Delta Winglet Pair Research Articles

Impact of Delta-Winglet Pair of Vortex Generators on the Thermal and Hydraulic Performance of a Triangular Channel Using Al2O3–Water Nanofluid

This paper presents the laminar forced convection of Al2O3–water nanofluid in a triangular channel, subjected to a constant and uniform heat flux at the slant walls, using delta-winglet pair (DWP) of vortex generator which is numerically investigated in three dimensions. The governing equations of mass, momentum, and energy are solved using the finite volume method (FVM). The nanofluid properties are estimated as constant and temperature-dependent properties. The nanoparticle concentrations and diameters are in ranges of 1–4% and 25–85 nm, respectively. Different attack angles of vortex generators are examined which are 7 deg, 15 deg, 30 deg, and 45 deg with range of Reynolds number from 100 to 2000. The results show that the heat transfer coefficient is remarkable dependent on the attack angle of vortex generators and the volume fraction of nanoparticles. The heat transfer coefficient increases as the attack angle increases from 7 deg to 30 deg and then diminishes at 45 deg. The heat transfer rate remarkably depends on the nanoparticle concentration and diameter, attack angle of vortex generator and Reynolds number. An increase in the shear stress is found when attack angle, volume fraction, and Reynolds number increase.

Numerical study of heat-transfer enhancement by punched winglet-type vortex generator arrays in fin-and-tube heat exchangers

The potential of punched winglet type vortex generator (VG) arrays used to enhance air-side heat-transfer performance of finned tube heat exchanger is numerically investigated. The arrays are composed of two delta-winglet pairs with two layout modes of continuous and discontinuous winglets. The heat transfer performance of two array arrangements are compared to a conventional large winglet configuration for the Reynolds number ranging from 600 to 2600 based on the tube collar diameter, with the corresponding frontal air velocity ranging from 0.54 to 2.3m/s. The effects of different geometry parameters that include attack angle of delta winglets (β=10deg, β=20deg, β=30deg) and the layout locations are examined. The numerical results show that for the punched VG cases, the effectiveness of the main vortex to the heat transfer enhancement is not fully dominant while the “corner vortex” also shows significant effect on the heat transfer performance. Both heat transfer coefficient and pressure drop increase with the increase of attack angle β for the side arrangements; the arrays with discontinuous winglets show the best heat transfer enhancement, and a significant augmentation of up to 33.8–70.6% in heat transfer coefficient is achieved accompanied by a pressure drop penalty of 43.4–97.2% for the 30deg case compared to the plain fin. For the front arrangements of VGs higher heat transfer enhancement and pressure drop penalty can be obtained compared to that of the side arrangement cases; the case with front continuous winglet arrays has the maximum value of j/f, a corresponding heat transfer improvement of 36.7–81.2% and a pressure drop penalty of 60.7–135.6%.

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.

Experimental study on heat transfer enhancement of wavy finned flat tube with longitudinal vortex generators

The performance of direct air-cooled condensers in power plant is affected significantly by air-side flow and heat transfer characteristics of the wavy finned flat tube. Experimental investigations were conducted for air-side flow and heat transfer with and without delta winglet pairs punched on the surface of the wavy fin. The different temperature fields of the heated wavy fin surface with and without delta winglet pairs were obtained by the infrared thermography technology. Both experiments and numerical simulations showed that a substantial increase in the heat transfer with six delta winglet pair generators on the wavy fin was obtained with the Reynolds number varying from 1500 to 4500, which was the range of the air flow in practical direct air-cooled condensers. The average Nusselt number increased by 21–60% with the Reynolds number varying from 1500 to 4500 and the average friction factor increased by 13–83% with the Reynolds number varying from 500 to 4500 in experiments. The average performance evaluation criteria, PEC, can be up to 1.31 with six delta winglet pairs punched on the wavy fin surface, indicating the high potential of heat transfer enhancement to direct air-cooled condensers by longitudinal vortex generators.

