## 1 Introduction  
Owing to the growing energy costs and manufacturing materials, researchers have given significant attention to building up efficient, compact, and economical thermal systems for different engineering applications such as nuclear reactors, chemical processing industries, solar heaters, etc. A highly efficient thermal system comes with augmented heat transfer using passive, active, or a combination of multiple passive or active techniques. The primary goals of these strategies are to increase the thermal efficiency of the existing systems whilst also actually reducing their size, weight, and operating costs. Consequently, the suitable design of these approaches constitutes a difficult problem in order to achieve heat transfer optimisation at reasonable increase in frictional forces. Using a passive strategy based on introducing turbulators in the flow route as augmentations, such as helical inserts [1, 2], it was found that with increasing pitch, vortex shedding frequencies also rise, and that the highest amplitude of the vortex generated by conical-ring turbulators occurred at low pitches. It is discovered that the Nu number grows with rising Re number, and the optimum heat transfer is reached for the arrangement with the smallest pitch, for corrugated and grooved geometries [3, 4, 5]. As the pitch length is increased, the frequency of vortex shedding is also observed to rise, and the peak amplitude of vortex generated by conical-ring turbulators occurs at small pitches. Findings were made to show increasing Nu with rising Re, and maximum heat transfer is achieved at the minimum pitch configuration for ribbed and grooved geometries [3; 4; 5]. The dimensions of the grooves, the rotational speed, and the axial velocity of the fluid show a significant impact on their heat transfer. The results of vortex generators [6; 7] show that increases in distance between two vortex generators increase the mean Nusselt number, and other types of turbulators [8; 9; 10] are commonly used to improve heat transfer inside thermal systems due to the simple manufacturing process, easily employed compared to the active technique. Employing conical-ring turbulators at three-pitch ratios (PR) 2, 3, and 4, [PERSON] et al. [11] demonstrated augmentations of 330, 419, and 765 percent, respectively. They also discovered that using the diverging conical-ring with a smaller setup can increase the enhancement effectiveness by 1.291 (PR). [PERSON] et al. [12] examined the effect of winglet vortex generators on a horizontal circular tube in a forced convection setting with a Re number range of 6000-34,000. They concluded that winglet tensor flow with a pitch ratio of 1.6, a barrier ratio of 0.3, and an attack angle of 45\({}^{\circ}\) boosted the heat transfer rate by up to 4.88 times under constant heat flux conditions. The perforated inserts are usually used to generate strong turbulence/vortex flows, which helps further improvement of turbulence intensity with reasonable increment in pressure loss. [PERSON] et al. [13] performed an investigation of the perforated vortex generators' influences on forced convection in a circular tube. They observed that the perforated insert improves the thermal performance by 1.65 times under constant pumping power conditions. In comparison with the conventional conical-ring, [PERSON] et al. [14] claimed that the recirculation flow by holes of perforated conical-rings with varying geometrical characteristics can improve the heat transfer rate by about 35.48% and decrease the pressure loss by about 88.06%. Attempts to enhance thermal efficiency took place by utilising the advantages of twisted-tape swirl generator inserts as cross hollow twisted-tape [15]. A plain tube's experimental data are compared to the standard validation. With the increase in the hollow width from 6 mm to 10 mm, the Nu number and friction factor (f) subsequently increase. Compared to the flat tube, Nu and f increase by 93 percent-120 percent and 883 percent-1042 percent, respectively, for the 6 mm hollow width rise.