Flow visualization of annular and delta winlet vortex generators in fin-and-tube heat exchanger application

This study presents flow visualization and frictional results of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Two types of vortex generators and a plain fin geometry were examined in this study. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and the horseshoe vortex separates into two streams as it flows across the second row and consequently loses its vortical strength. This phenomenon may supports the “maximum phenomenon” in low Reynolds number region reported by previous studies. With the presence of annular vortex generator, the presence of a pair of longitudinal vortices formed behind the tube is seen. The strength of the counter-rotating vortices increases with the annular height and the strength of the longitudinal vortices is so strong that may swirl with the horseshoe vortices and other flow stream. For the same winlet height, the delta winlet shows more intensely vortical motion and flow unsteadiness than those of annular winlet. This eventually leads to a better mixing phenomenon. However, it is interesting to know that the corresponding pressure drops of the delta winlet are lower than those of annular winlet. Compared to the plain fin geometry, the penalty of additional pressure drops of the proposed vortex generators is relatively insensitive to change of Reynolds number.     

Numerical study on a slit fin-and-tube heat exchanger with longitudinal vortex generators

A 3-D numerical simulation is performed on laminar heat transfer and flow characteristics of a slit fin-and-tube heat exchanger with longitudinal vortex generators. Heat transfer enhancement of the novel slit fin mechanism is investigated by examining the effect of the strips and the longitudinal vortices. The structure of the slit fin is optimized and analyzed with field synergy principle. The result coincides with the guideline ‘front coarse and rear dense’. The heat transfer and fluid flow characteristics of the slit fin-and-tube heat exchanger with longitudinal vortex generators are compared with that of the heat exchanger with X-shape arrangement slit fin and heat exchanger with rectangular winglet longitudinal vortex generators. It is found that the Colburn j-factor and friction factor f of the novel heat exchanger with the novel slit fin is in between them under the same Reynolds number, and the factor j/(f^1/3) of the novel heat exchanger increased by 15.8% and 4.2%, respectively.     

NUMERICAL ANALYSIS OF FLUID FLOW AND HEAT TRANSFER CHARACTERISTICS IN THREE-DIMENSIONAL PLATE FIN-AND-TUBE HEAT EXCHANGERS

This numerical study provides three-dimensional (3-D) time-dependent modeling of unsteady laminar flow and heat transfer over single- and multirow plate fin-and-tube heat exchangers. The complex nature of the flow field featuring a horseshoe vortex is investigated for both configurations. The time-dependent evolution of the horseshoe vortex mechanism on the forward part of the tube and its journey to the rear of the tube are studied to provide fundamental information on the local flow structure and the corresponding heat transfer characteristics. The effects of various governing parameters, such as fin spacing, Reynolds number, tube row number, and tube arrangement, on the heat transfer and flow characteristics are also studied for the Reynolds number range investigated. It is found that the local flow structure including formation and evolution of vortex systems and singular-point interactions correlates strongly with the heat transfer characteristics. The numerical results for the integral heat transfer parameters agree well with available experimental measurements.     

Air-side performance of herringbone wavy fin-and-tube heat exchangers under dehumidifying condition – Data with larger diameter tube

This study examines the airside performance of the herringbone wavy fin-and-tube heat exchangers in dehumidifying condition having a larger diameter tube (D_c = 16.59 mm) with the tube row ranging from 2 to 12. Test results are compared to that of dry conditions and plain fin geometry. Upon the influence of surface condition (dry or wet) on the heat transfer performance, the heat transfer performance in dehumidifying condition normally exceeds that in dry condition, and is more pronounced with the rise of tube row or reduction of fin pitch. By contrast, it is found that the heat transfer coefficient for plain fin geometry in dehumidifying condition is slightly lower than that in dry condition. The pressure drops in wet condition is much higher than that in dry condition. However, the difference in pressure drop amid dry and dehumidifying condition for wavy fin configuration is less profound as that of plain fin geometry.     

