WINGLET-TYPE VORTEX GENERATORS WITH COMMON-FLOW-UP CONFIGURATION FOR FIN-TUBE HEAT EXCHANGERS

This investigation stems from the area of augmentation of heat transfer by generating streamwise longitudinal vortices. The vortex generators are arranged in a common-flow-up configuration. Existing air-cooled condensers in geothermal power plants use fin-tube heat exchangers with circular tubes. The heat exchangers are huge, and often the cost of the condensers is more than one-third of the plant cost. The size of the condensers can be reduced through enhancement of heat transfer from fin surfaces. The enhancement strategy involves introduction of strong swirling motion in the flow field. The swirl can be generated by the longitudinal vortices. In this study, the longitudinal vortices are created by delta winglet-type vortex generators, which are mounted behind the tubes. An element of a heat exchanger has been considered for detailed study of the flow structure and heat transfer analysis. Biswas and colleagues have obtained significant enhancement of heat transfer by deploying the winglet pair behind each tube. In this study, a novel technique (Torii and colleagues [2]) has been utilized for the enhancement. The winglets are placed with a heretofore unused orientation for the purpose of augmentation of heat transfer. This orientation is called the common-flow-up configuration. The proposed method causes significant separation delay, reduces form drag, and removes the zone of poor heat transfer from the near wake of the tubes. The analyses of flow and heat transfer in the proposed configuration have been accomplished through a numerical solution of complete Navier-Stokes and energy equations.     

Heat transfer enhancement accompanying pressure-loss reduction with winglet-type vortex generators for fin-tube heat exchangers

This paper proposes a novel technique that can augment heat transfer but nevertheless can reduce pressure-loss in a fin-tube heat exchanger with circular tubes in a relatively low Reynolds number flow, by deploying delta winglet-type vortex generators. The winglets are placed with a heretofore-unused orientation for the purpose of augmentation of heat transfer. This orientation is called as “common flow up” configuration. The proposed configuration causes significant separation delay, reduces form drag, and removes the zone of poor heat transfer from the near-wake of the tubes. This enhancement strategy has been successfully verified by experiments in the proposed configuration. In case of staggered tube banks, the heat transfer was augmented by 30% to 10%, and yet the pressure loss was reduced by 55% to 34% for the Reynolds number (based on two times channel height) ranging from 350 to 2100, when the present winglets were added. In case of in-line tube banks, these were found to be 20% to 10% augmentation, and 15% to 8% reduction, respectively.     

An overview on heat transfer augmentation using vortex generators and nanofluids: Approaches and applications

The subject of heat transfer enhancement has significant interest to develop the compact heat exchangers in order to obtain a high efficiency, low cost, light weight, and size as small as possible. Therefore, energy cost and environmental considerations are going on to encourage attempts to invent better performance over the existence designs. Streamwise vortices can be generated using small flow manipulators or protrusions such as wings and winglets configurations. Single-pair, single row, or two dimensional array of vortex generators (VGs) can be punched, mounted, attached or embedded in the boundary layer of flow channel. VGs generate longitudinal and transverse vortices, while longitudinal vortices are more efficient for heat transfer enhancement than transverse vortices. A dramatic augmentation in thermal performance of the thermal system can be achieved but pressure drop penalty is existed. Several parameters have been overviewed in this paper, which have pronounced effect on the convective heat transfer coefficient and pressure drop penalty. These parameters are: attack angle of VG, geometry of VG, standard and novel types of VG, spacing between the VG tips, number of pairs of VGs in the flow direction, rectangular or circular array arrangement of VGs, common-flow upper (CFU) or common-flow down (CFD) configuration of VG, pointing up (PU) or pointing down (PD) arrangement of VG with flow direction, Re number, channel aspect ratio, number of tubes of fin-tube heat exchanges (HE), circular or oval tubes of fin-tube HE, and location of VG respect to the tube of HE or from leading edge of the channel. This paper gives an overview about the early studies done in order to improve the performance of thermal systems with minimal pressure losses to derive systems with less negative impact on the environment and high level of energy economic. This study also provides an outlook for future work using nanofluids with vortex generators.This article is also summarizes the recent experimental and numerical developments on the thermal conductivity measurements of nanofluids, thermal conductivity enhancement, convection and conduction heat transfer, some applications, main problems and suggestions for future works.     

