Advances in passive heat transfer enhancement for heat exchangers a comprehensive review

Abstract

Keywords: Air Bubble Injection; Extended Surfaces; Heat Transfer Enhancement; Micro-Surfaces; Nanofluids; Passive Methods; Phase Change Materials; Printed Circuit Heat Exchangers; Tape Inserts; Vortex Generators

Introduction

The primary objective of a power plant is to convert thermal energy into mechanical energy for electricity generation. Heat transfer is an essential process that has a direct impact

on the plant’s efficiency and environmental sustainability. However, achieving high-efficiency energy conversion faces challenges due to inherent limitations in heat transfer processes, particularly due to boundary layer formation. This

*Corresponding author. *E-mail address: faydogan@gmail.com This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

phenomenon establishes the existence of a laminar sub-layer with minimal fluid velocity in the heat exchanger (HX) tubes [1, 2]. The stagnant layer, dominated by thermal conduction, acts as a primary resistance to heat transfer, emphasizing the need for innovative techniques [3]. Efficient heat transfer is crucial for power generation and the longevity of industrial systems, ranging from power plants to electronic cooling [4]. Traditional HXs often face limitations in achieving optimal heat transfer rates, leading to increased energy consumption and reduced system efficiency. Exploring advanced techniques becomes imperative to address these challenges and develop sustainable and energy-efficient technologies [5]. Optimization of the heat exchange process remains an active area of research across various engineering applications. HXs are essential components in many industrial processes facilitating thermal energy transfer between two fluids without direct contact [6]. Their applications span a wide range of industries, including power generation [7], chemical processing [8], HVAC [9], automotive [10], and aerospace [11]. The widespread use of HXs has driven researchers towards technological advancements and innovative designs. These advancements focus on improving the heat transfer coefficient while minimizing pressure drop, which leads to higher efficiency and more compact equipment. Energy-efficient HXs play a key role in reducing energy consumption, lowering operational costs, and minimizing environmental impact. In critical applications such as nuclear reactors and advanced manufacturing, HXs are crucial for maintaining safe and stable operating conditions, increasing their significance in modern industrial systems. A lot of review papers are available on heat transfer enhancement techniques, with some focusing specifically on concentric tube [12-14] and plate type HXs [15], while others cover single phase [16, 17], two-phase [18] and phase change materials [19]. Some researchers examine specific heat enhancement techniques such as dimple surface [20], insert and nanofluid [21], porous media [22], extended surfaces [23], twisted tape [24-26], micro surface modification [27], swirl flow devices [28], nanosuspension [29] and microchannel heat sink [30]. Some reviewers also summarize various passive techniques [31, 32], active techniques [33, 34], and combined passive and active techniques [35-37]. The motivation for the present review paper stems from the fact that the available review papers lack the inclusion of the most recent developments across the entire spectrum of heat transfer enhancement techniques. It is thus required to conduct a comprehensive and up-to-date review of the different heat enhancement techniques. This review focuses on classifying and analyzing the latest research on passive heat enhancement techniques, thereby offering insights into their applications in the next generation of HXs. The review covers a range of HX

configurations, from traditional tubular designs to cutting-edge micro-channel HXs, characterizing them based on heat transfer rate, pressure drop, and performance evaluation criteria (PEC). It contributes to the understanding of heat enhancement using geometric parameters, air bubble injection, vortex generators, tape inserts, micro-surfaces, and baffles in tubular HX, as well as straight, zigzag, and airfoil channels in printed circuit heat exchangers (PCHE) and phase change materials. The innovative techniques supported by pictorial representations highlighting their salient features are summarized in tables. This review serves as a valuable resource for designers and engineers looking to develop HX designs tailored to specific system requirements. By presenting the advantages and limitations of various heat transfer enhancement techniques, the review facilitates the selection of the most suitable configurations for the development of more efficient, cost-effective, and sustainable HX designs. Heat Enhancement Using Geometric Parameters In this section, the heat transfer characteristics improved by changing the geometrical configurations in the tubular HX are reviewed. This approach focuses on optimizing the surface area and flow patterns within the HX to enhance heat transfer rates. Tubular HXs are the most important part of the power plant and process industry, where effective heat transfer is essential for achieving desired operational outcomes. The shell and tube and the double pipe HXs are the most commonly used heat transfer equipment in the industry [38]. However, these types of HX require large spaces and are less efficient in heat transfer. To address these limitations, a lot of researchers are working on new types of HXs that are compact, efficient, and easy to maintain as summarized in Table 1.

Figure 1. A single module of twined tube HX: (a) tube-1, (b) tube-2, (c) twined tubes, and (d) top view. [From Khan et al. [39], with permission from Elsevier.]

The design outperforms straight tube designs, achieving a peak performance improvement of 60%.

Enhances the heat transfer from ice by 30% compared to smooth tubes.

Experimental

Convective heat transfer is improved due to increased velocity fluctuations in the pipe, resulting in a higher ∆T.

Shows the highest Nu and friction factor compared to OCT and HCT and is 60% larger Nu compared to smooth tubes.

Exhibits the lowest Nu and friction factor compared to TCT and HCT.

Lower friction factor and high Nu show the highest PEC of 1.09 compared to TCT and OCT.

With a -30° rotation angle at the tube inlet, the PEC is improved by 11.14%. The maximum PEC of 1.25 is achieved with 13 twists and a Re of 1000.

Experimental

Is effective within a range from 1000 to 2250 Re range but is less applicable below 1000 Re. The maximum increase in PEC of 2.71 is observed.

At an eccentric tube lead length (s) of 50 mm and eccentric distance (a) of 4 mm, achieves a maximum PEC of 2.43.

EHTs with a pitch of 25 mm, eccentricity of 1.5 mm, and tube diameter of 6 mm show superior performance for heat transfer enhancement in sCO2 HXs.

6. LPM

CuO-water (0.25% 2 to 6 LPM Experimental The highest PEC of 1.24 to 1% vol. fraction) is achieved with a hole diameter ratio of 6 and 6 LPM air flow rate.

