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The thermal and hydraulic performance of a twisted circular tube with an obstacle attached to the internal surface along the tube length (L = 1000 mm) is numerically analyzed in this work. The considered configuration is investigated by varying obstacle height ratio (OHR = h/D) which is the obstacle height (h) to the tube diameter (D = 20 mm) while obstacle thickness is kept constant under turbulent flow regime, i.e. Reynolds number 4000 ≤ Re ≤ 10,000 for air as working fluid. Various cases of OHR are triggered 0.0, 0.125, 0.25, 0.375, 0.5 and 0.625 respectively. The results show that the larger OHR gives a higher Nusselt number and vice versa due to increasing swirling power to the air in the tube. However, a higher friction factor is the penalty associated with this large obstacle. Recorded thermal performance factor TPF is beyond unity for all cases. The optimal value of TPF is observed at OHR = 0.625 which is recommended for high heat transfer rate requirement. Corresponding TPF = 2.07 at Re = 4000 and TPF = 1.69 at Re = 10,000. The results are validated against the findings of the previous works and the comparisons show good agreements.

This numerical study examines the thermal-hydraulic performance of helical coil heat exchangers (HCHEs) using twisted tubes versus conventional circular tubes. The effects of Reynolds number (Re = 10,000 to 100,000), pitch length (11, 7.2, 5.5, 3.6 mm), and twisted tube orientation (co-current, counter-current) on heat transfer efficiency and pressure drop were investigated. The results show that higher Reynolds numbers can reduce the Resistance factor by approximately 25 % and improve heat transfer by approximately 35 %. An optimal pitch length of 5.5 mm was found to balance enhanced heat transfer and manageable pumping costs. Twisted tubes demonstrate significant performance advantages at lower Re, with 20–33 % higher Performance Evaluation Criterion (PEC) values compared to circular tubes. However, the benefit diminishes at higher Re, with less than 10 % PEC increase. Implementing counter-current twisted tube configurations leads to approximately 13 % higher Nusselt number and 11 % higher Resistance factor, resulting in a 10 % overall PEC improvement. These findings highlight the superior thermal-hydraulic performance of twisted tubes in HCHE applications, especially at lower Reynolds numbers. Future work could explore different fluids, advanced optimization, energy analysis, cost assessment, and additional geometric parameters to further enhance twisted tube heat exchanger design and performance.

This study investigates the influence of nanofluids on heat exchanger efficiency using 3-D computational fluid dynamics (CFD). The objective is to optimize the performance of twisted tubes by analyzing various pitch lengths (P = 180, 135, 90, 67.5, and 45 mm). The method's accuracy is validated by comparing experimental and numerical data from previous studies. The analysis focuses on key parameters such as heat transfer factors, outlet temperatures, and pressure drops, encompassing a wide range of flow rates (0.5 kg/s to 2 kg/s). The findings demonstrate that using nanofluids in twisted tubes significantly enhances heat transfer while slightly increasing pressure drop. Specifically, when compared to the smooth tube device with six baffles, employing 0.1 vol% Cu and 0.15 vol% Al2O3 nanoparticles in the twisted tube with a pitch length of 45 mm leads to heat transfer improvements of 1.04 and 1.12 times, respectively. Moreover, eliminating baffles favoring the optimized twisted tube configuration results in a notable reduction in pressure drop by approximately 1.55 times. These results highlight the potential of nanofluid implementation in enhancing heat exchanger efficiency and offer valuable insights for designing and optimizing heat transfer systems in various industrial applications.