不同纵向涡发生器流动传热的数值模拟

利用三维数值模拟的方法对带有3种异形纵向涡发生器的H型翅片椭圆管换热器的空气侧流动传热特性进行研究。基于H型翅片椭圆管束,讨论了在不同雷诺数下,纵向涡发生器的摆放位置、摆放攻角和形状对空气侧流动传热的影响。研究表明:纵向涡发生器能够将高能量的流体引向流速较低的壁面区域,使冷热流体之间的混合加剧,增强流体的湍流动能,进而达到强化传热的效果;与无纵向涡发生器的管束相比,带纵向涡发生器管束的传热效果有明显的提高;当纵向涡发生器后置时,换热器的传热效果最优;在雷诺数相同,攻角为30°时,流体的传热性能和阻力特性均达到最优;相同攻角摆放时,椭圆角矩形发生器的传热性能和阻力因子均优于其他两种形式的发生器。研究结果为烟气余热回收系统换热器传热性能强化提供理论依据。     

Enhancement of Heat Transfer Using Delta-Winglet Type Vortex Generators with a Common-Flow-up Arrangement

Simulations of a coolant air flowing in a heat exchanger with delta-winglet type vortex generators in common-flow-up configuration have been performed to unveil the salient heat transfer characteristics. The heat exchanger is approximated as a periodic rectangular channel with heated walls and a pair of built-in tubes near the inlet and outlet. The heat transfer characteristics of the heat exchangers with vortex generators near the inlet, outlet, and both inlet and outlet have been compared. The Navier-Stokes equations together with the energy equation are solved employing unstructured finite volume method. The simulations reveal a significant enhancement in heat transfer because of the strong swirling motion originating from the streamwise longitudinal vortices behind the pair of delta winglets. The spiraling flow entrains air into the core and causes intermixing of the fluid layers to disrupt the growth of the thermal boundary layer. A parametric study on the angles of attack identifies the conditions under which enhancement in heat transfer can lead to significant miniaturization of the heat exchangers. The analysis also reveals interesting flow structures behind the winglets and correlates them to the mechanism of heat transfer.     

Advection transport model of vortex enhanced heat transfer

The present investigation is an analysis to predict the kinematic behavior of a Lagrangian fluid particle and the associated enhancement in advection heat transfer in vortex dominated channel flows. The vortex systems considered are two and four parallel vortex filaments in different configurations, subjected to external translation and pulsation. Such vortex systems are typical of the kind generated by the vortex generators mounted on channel walls of fin-tube and fin-plate heat exchangers. The method of analysis is efficient in predicting the intensity of mixing in the flow field and provides a quantitative estimate of advection heat transfer due to different configurations of the vortex systems. The predictions are primarily based on an analytical approach and hence offer quantification of advection requiring reduced computational cost as compared to the actual numerical solutions of the flow and temperature fields.     

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.     

Heat Transfer and Fluid Flow Analysis for Plate-Fin and Oval Tube Heat Exchangers With Vortex Generators

This article investigates the effectiveness of embedded vortex generators in enhancing the heat transfer performance of a plate-fin heat exchanger with a four-row staggered oval tube bundle. Two different types of vortex generator are considered, namely annular and inclined block. Numerical simulations are performed to analyze the effects of the three-dimensional turbulence induced by the vortex generators on the heat transfer and fluid flow characteristics of the heat exchanger. The results indicate that compared to a plate-fin heat exchanger with circular tubes, the use of oval tube fins and vortex generators increases the heat transfer rate by 3 to 16% and reduces the pressure drop by 17 to 35% for inlet velocities in the range of 1 to 8 m/s. Furthermore, the vortex generators make possible an average area reduction ratio of 14 to 18%. Overall, the results show that the inclined block shape vortex generators yield the greatest improvement in the heat transfer performance at medium to high inlet velocities.     

Heat transfer enhancement and pressure drop for fin-and-tube compact heat exchangers with wavy rectangular winglet-type vortex generators

In the current work, heat transfer enhancement and pressure loss penalty for fin-and-tube compact heat exchangers with the wavy-up and wavy-down rectangular winglets as special forms of winglet are numerically investigated in a relatively low Reynolds number flow. The rectangular winglets were used with a particular wavy form for the purpose of enhancement of air side heat transfer performance of fin-and-tube compact heat exchangers. The effect of Reynolds numbers from 400 to 800 and angle of attack of 30° of wavy rectangular winglets are also examined. The effects of using the wavy rectangular winglet, conventional rectangular winglet configuration and without winglet as baseline configuration, on the heat transfer characteristics and flow structure are studied and analyzed in detail for the inline tube arrangements. The results showed that the wavy rectangular winglet can significantly improve the heat transfer performance of the fin-and-tube compact heat exchangers with a moderate pressure loss penalty. In addition, the numerical results have shown that the wavy winglet cases have significant effect on the heat transfer performance and also, this augmentation is more important for the case of the wavy-up rectangular winglet configuration.     