Airside performance of fin-and-tube heat exchangers in dehumidifying conditions – Data with larger diameter

This study presents the airside performance of the fin-and-tube heat exchangers having plain fin geometry with a larger diameter tube (D_c = 15.88 mm) under dehumidifying condition. A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch are made and tested. It is found that the effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row. However, there is a unique characteristic of fin pitch at a shallow tube row, the heat transfer performance is first increased at a wider pitch but a further increase of fin pitch lead to a falloff of heat transfer performance due to interactions amid flow development and bypass flow. The influence of tube row on the airside performance is rather small for both heat transfer and frictional characteristics at a fin pitch of 2.1 mm and when the Reynolds number is less than 4000. A slight deviation of this effect is encountered when fin pitch is increased to 2.54 mm or 3.1 mm due to condensate adhered phenomena.     

An experimental investigation of air cooling thermal module using various enhancements at low Reynolds number region

This study examines the airside performance of heat sinks having fin patterns of plate fin (Type I), interrupted fin geometry (Type II), dense vortex generator (Type III), and loose vortex generator (Type IV). Test results indicate that the heat transfer performance is strongly related to the arrangement of enhancements. The interrupted and dense vortex generator configurations normally contribute more pressure drop penalty than improvements of heat transfer. This is especially pronounced when operated at a lower frontal velocity. Actually the plain fin geometry outperforms most of the enhanced fin patterns such as of Type II and Type III at the fully developed region. This is because a close spacing prevents the formation of vortex, and the presence of interrupted surface may also suffer from the degradation by constriction of conduction path. The results suggest that the vortex generators operated at a higher frontal velocity is more beneficial than that of plain fin geometry. In association with the VG-1 criteria (same pumping power and same heat transfer capacity), the results show that effective reduction of surface area can be achieved when the frontal velocities are at 3–5 m s^?1 and the fin patterns are triangular, triangular attack, or two-groups dimple. The result from the present experiment suggests that the asymmetric combination such as using loose vortex generator (Type IV) can be quite effective. The triangular attack vortex generator is regarded as the optimum enhancement design for it could reduce 12–15% surface area at a frontal velocity around 3–5 m s^?1. The asymmetric design is still applicable even when the fin pitch is reduced to 1 mm.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.     

Fin Pattern Effects on Air-Side Heat Transfer and Friction Characteristics of Fin-and-Tube Heat Exchangers with Large Number of Large-Diameter Tube Rows

Air-side heat transfer and friction characteristics of nine kinds of fin-and-tube heat exchangers, with a large number of tube rows (6, 9, and 12, respectively) and large diameter of tubes (18 mm), are experimentally investigated. The test samples consist of three types of fin configurations: plain fin, slit fin, and fin with delta-wing longitudinal vortex generators. The working fluid in the tube is steam. Results show that when the number of tube is larger than 6, the heat transfer and friction performance for three kinds of fins is independent of the number of tube rows, and slit fin provides higher heat transfer and pressure drop than the other two fins. The heat transfer and friction factor correlations for all the heat exchangers were acquired with Reynolds numbers ranging from 4000 to 10000. The air-side performance of heat exchangers with plain fin, slit fin, and longitudinal vortex-generator fin were evaluated under three sets of criteria, and the results showed that the heat exchanger with slit fin has better performance than that with vortex-generator fin, especially at high Reynolds numbers.     

Correlations for wet surface ratio of fin-and-tube heat exchangers

Correlations are proposed for the wet surface ratio of a fin-and-tube heat exchanger with plain and wavy fin geometry under dehumidifying conditions. The ‘Finite Circular Fin Method’ (FCFM) is used for data reduction. It is found that the percentage of wet surface area increases with increasing fin spacing or number of tube rows and decreasing Reynolds number. Despite the addition of tube rows or reduced fin spacing the effective surface area is increased, and its influence on a partially wet surface is apparently the opposite. This is because adding tube rows will provide a more effective temperature drop in air flow than adding fins, Moreover, the heat and mass transfer characteristics of j_h and j_m increase with an increase in the area of wet surface. Correlations for prediction of the percentage of wet surface area are proposed. These correlations can describe 83.81% of the area of wet surface to within ±10%.     

