###### Abstract
[21]} & \multirow{2}{*}{2018} & \multirow{2}{*}{Experimental and \(\;  
Solar energy is considered as a heat flux for solar collectors. The survey of a solar heat transfer device is trusted for increasing the thermal performance of the available system to obtain an outcome [41]. [PERSON] et al. [42] used two passive techniques to ameliorate the efficiency of heat exchangers in solar collectors. The outcomes illustrate that the use of nanofluid went up the Nusselt number up to 4%. [PERSON] and [PERSON] [43] in the numerical verification on solar parabolic collector demonstrated that adding nanoparticles increase thermal performance and using Al\({}_{2}\)O\({}_{3}\) or CuO as nanofluid at 3% volume fraction enhances the heat exchange up to 28 and 35 percent, respectively. [PERSON] et al. [44] investigated the thermal analysis of parabolic trough collectors for various geometrical parameters such as porosity and fin aspect ratio. Their results pointed out that the porosity will increase the heat exchange by about 17%. According to the presented literature review of previous works (Table 1) related to heat transfer enhancement of the PTSC by various passive methods, it can be concluded that among different kinds of utilized methods, employing helical axial fins has not been considered and analyzed previously. In the present work, one parabolic trough solar collector with inner helical axial fins as swirl generator or turbulator is considered and analyzed. This work consists of two sections. In the first section, four various schematics of the collector are appraised. In the second section, the selected geometry of the collector \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Author** & **Year** & **Method (Exp/Num)** & **Inserts** & **Result** \\ \hline [PERSON] et al. [30] & 2020 & Numerical & Flexible fins & Due to the oscillation of the fins, the heat exchange rate goes up. \\ \hline [PERSON] et al. [31] & 2020 & Numerical & Central corrugated insert & The overall heat transfer performance was achieved in the range of 1.3–2.6. \\ \hline [PERSON] and [PERSON] [32] & 2020 & Experimental & Helically corrugated tube & The maximum obtained thermal performance (2.29) belongs to the case with pitch length and roughness height of 3 mm and 1.5 mm, respectively. \\ \hline [PERSON] et al. [33] & 2020 & Numerical & Helical absorber tube & The thermal efficiency and exergy rise by 1–10\% and 0.2–3.2\%, correspondingly. \\ \hline [PERSON] et al. [34] & 2020 & Numerical & Conical strip inserts & The overall thermal–hydraulic performance was obtained in the range of 0.679–1.107. \\ \hline [PERSON] et al. [35] & 2020 & Numerical & \begin{tabular}{c} Absorber tube with twisted tape insert and tube with longitudinal fins \\ \end{tabular} & The thermal efficiency of the cases with twisted tape insert, tube with internal fins, and plain tube are by 72.26%, 72.10\% and 71.09\%, respectively. \\ \hline [PERSON] et al. [36] & 2021 & Numerical & \
\begin{tabular}{c} Turbulence-Inducing elements \\ \end{tabular} & The maximum achieved thermal efficiency was by 29\%. \\ \hline [PERSON] and [PERSON] [37] & 2021 & Experimental &
\begin{tabular}{c} Angular twisted strip inserts \\ \end{tabular} & The obtained enhancement efficiency was in the range of 145–215\%. \\ \hline [PERSON] et al. [38] & 2021 & Numerical & Helical absorber tube & The thermal efficiency and exergy rises by 4–10\%, and 4–5\%, correspondingly. \\ \hline [PERSON] and [PERSON] [39] & 2021 & Numerical & Helically corrugated tube & The thermal performance shows a growth of 26–176\%. \\ \hline [PERSON] et al. [40] & 2021 & Experimental and Numerical & Semi-annular and fin shape metal foam hybrid structure & Performance evaluation criteria augmentation, total entropy generation decrease in addition to exergetic efficiency growth maximally reach 360, 93.3 and 10.2\%, correspondingly. \\ \hline \hline \end{tabular}
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