Arrangement Of Vortex Generators Research Articles

Effect of novel vortex generator on parabolic trough solar collectors using ionic liquid

In order to improve the overall thermal performance of parabolic trough solar collector systems, numerical simulation with different forms of vortex generators inserted in the absorber tube using [EMIM][BF4] ionic liquid is presented. The absorber tubes with eight vortex generators are compared with smooth absorber tubes and verified from the field synergy principle by combining the finite volume method and the Monte Carlo ray trace method. The thermal efficiency, Nusselt number, friction factor, performance evaluation criterion factor, and exergetic efficiency with varying parameters like Reynolds number, structure, and arrangement of vortex generators are discussed. The results show that the Nusselt number of common flow up double winglet pair structure increases by almost 77% when the Reynolds number is 5000, and its thermal efficiency and exergetic efficiency improvements are both the highest, which are 2.68% and 1.51%, respectively.

Heat transfer performance of internal flow by inserting punched and non-punched vortex generators

This article proposed a new arrangement of planar vortex generators (VGs) and punched VGs within a heated tube. The airflow and heat removal performance of the VGs with new configurations were studied by numerical simulation and experimental methods in turbulent flow (6060∼21,210), and compared with two commonly used arrangements of VGs in tubes (tube fitted with all planar VGs and all punched VGs). The punched VGs used were with three perforation indexes of PI=7.8%,15.2%,46.6%, and three-pitch ratios of PR=1.63,3.27,4.89. An interesting discovery in the case of tubes fitted with VGs in the new configuration was that the strength of main longitudinal vortices displayed a periodic change from weak to strong along the mainstream direction, which presented relative low-pressure loss and poor thermal performance as well. The results indicated that when VGs had the same area holes, there was little difference between the heat enhancement factor (TEF) value of tubes fitted with three arrangements VGs when Re>10,000. Moreover, the increased area of punched hole resulted in poor heat removal rate and lower pressure loss and the smaller the PI was, the larger the TEF. Additionally, the TEF value increased with the decrement of PR. In most Re number ranges, the TEF values were lower than 1.0. The highest TEF of 1.44 was reached by utilizing the punched VGs with PI=7.8%,PR=3.27 at Re=6060.

Thermal-hydraulic characteristic of a novel wavy fin-and-circle tube heat exchanger with concave curved vortex generators

This paper proposes a novel configuration of two pairs of concave curved vortex generators located beside each tube in a wavy fin-and-circle tube heat exchanger. The effect of the concave curved vortex generator pairs with different transverse locations and attack angles on the thermal-hydraulic characteristics of the heat exchanger is numerically reported. The studied range of attack angle is 20° - 50° for five different transverse locations of vortex generators. The results show that the combination of concave curved vortex generators and wavy fins can significantly improve the heat transfer performance of the wavy fin-and-tube heat exchanger. Nu of the proposed fin configuration is enhanced by 30.6%, and the maximum thermal performance factor is increased by 24.0% compared with the wavy fin-and-tube heat exchanger without vortex generators. The multi-objective optimization and a neural network are adopted to optimize the vortex generator arrangements for the best heat transfer capacity. At the optimal Pareto front, the optimal heat transfer capacity is increased by 35.73% under the same pumping power. Correlations of Nu, f, JF and the optimal Qv and Pv are presented as a function of Re, transverse location, and attack angle of the vortex generator.

Numerical Study on the Influence of Vortex Generator Arrangement on Heat Transfer Enhancement of Oil-Cooled Motor

At present, vortex generators have been extensively used in radiators to improve the overall heat transfer performance. However, there is no research on the effect of vortex generators on the ends of motor coils. Meanwhile, the current research mainly concentrates on the attack angle, shape and size, and lacks a detailed study on the transverse and longitudinal distance and arrangement of vortex generators. In this paper, the improved dimensionless number R is used as the key index to evaluate the overall performance of enhanced heat transfer. Firstly, the influence of the attack angle on heat transfer enhancement is discussed through a single pair of rectangular vortex generators, and the results demonstrate that the vortex generator with a 45° attack angle is superior. On this basis, we compare the effects of different longitudinal distances (2 h, 4 h, and 6 h, h meaning the height of vortex generator) on enhanced heat transfer under four distribution modes: Flow-Up (FU), Flow-Down (FU), Flow-Up-Down (FUD), Flow-Down-UP (FDU). Thereafter, the performances of different transverse distances (0.25 h, 0.5 h, and 0.75 h) of the vortex generators are numerically simulated. When comparing the longitudinal distances, FD with a longitudinal distance of 4 h (FD-4 h) performs well when the Reynolds number is less than 4000, and FU with a longitudinal distance of 4 h (FU-4 h) performs better when the Reynolds number is greater than 4000. Similarly, in the comparison of transverse distances, FD-4 h still performs well when the Reynolds number is less than 4000, and FU with a longitudinal distance of 4 h and transverse distance of 0.5 h (FU-4 h–0.5 h) is more prominent when the Reynolds number is greater than 4000.

