Rectangular Winglet Pairs Research Articles

Flow structures and heat transfer in repeating arrangements of staggered rectangular winglet pairs by Large Eddy Simulations: Effect of winglet height and longitudinal pitch distance

Large Eddy Simulations (LES) of the flow in repeating arrangements of staggered rectangular winglet pairs are presented to get a better understanding of the detailed flow and heat transfer enhancing mechanisms. Simulations are performed for various geometrical configurations with the winglet height h and longitudinal pitch distance Lp being varied in the ranges h/H∈[0.3;0.7] and Lp/H∈[3;7] at Re=ubH/ν=700 and Pr=0.71.The results show that three different types of vortices are generated by the winglet pairs: main longitudinal vortices, corner vortices and induced vortices, with the main longitudinal vortices being the main contributor to heat transfer enhancement. It is found that the heat transfer and pressure loss increase with increasing winglet height and decrease with increasing longitudinal pitch distance. The winglet height proves to have the highest impact on both the heat transfer and pressure loss. Furthermore, the results show that local heat transfer can effectively be increased on the fin side by utilising smaller winglet heights. For higher winglet heights local heat transfer is observed to be more equally distributed on both sides.Overall, pressure loss increments f/f0 between 3.3 and 33.5 and heat transfer enhancements Nu/Nu0 between 2.2 and 4.6 are found. When introducing the performance factor η=(Nu/Nu0)/(f/f0)1/3 as a measure for heat transfer enhancement relative to pressure loss, the optimal geometries generally have a combination of lower winglet heights and higher longitudinal pitch distances.

Enhancement of Film Cooling Effectiveness Using Rectangular Winglet Pair

This study deals with the film cooling enhancement in a combustion chamber by the use of rectangular winglet vortex generators (VGs). Rectangular winglet pair (RWP) in both the common-flow up and the common-flow down configuration is installed upstream of a coolant injection hole on the lower chamber wall. A three-dimensional numerical approach with complete solution of Navier–Stokes (NS) equations closed by the k–ɛ turbulence model is used for analyzing the effect of VG installation on film cooling effectiveness enhancement. The effect of RWP orientation is investigated to deduce the best configuration which is then optimized in terms of its geometrical parameters including its upstream distance from the hole and the angle it makes with the incoming flow. Results obtained show that a RWP located upstream of the coolant hole in common-flow down configuration gives the best effectiveness enhancement with certain other geometrical parameters specified. A novel “mushroom” adiabatic distribution scheme for film cooling effectiveness and temperature has been discussed in the paper. This characteristic scheme is developed as a result of RWPs' vortices interaction with the coolant inlet jet and the hot mainstream flow. A detailed discussion of the mechanisms and the flow field properties underlying the effectiveness enhancement and other phenomenon observed has also been presented in the paper.

Effect of winglet location on performance of fin-tube heat exchangers with inline tube arrangement

Heat transfer performance of fin-tube heat exchangers can be enhanced by introducing longitudinal vortices generated using vortex generators (VGs). Location of VGs with respect to tubes plays major role in deciding the enhancement in heat transfer. Three-dimensional numerical investigations have been carried out to study the effect of winglet location on heat transfer characteristics of fin-tube heat exchangers for inline tube arrangement. In this study, rectangular winglet pair (RWP) type VGs having common flow down configuration are considered. Initially, effect of different possible locations of RWP corresponding to tube centre have been investigated to analyse the heat transfer and flow field characteristics. Further, effects of angle of attack (β) ranging from 15° to 60° and Reynolds number (Re) in the range from 2000 to 4000 have also been studied for the location associated with maximum heat transfer corresponding to mean values of β and Re in the considered range. To illustrate the interesting flow features of vortices generated due to placement of RWPs and how those affect heat transfer, streamlines and temperature contours have been investigated. Enhancement in heat transfer has been quantified using Nusselt number while pressure loss is estimated with the help of friction factor. Role of secondary flow induced by RWPs is quantified in terms of secondary flow intensity. Thermal performance factor (JF) is used to evaluate the combined effect of enhancement in heat transfer and increase in pressure drop. For upstream located RWPs, the value of JF is found to be higher due to lower pressure loss as compared to RWPs mounted in other regions. The value of JF is maximum for RWP located at ΔX = −2.0, ΔY = ±1.25 and correspondingly 37.6% enhancement in heat transfer is observed as compared to the case for the arrangement in absence of RWPs.