Shell and tube Single Water HX segmental baffle, disc and ring baffle, and disc and ring with circular holes

10x10-5 m3/s Experimental The disc and ring with circular holes exhibit the highest effectiveness, ranging from 202% to 231%. The maximum PEC of 4.5 is achieved at a mass flow rate of 0.23 kg/s.

Hshte

1 to 5 LPM Experimental An increase in air bubble volume fraction increases the thermal performance of the HX.

2 to 5 LPM Experimental Increases heat transfer by 156%. The PEC peaks at 1.28.

Figure 9. Wavy strip turbulator. [From Zhao et al. [49], with permission from Elsevier.]

Figure 10. Perforated wavy strip turbulator. [From Luo et al. [50], with permission from Elsevier.] side of the fluid. Experiments are conducted with varying cold-water flow rates and different baffle configurations. Figure 11 illustrates three configurations of baffles for shell and tube HX: (a) Single segmental baffle, (b) Disc and ring baffles, and (c) Disc and ring baffles having circular holes. The results show that the effectiveness improves with an

increase in cold-water mass flow rates. With an air injection flow rate of 10-5 m3/s, an improvement of 202% to 231% in effectiveness is observed for the configuration (c). The maximum PEC of 4.5 is achieved at a mass flow rate of 0.23 kg/s [52]. Zhou et al. introduce air bubbles from a 3 mm diameter tube into the horizontal shell and helically coiled

tube exchanger (HSHTE). The increase in volume fraction results in a 4.5-fold increase in the number of thermal units, a 2.4-fold increase in first-law efficiency, and a 12.5-fold increase in exergy destruction [53].

Figure 11. Three baffle configurations for shell and tube HX (a) single segmental baffle, (b) disc and ring baffle, and (c) disc and ring with circular holes baffle. [From Al-darraji et al. [52], with permission from Elsevier.]

Heat Enhancement Using Vortex Generators In this section heat enhancement techniques based on the use of extended surfaces positioned in the path of flow are reviewed. These additional surfaces or vortex generators (VG) disturb the uniform flow of the fluid, reducing boundary layer thickness and increasing heat transfer. The method involves strategically positioning the VGs to maximize turbulence and improve heat transfer efficiency. There are different types of VGs depending upon their size and shape, as mentioned in Table 3. Rastan et al. examine the use of longitudinal VGs in mini-channel HXs. The experimental and numerical studies exhibit a three-times improvement in the convection heat transfer as compared to without VRs [54]. Aridi et al. study the trapezoidal VR in a concentric tube HX. Through

Experimental and Improves convective heat transfer by up to computational threefold.

The 2.5 mm wide square-cut turbulator demonstrates a 271.7% increase in Nu value, while the hot water temperature difference rises by 113.7%.

The ring inserts show the highest increase in the Nu, achieving 2.92 times that of the smooth pipe.

Decreasing the inter-turbulator distance increases both the Nu and friction factor. Among different pitches, the 48 mm pitch demonstrates the highest PEC of 2.71.

Nu and friction increase by 52.3% to 65.8% and 92.7% to 202.3% across the range of Re. The maximum PEC of 1.335 is achieved at a pitch ratio of 0.5.

Nu and friction factor increase by 43.3% to 57.9% and 78.7% to 263.9% across the range of Re. Computational

Smaller offsets exhibit superior comprehensive performance. Larger attack angles lead to increased heat transfer intensity and greater flow drag force.

Experimental and The flexible VG enhances Nu, mixing index, and computational PEC by 18.46%,16.86%, and 42%, respectively, and reduces the friction factor by 42.33%, compared to rigid VG.

Experimental

The maximum PEC of 1.91 is achieved at a Re of 6000, a thickness of 0.3, and a pitch ratio of 1.11.

numerical simulation, they explore multiple positions of the VG in the annulus and tube region. The best performance is observed when VGs are inserted into the inner wall of the tube (Figure 12). This configuration leads to a 97% enhancement of the heat transfer ratio at a Re of 8000 for the cold fluid and 2000 for the hot water. A PEC of 2.10 is achieved under these conditions [55]. Hassan et al. test new copper VGs in double-tube HXs. The experimental results show that reducing the width of the turbulator strip fins improves heat transfer characteristics in turbulent flow regimes. The 2.5 mm wide square-cut turbulator (Figure 13), shows a 271.7% increase in the Nu, and a 113.7% increase in hot water temperature difference compared to the smooth tube [56]. Vahidifar et al. perform a numerical simulation on different shapes of turbulators in a pipe with an equal blocking area (Figure 14). The ring-shaped turbulator shows a significant impact on enhancing heat transfer characteristics inside the tube. The highest increase in the Nu with the ring,

disk, and O-ring is 2.92, 1.8, and 2.19, respectively. The overall performance increases by 16.2% at a Re of 24,000 [57]. Salhi et al. introduce small baffle turbulators to enhance heat transfer from hot air to cold water (Figure 15). The numerical simulation shows that reducing the inter-turbulator distance increases the Nu and friction factor. The turbulators with pitches of 260 mm, 108.57 mm, 67.27 mm, and 48 mm yield PECs of 2.36, 2.42, 2.53, and 2.71, respectively [58]. Zhu et al. introduce two bionic rib designs: triangular concave rib (TCR) and triangular convex rib (TVR) (Figure 16). The numerical simulation using the SST k-ω model shows that bionic ribs stimulate the development of a longitudinal and transverse swirl, thereby enhancing fluid mixing and temperature distribution. For a pitch ratio of

Experimental

The laminar flow regime shows a maximum PEC of 1.9 at a heat flux of 3 kW/m2.

Experimental

Twisted tubes with inserts show higher Nu and friction factors. The maximum PEC of 2.28 is attained using a lower twist ratio tape in the tube at a minimal Re.

SSTT shows a maximum PEC of 1.22 with a 2 mm gap distance. The SSTT and HCW combination shows a maximum PEC of 1.31.

Enhances heat transfer but is unfavorable during laminar-turbulent regime transitions.

Experimental

Imperforated twisted tape yields a maximum PEC of 1.55 in laminar flow at Re 2000.

Y-shape inserts with hexagonal, trapezoidal, kite, and hybrid shapes geometries and PI

The trapezoidal configuration with a PI of 20% achieves the highest PEC of 3.68 with air as the working fluid.