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 on the performance of a novel fin-tube air heat exchanger with punched longitudinal vortex generator

To enhance the heat transfer of fin-tube surface, two kinds of novel fin-tube surfaces with two rows of tubes in different diameter, arranged in staggered pattern are presented. Along the direction of air flow, the diameter of the first transverse row of tubes is smaller than that of the second. Delta winglet pairs working as longitudinal vortex generators are punched only around the first transverse row of tubes in reduced size, with either “common flow up” or “common flow down” orientation. The performances of two new types of air heat exchangers using the novel fin-tube surfaces (referred as SA and SB) and one traditional plain fin-tube heat exchanger (named BL) acting as baseline in contract are experimentally studied under dry-surface conditions. On the condition of the minimum cross velocity in heat exchanger umax=4.0m/s, air-side convective heat transfer coefficients of SA and SB increases by 16.5% and 28.2%, respectively, in comparison with that of BL. The air-side pressure drop decreases less than 10% for SA, and slightly decreases for SB, compared with the baseline heat exchanger. The present results show that one can achieve heat transfer enhancement accompanying with a reduction of pressure loss using the strategies of fin-tube surface, longitudinal vortex generator design and appropriate placement on the fin-tube surfaces.

Effect of longitudinal vortex generator on heat transfer in rectangular channels

Abstract Average convective heat transfer on the top and bottom surfaces of a plain plate and four plates with a pair of delta winglet longitudinal vortex generator punched directly from the plates at attack angles of 15°, 30°, 45° and 60° respectively, is experimentally and numerically studied in the present paper. The plate fixed horizontally in the center of the tested channel is designed to simulate the forced heat transfer on the both surfaces of a single piece of fin in a fin-tube heat exchanger. Experimental results show that the average Nusselt number on the surfaces of plate increases with the increase of the attack angle of delta winglet pair compared with that of plain plate without delta winglet pair in the test range. The average Nusselt number of the plate with attack angle of 60° is slightly higher than that of plate with attack angle of 45°, yet may bring larger pressure drop. Strong longitudinal vortices are visualized when air flow across the leading-edges of the winglet pair. And a portion of air stream flows into the lower channel through the punched holes on the plates after rushing at the frontal face of delta winglet, and disturbs the flow field of the lower channel further. The heat transfer in the tested channel of five cases is also numerically investigated. Variance of average Nusselt number between the numerical results and experimental results for the five cases are all less than 10%, validating the present models and methods used for the simulation of fin channel flow with punched vortex generators feasible and reliable. The computed velocity and temperature fields are analyzed to understand the details of fin channel flow with longitudinal vortex generator. The experimental and numerical results reveal that the transverse flow of air stream through the punched holes disturbs the air flow in the lower channel, enhancing the heat transfer on the under surface of fin.

Experimental investigations of thermal and flow characteristics of curved trapezoidal winglet type vortex generators

The performance of a pair of new vortex generators – curved trapezoidal winglet (CTW) has been experimentally investigated and compared with traditional vortex generators – rectangular winglet, trapezoidal winglet and delta winglet using dimensionless factors – j / j 0 , f / f 0 and R = ( j / j 0 )/( f / f 0 ). The results showed that delta winglet pair is the best in laminar and transitional flow region, while curved trapezoidal winglet pair (CTWP) has the best thermohydraulic performance in fully turbulent region due to the streamlined configuration and then the low pressure drop, which indicates the advantages of using this kind of vortex generators for heat transfer enhancement. Parametric study on CTWP showed that smaller attack angle ( β = 0° = and 15°), larger curvature ( b / a = 1/2) and larger angle of inclination ( α = 20°) gives better thermohydraulic performance under the present conditions. An appropriate spacing between the leading edges of a pair of CTW VG should be considered for different flow regions. In addition, double rows of CTWP do not show better thermohydraulic performance due to the larger pressure drop and the spacing between the two rows of CTWP should also be optimized. ► New VG-curved trapezoidal winglet has high thermohydraulic performance particularly for turbulent region. ► Curved trapezoidal winglet has low flow resistance due to the streamlined configuration. ► For present conditions, smaller attack angle, larger curvature and angle of inclination give better performance. ► Appropriate spacing between the leading edges of the new VG pair is important for overall performance.