Numerical investigation for the optimal placements of rectangular vortex generators for improved thermal performance of fin-and-tube heat exchangers

In the present work, a numerical investigation has been performed to find out the best emplacement of rectangular winglet pairs (RWPs) for the improvement of the heat transfer and flow characteristics of a fin and tube compact heat exchanger at low Reynolds number ranges from 500 to 900. The RWPs are placed at two different locations i.e., adjacent to the tubes and behind the tubes namely, common-flow-up (CFU) and common-flow-down (CFD) orientations respectively. The effect of attack angle, span angle and location of RWPs on air-side thermal hydraulic performance is investigated numerically. Area goodness factor and heat transfer rate per unit fan power are considered as the performance evaluation criteria. It is found that with the increase in attack angle and span angle heat transfer rate increases causing an increase in pressure drop penalty as well. Vortex generators behind the tube can reduce the pressure drop but with a significant reduction in heat transfer. Furthermore, MOORA (Multi-objective optimization by ratio analysis) method is employed to obtain the most expedient position and orientation of RWPs.

Numerical simulation of heat transfer augmentation in fin-and-tube heat exchanger with various number of rows of concave rectangular winglet vortex generator

The passive method by using a vortex generator (VG) is an effective method for the improvement of convective heat transfer. This study is focused on usage of concave rectangular winglet vortex generator (CRW VG) for improving convective heat transfer in a fin-and-tube heat exchanger using numerical simulation. Concave rectangular winglet pairs (CRWP) and rectangular winglet pairs (RWP) VGs were mounted inside the gap between fins (gas side) with variations of the number of VG pairs of rows. Inlet air velocity variations expressed by the Reynolds numbers were ranged from 364 to 689. Augmentation of heat transfer is indicated by the ratio value of heat transfer convection coefficient between cases using VG and that without using VG (baseline). The results show that the convection heat transfer coefficient for cases using CRWP VG is higher than that using RWP VG. Convection heat transfer coefficient increases up to 102% by mounting CRWP VG at Re = 364. However, the increase in convection coefficient is accompanied by a rise in pressure drop to 216.8%.

Effect of Rectangular Winglet Pair in Common Flow Down Configuration on Heat Transfer from an Isothermally Heated Plate

ABSTRACTPresent three-dimensional numerical study aims to investigate the effect of mounting rectangular winglet pair (RWP) on heat transfer enhancement in flow over a flat plate. Computations for incompressible flow of air have been carried out using commercial software ANSYS Fluent. Flow of air has been considered over the surface of an isothermally heated horizontal plate in presence of RWP in the range of Reynolds number from 400 to 2000. Common flow down configuration of RWP has been considered to study the effect of various geometric parameters, such as length of RWP, spacing between leading edges of the winglets and angle of attack of RWP, on flow characteristics and enhancement in heat transfer. Flow and temperature field characteristics have been presented using streamlines and temperature contours near the plate surface and streamlines in cross-stream planes. Enhancement in heat transfer in presence of RWP has been quantified using cross-stream variation of local Nusselt number, streamwise variation of span-averaged Nusselt number and surface-averaged overall Nusselt number.

Effect of winglet location on heat transfer of a fin-and-tube heat exchanger

Heat transfer enhancement and pressure loss of a fin-and-tube heat exchanger with rectangular winglet pairs has been numerically investigated by solving the conservation equations of mass, momentum and energy. The flow is assumed to be laminar at different Reynolds numbers. The winglets are placed in common flow up configuration which increases the heat transfer in the wake region of the tube. It was found from the numerical investigation the rate of heat transfer is increasing with number of winglets. It was also found that the placement of winglets near the central tube is more effective in transferring heat compared to entrance and exit of the heat exchanger. Keeping other parameters fixed it was found from the numerical experiment that the heat transfer performance was increasing significantly with the attack angle (β) of winglets. There also exists an optimum stream wise distance (ΔX) for placement of winglets for which the heat transfer is found to be highest. An optimum value of span wise distance (ΔY) could be decided where the heat transfer to the fluid was found to be maximum.