Optimization of heat exchangers with vortex-generator fin by Taguchi method

A comparative study of effects of attack angle, length of vortex generator, height of vortex generator, fin material, fin thickness, fin pitch and tube pitch on fin performance of vortex-generator fin-and-tube heat exchanger is conducted by numerical method. The parameters of vortex-generator fin-and-tube heat exchangers are optimized by the Taguchi method. Eighteen kinds of models are made by compounding levels on each factor, and the heat transfer and flow characteristics of each model are analyzed. The results show that these six factors (fin pitch, longitudinal tube pitch, transverse tube pitch, length of vortex generator, height of vortex generator, and attack angle of vortex generator) have great influences on the JF-factor. The fin material and fin thickness have trifling effects on the JF-factor. The two optimal conditions (A1B3C3D2E1F2G1H3 and A2B2C2D3E1F2G1H3) are acquired, and the reproducibility of the results is verified by two analytical results.     

Application of Combined Enhanced Techniques for Design of Highly Efficient Air Heat Transfer Surface

In order to reduce the size and cost of heat exchangers, an air-side wavy fin-and-tube heat transfer surface with three-row tubes needs to be replaced by two-row tubes with some appropriate enhancing techniques. The major purpose of the present paper is to search for such new structure by numerical simulation. First, longitudinal vortex generators of Delta-winglet type are tried. The influence of number and of arrangement of the winglets on the performance of the heat transfer surface is studied in detail. The numerical results show that the fin with two winglets aligned spanwise in the front and rear of each tube (Fin W6) has higher heat transfer capability than other enhanced structures with vortex generators, but it still unable to meet the heat transfer requirement. Then a combination design of the longitudinal vortex generator with slotted protruding parallel strips is proposed and different variations of their arrangement are tried. Finally we come to such a combination (C3), which is based on Fin W6 with additional eight protruding strips situated at five positions (grouped by 1, 2, 2, 2, and 1) along the flow direction. Fin C3 can satisfy the requirements for heat transfer rate of the original wavy fin of three-row tubes with a mild increase in pressure drop, and its volume and material reduce to about 67% of the original one.     

Finite circular fin method for heat and mass transfer characteristics for plain fin-and-tube heat exchangers under fully and partially wet surface conditions

This study proposes a new method, namely the “finite circular fin method” (FCFM), to analyze the performance of fin-and-tube heat exchangers having plain fin configuration under dehumidifying conditions. The analysis is done by dividing the heat exchanger into many tiny segments (number of tube rows × number of tube passes per row × number of fins). The tiny segments are distinguished into three types: the fully dry, partially wet or fully wet surface conditions. The proposed method is capable of handling fully and partially wet surfaces. From the test results, it is found that the sensible heat transfer performance and the mass transfer performance are insensitive to changes of fin pitch. The influence of inlet relative humidity on the sensible heat transfer performance is small, and is almost negligible when the number of tube rows is above four. For one and two row configurations, considerable increase of mass transfer performance is encountered when partially wet condition takes place. The sensible heat transfer coefficient is about the same for those in fully wet and partially wet conditions provided that the number of tube row is equal or greater than four. Correlations applicable for both fully wet and partially wet conditions are proposed to describe the heat and mass performance for the present plain fin configuration.     

Three-Dimensional Numerical Study of Fluid and Heat Transfer Characteristics of Dimpled Fin Surfaces

In the present study, a code based on the nonorthogonal curvilinear coordinates is developed with a collocated grid system generated by the two-boundary method. After validation of the code, it is used to compare simulated results for a fin-and-tube surface with coupled and decoupled solution methods. The results of the coupled method are more agreeable with the test data. Simulation for dimpled and reference plain plate fin-and-tube surfaces are then conducted by the coupled method within a range of inlet velocity from 1.0 m/s to 5 m/s. Results show that at identical pumping power the dimpled fin can enhance heat transfer by 13.8–30.3%. The results show that relative to the reference plain plate fin-and-tube surface, heat transfer rates and pressure drops of the dimpled fin increase by 13.8%–30.3% and 31.6%–56.5% for identical flow rate constraint. For identical pumping power constraint and identical pressure drop constraint, the heat transfer rates increase by 11.0%–25.3% and 9.2%–22.0%, respectively. By analyzing the predicted flow and temperature fields it is found that the dimples in the fin surface can improve the synergy between velocity and fluid temperature gradient.     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