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.

Air-Side Heat-Transfer Enhancement by a New Winglet-Type Vortex Generator Array in a Plain-Fin Round-Tube Heat Exchanger

The impact of a vortex-generation technique for air-side heat-transfer improvement is experimentally investigated through full-scale wind-tunnel testing of a plain-fin round-tube heat exchanger under dry-surface conditions. Inspired by the formation locomotion of animals in nature, a new vortex generator (VG) array deployed in a “V” is proposed in the present work, aiming to create constructive interference between vortices. The array is composed of two delta-winglet pairs and placed at an attack angle of 10 deg or 30 deg. Its effectiveness is compared with a baseline configuration and two conventional single-pair designs placed at 30 deg, a small pair with half the area of the array and a large pair with the same area as the array. The frontal air velocity considered ranges from 2.3 m/s to 5.5 m/s, corresponding to a Reynolds number range based on the hydraulic diameter of 1400–3400. The experimental results show little impact of the 10 deg array and a moderate heat-transfer improvement of up to 32% for the small pair, both introducing additional pressure loss of approximately 20–40%. For the 30 deg array and the large pair, similar augmentation of 25–55% in air-side heat-transfer coefficient is obtained accompanied by average pressure drop penalties of 90% and 140%, respectively. Performance evaluation using the criteria of the modified area goodness factor and the volume goodness factor indicates the superiority of the heat exchanger enhanced by the 30 deg array among all the investigated VGs. The VG array is found more effective at comparatively low Reynolds numbers, representative of many heating, ventilation, air-conditioning, and refrigeration applications and compact heat-exchanger designs.

C111 ボルテックスジェネレータを用いた伝熱促進 : 渦構造と熱伝達の関係(OS-7:電子機器における熱流動現象(I))

Heat transfer enhancing method is more and more needed to decrease power consumption. This study was carried out to develop a high performance vortex generator, and find out the relationship between vortex structure and heat transfer. Double delta winglet pair was developed to reduce pressure drop with keeping up vortex strength. It was clarified that double delta winglet pair had higher performance than delta winglet pair by 10% at flow velocity of 1.4m/s by measuring average heat transfer coefficient and pressure drop. It was found out that double delta winglet pair had the position of vortex core at short distance from heat transfer surface than delta winglet pair. Therefore, the heat transfer coefficient was larger than delta winglet pair's one, even though it's vortex strength was smaller than that.

Numerical study of fluid flow and heat transfer in a flat-plate channel with longitudinal vortex generators by applying field synergy principle analysis

Three dimensional numerical simulations are performed on laminar heat transfer and fluid flow characteristics of a flat-plate channel with longitudinal vortex generators (LVGs). The effects of two different shaped LVGs, rectangular winglet pair (RWP) and delta winglet pair (DWP) with two different configurations, common-flow-down (CFD) and common-flow-up (CFU), are studied. The numerical results indicate that the application of LVGs effectively enhances heat transfer of the channel. According to the performance evaluation parameter, ( Nu/ Nu 0)/( f/ f 0), the channel with DWP has better overall performance than RWP; the CFD and CFU configurations of DWP have almost the same overall performance; the CFD configuration has a better overall performance than the CFU configuration for RWP. The basic mechanism of heat transfer enhancement by LVGs can be well described by the field synergy principle.