Table 5. Heat enhancement using tape insert with nanofluid Author

TiO2 nanofluid with inserts shows a maximum PEC of 1.55 in the laminar region.

Computational Carbon nanotubes/ H2O, Al2O3/H2O, and SiO2/H2O

Carbon nanotubes/H2O exhibit the maximum Nu of 143, followed by SiO2/H2O at 140.12, and Al2O3/H2O at 136.68. A 3% volume concentration of nanofluid is used.

The combinations of DTTI with THNF (Al2O3 + Graphene + MWCNT) are the most effective.

The maximum increase in the Nu is 11.04% when 0.2 wt.% nanofluid is used instead of water, and up to 105% with the addition of RRTT compared to a plain tube.

compared to a smooth tube. The maximum PEC of 1.22 is achieved by the SSTT turbulator with a 2 mm opening distance. Combining both turbulators increases heat transfer and pressure drop by 2.42 times and 9.6 times, respectively, thereby representing a PEC of 1.31 [65]. Forooghi et al. explore the impact of non-symmetrically distributed bumps on the inserts, as shown in Figure 22. The numerical simulation finds that the non-symmetric insert distribution is unfavorable during laminar-turbulent regime transitions. Moreover, the study shows that the flow is fully laminar at 1000 Re, transitions at 1500 Re, and becomes turbulent at 2710 Re. The presence of a bypass region diminishes the potential for heat transfer enhancement [66]. Heeraman et al. investigate the impact of dimples on twisted tape inserts, as shown in Figure 23. The study examines dimples and protrusions with varying

Figure 19. Wavy tape insert for circular pipe. [From Vishwakarma et al. [63], with permission from Elsevier.]

diameters and diameter-to-depth (D/H) ratios over a tube length of 1500 mm. The results show that the Nu reaches its peak at a D/H ratio of 3, with a value of 111 at a Re of 13987. The lowest value of friction factor is achieved at a D/H of 4.5, measured at 0.1033 at a Re of 13987. The maximum PEC of 1.088 occurs at a dimple diameter of 4 mm with a D/H of 4.5 at a Re of 6180 [67]. Farhadi et al. investigate the perforated and imperforated twisted tape insert (Figure 24). The best performance is attained in the laminar regime at a Re of 2000 using an imperforated twisted tape with a twist ratio of 15.73. The highest PEC in the laminar regime is 1.55, and in the turbulent regime, it is 0.99 [68]. Ifraj et al. examine Y-shape inserts with hexagonal, trapezoidal, and kite designs, as well as a combination of all three shapes, using perforation indexes (PI) of 10%, 20%, and 30% (Figure 25). The numerical simulations are performed using air, with Re varying from 3000 to 21000. The trapezoidal designs show a maximum Nu of 225 and a minimum friction factor of 0.07 with a PI of 20% at Re of 21000. Moreover, the trapezoidal insert achieves the highest PEC of 3.68 [69]. Some of the researchers study the tape inserts along with nanofluid to enhance the heat transfer. Gnanavel et al. modify the existing geometry of the twisted tape insert by adding the rectangular cuts (Figure 26). The thermal conductivity of the fluid medium is increased using nanofluids. The results show that the heat transfer increases with the increase in flow velocity. The TiO2 nanofluid is found to be the most effective option for the HX in combination with a twisted tape

Figure 20. Straight and twisted tubes with twisted tapes of lower and higher twist ratios. [From Farnam et al. [64], with permission from Elsevier.]

Figure 24. (a) Imperforated and (b) perforated twisted tape insert. [From Farhadi et al. [68], with permission from Elsevier.]

Figure 21. Geometric configuration of the special shape twisted tape turbulator (SSTT) and the helical coiled wire (HCW) turbulator. [From Luo et al. [65], with permission from Elsevier.] Figure 25. The geometric configuration of various metallic inserts: (a) hexagonal, (b) trapezoidal, (c) kite, and (d) hybrid. [From Ifraj et al. [69], with permission from Elsevier.]

Figure 22. Non-symmetric distribution of inserts. [From Forooghi et al. [66], with permission from Elsevier.]

Figure 23. Twisted tape insert with variable hole diameter and depth for dimples. [From Heeraman et al. [67], with permission from Elsevier.]

insert yielding a maximum PEC of 1.55 in laminar flow [70]. Azhari et al. numerically evaluate twisted tape inserts, using the CuO/water-based nanofluid. The study finds that Nu and frictional entropy generation increase, whereas friction factor and thermal irreversibility decrease with an increase in Re. At a Re of 20000, the maximum Nu is 285.8 for the twisted tape insert and 156.1 for the plain tube. The results indicate that nanofluids and twisted tapes decrease the exergy losses and enhance heat transfer performance in HXs [71]. Saini et al. explore the use of twisted tape inserts with nanofluid. The results show a 6.03%, 16.74%, and 6.74% increase in the overall heat transfer coefficient with a 3% volume concentration of carbon nanotubes, Al2O3, and SiO2 nanofluids in water, respectively. The maximum Nu is found to be 143 for carbon nanotubes, 136.68 for Al2O3, and 140.12 for SiO2 [72].

Figure 26. Twisted tape with rectangular insert. [From Gnanavel et al. [70], with permission from Elsevier.]

Figure 27. Geometric configuration of twisted turbulator insert (TTI), perforated twisted turbulator insert (PTTI), and dimpled twisted turbulator insert (DTTI). [From Kumar et al. [73], with permission from Elsevier.]