纵向涡强化换热的优化设计及机理分析

对带纵向涡发生器的椭圆管翅片换热器空气侧表面的换热和流动特性进行了三维数值模拟.深入分析了纵向涡对流场和温度场的影响,并通过场协同原理揭示了纵向涡强化换热的根本机理,即减小了速度和温度梯度之间的夹角,改善了速度场和温度场的协同性.在此基础上,对纵向涡发生器的布置位置(上游布置和下游布置)和纵向涡发生器的攻角α(15°,30°,45°,60°)进行了优化设计.结果表明:当纵向涡发生器布置于换热管下游时,具有更好的强化换热能力;在纵向涡发生器采用下游布置的前提下,当纵向涡发生器的攻角α=30°时,具有最佳的强化换热能力.     

Comparisons of Local Experimental Results with Numerical Results of Heat Transfer Enhancement of a Flat Tube Bank Fin with Vortex Generators

The common way to obtain the fin pattern of a tube bank fin heat exchanger with good heat transfer performance is through experiments. Such experiments are complex, expensive, and very difficult to carry out. Recently, numerical analysis has become a powerful method for selecting the fin pattern of tube bank fin heat exchangers. In this article, we focus on testing the reliability of the numerical method by comparing local numerical results with local experimental results obtained through naphthalene sublimation. The target of numerical analyses carried out here is a flat tube bank fin heat exchanger mounted with vortex generators. The results show that using body-fitted coordinates, with proper treatment of vortex generators penetrating the fluid flow, leads to reliable local and average results.     

Effect of a delta-winglet vortex pair on the performance of a tube–fin heat exchanger

The experimental analysis of the effects of delta-winglet vortex generators on the performance of a fin and tube radiator is presented. The winglets were arranged in flow-up configuration, and placed directly upstream of the tube. This is a hitherto untested configuration, but is thought to have certain advantages. In addition to vortex generation the flow is guided onto the tube surface increasing the localised velocity gradients and Nusselt numbers in this region. The study includes dye visualisation and full scale heat transfer performance measurements. The results are compared to a standard louvre fin surface. It was found that the winglet surface had 87% of the heat transfer capacity but only 53% of the pressure drop of the louvre fin surface.     

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.     

Experimental study of the effect of winglet location on heat transfer enhancement and pressure drop in fin-tube heat exchangers

Local heat transfer coefficients were measured on fin-tube heat exchanger with winglets using a single heater of 2 inch diameter and five different positions of winglet type vortex generators. The measurements were made at Reynolds number about 2250. Flow losses were determined by measuring the static pressure drop in the system. Results showed a substantial increase in the heat transfer with winglet type vortex generators. It has been observed that average Nusselt number increases by about 46% while the local heat transfer coefficient improves by several times as compared to plain fin-tube heat exchanger. The maximum improvement is observed in the re-circulation zone. The best location of the winglets was with ΔX = 0.5D and ΔY = 0.5D. The increase in pressure drop for the existing situation was of the order of 18%.     

Analysis of coiled-tube heat exchangers to improve heat transfer rate with spirally corrugated wall

Steady heat transfer enhancement has been studied in helically coiled-tube heat exchangers. The outer side of the wall of the heat exchanger contains a helical corrugation which makes a helical rib on the inner side of the tube wall to induce additional swirling motion of fluid particles. Numerical calculations have been carried out to examine different geometrical parameters and the impact of flow and thermal boundary conditions for the heat transfer rate in laminar and transitional flow regimes. Calculated results have been compared to existing empirical formulas and experimental tests to investigate the validity of the numerical results in case of common helical tube heat exchanger and additionally results of the numerical computation of corrugated straight tubes for laminar and transition flow have been validated with experimental tests available in the literature. Comparison of the flow and temperature fields in case of common helical tube and the coil with spirally corrugated wall configuration are discussed. Heat exchanger coils with helically corrugated wall configuration show 80–100% increase for the inner side heat transfer rate due to the additionally developed swirling motion while the relative pressure drop is 10–600% larger compared to the common helically coiled heat exchangers. New empirical correlation has been proposed for the fully developed inner side heat transfer prediction in case of helically corrugated wall configuration.     

