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Numerical Investigation of Thermodynamic Performance in Gradient-Pitch Twisted Square Ducts with Variable Aspect Ratio
(2026-04-01)
Samruaisin, Prachya
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Liengsirikul, Sathaporn
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Phila, Arnut
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Maruyama, Naoki
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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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Influence of Wired Twisted Tape on Heat Transfer Enhancement, Friction Factor and Thermal Performance Behaviors in a Heat Exchanger Tube
(2026-03-01)
Lin, Jianyu
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Laphirattanakul, Ponepen
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Bhattacharyya, Suvanjan
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Thapmanee, Piphatpong
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Wongcharee, Khwanchit
This study experimentally investigates the thermal–hydraulic performance of heat exchanger tubes fitted with wired twisted tapes, with particular emphasis on the effects of the hole spacing-to-width ratio (s/W) and edge margin-to-width ratio (e/W). Experiments were conducted over a Reynolds number range of 6000–20,000, and the results were compared with those of plain tubes and tubes equipped with conventional twisted tapes. The findings revealed that the incorporation of wires significantly enhanced heat transfer due to the combined action of longitudinal eddies generated by wire protrusions and swirling flow induced by the twisted tape. At identical Reynolds numbers, tubes with a smaller hole spacing (s/W = 0.16) exhibited superior heat transfer performance, achieving Nusselt number enhancements of up to 107.7% relative to plain tubes and 51.6% relative to conventional twisted tapes. Similarly, reducing the edge margin ratio intensified near-wall eddies and further disrupted the boundary layer. The friction factor was found to increase with decreasing hole spacing and edge margin, primarily due to additional flow obstructions and enhanced near-wall shear stresses. For wired twisted tapes with s/W = 0.16, the friction factor reached nearly six times that of a plain tube. Despite this penalty, the thermal performance factor (TPF) remained favorable, with values of up to 1.2, indicating that the heat transfer benefits outweighed the corresponding pressure losses.
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Path Optimization Using an Improved APF-RRT* Algorithm
(2026-01-01)
Zheng, Yongyang
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Pimsarn, Monsak
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Chuwattanakul, Varesa
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Chokphoemphun, Suriya
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Eiamsa-Ard, Smith
Path planning remains a critical research area in mobile robotics, yet current approaches often suffer from suboptimal path quality, limited sampling efficiency, and inadequate adaptability across diverse operational scenarios. To address these issues, this paper proposes an improved algorithm combining Artificial Potential Field (APF) and Restricted Path Time (RRT*) approaches. This algorithm employs an optimization model that combines dynamic sampling with potential field guidance, constructing a two-stage dynamic sampling mechanism. During sampling, candidate nodes with Gaussian noise are generated along the resultant force direction. Finally, path cost comparison and parent node reselection are performed within the dynamic optimization radius to ensure asymptotic optimality of the path. Experimental results show that in complex maps, path length is reduced by 33.41% and 26.64%, respectively, and planning time is reduced by 21.36% and 86.32%, respectively; in narrow passages, path length is reduced by 49.6% and 49.8%, respectively. The results confirm the effectiveness of the two-stage dynamic sampling mechanism, which not only preserves the probabilistic completeness of the RRT* algorithm but also adaptively adjusts the sampling strategy, improving both planning length and time.
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Thermal performance evaluation of a diamond-shaped roughened tube
(2025-12-01)
Chokphoemphun, Suriya
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Kamma, Panit
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Promvonge, Pongjet
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Promthaisong, Pitak
Thermal performance evaluation was examined numerically in a diamond-shaped roughened tube, which created recirculation and pair counter-rotation flows, which helped to disrupt the boundary layer, and increased fluid mixing led to improving the rate of heat transfer. The parameters studied, including relative depth ratio, e/D, DR, from 0.02 ≤ DR ≤ 0.14, and relative pitch ratio, p/D, PR, from 0.25 ≤ PR ≤ 1.5, under turbulent flow conditions, 3000 ≤ Re ≤ 20,000. Computed results included heat transfer (Nu/Nu<inf>0</inf>), frictional loss (f/f<inf>0</inf>) and thermal performance (in terms of thermal enhancement factor, TEF). The simulations showed that the velocity and heat transfer became fully developed periodic at around x/D ≈ 6–7.5. The pair counter-rotation flows increased the level of both the flow strength and the mixing of fluid, and disrupted the boundary layer, leading to an increase in heat transfer rate. The Nu/Nu<inf>0</inf>, f/f<inf>0</inf> and TEF were achieved in a range of 1.00–3.34, 1.21–24.00 and 0.69–1.56. The maximum TEF was found at 1.56 for DR = 0.08, PR = 0.50 and Re = 5000.