Kumar et al. study the compact air HXs using waterbased tripartite hybrid nanofluids (THNFs) with multiple inserts like twisted turbulator insert (TTI), perforated twisted turbulator insert (PTTI), dimpled twisted turbulator insert (DTTI) as shown in Figure 27. The study shows that the nanofluids alone do not significantly enhance the performance of HX, but the combination of THNFs (Al2O3 + Graphene + MWCNT) with turbulator inserts improves thermohydraulic performance at a lower Re. The DTTI with THNF is the most efficient for heat transfer enhancement, while the Al2O3 + CuO + Graphene combination of THNF should be avoided due to higher pressure drop [73]. Das et al. numerically examine the triangular and rectangular ribbed twisted tape (TTT and RRTT) inserts (Figure 28) with Therminol55(TH55)/ MXene + Al2O3 nanofluid. An increase of 11.04% in the Nu is observed using the nanofluid as compared to water. The combination of nanofluid with RRTT increases the Nu up to 105% compared to a plain tube. The highest PEC of 1.67 is achieved with the combination of TH55 + Al2O3 and RRTT insert [74]. Heat Enhancement Through Surface Modifications Heat enhancement is achieved by creating microstructures or fins on the heated surface, which induce turbulence in the fluid flow and reduce the thermal boundary layer as mentioned in Table 6. This approach focuses on designing and testing these microstructures or fins through experimental and numerical methods to enhance PEC. There are different methods of creating microstructure on the surface, Nguyen et al. use electrochemical etching to create micro-surfaces on the plate of the HX. This method produces a uniform structure with an accurate thickness. The results show that at a roughness of 1.1 μm, there is a 10% to 18% enhancement in the overall heat transfer coefficient,

Figure 28. Triangular and rectangular ribbed twisted tape (TRTT& RRTT) inserts. [From Das et al. [74], with permission from Elsevier.]

Experimental

The combined technique outperforms single techniques, with enhancements of 51% to 92% in pool boiling.

Experimental

Microstructure on tube bundles outperforms smooth tube bundles by over 100%.

More effective as the Re increases but consumes more power, with a maximum increase in effectiveness of 84% for Cr = 1 and 66% for Cr = 0.5.

Inward-facing elements, a wire coil, and spiral corrugations

Inward-facing ellipsoidal elements show the highest performance with a 22 to 42% increase in heat transfer alongside a 41 to 97% rise in pressure drop across the range of Re.

The gyroid structure improves the HX effectiveness by 35% with a maximum pressure drop of 4 Pa compared to the primitive structure.

The gyroid structure shows better PEC compared to offset strip fin, 15° zigzag channel PCHE, circular tube, and chevron-type plate HX and is comparable to offset strip fin HX.

At 14500 Re, increasing the fin angle from 2° to 10° results in a 13% increase in Nu and a 51% increase in pressure drop.

The presence of three slots on the fins reduces the drag coefficient by 23% and increases the Nu by 76%. Oscillation of tubes and slotted fins further improves heat transfer up to 2.5 times.

A 14% improvement in the Nu is observed at an equal friction factor and a 4-time reduction in the friction factor is noticed at equal Nu.

Adding nanoparticles enhances the effectiveness by a maximum of 7%. Reducing the speed of hot fluid from 6 m/s to 2 m/s increases effectiveness by approximately 33.3%.

known as triply periodic minimal surfaces (TPMS) (Figure 33). Numerical simulation shows that the complex topology of the gyroid structure increases the effectiveness of the compact HX by 35% compared to the primitive structure and exhibits a maximum pressure drop of 4 Pa [80]. Yan et al. use a 3D printer to develop a Gyroid-structured from stainless steel using a laser melting technique (Figure 34). Experimental and numerical evaluations are conducted using water as the working medium. The heat transfer performance of the Gyroid-structured HX exceeds that of

the offset strip fin, 15° zigzag channel printed circuit heat exchanger (PCHE), circular tube, and chevron-type plate HX. The Gyroid-structured HX shows a higher pressure drop with an increase in Re, resulting in a PEC comparable to the offset strip fin HX and higher than the rest of the HX designs [81]. Liu et al. develop a reversed trapezoidal profile fins for staggered tube HX (Figure 35). The model is tested with different fin angles (2° to 10°), lengths (0.5D to 2D), and thicknesses (1 mm to 5 mm) over a range of Re (5500 to

Figure 31. Microchannel square in-line pin-fins HX. [From Alnaimat et al. [78], with permission from Elsevier.]

Figure 29. Different images of microstructured surfaces on copper plate. [From Khalaf-Allah et al. [76], with permission from Elsevier.]

Figure 32. Different types of pattern structures: (a) ellipsoidal inward-facing elements, (b) a wire coil, and (c) spiral corrugations. [From Dizjeh et al. [79], with permission from Elsevier.]

Figure 30. Semi-closed microstructure (a) top view and (b) side view. [From Moharana et al. [77], with permission from Elsevier.] 14500). The numerical simulation shows that the larger fin lengths achieve higher heat transfer rates and lower flow resistance. In contrast, increasing fin thickness leads to a larger pressure drop than Nu improvement. For a

Re of 14500, an increase in fin angle from 2° to 10° leads to a 13% increase in Nu and a 51% increase in pressure drop [82]. Soheibi et al. study the oscillation of tubes with slot fins on the tube to enhance heat transfer (Figure 36). Three slots on the fins reduce the drag coefficient by 23% and increase the Nu by 76%. The tube oscillation increases the heat transfer by 3% as compared to a fixed tube. Adding slotted fins to the oscillating tube bank increases heat transfer up to 2.5 times [83]. Searle et al. perform an experimental study of pin fin in a helical pattern inside a tube, as shown in Figure 37. The results show a four-time reduction in the friction factor at the equal Nu and a 14%

Figure 33. Triply periodic minimal surfaces (TPMS) structures-based compact HXs: (a) gyroid, and (b) primitive. [From Alteneiji et al. [80], with permission from KeAi.]

Figure 34. Geometric configuration of gyroid-structured HX: (a) computational domain, (b) complete numerical model, and (c) cut gyroid-structured. [From Yan et al. [81], with permission from Elsevier.]

Figure 35. Reversed trapezoidal profile fins on the HX tube. [From Liu et al. [82], with permission from Elsevier.]

Figure 36. Geometric configuration of slots on the tube fin. [From Soheibi et al. [83], with permission from Elsevier.]

Figure 37. Helically coiled fin tube design. [From Searle et al. [84], with permission from Elsevier.]

