Recent Progress in Heat Transfer and Fluid Flow Characteristics within Cross-Flow Heat Exchangers: A Passive Approach

Abstract

Efficient heat transfer in industry and commercial sectors is vital if saving energy is a concern. Heat exchangers are devices that are built for this aim. Here, cross-flow heat exchangers are analyzed to increase the heat transfer rate and reduce the pressure drop simultaneously. Cross-flow heat exchangers possess a simple geometry, yet a complex flow behavior. The reason lies behind the von Karman vortex structures that are created due to the presence of an initial row of tubes. This will affect the next stages of tube bundles and form an intricate thermo-hydraulic interplay. In general, there are various passive techniques to enhance the efficiency of cross-flow heat exchangers such as tube orientation, adding vortex generators, and addition of fins. Each technique has its strengths and weaknesses that try to address them individually in this chapter.

1. Introduction

Heat exchangers play a crucial role in various industries and commercial sectors by efficiently transferring heat between two or more fluids at different temperatures while keeping them physically separate [1, 2]. Their applications range from heating, ventilation, and air conditioning (HVAC) systems in buildings to large-scale industrial processes like power generation, chemical manufacturing, petroleum refining, and food processing [3, 4]. One of their primary functions is to either cool or heat fluids, depending on the process requirements, to maintain optimal operating conditions and enhance energy efficiency.

There are several types of heat exchangers commonly used across industries, each with its advantages and limitations. One widely used type is the shell and tube heat exchanger, which consists of a bundle of tubes enclosed within a cylindrical shell [5]. This design allows for high heat transfer efficiency and versatility in handling a wide range of fluids and operating pressures. Another common type is the plate heat exchanger, which utilizes a series of thin metal plates to facilitate heat transfer between fluids. Plate heat exchangers with high performance-to-volume ratios are compact, lightweight, and offer excellent thermal performance, making them suitable for applications where space and weight are limited [6].

Cross-flow heat exchangers are a specific type of heat exchanger where fluids flow in orthogonal directions. They consist of fluid flow across a bundle of tubes, leading to the formation of boundary layers, flow separation, and vortices.

These heat exchangers are widely adopted in the industry due to their simple and compact design, efficient heat transfer, reduced fouling, ease of maintenance, versatility across a wide range of operating conditions, cost-effectiveness, lower pressure drop, and flexibility in fluid selection [7]. Although cross-flow heat exchangers possess a seemingly uncomplicated architecture, their fluid dynamics exhibit intricate complexities owing to Von Kármán-like oscillations occurring in the wake of tubes and subsequent collisions. Notably, the flow and thermal characteristics within the initial row closely resemble those surrounding a solitary cylinder in cross-flow conditions [8]. Nevertheless, as the flow progresses through successive rows, instabilities manifest and propagate downstream. These instabilities precipitate heightened turbulence levels, consequently augmenting heat transfer rates within the inner rows [9]. Improving the efficiency of heat exchangers stands out as the paramount priority for maximizing their operational effectiveness across all varieties. A range of methods, both active and passive, can be employed to enhance heat transfer efficiency [10]. Numerous research studies have been conducted on improving the performance of cross-flow heat exchangers using passive techniques that require no extra pumping power compared to active methods [11]. These techniques include the use of vortex generators, winglets, or the addition of fins [12]. Additionally, implementing oval-shaped [13] or cam-shaped tubes [14], as shown in Figure 1, is another option for reducing the pressure drop. Since increasing the heat transfer rate is often accompanied by an increase in pressure drop, many researchers have investigated and optimized heat exchanger designs to achieve optimum performance.

Figure 1.

Pictures of oval-shaped tubes [15] and cam-shaped tubes [16].

Numerous review articles are published, aiming to consolidate passive techniques. When aiming for optimal heat exchanger selection, key considerations include design and operational parameters. This review exclusively delves into passive heat transfer techniques, addressing significant areas of interest and performance aspects. The following section discusses various types of heat transfer enhancement methods for cross-flow heat exchangers.

2. Passive heat transfer enhancement methods

Passive techniques implemented to enhance the heat transfer rate of a cross-flow heat exchanger include tube geometry orientation, vortex generators, and the addition of fins to the tube bundle. In this section, we will review the relevant research conducted in each area of passive cooling methods applied to cross-flow heat exchangers. Although many of the techniques mentioned earlier offer the common advantage of enhancing heat transfer rates, their drawbacks include increased complexity and manufacturing costs, as well as elevated pressure drops.