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Effect of arc-shaped twisted-baffles on augmented heat transfer in a rectangular duct
(2023-02-01)
Promvonge, Pongjet
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Phila, Arnut
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Chuwattanakul, Varesa
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Chokphoemphun, Suriya
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Eiamsa-Ard, Smith
In this current work, the effect of utilizing arc-shaped twisted-baffles (T-ABs) in a rectangular air channel on thermal performance factor (TPF) has been experimentally studied. In these experiments, the influence of the changes in parameters such as dimensions of pitch ratio (p/w), attached arc-shape angle (α), and Reynolds number (Re) are explored. The comparisons demonstrate that a channel mounted with arc-shaped twisted-baffles yielded considerably greater Nusselt numbers than a smooth channel, possibly attributable to multiple-impinging jets near the channel surface. Heat transfer enhancements of twisted arc-shaped baffles (T-AB) having larger attack angles were superior to those having smaller attack angles. The one with α = 90o offered greater heat transfer rates than the ones with α = 20o, 40o, 60o, and 80o by approximately 8%, 7%, 4%, and 2%, respectively. The superior heat transfer was attributed to the better contact between the working fluid and heat transfer surfaces. In addition, utilizing arc-shaped twisted-baffles with the lowest p/w of 4.0, in a channel produced stronger vortices and multiple impinging jets, which caused better fluid mixing than other p/w. The optimum condition is achieved using T-ABs at an attached arc-shape angle of α = 90o, p/w = 4.0 and Re = 4000, where the heat transfer rate (Nu), friction factor (f) and TPF are found to be, respectively, 3.31, 4.68 and 1.98 times greater than those of a plain channel.
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Investigation of Combustion Characteristics in a Vortexing Fluidized-bed Combustor with a Contraction Chamber
(2023-01-01)
Manop, Chalermchat
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Chokphoemphun, Suriya
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Chuwattanakul, Varesa
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Promvonge, Pongjet
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Eiamsa-Ard, Smith
This investigation was undertaken to obtain information on the effects of a contraction chamber and secondary air ratio (λ) on combustion temperature distribution, gas emissions of rice husk combustion and flow behaviours in a vortexing fluidized-bed combustor with a contracted chamber (VFBCC).The combustor chamber was modified by reducing the diameter chamber size (contraction) near the top part of the chamber to create a large central recirculation zone. In the experiment, the effects of secondary air ratio (λ) of 0.0, 0.25 and 0.4, and the equivalent ratio, (ϕ) of 0.2, 0.25, 0.3 on combustion characteristics and temperature distribution were examined. The results indicated that the temperature profile inside the combustor between a typical vortexing fluidized-bed combustor (VFBC) and VFBCC was slightly different.The emission of VFBCC was lower than in the original VFBC. At the equivalent ratio of 0.3 and secondary air ratio of 0.4, the average concentrations of O2, CO2, CO and NOx of the flue gases from the original VFBC were 10.5%, 6.7%, 717 ppm and 287 ppm, respectively while those from the modified VFBCC were 9.7%, 6.3%, 357 ppm, and 250 ppm, respectively. In addition, flow visualization results showed the presence of a contracted part led to the recirculation formation in the top chamber.
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Heat Transfer Intensification in a Heat Exchanger by Means of Twisted Tapes in Rib and Sawtooth Forms
(2022-12-01)
Poonpakdee, Pasu
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Samutpraphut, Boonsong
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Thianpong, Chinaruk
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Chokphoemphun, Suriya
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Eiamsa-ard, Smith
This experimental study aimed to intensify the aerothermal performance index (API) in a round tube heat exchanger employing twisted tapes in rib and sawtooth forms (TTRSs) as swirl/vortex flow generators. The TTRSs have a constant twist ratio of 3.0, a constant rib pitch ratio (p/e) of 1.0, and six different sawtooth angles (α = 20°, 30°, 40°, 50°, 60°, and 70°). Experiments were carried out in an open flow using air as the working fluid for Reynolds numbers between 6000 and 20,000 in the current study, which was conducted in a heated tube under conditions of uniform wall heat flux. A typical twisted tape (TT) was also tested for comparison. The experimental results suggest that TTRSs yield Nusselt numbers ranging from 1.42 to 2.10 times of those of a plain tube. TTRSs with larger sawtooth angles (α) offer superior heat transfer. The TTRSs with α = 20°, 30°, 40°, 50°, 60°, and 70° respectively, enhance average Nusselt numbers by 158%, 162%, 166%, 172%, 180%, and 187% with average friction factors of 3.51, 3.55, 3.60, 3.67, 3.75 and 3.82 times higher than a plain tube. Additionally, TTRSs with sawtooth angles (α) of 20°, 30°, 40°, 50°, 60°, and 70° offer APIs in the ranges of 0.99 to 1.19, 1.01 to 1.21, 1.03 to 1.26, 1.05 to 1.31, 1.07 to 1.42, and 1.09 to 1.48, respectively, which are higher than those of the typical twisted tape (TT) by around 5%, 7%, 11%, 16%, 25%, and 31%, respectively. This demonstrates that twisted tapes in rib and sawtooth form (TTRSs), with appropriate geometries, give a promising trade-off between enhanced heat transfer and an increased friction loss penalty.