Figure 38. Finned tube HX (solid side). [From Tariq et al. [85], with permission from Elsevier.]

improvement in the Nu at an equal friction factor compared to the smooth tube [84]. Tariq et al. develop a new design for a finned HX (Figure 38) and use graphene oxide nanoparticles in the working fluid. The results show that reducing the speed of the hot fluid from 6 m/s to 2 m/s increases the effectiveness of the HX by 33.3%. Moreover, the study finds that the insertion of nanoparticles at a concentration of 0.1% enhances the effectiveness of the HX by a maximum of 7% [85]. Heat Enhancement Using the Baffle Another method to enhance heat transfer involves introducing baffles into the fluid flow path as summarized in Table 7. The baffle plates produce a rotary flow pattern, enhancing the axial fluid flow and boosting the heat transfer rate [86]. In this approach, different shapes, angles, and positions of the baffles are analyzed to determine their effect on heat transfer enhancement. Mazharmanesh et al. study heat enhancement on vertically heated surfaces using a flexible baffle. The moving baffle produces turbulence in the fluid with its to-and-fro motion, resulting in vortices in the natural convection path. The 2D numerical simulation shows that the heated wall with a flexible baffle enhances convection heat transfer by 12% compared to a rigid baffle. They also study the effect of attaching a baffle on the heated surface at different elevations from the bottom. The results show that changes in the normalized baffle elevation to 0.2, 0.3, and 0.5 across the range of Re result in 8% to 10%, 9% to 12%, and 7% to 8% enhancement in convection heat transfer, respectively [87]. Boonloi et al. study “X” and “V” shaped baffles in the laminar flow, as shown in Figure 39. Numerical simulations are performed to find the thermal-hydraulic performance of the two baffles. The V-shaped baffle shows the highest PEC of 4.07 [88]. Rahman et al. explore different angles of deflector baffle plates fitted in a smooth air duct (Figure 40). The experiment shows that increasing the angle of the baffles decreases the flow velocity and pressure drop. The results indicate that baffle angles of 30°, 40°, and 50° resulted in 70.8%, 66.3%, and 61.3% increases in fluid velocity

compared to the smooth duct. The 30° baffle shows the highest PEC of 1.449, while the 40° and 50° baffles show PEC values of 1.148 and 1.141, respectively, compared to the smooth duct [89]. Abidi et al. use angled baffles to enhance heat transfer in a shell and tube HX. As the baffle angle increases, the Nu also increases. The maximum PEC of 1.55 is achieved with a baffle angle of 180° at a Re of 7500 [90].

Figure 39. Geometric configuration of X-V baffles in the HX. [From Boonloi et al. [88], with permission from Elsevier.]

Figure 40. Pictorial view of the deflector baffle plate. [From Rahman et al. [89], with permission from Elsevier.]

Replacing rigid baffles with flexible ones enhances heat transfer by up to 12%.

Experimental

30° baffle shows the highest PEC of 1.449 compared to 40° and 50° baffle.

The 180° baffle angle exhibits the maximum PEC of 1.55 at Re of 7500.

Fe3O4-MWCNT water hybrid nanofluid with a 4% volume fraction

Printed Circuit Heat Exchanger PCHEs are designed to have a higher heat transfer rate area per unit volume compared to tube-type HXs. The higher power density of the PCHE results in the smaller size of the HX [91]. This approach focuses on optimizing the flow paths and adjusting geometric parameters to enhance heat transfer efficiency while minimizing the overall volume and pressure drop. There are different types of PCHE designs based on the geometry configuration, as described below: Straight Channel Table 8 shows the summary of the straight channel PCHE. In the straight channel PCHE, a straight flow path is configured with multiple possibilities for arranging the cold and hot plates in the assembly. Pandey et al. study the cooling of sCO2 with water using various banking configurations as shown in Figure 41. They use a numerical tool

to study the PCHE for a range of Re for the hot and cold fluids. The results reveal that the hot-cold-hot assembly of the plates provides the smallest possible volume (0.74 m3) and the highest effectiveness of 99.7% compared to other banking configurations [92]. Zhong et al. introduce a novel PCHE with straight rectangular micro-channels for sCO2 (Figure 42) and study the effect of primary and secondary fluid inlet temperatures on the system’s heat transfer. Experiments are performed with a 10.3% rise in primary fluid inlet temperature. As a result, an increase of 89.0% is observed in the heat transfer rate. It is found that the effectiveness of the system is a function of the inlet temperature of the secondary fluid and increases by 23.3% with a 5.9% rise in the cold fluid inlet temperature [93]. Xu et al. investigate a double semicircular straight channel PCHE for Lead-Bismuth Eutectic (LBE) and sCO2. The overall heat transfer coefficient is significantly enhanced with the change in the mass flux of the sCO2 as compared to that

Table 8. Heat enhancement using straight channel PCHE Author

The hot-cold-hot banking configuration provides the smallest possible volume (0.74 m3) and the highest effectiveness of 99.7% compared to other banking configurations.

Experimental

Heat transfer rate increases by 89.0% with a 10.3% rise in primary side inlet temperature, while effectiveness rises by 23.3% with a 5.9% increase in secondary side inlet temperature.

The smaller tube diameter and reduced wall thickness improve overall heat transfer efficiency in PCHE.

Increasing the channel length is favorable for CO2 and unfavorable for LBE due to higher pressure drop. At a maximum channel length of 900 mm, LBE and sCO2 exhibit PEC values of 0.9 and 1.4, respectively.

Heat transfer peaks near the pseudo-critical point and decreases in film boiling.

PCHE shows a power density of 181 MW/m³, which is eight times higher than that of shell-andtube HX.

A 66% improvement in the heat transfer rate is achieved without any increase in the pressure drop of the HX.

NACA 0025 airfoil fin, rectangular fin, rhombic fin, and elliptical fin

The rhombic fin demonstrates the highest heat transfer rate and the lowest synergy field.

The Nu increases with the increase in velocity, while molten salt exhibits a higher pressure drop than sCO2.

