###### Abstract  
In the present work, a parabolic trough solar (PTC) collector with inner helical axial fins as swirl generator or turbulator is considered and analyzed numerically. The three-dimensional numerical simulations have been done by finite volume method (FVM) using a commercial CFD code, ANSYS FLUENT 18.2. The spatial discretization of mass, momentum, energy equations, and turbulence kinetic energy has been obtained by a second-order upwind scheme. To compute gradients, Green-Gauss cell-based method has been employed. This work consists of two sections where, first, four various geometries are appraised, and in the following, the selected schematic of the collector from the previous part is selected, and four various pitches of inner helical fins including 250, 500, 750 and 1000 mm are studied. All the numerical results are obtained by utilizing the FVM. Results show that the thermal performance improvement by 23.1% could be achieved by using one of the proposed innovative parabolic trough solar collectors compare to the simple one. Additionally, the minimum and maximum thermal performance improvement (compare to the case without fins) belong to the case with P = 250 mm by 14.1% and, to the case with P = 1000 mm by 21.53%, respectively. solar energy; parabolic trough collector; thermal performance; turbulator; swirl generator; numerical simulation 20 May 2021  
Accepted: 2 August 2021  
12 August 2021  
[PERSON] et al. [10] checked the heat transfer in the heat exchanger. According to their results, an increase of length enhances the performance of the vortex-generator fin. [PERSON] et al. [11] checked the heat exchange in a heterogeneous circular microchannel, the results show the maximum heat exchange rates are got when the boundary conditions are symmetrical. [PERSON] et al. [12] have performed an experimental probing in a corrugated tube with nanofluid, they deduced that nanofluid with corrugated tubing could increase heat exchange by 330% compared to net liquid. [PERSON] et al. [13] numerically have worked on tube layouts, they demonstrated that the comprehensive efficiency of the optimized heat transfer device is ameliorated by about 50%. [PERSON] et al. [14] have checked on a triple-tube that their outcomes show that, based on heat exchange efficiency, an increase in height and reduced pitch are suggested. [PERSON] et al. [15] have checked the impact of various geometries in twisted tapes, and the results pointed out that connection angle increases heat transfer. They found that when the wavelength is smaller, the temperatures near the walls increased. [PERSON] et al. [16] checked the thermal performance in corrugated pipes. According to the outcomes, the corrugated pipe ameliorated the heat transfer rate. Therefore, the other passive methods containing turbulator, etc., are usually better in the turbulent flow [17]. Several of the new works related to the utilization of inserts and the other passive methods to improve the thermal performance of the heat exchangers are displayed and listed in Table 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Author** & **Year** & **Method (Exp/Num)** & **Inserts** & **Result** \\ \hline \multirow{2}{*}{[PERSON] et al. [18]} & \multirow{2}{*}{2015} & Experimental and & \multirow{2}{*}{Spirally corrugated tube} & The tube of higher severity index \\  & & Numerical & & has the best thermal \\  & & & & performance. \\ \hline \multirow{2}{*}{[PERSON] et al. [19]} & \multirow{2}{*}{2016} & \multirow{2}{*}{Numerical} & \multirow{2}{*}{Vortex generator} & Their results pointed out that \\  & & & & holes close to the leading-edge \\  & & & & give better efficiency. \\ \hline \multirow{2}{*}{[PERSON] et al. [20]} & \multirow{2}{*}{2017} & \multirow{2}{*}{Experimental} & \multirow{2}{*}{Helically coiled} & Findings showed that the use of \\  & & & & a turbulator increases Nusselt \\  & & & & number around 8–32\%. \\ \hline \multirow{2}{*}{[PERSON] et al.