Twisted tubes and twisted tapes are swirl flow generators utilized for increasing thermal performance. The combined effects of twisted tubes and twisted tapes on heat transfer, pressure drop and thermal performance were experimentally investigated. Experiments were performed using a trapezoidal shaped twisted tube and twisted tapes having various twist ratios (y/w = 2.0, 3.0, 4.0 and 5.0). Water was used as the test fluid in a turbulent regime (4500

Twisted tri-lobed tubes have garnered attention due to their exceptional heat transfer efficiency and straightforward fabrication. Existing literature lacks comprehensive assessments of the overall heat transfer performance of twisted tri-lobed tubes from the perspective of energy loss and irreversibility. This research aims to investigate entropy generation during turbulent water flow within twisted tri-lobed tubes, examining the influence of geometric parameters on local and average entropy production. Findings indicate that larger small circle radius (r) and straight lengths (l), coupled with smaller transition circle radius (R) and twist pitch lengths (p), result in diminished local and average heat transfer entropy production while enhancing local and average frictional entropy production, with heat transfer entropy generation dominating the overall entropy production. Additionally, with increasing Reynolds numbers, all twisted tubes demonstrate an increasing trend in average frictional entropy production, except for some cases (Case 1-Case 4) that exhibit an initial rise followed by a decline in average heat transfer entropy generation. Among the examined Reynolds range, Case 4 displays lower overall irreversibility compared to a plain tube. Following the second law of thermodynamics, Case 4 is preferred. The findings and methodology contribute to enhancing the thermodynamic evaluation of convective heat transfer in twisted tri-lobed tubes.

Augmentation of heat transfer performance has long been the main interest in thermal fluid engineering sector. Some commonly adopted thermal enhancement methods are adoption of helical tubes, twisted tubes and multilobe cross-sections. Several studies reported further enhancement when combination of these methods were implemented. Despite their promising potential, no investigation on the performance of heat transfer in helical twisted multilobe tubes has been reported. Therefore, present study is conducted with the main objective to study the laminar convective heat transfer of Newtonian fluid in a helical twisted multilobe tube through computational fluid dynamics (CFD) simulations. A three-dimensional model was developed with accordance to principles of fundamental conservation. The model was validated against available experimental data for which a good agreement was achieved. The model was thus utilised to investigate the fluid flow and heat transfer in the studied tube within a range of certain parameters, such as, number of lobes, number of twists, inlet Reynolds number as well as the geometry of the pipe (straight and helical). The performances of heat transfer of the investigated configuration are evaluated using the typical Nusselt number, friction factor and performance index (PI), which is a ratio between average Nusselt number and friction factor under constant pumping work condition. The results reveal that adding the number of lobes alone has negligible effect on the performance. However, combination of number of lobes and tube twisting result in considerable changes on the heat transfer coefficient with marginal effect on the friction factor. This is especially pronounced for tube with even number of lobe (bilobe and quadrilobe). Overall, helical coiled tube performs better within the range of Re 500 to Re 2000 as the performance index averagely increase by 18.4% as compared to straight tube, whereas below Re 500, straight tube is slightly outperforming the helical tube (3.65%). Twisting of the tubes result in the opposite effect, i.e. it improves the performance of straight tube but deteriorate the performance of helical tube. Thus, twisting is recommended primarily for straight tubes. Highest performance index yielded is 7.01 by bilobe helical tube without twist at Re 2000. Lastly, correlations for friction factor and Nusselt number are developed for straight and helical multilobe twisted tube within studied range. This correlation can serve as quick design tools for non-conventional heat exchanger utilizing twisting multilobe tubes.

In order to further improve the heat transfer performance inside tubes, a three-start twisted tube combined with oval dimples is proposed. The effects of dimple depths on the characteristics of fluid flow and heat transfer are numerically studied. The results indicate that the fluid mixing inside the tube is greatly improved by the longitudinal vortex generated behind the dimple compared with the corresponding three-start twisted tube without dimples. The maximum increases in Nusselt number and friction factor of the three-start twisted tube with dimples are 26.68% and 19.96% compared with the three-start twisted tube without dimples at Re = 2000, respectively. The maximum thermal performance factor of the three-start twisted tube with oval dimples is 1.19 compared with the three-start twisted tube without dimples. The thermal performance factor of the three-start twisted tube with oval dimples is up to 1.38 compared with the straight tube, which is larger than the reported twisted tubes, such as twisted tube, multi-start spirally corrugated tube, and twisted square duck reported in the open literature. The proposed three-start twisted tube with dimples provides a new choice for the heat transfer enhancement inside tubes.