 15]. Table 2 shows some of the active and compound AHT types used in the past and their corresponding effect on heat transfer rate. Previous studies indicate that each AHT method contributes to the improvement of heat transfer rate. It is therefore important to study the effect of combining different methods: passive, active, and compound AHT methods. As previous literature focused on isolated passive enhancement techniques, the understanding of the application of active and compound AHT methods in heat transfer applications, particularly rotating circular perforated inserts and rotating inner pipes, respectively, remains limited and insufficiently explored. The novelty of this research lies in the integrated approach to enhancing heat exchanger performance by assessing all heat transfer methods--passive, compound, and active AHT--through the strategic incorporation of uncomplicated inserts that are not difficult to fabricate. Specifically, this study initiated the integration of a rotating circular perforated insert with an actively rotating inner pipe, which has not been systematically \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Author** & **Insert Type** & **Effect on Heat Transfer Rate** & **Reference** \\ \hline [PERSON] et al. (2021) & Double-perforated inclined elliptic insert & Increase by 39.4\% in heat transfer rate & [8] \\ [PERSON] et al. (2020) & Louvered Strip Insert & A stronger axial velocity near the pipe wall, thereby increased the heat transfer rate by 45.8\%. & [9] \\ [PERSON] et al. (2018) & Bidirectional conical strip insert & Increased in Nusselt number (\(Nu\)) by 2.35–9.85 times & [10] \\ [PERSON] and [PERSON] (2018) & Equilateral triangle cross-sectioned coiled-wire insert & A maximum increase of 1.67 times in thermal performance & [11] \\ [PERSON] et al. & Helical insert of 5 mm and (2021) & Increase by 63.91\% and 31.39\% in heat transfer rate, respectively & [12] \\ [PERSON] et al. (2021) & Y-shaped insert & Increase by 5.05 times in heat transfer rate & [13] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Type of inserts used and the effect on heat transfer rate.