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    Evaluation of Heat Transfer Augmentation in a Tube Fitted with Grooved Twisted Tapes: A Comparative Thermal-Hydraulic Performance Study
    (2026-06-01)
    Du, Yuexiang
    ;
    Liengsirikul, Sathaporn
    ;
    Phila, Arnut
    ;
    Wongcharee, Khwanchit
    ;
    Pimsarn, Monsak
    A computational fluid dynamics (CFD) analysis is conducted to systematically investigate heat transfer enhancement in tubes fitted with grooved twisted tapes and to identify the groove geometry that provides the best thermo-hydraulic performance. Three grooved twisted tape configurations—circular-grooved twisted tapes (CGTT), rectangular-grooved twisted tapes (RGTT), and triangular-grooved twisted tapes (TGTT)—are evaluated and compared with a smooth tube and a conventional twisted tape over a Reynolds number range of 5000–20,000 under isothermal wall conditions. The grooved twisted tapes enhance heat transfer through the combined effects of swirl-induced secondary flows and groove-generated flow disturbances, which intensify turbulent mixing and reduce the thickness of the thermal boundary layer. Compared with the plain tube, the grooved configurations increase the Nusselt number by 1.472–1.98 times while increasing the friction factor by 3.21–3.58 times. Relative to the conventional twisted tape, the grooved designs provide an additional 8.0–12.1% enhancement in heat transfer with only a marginal increase of 0.2–1.5% in friction factor. The thermodynamic analysis indicates that the CGTT configuration exhibits the lowest entropy generation rate and exergy loss throughout the investigated Reynolds number range. In particular, the CGTT achieves a Bejan number of 0.999841 at Re = 5000, demonstrating an excellent balance between heat transfer enhancement and frictional losses. Furthermore, the CGTT attains the highest thermal performance factor (TPF) of 1.294 at Re = 5000 and maintains TPF > 1.0 over the entire Reynolds number range. The overall performance ranking is consistently established as CGTT > TGTT > RGTT based on comprehensive analyses of velocity fields, streamline patterns, turbulent kinetic energy distributions, temperature contours, and thermodynamic characteristics. Although the present study identifies the circular-groove configuration as the optimal design for a twist ratio (y/W) of 3.0, further parametric investigations involving variations in twist ratio, groove dimensions, and groove pitch are required to develop generalized design guidelines.
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    Numerical Investigation of Thermodynamic Performance in Gradient-Pitch Twisted Square Ducts with Variable Aspect Ratio
    (2026-04-01)
    Samruaisin, Prachya
    ;
    Liengsirikul, Sathaporn
    ;
    Phila, Arnut
    ;
    Maruyama, Naoki
    ;
    Shoon Wai, Thiri
    This study numerically investigates heat transfer and thermodynamic behavior in twisted square and rectangular air ducts while keeping a constant hydraulic diameter (D<inf>h</inf> = 30 mm). Three aspect ratios are considered (AR = 1.00, 0.75, and 0.50). The heated test section (900 mm) is divided into three equal segments, and three pitch patterns are examined: a uniform pitch (400–400–400 mm, P444) and two axial gradients (300–400–500 mm, P345; 500–400–300 mm, P543). All results are compared to a standard reference, the straight square duct (SD-AR1.00), to ensure fair comparisons across all cases with Reynolds numbers between 5000 and 20,000. Among the twisted ducts, the strongest rectangularity combined with the increasing pitch sequence, TSD-AR0.50-P345, provides the best overall balance. Its heat transfer rises from Nu = 39.39 to 88.62, giving Nu/Nu<inf>0</inf> = 1.493 → 1.433, while the pressure penalty increases to f/f<inf>0</inf> = 1.345 → 1.405. Under cube-root weighting of friction, this case maintains the highest thermal performance factor, TPF = 1.352 at Re = 5000 and TPF = 1.279 at Re = 20,000. Second-law trends support the same ranking: exergy destruction decreases from 12.81 W (baseline) to 8.44 W at Re = 5000 (≈34% reduction) and from 6.54 W to 4.84 W at Re = 20,000 (≈26% reduction). The Bejan number remains high at low Reynolds numbers (≈0.998), indicating heat-transfer irreversibility dominance, but drops at higher Reynolds numbers (≈0.87) as frictional effects become more important. In general, the results show that adding a small axial pitch increase to rectangularity can improve near-wall mixing while reducing losses downstream. This leads to a clear improvement in both first-law performance and exergy-based measures.
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    Thermodynamic performance analysis of a round tube fitted with gradient quadruple twisted tapes
    (2026-03-01)
    Samruaisin, Prachya
    ;
    Liengsirikul, Sathaporn
    ;
    Thianpong, Chinaruk
    ;
    Chuwattanakul, Varesa
    ;
    Chamoli, Sunil
    Air-cooled heat exchangers employing twisted-tape passive inserts exist in many forms; however, designing tapes that are practical to manufacture while enhancing heat transfer without incurring excessive pressure drop remains challenging. This study therefore introduces gradient quadruple twisted tapes (GQTT), which gradually vary the pitch across four tapes to control swirl and mix along the tube. In addition, the turn count changes in steps along the four tapes, which triggers extra turbulence near each change and keeps the flow well mixed downstream. This simple and manufacturable pattern aims to boost heat transfer while keeping the added friction under control. Our objective is to evaluate whether GQTT can improve overall performance while keeping the pressure drop within a practical range, in comparison with a plain tube and a constant-turn tape. A realizable k-ε turbulence model, rigorously validated against benchmark data, was applied to eight GQTT variants and benchmarked against a plain tube and a constant-turn tape for Reynolds numbers (Re) between 5000 and 19,000. Spatial second-order schemes, grid-independence testing, and strict residual criteria ensured solution accuracy. Among the candidates, the descending-opposite-pitch configuration (Ds-OPSD) consistently delivered the highest performance. First-law analysis reveals a peak thermal performance factor (TPF) of 1.42 at Re = 5000, equating to a 42 % gain in overall thermo-hydraulic efficiency over the plain tube, with the Nusselt number (Nu) climbing from 57.5 to 100 across the examined Re window. Second-law metrics corroborate this superiority. At the same Re of peak TPF, the Ds-OPSD cuts exergy destruction from 291.4 to 62 W (≈79 % reduction versus the plain tube and ≈22 % versus the constant-turn tape). Total entropy generation remains minimal and nearly constant (S<inf>total</inf> ≈ 0.206–0.212), while the Bejan number stays high (≈0.999–0.971), indicating that the enhancement is achieved without excessive frictional penalties.