2.1 Tube geometry orientation

In the realm of thermal dynamics, there has been a concerted focus on innovating tube cross-sections to optimize heat transfer efficiency. While circular shapes have long been the norm, researchers have ventured into uncharted territories, exploring streamlined tube designs for improved energy exchange. Shapes ranging from elliptical to tubular configurations have been under scrutiny, each promising potential advancement in cross-flow scenarios.

A seminal study by Buyruk [17] delved into the intricacies of heat transfer dynamics amidst cross-flow scenarios, employing advanced numerical simulations through finite element methodologies. The experimentation involved manipulating tube pitch ratios across a spectrum, from 1.13 to 6, all while holding the Reynolds number constant at 400. The computational framework of this investigation is illustrated in Figure 2, shedding light on the intricacies of energy interaction in this domain.

Figure 2.

Computation domain of reference [17] study.

Beyond the confines of traditional circular cross-sections, this exploration aims to revolutionize thermal engineering practices, offering a pathway toward unprecedented efficiency gains. Through meticulous analysis and experimentation, researchers endeavor to unlock the full potential of alternate tube geometries, ushering in a new era of thermal optimization.

Merker et al. [18] undertook an experimental exploration concerning heat transfer and pressure drop in an offset tube array employing elliptical tubes. Their approach involved employing the mass transfer analogy, utilizing naphthalene as the medium for analysis. Oval tubes exhibited relatively lower pumping power requirements compared to traditional circular tubes, owing to their reduced frontal area.

In a parallel study, Aiba [19] delved into the heat transfer dynamics of straight round tubes positioned near a plane wall, covering a broad spectrum of Reynolds numbers from 8000 to 40,000. Nusselt numbers were derived by manipulating variables such as clearance between the walls and the ratio of tube spacing to tube diameter.

Likewise, Toolthaisong and colleagues [20] investigated the impact of attack angles on the thermal performance and pressure differential on the air side in a cross-flow heat exchange setup featuring an offset arrangement of flat tubes, illustrated in Figure 3.

Figure 3.

Various attack angles and aspect ratios studied in Ref. [20].

Circular tubes exhibit superior heat transfer rates compared to flat tubes, whereas flat tubes are favored for their ability to minimize pressure drop. This necessitates an investigation into the optimal aspect ratio and tube pitch ratio for flat tubes to optimize overall thermal performance. This examination should consider the integrated effects of both heat transfer and pressure drop amidst the tube clusters. In this context, Horvat et al. [21] examined different geometries of tube cross-sectional shapes to investigate their impact on heat transfer rates. Utilizing the documented temporal patterns of speed and heat, the average temporal values of the Reynolds figure, resistance factor, and Stanton factor were computed. Their findings indicated reduced resistance factor and Stanton factor for ellipsoid tubes in contrast to cylindrical counterparts. Nonetheless, with the augmentation of the hydraulic dimension, the distinction diminishes between the dual tube configurations. The universal pattern for resistance factor and Stanton quotient remains steady throughout all structures. Nevertheless, both the resistance factor and Stanton quotient dwindle with the surge in Reynolds magnitude. The results suggest that ellipsoidal and wing-shaped tubes exhibit decreased resistance factor and Stanton quotient in comparison to cylindrical tubes.

In an exhaustive study by Berbish [22], the intricate interplay of heat transfer and flow dynamics surrounding four staggered elliptic cylinders in crossflow was meticulously examined, as shown in Figure 4. These cylinders, characterized by an aspect ratio of 1:2, were meticulously arranged at zero attack angle relative to the incoming flow. Heat transfer data were meticulously gathered solely from the downstream elliptical cylinder, termed the test cylinder, under meticulously controlled heat flux conditions, with air serving as the primary test medium. An expansive range of Reynolds numbers, spanning from 4000 to 45,570, predicated on the major axis length (c), was meticulously scrutinized, alongside longitudinal spacing ratio (S_x/c) and lateral spacing ratio (S_y/b) meticulously varied between 1.5 and 4.0. Employing state-of-the-art computational fluid dynamics (CFD) software, intricate airflow patterns and temperature fields enveloping the cylinders were meticulously simulated, while meticulous flow visualization techniques were meticulously employed to vividly illustrate the complex flow features. Surpassing mere observation, the study unveiled remarkable insights, notably revealing that, barring Reynolds number 4000, the Nusselt number (Nu) of the downstream elliptic cylinder in the four staggered arrangement conspicuously surpassed that of the three inline cylinders across all meticulously tested spacing ratios and Reynolds numbers. Intriguingly, at lower Reynolds numbers (Re < 14,100), the average Nusselt number of the downstream elliptic cylinder in the three staggered arrangement markedly exceeded that of the downstream cylinder in the four staggered layout, across all meticulously tested spacing ratios. Yet, as the Reynolds numbers scaled to greater than or equal to 14,100, a conspicuous shift occurred, with the staggered layout boasting discernibly elevated mean Nusselt number values for the scrutinized elliptical cylinder. This meticulous delineation underscores the profound influence wielded by cylinder arrangement and Reynolds numbers on the intricate tapestry of heat transfer characteristics in such convoluted configurations.