Impact of delta winglet vortex generators on the performance of a novel fin-tube surfaces with two rows of tubes in different diameters

To achieve heat transfer enhancement and lower pressure loss penalty, even pressure loss reduction, two novel fin-tube surface with two rows of tubes in different diameters are presented in this paper. Numerical simulation results show that the fin-tube surface with first row tube in smaller size and second row tube in larger size can lead to an increase of heat transfer and decrease of pressure drop in comparison with the traditional fin-tube surface with two rows of tubes in the same size. Based on this understanding, delta winglet pairs are punched out only from the larger fin area around the first transverse row of tubes in smaller size in the novel fin-tube surfaces. Delta winglet pairs used as longitudinal vortex generator are arranged either in “common flow up” or “common flow down” configurations. Numerical simulation results show that delta winglet pairs can bring about a further heat transfer enhancement and pressure drop decrease through the careful arrangement of the location, size and attack angle of delta winglet pairs either in “common flow up” or “common flow down” configurations. The traditional knowledge of heat transfer enhancement with necessary pressure drop increase is challenged by the present conclusion. The present work will be helpful to develop more compact, higher heat transfer efficiency, lower fan power and quieter heat exchanger of refrigeration and air condition system.     

WINGLET-TYPE VORTEX GENERATORS FOR PLATE-FIN HEAT EXCHANGERS USING TRIANGULAR FINS

Vortex generators in the form of delta winglet pairs have already been proposed by many researchers for enhancement of the heat transfer rate in plate-fin heat exchangers. In this work, the enhancement potential of triangular fins (which are widely used inserts between the plates of the plate-fin heat exchanger) having delta winglets mounted on their slant surfaces has been computed. The performance of this combination is evaluated for varying angles of attack of the winglet and different thermal boundary conditions. The performance of the combination of triangular fins and winglets with stamping on the slant surfaces also has been evaluated.     

Heat transfer and pressure drop performance of twisted oval tube heat exchanger

Twisted oval tube heat exchanger is a type of heat exchanger that aims at improving the heat transfer coefficient of the tube side and also decreasing the pressure drop of the shell side. In the present work, tube side and shell side heat transfer and pressure drop performances of a twisted oval tube heat exchanger has been experimentally studied. The tube side study shows that the tube side heat transfer coefficient and pressure drop in a twisted oval tube are both higher than in a smooth round tube. The shell side study shows that the lower the modified Froude number Fr_M, the higher the shell side heat transfer coefficient and pressure drop. In order to comparatively analyze its shell side performance of the heat exchanger, a rod baffle heat exchanger with similar size of the twisted oval tube heat exchanger is designed and its performance is calculated with Gentry's method. The comparative study shows that the heat transfer coefficient of the twisted oval tube heat exchanger is higher and the pressure drop is lower than the rod baffle heat exchanger. In order to evaluate the overall performance of the twisted oval tube heat exchanger, a performance evaluation criterion considering both the tube side and shell side performance of a heat exchanger is proposed and applied. The analyze of the overall performance of the twisted oval tube shows that the twisted oval tube heat exchangers works more effective at low tube side flow rate and high shell side flow rate.     

Thermo-Hydraulic Performance Enhancement of Finned Elliptical Tube Heat Exchangers by Utilizing Innovative Dimple Turbulators

Abstract

As a new type of fin structure in finned tube heat exchangers, dimple turbulators exhibit excellent potential for thermo-hydraulic performance enhancement. A three-dimensional numerical simulation study was conducted to investigate the influences of five kinds of innovative concave dimple turbulators (CDTs), namely – elliptical dimple, conical frustum dimple, trapezoidal prism dimple, leeward triangular dimple and upward triangular dimple (UwTD) on the thermo-hydraulic performance enhancement in a plate fin-and-elliptical tube (PFET) heat exchanger, where CDTs are textured on the fin surface transversely between the elliptical tubes. The computational results are analyzed by considering the performance evaluation criterion for the PFET heat exchangers with different types of CDT shapes. The present investigation demonstrates that the heat transfer enhancement is intimately pertained to ejection with longitudinal counter-rotating flow, strengthened secondary flow and vortex structures at the downstream rim of CDT. A parametric study on the CDTs indicated that the UwTD vortex turbulators give better thermo-hydraulic performance under the present conditions. The numerical simulation results illustrated different secondary flow structures and heat transfer characteristics of the CDTs with various shapes, which disclosed the influential mechanisms of differently shaped dimple turbulators on the heat transfer augmentation in PFET heat exchangers.     

Flow visualization of wave-type vortex generators having inline fin-tube arrangement

This study presents visual observation of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Three samples of fin-and-tube heat exchanger having inline arrangements are examined, including one plain fin and two wave-type vortex generators. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and a very large secondary flow circulation is seen between the first and second row. This flow re-circulation phenomenon is almost disappeared with the presence of proposed vortex generators. The presence of vortex generators significantly increase the vortrical motions of the horseshoe vortices hitting on the tubes. A much better mixing characteristics is seen by introducing the vortex generators. The frictional penalty of the proposed vortex generators are about 25-55% higher than that of the plain fin geometry. The penalty of pressure drops of the proposed vortex generators relative to plain fin geometry is relatively insensitive to change of Reynolds number.