Achieves three times the heat transfer rate compared to a shell and tube steam generator.

of LBE. Additionally, geometric parameters such as smaller tube diameter and reduced wall thickness also increase the system’s heat transfer [94]. Li et al. analyze the heat transfer performance of a straight-channel PCHE for sCO2 and LBE. An increase in the sCO2 mass flow rate enhances the overall heat transfer performance, and higher pressure on the sCO2 side improves thermo-hydraulic performance. Sensitivity to geometric parameters shows that increasing the channel width increases the heat transfer but also increases the HX volume. The channel length is favorable for sCO2 but unfavorable for LBE due to the higher pressure drop. At a maximum channel length of 900 mm, LBE and sCO2 exhibit PEC values of 0.9 and 1.4, respectively [95]. Li et al. investigate a 1.2 mm diameter semicircular channel for supercritical methane vaporization at the pseudo-critical point. Their numerical simulation reveals a peak in heat transfer at this point, followed by a decrease due to film boiling [96]. Xu et al. design a semicircular channel PCHE for a 280 MWth lead-bismuth HX for advanced nuclear reactors (Figure 43). The design demonstrates a power density of 181 MW/m3, which is eight times greater as compared to shell and tube HX. Larger LBE inlet velocities or smaller channel diameters resulted in a reduced total volume and mass of the PCHE [97]. Lee et al. introduce a novel design for a PCHE using a mathematical model to optimize the layout of the channels. The model replaces the straight wall channel design with a dotted channel design, allowing cross-flow along the flow path (Figure 44). Numerical simulation shows a 66% improvement in heat transfer rate without any increase in the pressure drop of the HX [98]. Tong et al. study NACA 0025 airfoil fin, rectangular fin, rhombic fin, and elliptical fin design in the molten salt and sCO2 PCHE (Figure 45). Numerical simulations reveal that the heat transfer rate increases with velocity. Among the designs, the rhombic fin exhibits the highest heat transfer rate, followed by the elliptical and NACA 0025 airfoil

Figure 41. Different configurations of hot and cold plates in the PCHE. [From Pandey et al. [92], with permission from Elsevier.]

fins, which show similar values, with the rectangular fin showing the lowest. Additionally, the rhombic fin design displays the lowest synergy field. The molten salt achieves a maximum heat transfer rate of 210 kW, compared to 45 kW for sCO2 at a velocity of 0.5 m/s with the rhombic fin [99]. Lao et al. introduce discontinuous fins into the flow path of molten salt and sCO2 PCHE (Figure 46). Numerical simulations demonstrate that these discontinuous fins cause

Figure 42. Novel PCHE with rectangular channels: (a) shape of the channels, (b) configuration of the different plates, and (c) flow path of the cold and hot fluid. [From Zhong et al. [93], with permission from Elsevier.]

flow separation and the generation of vortices within the fluid, leading to fluctuations in the heat transfer coefficient. Both the Nu and pressure drop increase with the velocity of the fluid; however, molten salt exhibits a greater pressure drop compared to sCO2 [100]. He et al. explore printed circuit steam generators for high-temperature applications

for molten salt thermal power systems (Figure 47). They improve the design by utilizing a combination of semi-circular and circular channels for a 50 MWth printed circuit steam generator. Numerical simulations show that this design achieves three times more heat transfer compared to a shell and tube steam generator [101].

Figure 43. Semicircular PCHE design for a lead–bismuth fast reactor. [From Xu et al. [97], with permission from Elsevier.]

Figure 46. Discontinues fin in the PCHE: (a) side view, (b) fin top view, and (c) computational domain. [From Lao et al. [100], with permission from Elsevier.]

Figure 44. Geometric configuration of a novel PCHE design utilizing dual-fluid topology optimization. [From Lee et al. [98], with permission from Elsevier.]

Figure 47. Cross-sectional view of the unit cell of the printed circuit steam generator for a molten salt reactor. [From He et al. [101], with permission from Elsevier.]

Figure 45. Geometric configuration of fin designs: (a) NACA 0025 airfoil fin, (b) rectangular fin, (c) rhombic fin, and (d) elliptical fin. [From Tong et al. [99], with permission from Elsevier.]

Zigzag Channel Recently, researchers have designed the PCHE channels in a wavy or crisscrossed pattern to enhance the disturbance in the fluid flow path and consequently increase heat transfer, as summarized in Table 9. Liu et al. experimentally investigate zigzag channels for sCO2 for Gen-IV nuclear reactors (Figure 48). Experimentation is performed on a 30° zigzag angle, 7.24 mm pitch, and 1.95 mm diameter of the channel for Re 3201 to 43258 and Pr 1.1 to

Experimental

Enhances heat transfer by two times and friction factor by seven times compared to straight channels

The HX size is reduced by three times, with a slight decrease in cycle efficiency due to the higher pressure drop.

Heat transfer improves by reducing water side inlet temperature and sCO2 working pressure.

A novel cellular arrangement enhances overall heat transfer coefficients by 8.6% compared to traditional designs.

The PEC peaks at 1.45 with a bend angle of 35° and adding a straight section at the bend further enhances it to 1.5.

The addition of a straight section at each bend, decreases the friction factor by 33.1% to 84.7%, and Nu by 3.6% to 30.3%, resulting in improved PEC with a maximum enhancement of 45.9%.

A 120° zigzag channel combined with a straight channel, exhibits a higher performance with a 12.6% increase in heat flux compared to straight channels on both sides.

Experimental

The elliptical zigzag channel shows a friction factor of 3 to 5 times higher for molten salt and higher Nu compared to a straight circular channel.

Two-way corrugated with backward/forward-facing channels with an amplitude of 0.8 and angle of attack of 60° achieve a maximum Nu improvement of 78% and the highest PEC of 1.43.

Experimental

The airfoil fin exhibits a 1/5 times lower pressure drop than that of the zigzag channel.

Experimental and The PEC of the airfoil is 6.6% to 15.5% and 12.8% to 27.8% computational higher compared to the straight and zigzag channels.

The performance of the airfoil is better than straight PCHE, and zigzag PCHE, and it achieves a 21% reduced volume compared to zigzag PCHE.

Channels with consistent distributions exhibit enhanced overall performance compared to inconsistent distributions. Airfoil fin angles of 15° to 25° demonstrate the highest performance.

Increasing airfoil fin thickness improves heat transfer but weakens flow characteristics. Fin-5 (Wf =1.5 mm, Df =1.2 & Rf =0.26 mm) outperforms Fin-3 (Wf =1 mm, Df =1.2 & Rf =0.26 mm) by 2.0–6.3%.