Figure 4.

Computational domain and arrangements of elliptic tubes in Ref. [22].

2.2 Vortex generators

Winglets and vortex generators are designed to manipulate the flow field around surfaces, mitigating flow separation. By controlling the formation and behavior of vortices, they help maintain attached flow, which improves overall efficiency and performance. In heat exchangers, winglets or vortex generators can enhance heat transfer rates by promoting mixing and increasing convective heat transfer coefficients. They disrupt the boundary layer, enhancing the exchange of heat between the fluid and the solid surface.

Xu et al. [23] devised an elliptical fin-and-tube heat exchanger incorporating X-VGs (cross-shaped vortex generators) to enhance both hydraulic and heat performance, as shown in Figure 5. Their findings indicate that increasing the streamwise distance and the main flow angle leads to a reduction in thermal-hydraulic performance. However, there exists a non-linear correlation between longitudinal placement and thermal-hydraulic performance.

Figure 5.

Geometric depiction of 3d and 2d views of reference [23].

To determine the optimal layout of X-VGs, the researchers employed a Genetic Algorithm combined with a radial basis function neural network prediction model. The best layout was achieved with a streamwise space ratio of 1.995, longitudinal placement ratio of 0.387, and main attack angle ratio of 0.053. This layout yielded a remarkable 109.681% enhancement in performance evaluation criteria compared to the baseline case.

Torii et al. [24] undertook empirical investigations to analyze the heat transfer and pressure differential attributes linked with winglet-style vortex generators integrated into fin-tube heat exchangers. Both conventional upward flow and downward flow configurations were explored. Figure 6 illustrates a schematic depiction of the vortex generators. The Reynolds number was consistently kept within the range of 250–2100 throughout the experiment. Comparing the in-line and staggered configurations of the tube bank, there was observed a notable enhancement in heat transfer, approximately ranging from 10 to 20% and 10 to 30%, respectively. Moreover, an increase in pressure drop was also noted, with percentages varying from 8 to 15% for the inline configuration and from 34 to 55% for the staggered configuration.

Figure 6.

Configuration of winglet-type vortex generator on the fin surface-tube bank [24].

Saini et al. [25] conducted a numerical investigation on a three-dimensional fin-and-tube heat exchanger, incorporating curved delta winglets as vortex generators (Figure 7). The Reynolds number ranged between 400 and 2000. The study analyzed the pressure distribution, temperature distribution, and flow structure distribution of the heat exchanger with a four-in-line circular tube configuration, comparing it with a heat exchanger without vortex generators.

Figure 7.

Three-dimensional computational domain of fin-and-tube heat exchanger as described in Ref. [25].

The heat exchanger equipped with vorticity inducers experiences a significant increase in the Nusselt number, with enhancements of 77.25 and 42.51% observed at Reynolds numbers of 400 and 2000, respectively, compared to a fin-and-tube heat exchanger without vortex generators. Additionally, friction is reduced by 5.11%.

In another study focusing on winglet geometries, Zhao et al. [12] investigated nine different cases involving curved and straight rectangular winglet vortex generators integrated into a fin-and-tube cross-flow heat exchanger. When comparing the cases with winglet vortex generators to those without, the Colburn factor was consistently higher across all case studies. Furthermore, the findings indicated that curved winglets with smaller offsets exhibited better overall performance (Figure 8).

Figure 8.

Schematic view of a heat exchanger with rectangular winglet vortex generator described in Ref. [12].