Numerical study on laminar convection heat transfer in a channel with longitudinal vortex generator. Part B: Parametric study of major influence factors

This paper presents the influences of main parameters of longitudinal vortex generator (LVG) on the heat transfer enhancement and flow resistance in a rectangular channel. The parameters include the location of LVG in the channel, geometric sizes and shape of LVG. Numerical results show that the overall Nusselt number of channel will decrease with the LVGs’ location away from the inlet of the channel, and decrease too with the space between the LVG pair decreased. The location of LVG has no significant influence on the total pressure drop of channel. With the area of LVG increased, the average Nusselt number and the flow loss penalty of channel, especially when β = 45° will increase. With the area of LVG fixed, increasing the length of rectangular winglet pair vortex generator will bring about more heat transfer enhancement and less flow loss increase than that increasing the height of rectangular winglet pair vortex generator. With the same area of LVG, delta winglet pair is more effective than rectangular winglet pair on heat transfer enhancement of channel, and delta winglet pair-b is more effective than delta winglet pair-a. Delta winglet pair-a results in a higher pressure drop, the next is rectangular winglet pair and the last is delta winglet-b. The increase of heat transfer enhancement is always accompanied with the decrease of field synergy angle between the velocity and temperature gradient when the parameters of LVG are changed. This confirms again that the field synergy is the fundamental mechanism of heat transfer by longitudinal vortex. The laminar heat transfer of the channel with punched delta winglet pair is experimentally and numerically studied in the present paper. The numerical result for the average heat transfer coefficient of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.

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.

A note on the flow and heat transfer enhancement in a channel with built-in winglet pair

Counter rotating longitudinal vortices produced by winglet in a channel are known to enhance heat transfer. In the present investigation the flow structure and heat-transfer enhancement by a winglet pair of non-zero thickness has been studied. A delta winglet pair type vortex generator is placed in a hydrodynamically developed and thermally developing laminar channel flow. Computations are done by solving the unsteady, three-dimensional, incompressible Navier–Strokes equations and energy equation using a modified Marker-and-Cell (MAC) method. The flow structure is complex and consists of main, corner and induced vortices. It is observed that as compared to a channel without winglets, the heat transfer is enhanced by 33% when single winglet is used and by 67% when a winglet pair is employed. Effects of thickness of the winglets and Reynolds number on the heat transfer augmentation are presented.

Intensification of heat-transfer and mixing in multifunctional heat exchangers by artificially generated streamwise vorticity

Compact heat exchangers are well known for their ability to transfer a large amount of heat while retaining low volume and weight. The purpose of this paper is to study the potential of using this device as a mixer as well as a chemical reactor, generally called a multifunctional heat exchanger (MHE). Indeed, the question arises: can these geometries combine heat transfer and mixing in the same device? Such a technology would offer many potential advantages, such as better reaction control (through the thermal aspect [S. Ferrouillat, P. Tochon, H. Peerhossaini, D. Della Valle, Open-loop thermal control of exothermal chemical reactions in multifunctional heat exchangers, Int. J. Heat Mass Transfer, in press]), improved selectivity (through intensified mixing, more isothermal operation and shorter residence time, and sharper residence time distribution (RTD)), byproduct reduction, and enhanced safety. Several geometries of compact heat exchanger based on turbulence generation are available. This paper focuses on one type: vortex generators. The main objective is to contribute to the determination of turbulent flow inside various geometries by computational fluid dynamics methods. These enhanced industrial geometries are studied in terms of their thermal-hydraulic performance and macro-/micro-mixing ability [S. Ferrouillat, P. Tochon, H. Peerhossaini, Micromixing enhancement by turbulence: application to multifunctional heat exchangers, Chem. Eng. Process., in press]. The longitudinal vortices they generate in a channel flow turn the flow perpendicular to the main flow direction and enhance mixing between the fluid close to the fin and that in the middle of the channel. Two kinds of vortex generators are considered: a delta winglet pair and a rectangular winglet pair. For both, good agreement is obtained between numerical results and data in the literature. The vortex generator concept is found to be very efficient in terms of heat-transfer enhancement and macro-mixing. Nevertheless, the micro-mixing level is poor due to strong inhomogeneities: the vortex generator must be used as a heat-transfer enhancement device or as a static mixer for macro- and meso-mixing.