Effect of rectangular winglet pair roll angle on the heat transfer enhancement in laminar channel flow

Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions in the system. One method involves the use of vortex generators to produce streamwise and transverse vortices on the top of the main flow. This study presents numerical computation results on laminar convection heat transfer in a rectangular channel which bottom wall is equipped with rectangular winglet pair vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible fluid. Three dimensional numerical simulations are performed to study the effect of the generators' roll-angle β on the flow and heat transfer characteristics. Different values of roll-angle β in the range [20°–90°] are considered, while maintaining a constant angle of attack (α = 30°) for all the cases. The influence of the Reynolds number is also studied for values 456 and 911 (based on the channel hydraulic diameter). Both local and global analyses of the thermal performances are carried out using parameters such as the Nusselt number and the friction coefficient. In addition, the position and strength of the longitudinal vortices created are presented and discussed, highlighting their effect on the heat transfer rates throughout the duct, for the various generators' roll-angles. Finally, from both local and global investigations, it is found that the optimal values of the roll-angle, determined for each Reynolds number, are not necessarily 90°, which corresponds to the widely used configuration in the open literature.

Enhancement of heat transfer in a fin-tube heat exchanger using rectangular winglet type vortex generators

The present study simulates the air flow through fin-tube type heat exchangers with rectangular winglet pairs (RWP) of half the channel height as vortex generators (VG). The heat exchanger is approximated as a periodic rectangular channel with heated walls and three rows of built-in tubes placed at an appropriate interval. Two different orientations of the tubes in the heat exchangers are considered here – one with inline arrangement of three tube rows and the other with staggered arrangement of three tube rows. Further, the angles of attack in each orientation are varied. The heat transfer characteristics of the heat exchangers with vortex generators located near the tubes have been compared among the cases with varied angles of attack and orientations of tubes. The Navier–Stokes and energy equations along with the appropriate boundary conditions are solved using the ANSYS FLUENT 14.5 solver. Performance parameters in terms of Nusselt number, vorticity and quality factor (a ratio between the Colburn factor to apparent friction factor, also refer to as area goodness factor with slight modification) were evaluated. The results show significant improvement in the heat transfer performance due to the nozzle-like flow passages created by the winglet pair and the region behind the circular tube which promote accelerating flow. There is an increasing trend of the above for the in-line row of tubes; whereas with the staggered row of tubes, there is slight deviation of this trend. Due to the alternate CFD–CFU orientations of the VG, the performance improves with increase in angle of attack up to a certain point and afterwards it is going down.

Numerical study of heat transfer enhancement by rectangular winglet vortex generator pair in a channel

A numerical simulation was performed to investigate the effects of longitudinal vortices on the heat transfer enhancement of a laminar flow in a rectangle duct mounted with rectangular winglet pair on the bottom wall. The studied Reynolds number which was defined using the hydraulic diameter twice the channel height ranges from 500 to 7000. The comparisons of the fluid flow and heat transfer characteristics for the cases with and without rectangular winglet pair were carried out. The effects of the height and attack angle of vortex generator pair on the heat transfer performance were investigated. The results show that mounting rectangular winglet pair on the bottom wall of the channel can significantly enhance heat transfer. The distributions of secondary flow on the cross sections are consistent with the distributions of Nu and J for different attack angles. The maximum heat transfer performance is obtained when the attack angle is 29° due to the maximum value of secondary flow generated by rectangular winglet pair.

Numerical Investigation of Heat Transfer Enhancement Using Hybrid Vortex Generator Arrays in Fin-and-Tube Heat Exchangers

This is a novel study for assessing the heat transfer enhancement in a multi-row inline-tube heat exchanger using hybrid vortex generator (VG) arrays, i.e., rectangular winglet pairs (RWPs) with different geometrical configurations installed in coherence for enhanced heat transfer. The three-dimensional numerical study uses a full scale seven-tube inline heat exchanger model. The effect of roll of rectangular winglet VG on heat transfer enhancement is analyzed and optimized roll angle is determined for maximum heat transfer enhancement. Four different configurations are analyzed and compared in this regard: (a) single RWP (no roll); (b) 3RWP-inline array(alternating tube row with no roll of VGs); (c) single RWP (with optimized roll angle VGs); and (d) 3RWP-inline array(alternating tube row with all VGs having optimized roll angle). It was found that the inward roll of VGs increased the heat transfer from the immediately downstream tube but reduced heat transfer enhancement capability of other VG pairs downstream. Further, four different hybrid configurations of VGs were analyzed and the optimum configuration was obtained. For the optimized hybrid configuration at Re = 670, RWP with optimized roll angle increased heat transfer by 17.5% from the tube it was installed on and by 42% from the immediately downstream tube. Increase in j/ƒ of 36.7% is obtained by use of hybrid VGs in the optimized hybrid configuration. The deductions from the current study are supposed to well enhance the performance of heat exchangers with different design configurations.