While numerical calculations are more cost-effective than expensive experimental tests, achieving the optimal performance of a cross-flow heat exchanger at a specific design configuration entails a substantial computational cost. Hence, Xie et al. [26] employed response surface methodology in conjunction with artificial neural networks to determine the optimal design of a cross-flow heat exchanger equipped with vortex generators. According to their findings, both optimization techniques concluded that minimizing the attack angle to 160 degrees and maximizing the arc angle to 10 degrees would result in optimal performance, taking into account both heat transfer and friction factor simultaneously.

2.3 Addition of fins

Enhancing the effective heat transfer area is a traditional approach that leads to increased heat transfer rates. The widespread use of extended surfaces is well-known across industries, particularly in cross-flow heat exchangers. Different types of fins, including plate fins, rectangular straight fins, helical fins, and others, are commonly employed in cross-flow heat exchangers. In this review, we will endeavor to examine research conducted on cross-flow heat exchangers equipped with extended surfaces or fins.

Mitra et al. [27] conducted a study on the conjugate heat transfer of a finned tube, as depicted in Figure 9, specifically focusing on circular tube configurations. Their analysis encompassed the evaluation of flow patterns, pressure distributions, Nusselt numbers, and fin efficiencies. This setup reflects the conventional alignment of fins on the tube surface, depicting the directions of hot and cold fluids, which mirrors the simplest form of fin tube configuration.

Figure 9.

Cross-flow heat exchanger equipped with plate fins [27].

Sparrow and Kang [28] presented a simplified approach, suggesting the placement of a single fin at the front, back, or both ends for circular tubes. The diagram illustrating the arrangement of the fins is presented in Figure 10. The Reynolds number for the flow across the fluid ranged up to 8000. The results suggested that adding longitudinal fins to the tube improves the heat transfer rate while simultaneously decreasing pressure drop, especially for fins affixed to the rear of the cylinder in contrast to tubes without fins, conversely when fins are attached to both the front and rear ends, both the heat transfer rate and pressure drop rise.

Figure 10.

Finned tube heat exchanger used in work [28] with different configurations of (a) front, (b) back, and (c) front and back.

Martinez et al. [29] analyzed four semi-empirical models of heat transfer and pressure drop for helically segmented finned tubes in a staggered layout. Their study focused on assessing the performance of a helically segmented finned tubes heat exchanger on an industrial scale and comparing predictions with experimental data. Utilizing the logarithmic mean temperature difference (LMTD) method for thermal analysis, they found that a combination of the Kawaguchi and Gnielinski models achieved a precision greater than 95% in heat transfer prediction at a flue gas Reynolds number of approximately 10,000. Regarding pressure drop, the Weierman model exhibited a precision of around 90% at the same Reynolds number. Overall, their results indicated that the optimal flow regime, achieving optimal heat transfer and pressure drop, was observed at a Reynolds number of about 10,000 based on the outside bare tube. Their schematic view of the helical segmented tube is shown in Figure 11.

Figure 11.

The schematic representation of the helically segmented tube by Martinez et al. [29].

3. Lessons summary

Given the amount of research done on ways to improve heat transfer passively in cross-flow heat exchangers, it is important to summarize all the articles mentioned in Section 2. Table 1 summarizes the papers, noting their main findings, their study type, and the parameters they tested. Plus, since we mention lots of case studies in the next table, it might be helpful to quickly go over the design details and what they achieved, as well as how we could combine different passive methods for future work. This next table aims to make it easier to compare and classify different ways to cool passively.