The novel DAVG increases the local heat transfer coefficient by up to 120%, outperforming traditional DASVG and NACA0024.

Resulting in 25.5% to 30.8% higher Nu and over 10% increase in PEC.

Integrating airfoil fins with rectangular VGs enhances PEC by 5.95% to 19.47%.

Flow-induced pitching of airfoils in rectangular channels results in the highest PEC of 1.4, which is 6% higher than that of active ones.

The novel header reduces flow non-uniformity by 39.4% and 61.8% at different operating conditions while enhancing overall performance by 5% and 8.5% compared to free and straight channel manifolds.

following airfoil fin design: Wf =1.5 mm, Df =1.2 mm & Rf =0.26 mm; Wf =1 mm, Df =0.6 mm & Rf =0.26 mm and Wf =1 mm, Df =1.2 mm & Rf =1.46 mm airfoil fin channels, which offer PEC improvements of 2.0% to 6.3%, 0.9% to 5.6%, and 0.13% to 1.81% across the range of Re, respectively, higher than the Wf =1 mm, Df =1.2 & Rf =0.26 mm airfoil fin channel [119]. Liu et al. study three different airfoil fin PCHE configurations: delta winglet vortex generator (DWVG), delta airfoil vortex generator (DAVG), and delta airfoil staggered vortex generator (DASVG), with NACA0024 airfoil fins as a reference design as shown in Figure 62. The DAVG configurations display a 120% rise in the local heat transfer coefficient [120]. Wu et al. propose a novel crossed airfoil fin design for PCHEs in sCO2 applications (Figure 63). Numerical simulations show that the crossed airfoil fin induces a longitudinal vortex flow, disrupting the flow near the surface and enhancing heat transfer performance. The crossed airfoil fin design demonstrates a 25.5% to 30.8% higher Nu across the range of Re and a 10% increase in the PEC compared to the traditional airfoil fin design [121].

Li et al. combine airfoil fin channels with VG (HVG: 0.2 mm, LVG: 0.8 mm, and WVG: 0.04 mm) as shown in Figure 64. The combined design improves heat transfer, resulting in an increase of 5.95% to 19.47% in PEC compared to conventional airfoil fin channels [122]. Ke et al. study the pitching motion of airfoil fins in a rectangular channel. The study finds that after the Re reached 1000, a self-fluttering of the airfoil fin increases the Nu. The highest PEC of 1.4 is attained when the rotational axis is positioned at

Experimental

The 1% addition of biochar increases the energy storage by 32%.

The 2%, 5%, and 8% volume fraction of nanoparticles enhance the heat transfer by 8.5%, 9.3%, and 10.3%. The addition of a branch fin further enhances the heat transfer rate.

Experimental

The 1:3 of Mg-Al: PW-EG brick increases thermal storage efficiency by 95.6%. Replacing paraffin wax with Xylitol and Erythritol further increases the heat storage capacity.

Experimental

Wedge fin performs 12.8% better transfer heat rate than annular fin tube and 84.3% better than tube HX.

Figure 67. Finned tube HX design for PCM. [From Pignata et al. [127], with permission from Elsevier.]

Figure 68. Radial fin design for PCM. [From Arqam et al. [128], with permission from Elsevier.]

of 18%, reduces the melting time from 193 minutes to 163 minutes and the solidification time from 630 minutes to 425 minutes [130]. Joybari et al. develop a pillow plate HX for low-temperature thermal storage applications, using CO2 as the PCM (Figure 71). Numerical simulations show that a 25 mm center-to-center plate distance and aluminum as the construction material provide the best thermal performance at minimal operating costs [131]. Khader et al. propose a novel HX design integrated with PCM (Figure 72). Experimental results show that adding paraffin PCM to the HX increases heat discharge time by 438%. Numerical simulations reveal that replacing paraffin with ClimSel C58 increases storage capacity by 2.35 times [132]. Asgari et al. examine a branch-shaped fin design (Figure 73) combined with a hybrid nanofluid in PCM. Numerical results show that a 0.04 volume fraction of a hybrid nanofluid (Al2O3-Cu) with a thin fin design maximizes heat transfer [133]. Yazdani et al. design a lightweight square grid HX using a 3D printer (Figure 74). This compact HX uses decanoic acid as PCM. It exhibits a 32% increase in energy storage with a 1% biochar additive. However, further increases in biochar concentration cause particle settlement and reduce heat transfer [134].

Figure 70. Triplex tube HX with heat transfer fluid (HTF) flow channel. [From Abdulateef et al. [130], with permission from Elsevier.]

Figure 69. Thermal energy storage systems: (a) plate-type and (b) roll-bounded. [From Taghavi et al. [129], with permission from Elsevier.]

Figure 71. Pillow plate HX for the thermal storage application. [From Joybari et al. [131], with permission from Elsevier.]

Figure 72. HX design integrated with PCM. [From Khader et al. [132], with permission from Elsevier.]

Figure 74. Lightweight HX design for thermal storage system. [From Yazdani et al. [134], with permission from Elsevier.]

Figure 73. Branched shape fin design for PCM. [From Asgari et al. [133], with permission from Elsevier.]

Zhang et al. introduce a novel branch fin design for a triple tube HX, adding Al2O3 nanoparticles to paraffin to enhance heat transfer (Figure 75). Numerical simulations indicate that increasing the nanoparticle volume fraction in PCM by 2%, 5%, and 8% enhances heat transfer by 8.5%, 9.3%, and 10.3%, respectively. The branch fin further enhances heat transfer, with cases B, C, D, and E showing 83%, 80.7%, 80.8%, and 82.9% improvements, respectively [135]. Fan et al. combine magnesium-aluminum (Mg-Al) bricks with three different PCMs to enhance thermal storage capacity. Experiments and simulations show that a 1:3 Mg-Al to paraffin wax-expanded graphite (PW-EG) brick configuration improves thermal storage efficiency by 95.6% compared to PW-EG (Figure 76). Replacing paraffin wax with Xylitol and Erythritol further increases the heat storage capacity [136]. Safari et al. propose a novel wedge fin and annular fin HX tube design (Figure 77). Experiments show that the wedge fin tube HX outperforms the annular fin tube HX by 12.8% and the standard tube HX by 84.3% in heat transfer rate [137].