Numerical investigation of a finned-tube heat exchanger with novel longitudinal vortex generators

A novel longitudinal vortex generator (LVG), which is comprised of a modified rectangular wing and an accessory trapezoidal wing, was proposed. The performance of finned-tube heat exchanger with a single row of novel combined winglet pairs (NCWPs) was analyzed. Numerical simulation shows that compared with rectangular winglet pair (RWP), the NCWP generates main vortex with larger centric intensity. Furthermore, the accessory vortex has a lower core and flows into the back of tubes compressing wake regions' size behind tubes due to the influence of accessory wing. It can disturb the developing of boundary layers on the bottom fin and enhance effectively the heat transfer of the wake region behind the tube. Therefore, the performance of heat transfer for finned-tube heat exchanger is significantly enhanced by NCWP with a moderate pressure drop penalty. Compared with the heat exchanger without LVGs, the heat transfer for finned-tube heat exchanger with NCWPs is improved by 1.8–24.2%, with a pressure penalty increase of 1.3–29.1%. In addition, theory analysis and numerical simulation well agree with the experiment results, indicating the reliability of the research.

Comparison of winglet-type vortex generators periodically deployed in a plate-fin heat exchanger – A synergy based analysis

The objective of the present investigation is to assess the performance of a plate-fin heat exchanger with an emphasis on acquiring fundamental understanding of the relation between local flow behavior and heat transfer augmentation mechanism. Numerical simulations are performed in a rectangular channel containing built-in longitudinal vortex generators on the bottom wall arranged periodically both in the streamwise and spanwise directions. Two types of vortex generators, namely, rectangular winglet pair (RWP) and delta-winglet pair (DWP) with two different flow arrangements, common-flow-up (CFU) and common-flow-down (CFD) have been explored to assess the influence of shape and flow arrangements on heat transfer enhancement. The basic mechanisms of flow structure and heat transfer characteristics have been examined with the help of secondary velocity vectors, streamlines, and temperature contours. Additionally, the mechanism of the local heat transfer augmentation has been explained using a novel concept called the field synergy principle. The performance of the vortex generators has been compared based on integral quantities such as Nusselt number, pressure loss, performance evaluation factor and domain averaged synergy angle. The computations reveal enhanced mixing of fluid between the wall layer and the core due to strong secondary flows produced by vortex generators. The performance analysis indicates that the RWP is more effective in terms of heat transfer enhancement as compared to DWP. The field synergy analysis has shown that the sites with higher Nusselt number are associated with smaller synergy angle or better coordination between the velocity vector and the temperature gradient.

Fluid mixing in a microchannel with longitudinal vortex generators

A heat transfer enhancement technique based on longitudinal vortex generators (LVGs) has been well established for large-scale heat exchangers. Motivated by the success of the LVGs, this paper is intended as an investigation of micromixers based on the T-shaped channel with rectangular winglet pairs (RWPs) mounted on the bottom of the main channel. The RWPs stay with an angle of attack to the main flow direction and generate longitudinal vortices to enhance fluid mixing. The effects of geometrical parameters on the performance of micromixers with micro-scale LVGs are investigated by numerical simulations and the Taguchi method. The validity of numerical simulations is examined by comparing the numerical and experimental results. The results obtained for a wide range of Reynolds numbers show that the mixing efficiency of the micromixer with divergent RWPs is greater than that of the micromixer without RWPs for convection-dominant cases as well as diffusion-dominant cases. A static Taguchi analysis shows that the relative effectiveness of the geometrical parameters can be ranked as: asymmetry index>angle of attack>winglet height>winglet spacing. Based on the relative influence of the geometric parameters, we can obtain an optimal parameter group on the parameter selected range.

Numerical simulation of flow and heat transfer characteristics in wavy fin-and-tube heat exchanger with combined longitudinal vortex generators

The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger punched with combined rectangular winglet pairs (CRWPs) were numerically analyzed. The influences of several main parameters of the CRWP on heat transfer and friction were examined and compared to rectangular winglet pairs (RWP). The numerical results showed that the wavy fin-and-tube heat exchanger has a better heat transfer performance when the winglets punched on the suction side of the wavy fin. For the wavy fin punched with CRWPs, the Nusselt number and friction factor increase with increases of secondary attack angle, accessory winglet length or accessory winglet width. In comparison with RWP, the vortices generated by CRWP are more, larger and stronger. Hence, CRWP can more effectively improve heat transfer and increase friction.