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Rice husk combustion characteristics in a rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls
(2019-09-01)
Chokphoemphun, Suriya
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Eiamsa-Ard, Smith
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Promvonge, Pongjet
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Chuwattanakul, Varesa
An experimental combustion work using the rice husk as a fuel was performed in a rectangular fluidized bed combustor with triple pairs of chevron-shaped discrete ribbed walls. The effect of percent excess air (EA = 40%-70%) on temperature distribution and gas emissions in a rectangular fluidized-bed combustor on combustion behaviors (temperature distributions inside the bed, exhaust gas emissions and combustion efficiency) is reported. The free-board had a rectangular shape which modified by increasing the chamber size from 300 × 100 mm<sup>2</sup> to 600 × 200 mm<sup>2</sup> or two times of the bottom chamber and 2400 mm in height. Triple pairs of rib configurations, namely, 30° chevron-shaped discrete ribbed walls were introduced and placed in the bottom part of the combustion chamber to generate counter-rotating vortices in the chamber. The results show that the fluidized bed combustor with 30° chevron-shaped discrete ribbed walls at 40% percent excess air provides the higher temperature than other of 861 °C. Among the studied excess air percentages, the highest combustion efficiency of 99.2% is obtained at EA = 60%. In addition, combustion at EA = 60% shows the lowest emissions CO, CO<inf>2</inf>, O<inf>2</inf> and NO<inf>x</inf> of 236.8%Vol, 2.55 ppm, 13.53 %Vol, 110.2 ppm, respectively.
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Investigation on Rice Husk Combustion in a Fluidized Bed with Longitudinal Vortex Generators
(2019-05-16)
Eiamsa-Ard, Smith
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Wongcharee, Khwanchit
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Chokphoemphun, Suriya
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Chuwattanakul, Varesa
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Promvonge, Pongjet
Effects of wavy-ribbed surface chamber on combustion temperature/efficiency and exhaust gas behaviours in a fluidized bed combustor were investigated. In the experiments, the five wavy-rib pairs were placed at the bottom chamber for inducing longitudinal vortex flow which enhanced the fuel and fluid/combustion-air mixing in the top chamber for giving better combustion efficiency. The experiments were examined at five percent excess airs of EA = 15%, 30%, 45%, 60%, and 75% and were tested at the constant air mass flow rate of 95 kg/hr. From experimental results, it can be showed that a fluidized bed combustor with wavy-ribbed surface chamber can be promoted the high combustion efficient for all test runs especially at the percent excess airs of EA = 75%.
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Thermal performance of tubular heat exchanger with multiple twisted-tape inserts
(2015-05-01)
Chokphoemphun, Suriya
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Pimsarn, Monsak
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Thianpong, Chinaruk
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Promvonge, Pongjet
The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fluxed wall. The investigation has been conducted in the heat exchanger tube inserted with various twisted-tape numbers for co- and counter-twist arrangements for the turbulent air flow, Reynolds number (Re) from 5300 to 24000. The typical single twisted-tape inserts at two twist ratios, y/w = 4 and 5, are used as the base case, while the other multiple twisted-tape inserts are at y/w = 4 only. The experimental results of heat transfer and pressure drop in terms of Nusselt number (Nu) and friction factor (f), respectively, reveal that Nu increases with the increment of Re and of twisted-tape number. The values of Nu for the inserted tube are in a range of 1.15-2.12 times that for the plain tube while f is 1.9-4.1 times. The thermal enhancement factor of the inserted tube under similar pumping power is evaluated and found to be above unity except for the single and the double co-twisted tapes. The quadruple counter-twisted tape insert provides the maximum thermal performance.

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