Literature review on the enhancement of thermal performance of straight wickless heat pipes (two-phase closed thermosyphon): Geometry, surface, and internal structure modifications

Straight wickless heat pipes, two-phase closed thermosyphons (TPCTs), are emerging as key passive thermal devices for renewable energy, desalination, and waste heat recovery systems. However, their performance strongly depends on the geometric design, internal surface features, and structural configuration. This systematic literature review synthesizes 65 peer-reviewed TPCT studies (2015–2025) identified through the SPAR-4-SLR protocol (Scopus Q1-Q2 journals) and classifies enhancement strategies into three domains: (i) external geometry, (ii) internal surface, and (iii) internal structure. A vote-counting analysis across heterogeneous datasets (working fluids, filling ratios, operating scale) shows that 88 % of studies reported performance improvement in thermal resistance (Rth) and heat transfer coefficient (HTC). Mean effect sizes reveal a performance hierarchy: internal structure (39.5 ± 11.6 %) > internal surface (33.6 ± 10.4 %) > external geometry (27.4 ± 8.1 %). Structural modifications such as axial grooves, fins, vapor-liquid separators, and vortex generators yielded the largest gains, while nanostructured and wettability-engineered surfaces produced stable, repeatable enhancement. Geometric optimization, such as curvature, inclination, fin arrays, and corrugation, offered cost-effective, scalable improvements validated in modular kilometer-scale systems. Experimental observations align with Rohsenow's nucleate boiling and Nusselt's film condensation theories, though two-phase pressure-drop correlations often under-predict results, highlighting the need for revised data-driven models. To address reporting inconsistency, this review introduces TPCT-PRMS, a 12-element performance reporting minimum set, and a techno-economic lens linking performance, cost, and scalability. The study also emphasizes manufacturability, quality assurance, and long-term reliability under fluctuating thermal loads, providing a comprehensive foundation for sustainable, high-efficiency TPCT design.