Numerical investigation of turbulent flow and heat transfer in a channel with novel longitudinal vortex generators

A novel combined longitudinal vortex generator (LVG), comprising a rectangular wing mounted with an accessory rectangular wing, is developed and the turbulent flow and heat transfer characteristics are numerically analyzed. The influences of six main parameters of the combined rectangular winglet pair (CRWP) on heat transfer enhancement and fluid flow resistance characteristics in a rectangular channel are examined. The parameters include the location of accessory wing on the main wing and geometric sizes of the accessory wing. The Reynolds number range is from 2000 to 16,000. The numerical results show that in the range of the present study, the increase of the six parameters can result in the increase of heat transfer and pressure drop. Specially, the pressure drop decreases at large value of the distance of accessory wing from the channel bottom. In comparison with RWP, the CRWP generates vortices with larger area and lower core. Furthermore, the accessory wings generate vortices that swirl downward the channel bottom and disturb the boundary layer growth more effectively. Hence the heat transfer is enhanced. The numerical result for the Nusselt number of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.

Numerical study of heat transfer enhancement and fluid flow with inline common-flow-down vortex generators in a plate-fin heat exchanger

Three-dimensional numerical simulations are performed on a plate-fin heat exchanger (with triangular fins as inserts between the plates) to evaluate the laminar heat transfer and fluid flow characteristics with longitudinal vortex generators (LVGs). The effect with an inline rectangular winglet pair (RWP) with a common-flow-down (CFD) configuration is studied. The numerical results indicate that the application of inline LVGs effectively enhances the heat transfer of the channel. The heat transfer further increases with the increase in the Reynolds number from 200 to 500 and angle of attack from β = 15° to 22.5°. The computations are also performed to find the best location for the second RWP. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20414

Analysis of heat transfer and pressure drop for fin-and-tube heat exchangers with rectangular winglet-type vortex generators

In present work, heat transfer enhancement and pressure loss penalty for fin-and-tube heat exchangers with rectangular winglet pairs (RWPs) were numerically investigated in a relatively low Reynolds number flow. The purpose of this study was to explore the fundamental mechanism between the local flow structure and the heat transfer augmentation. The RWPs were placed with a special orientation for the purpose of enhancement of heat transfer. The numerical study involved three-dimensional flow and conjugate heat transfer in the computational domain, which was set up to model the entire flow channel in the air flow direction. The effects of attack angle of RWPs, row-number of RWPs and placement of RWPs on the heat transfer characteristics and flow structure were examined in detail. It was observed that the longitudinal vortices caused by RWPs and the impingement of RWPs-directed flow on the downstream tube were important reasons of heat transfer enhancement for fin-and-tube heat exchangers with RWPs. It was interesting to find that the pressure loss penalty of the fin-and-tube heat exchangers with RWPs can be reduced by altering the placement of the same number of RWPs from inline array to staggered array without reducing the heat transfer enhancement. The results showed that the rectangular winglet pairs (RWPs) can significantly improve the heat transfer performance of the fin-and-tube heat exchangers with a moderate pressure loss penalty.

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.

Compound heat transfer enhancement for shell side of double-pipe heat exchanger by helical fins and vortex generators

To improve heat transfer performance of shell side of double-pipe heat exchanger with helical fins on its inner tube, some vortex generators (VGs) were installed along the centerline of the helical channel. Heat transfer performance and pressure drop characteristic of the enhanced heat exchangers were investigated using air as the working fluid and steam as the heating medium. The helical fins were in the annulus and span its full width at different helical pitch. Wing-type VGs (delta or rectangular wing) and winglet-type VGs (delta or rectangular winglet pair) were used to combine with helical fins. The friction factor and Nusselt number can be well correlated by power-law correlations in the Reynolds number range studied. In order to evaluate the thermal performance of the shell side enhanced over the shell side without enhancement, comparisons were made under three constraints: (1) identical mass flow rate, IMF; (2) identical pressure drop, IPD and (3) identical pumping power, IPP. The results show the shell side enhanced by the compound heat transfer enhancement has better performance than the shell side only enhanced by helical fins at shorter helical pitch under the three constraints.