Augmentation of Heat Transfer by Creation of Streamwise Longitudinal Vortices Using Vortex Generators

This paper summarizes the current state of the art related to improvement of the heat exchanger surfaces using streamwise longitudinal vortices. Primarily, the improvements related to fin-tube cross-flow heat exchangers and the plate-fin heat exchangers have been addressed. Protrusions in certain forms, such as delta wings or winglet pairs, act as vortex generators, which can enhance the rate of heat transfer from the heat-exchanger surfaces that may be flat or louvered. The strategically placed vortex generators create longitudinal vortices, which disrupt the growth of the thermal boundary layer, promote mixing between fluid layers, and hence lead to augmentation in heat transfer. The flow fields are dominated by swirling motion associated with modest pressure penalty. Heat transfer is augmented substantially for all the proposed configurations of the longitudinal vortex generators, such as delta wings, rectangular winglet pairs, and delta winglet pairs, with varying degree of pressure penalty. Both computational and experimental investigations on flow and heat transfer in the heat exchanger passages with built-in vortex generators are revisited and summarized.     

Airside performance of herringbone wavy fin-and-tube heat exchangers – data with larger diameter tube

This study examines the airside performance of the wavy fin-and-tube heat exchangers having a larger diameter tube (D_c = 16.59 mm) with the tube row ranging from 1 to 16. It is found that the effect of tube row on the heat transfer performance is quite significant, and the heat transfer performance deteriorates with the rise of tube row. The performance drop is especially pronounced at the low Reynolds number region. Actually more than 85% drop of heat transfer performance is seen for F_p ～ 1.7 mm as the row number is increased from 1 to 16. Upon the influence of tube row on the frictional performance, an unexpected row dependence of the friction factor is encountered. The effect of fin pitch on the airside performance is comparatively small for N = 1 or N = 2. However, a notable drop of heat transfer performance is seen when the number of tube row is increased, and normally higher heat transfer and frictional performance is associated with that of the larger fin pitch.     

A comparison of the airside performance of the fin-and-tube heat exchangers in wet conditions; with and without hydrophilic coating

An experimental study was performed on compact fin-and-tube heat exchangers in wet conditions. Airside performance for both hydrophilic coated and un-coated surface is examined. It is found that the effect of inlet relative humidity on the heat transfer performance is small. For un-coated surfaces, the effect of inlet relative humidity has a pronounced effect on pressure drops. It is likely that this phenomenon is related to the condensate flow pattern along the fin surface. The heat transfer performance for the hydrophilic coating surface is lower than the corresponding un-coated surface tested at the same wet condition. Further, the degradation of heat transfer performance may be up to 20% for fin pitches of 1.2 mm. The pressure drops for the hydrophilic coated surface are also lower than the corresponding un-coated surfaces. A maximum 40% reduction is observed for plain fin geometry. The effect of inlet condition on frictional performance is more pronounced in the enhanced slit geometry.     

Investigation on laminar convection heat transfer in fin-and-tube heat exchanger in aligned arrangement with longitudinal vortex generator from the viewpoint of field synergy principle

3-D numerical simulation results are presented for laminar flow heat transfer of the fin-and-tube surface with vortex generators. The effects of Reynolds number (from 800 to 2000), the attack angle (30° and 45°) of delta winglet vortex generator are examined. The numerical results are analyzed from the viewpoint of field synergy principle. It is found that the inherent mechanism of heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, the second flow generated by the vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. In addition, the heat transfer enhancement of delta winglet with the attack angle of 45° is larger than that of 30°, while the delta winglet with the attack angle of 45° results in an increase of the pressure drop, however, the delta winglet with the attack angle of 30° results in a slight decrease.     

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. Translated from Journal of Xi’an Jiao Tong University, 2006, 40(7): 757–761 [译自: 西安交通大学学报]