Researcher	Type of study	Tested parameters	Results
Sparrow and Kang [28]	Experimental; (addition of longitudinal fins)	Placement of the fins, fin tip shape, and the fin thickness	Compared to greater diameter tubes, finned tubes result in a higher heat transfer rate at a given pressure drop
Merker and Hanke [18]	Experimental; (tube geometry orientation)	Varying transversal and longitudinal pitches of staggered tubes	Oval-shaped tube banks are desired compared to circular tube banks
Aiba [19]	Experimental; (tube geometry orientation)	Cylinder diameter, clearance between the cylinders, and pitch between cylinders	A correlation for overall heat transfer was yielded based on the given parameters
Chen et al. [27]	Numerical; (tube geometry orientation and addition of fins)	Reynolds number and fin parameter	A horseshoe vortex was observed near the oval-shaped tube, and the formed drag due to the tube shape was small
Buyruk [17]	Numerical; (tube geometry orientation)	Longitudinal distance of cylinders and Reynolds number	Flow acceleration resulted in a higher local heat transfer coefficient for the second cylinder compared to the initial row
Torii et al. [24]	Experimental; (addition of fins and vortex generators)	Reynolds number	Heat transfer was enhanced by 10–30% while pressure loss was reduced by 34–55%
Horvat and Mavko [21]	Numerical; (tube geometry orientation)	Varying diagonal pitch-to-diameter ratio	The drag coefficient and Stanton number decrease with an increase in Reynolds number
Martinez et al. [29]	Experimental; (addition of fins)	Reynolds number	Weierman model came up with the best accuracy for Reynolds number above 10,000
Berbish [22]	Experimental and numerical; (tube geometry orientation)	Longitudinal spacing ratio, transversal spacing ratio, and Reynolds number	The average Nusselt number is greater for the single elliptic cylinder compared to the circular one
Toolthaisong and Kasayapanand [20]	Experimental; (tube geometry orientation)	Air attack angles, tube aspect ratios, and Reynolds number	By increasing the attack angle, both heat transfer and pressure drop were increased
Xie et al. [26]	Numerical; (addition of fins and vortex generators)	Length, arc angle, and attack angle of vortex generator	At an attack angle of 160 degrees and arc angle of 10 degrees, the heat transfer was maximized
Saini et al. [25]	Numerical; (addition of fins and vortex generators)	Number of punch holes in vortex generators and Reynolds number	The curved delta winglet vortex generator addition dwindled the size of the wake downstream
Zhao et al. [12]	Numerical; (addition of fins and vortex generators)	Arc angle, attack angle, offset, and Reynolds number	The heat exchanger performance was improved by combining straight and curved winglets
Xu et al. [23]	Numerical; (addition of fins and vortex generators)	Main angle, streamwise distance, longitudinal placement, and Reynolds number	A 10-degree main angle showed the optimum performance based on the radial basis function neural network

Table 1.

The passive methods applied to cross-flow heat exchanger.

4. Conclusion

Following an in-depth analysis of recent developments in cross-flow heat exchangers, numerous conclusions and recommendations emerge:

1. Tube geometry orientation:

   • Employing non-circular tubes such as elliptical, wing, or cam-shaped configurations, particularly when arranged in an offset pattern with a decreased angle of fluid flow, enhances heat transfer efficiency.

   • Elliptic or oval tubes, available commercially with varying aspect ratios, offer a promising alternative to circular tubes, providing superior thermal performance and lower air side pressure drop.

   • While more advanced tube geometries like cam-shaped and lenticular tubes offer enhanced streamlined surfaces, their complex manufacturing processes may limit widespread industrial application.

   • Optimal thermal performance, balancing heat transfer and friction factor, is achieved at lower attack angles. Excessive streamlined tube designs should be avoided to ensure sufficient turbulence generation.

2. Vortex generators:

   • Vortex generators, including delta and rectangular winglets, significantly augment heat transfer by inducing additional turbulence in the cross-fluid path.

   • Delta winglet vortex generators are particularly effective, generating secondary flows alongside the mainstream flow. They can be easily mounted on fin surfaces, simplifying manufacturing processes.

   • Curved triangular and box-shaped winglet vorticity inducers offer the additional advantage of directing fluid movement over tubes, simultaneously diminishing wake dimensions and creating secondary flow.

   • Control of higher pressure drops can be achieved by implementing winglets on alternate rows, especially in staggered tube configurations.

3. Addition of fins:

   • Utilizing fin surfaces enhances heat transfer by expanding the surface area available for heat exchange. Helical, plate and longitudinal fins represent commonly utilized options, with continuous plate fins being favored for larger tube banks owing to their lower pressure drop.

   • Thin plate continuous fins are favored by researchers for their simplicity and ease of mounting, allowing for additional passive heat transfer methods if needed. Conversely, helical fins may present constraints due to tube pitch ratios.

   • Thinner fin thicknesses offer advantages, as both fins and tubes maintain similar temperatures, thereby enhancing heat transfer rates while reducing pressure drop. However, pressure drop increases with thicker fin thicknesses.

In summary, optimizing tube geometry orientation, vortex generator implementation, and fin configurations are vital strategies for enhancing heat transfer efficiency in cross-flow heat exchangers. Each approach offers unique advantages and considerations, influencing overall thermal performance and pressure drop characteristics.

Acknowledgments

The author acknowledges the use of ChatGPT 3.5 for language polishing of the manuscript.