Heat Transfer Enhancement for Finned-Tube Heat Exchangers With Winglets

This paper presents the results of an experimental study of forced convection heat transfer in a narrow rectangular duct fitted with a circular tube and/or a delta-winglet pair. The duct was designed to simulate a single passage in a fin-tube heat exchanger. Heat transfer measurements were obtained using a transient technique in which a heated airflow is suddenly introduced to the test section. High-resolution local fin-surface temperature distributions were obtained at several times after initiation of the transient using an imaging infrared camera. Corresponding local fin-surface heat transfer coefficient distributions were then calculated from a locally applied one-dimensional semi-infinite inverse heat conduction model. Heat transfer results were obtained over an airflow rate ranging from 1.51×10−3 to 14.0×10−3kg/s. These flow rates correspond to a duct-height Reynolds number range of 670–6300 with a duct height of 1.106 cm and a duct width-to-height ratio, W/H, of 11.25. The test cylinder was sized such that the diameter-to-duct height ratio, D/H is 5. Results presented in this paper reveal visual and quantitative details of local fin-surface heat transfer distributions in the vicinity of a circular tube, a delta-winglet pair, and a combination of a circular tube and a delta-winglet pair. Comparisons of local and average heat transfer distributions for the circular tube with and without winglets are provided. Overall mean fin-surface Nusselt-number results indicate a significant level of heat transfer enhancement associated with the deployment of the winglets with the circular cylinder. At the lowest Reynolds numbers (which correspond to the laminar operating conditions of existing geothermal air-cooled condensers), the enhancement level is nearly a factor of 2. At higher Reynolds numbers, the enhancement level is close to 50%.

Local Heat Transfer and Pressure Drop for Finned-Tube Heat Exchangers Using Oval Tubes and Vortex Generators

This paper presents the results of an experimental study of forced convection heat transfer in a narrow rectangular duct fitted with an elliptical tube and one or two delta-winglet pairs. The duct was designed to simulate a single passage in a fin-tube heat exchanger. Heat transfer measurements were obtained using a transient technique in which a heated airflow is suddenly introduced to the test section. High-resolution local fin-surface temperature distributions were obtained at several times after initiation of the transient using an imaging infrared camera. Corresponding local fin-surface heat transfer coefficients were then calculated from a locally applied one-dimensional semi-infinite inverse heat conduction model. Heat transfer results were obtained over a Reynolds number range based on duct height of 670–6300. Pressure-drop measurements have also been obtained for similar elliptical-tube and winglet geometries, using a separate single-channel, multiple-tube-row pressure-drop apparatus. The pressure-drop apparatus includes four tube rows in a staggered array. Comparisons of heat transfer and pressure-drop results for the elliptical tube versus a circular tube with and without winglets are provided. Mean heat transfer results indicated that the addition of the single winglet pair to the oval-tube geometry yielded significant heat transfer enhancement, averaging 38% higher than the oval-tube, no-winglet case. The corresponding increase in friction factor associated with the addition of the single winglet pair to the oval-tube geometry was very modest, less than 10% at ReDh=500 and less than 5% at ReDh=5000.

NUMERICAL PREDICTION OF HEAT TRANSFER IN A CHANNEL WITH A BUILT-IN OVAL TUBE AND TWO DIFFERENT SHAPED VORTEX GENERATORS

Existing air-cooled condensers in geothermal power plants use fin-tube heat exchangers with circular tubes. The size of the air-cooled condensers is huge, and often the cost of the condensers is about one third of the plant cost. An effort is being made to reduce the size of the air-cooled condensers by enhancing the transport coefficients on the heat exchange surfaces. Improvement over the common fin-tube heat exchangers can be brought about by a strategic modification. First, the oval tubes may replace the circular tubes. Second, the delta-winglet type vortex generators can be added on the flat surfaces to improve the heat transfer even further. Finally, a rectangular winglet pair can be deployed instead of a delta winglet pair and the performance of two different shaped vortex generators can be compared. A computational study has been undertaken to show the promise of above-mentioned combinations on a representative element of a heat exchanger.

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.