Figure 75. Various layouts of branch fin triple tube HX. [From Zhang et al. [135], with permission from Elsevier.]

Figure 76. Combination of Mg-Al and PW-EG brick for the thermal storage unit. [From Fan et al. [136], with permission from KeAi.].

reactors. The printed circuit heat exchangers with zigzag channels exhibit a 50% enhancement in thermal performance compared to the straight channel. This comprehensive review of recent advancements in heat transfer techniques covers a wide spectrum of passive heat transfer techniques. It serves as a valuable resource for designers and engineers seeking to optimize heat exchanger designs for specific system requirements. It provides practical insights essential for advancing the next generation of heat exchangers across a wide range of industrial applications. By providing a detailed analysis of various heat enhancement techniques, this paper serves as a key reference point for future research and development of more efficient and effective heat exchanger systems.

Future Direction

Figure 77. Geometric detail of tube HX for PCM: (a) tube HX, (b) annular fin tube HX, and (c) wedge fin tube HX. [From Safari et al. [137], with permission from Elsevier.]

Conclusion

This paper explores the latest advancements in passive heat transfer enhancement techniques specifically designed for power plant applications. The main findings from this review are summarized below: No single technique is universally better as it highly depends on the specific requirements of the application, including the type of heat exchanger, the fluids properties, and space constraints. However, if we consider the overall effectiveness and flexibility, it is found from this review that gyroid structure, air bubble injection in shell and tube heat exchanger with disc and ring arrangement, v-baffle tubular heat exchanger, and tape insert with trapezoidal geometry are better choices for tubular heat exchangers. The gyroid structure is very complex and challenging to fabricate. The air bubble injection technique has the highest performance evaluation criteria of 4.5. However, it can’t be implemented in most of the applications due to process constraints. V-baffle within a tube has a performance evaluation criteria of 4.07 for air. The Y-shaped tape insert with a trapezoidal configuration has a performance evaluation criteria of 3.68. It is easier to implement in existing and new designs. The printed circuit heat exchangers offer high heat transfer efficiency, compact design, and suitability for high-pressure and high-temperature applications in advanced nuclear reactors. Printed circuit heat exchangers are prominent in advanced applications requiring compact size and high efficiency, making them a preferred choice for cutting-edge industrial applications and advanced nuclear

The future of heat exchangers lies in the development of highly efficient, compact, corrosion-resistant, and fouling-resistant designs that can withstand high temperatures and pressures. Achieving this requires focused research and innovation across several key areas. Innovative approaches and technologies are needed to increase the convection heat transfer coefficient, thereby improving the overall efficiency of heat exchangers and ensuring that future designs can operate at peak performance. Additionally, the search for new materials that are more resistant to corrosion, can withstand extreme temperatures and pressures and are robust to thermal shock is essential for extending the lifespan of heat exchangers and reducing maintenance frequency. Advancements in surface treatments and nano-coatings are also crucial, as they improve heat transfer efficiency by maintaining cleaner surfaces over longer periods. Once these issues are addressed, future heat exchangers could meet the demanding requirements of modern industrial applications, paving the way for more efficient, durable, and environmentally friendly thermal management solutions.

Dasvg DAVG

Brayton cycle Backward/forward-facing wavy channels Computational fluid dynamics Coiled flow reverser Delta airfoil staggered vortex generator Delta airfoil vortex generator Dimpled twisted turbulator insert Deionized water Delta winglet vortex generator Ethylene glycol Eccentrical helical tube Electromagnetic vibration Flat plate heat exchanger Helical coiled wire Helically corrugated tube

Hshte

Heat transfer fluid Horizontal shell and helically coiled tube exchanger HX Heat exchanger LBE Lead-bismuth eutectic LPM Liter per minute Mg-Al Magnesium-aluminum MWCNTs Multi-walled carbon nanotubes N2 Nitrogen OCT Outward corrugated tube PCB Printed circuit board PCHE Printed circuit heat exchanger PCM Phase Change Material PEC Performance evaluation criteria PI Perforation indexes PTTI Perforated twisted turbulator insert PW-EG Paraffin wax-expanded graphite PWST Perforated wavy strip turbulator RRTT Rectangular-ribbed twisted tape RWVG Rectangular winglet vortex generator sCO2 Supercritical carbon dioxide SHCHEs Shell and helically coiled heat exchanger SMRs Small modular reactors SSTT Special shape twisted tape turbulator STST Smooth tri-lobed spiral tubes TCT Transversely corrugated tube TCR Triangular concave rib THNFs Tripartite hybrid nanofluids TPMS Triply periodic minimal surfaces TTI Twisted turbulator insert TTT Triangular twisted tape TTST Twisted tri-lobed spiral tubes TVR Triangular convex rib VG Vortex generator WST Wavy strip turbulator Dimensional less number Cr Cp min/ Cp max D/H Diameter-to-depth e/d Dimensionless height Nu Nusselt number p/d Dimensionless pitch Pr Prandtl number Re Reynold number English Symbol A Amplitude (m) a Eccentric distance (mm) ax Elastic axis (mm) C Chord length (mm) Cp Heat capacity (J/kg.K) D Diameter (mm) Df Leading edge thickness of the airfoil (mm) e Eccentricity (mm) H Depth (mm) h Height (mm)

Height of vortex generator (mm) Straight RWVGs length (mm) Length of vortex generator (mm) Radius of the leading edge of the airfoil fins (mm) Pitch (mm) Thickness of the airfoil fins (mm) Longitudinal position of straight RWVGs (mm) Transverse position of straight RWVGs (mm) Offset between tube center and center of curved RWVG (mm)

Greek Symbol Ɵ Angle of attack (°) α Arc angle of curved RWVG (°) φ Attack angle of straight RWVGs (°) Φ Concentration of nanoparticle (%age) λ Period (mm) Ω Rotational speed (revolution/ minute)

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

Statement On The Use Of Artificial Intelligence

Artificial intelligence was not used in the preparation of the articl

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