Combined experimental and numerical studies on flow characteristic and heat transfer in ribbed channels with vortex generators of various types and arrangements

This work numerically and experimentally studies the heat transfer performances and flow characteristics of vortex generators (VGs) in a high aspect ratio rectangular ribbed channel with the Reynolds number ranging from 20000 to 100000. The steady liquid crystal thermography (LCT) method is adopted to link the images captured by experiments and the Nusselt numbers. Four different cases combining ribs and VGs are considered, and they are compared with a case with five continuous ribs in the channel. For the first time, the effect of winglet VGs and the tetrahedral VGs on heat transfer performance are compared. Simultaneously, two different arrangements of VGs, i.e. common-flow-down VGs and common-flow-up VGs are also studied to figure out which arrangement is better for improving the heat transfer performance. Simulations are conducted to systematically compare the fluid flow and local heat transfer characteristics to reveal the underlying thermo-fluid mechanisms of the considered cases by using three-dimensional CFD numerical simulations with a verified turbulence model, i.e., the SST k-ω model in the transition state. Results indicate that in the range of 0 < x/P < 1, the Nusselt number is enhanced to a large extent due to larger space for the flow reattachment. In addition, it is found that the winglet VGs perform better than the tetrahedral VGs. The winglet VGs with the common-flow-down arrangement give the best overall thermal performance at all Reynolds numbers in most cases. More specifically, the heat transfer performance can be improved up to 5.51% by adopting the common-flow-down VGs compared with the case having five continuous ribs. Furthermore, the flow characteristics reveal that longitudinal VGs can generate vortices to disturb the boundary layers, which can enhance the flow mixing and augment the heat transfer performance.

Heat transfer enhancement of annular finned tube exchanger using vortex generators: The effect of oriented functional circumferential arrangement

Vortex generators (VGs) are widely employed to improve the heat transfer performance of tube-fin heat exchangers. In this study, a novel arrangement of VGs with ten triangular VGs deployed around the circular tubes regularly on both sides of fins is proposed. The influences of the fins on fluid flow and heat transfer characteristics are investigated numerically using the k-epsilon model. The results show that the novel configuration avoids interactive influence between VGs and also diverts the fluid to the near-wake of the tube. With the favour of VGs, the wake regions behind circular tubes are diminished and the mixing between hot and cold fluid is enhanced. The number of the vortices is increased for the arrangement with VGs compared to that without VGs, and the secondary flow is enhanced. Simultaneously, the intensity of secondary flow in the case with VGs is also increased by 27.8%–67.5%. The value of Nu increases with increasing Re, of which the case with VGs is improved by 37.7%–42.2%. The heat transfer is enhanced with aspect ratio decreasing from 3 to 1. When the AR = 1 and the b/L is 0.36, the optimal heat transfer performance can be obtained.

Reduction of Pressure Gradient and Turbulence Using Vortex Generators in Prosthetic Heart Valves.

Blood damage and platelet activation are inherent problems with present day bi-leaflet mechanical heart valve designs. Passive flow control through different arrangements of vortex generators (VG) as means of improving pressure gradients and reducing turbulence are investigated. Rectangular VG arrays were mounted on the downstream surfaces of a 23mm 3D printed mechanical valve. The effect of VGs on the resulting flow structures were assessed under pulsatile physiological flow conditions where high resolution particle image velocimetry measurement was performed. The co-rotating VGs showed lower Reynolds shear stresses and improved pressure gradients (PG) compared with the counter-rotating ones and the no-VG control one (that showed higher turbulence). RSS was found 38.13 ± 0.89, 12.95 ± 0.32, 15.75 ± 0.71, 24.54 ± 0.84 and 16.33 ± 0.58Pa for the control, co-rotating VGs, 8 counter-rotating VGs, 4 far-spaced VGs and 4 closely-spaced VGs, respectively. PG of 10.45 ± 0.94mmHg was obtained with co-rotating VGs and the difference was significant compared with the other configurations (control 14.88 ± 0.4mmHg; 8 counter-rotating VGs 13.76 ± 0.51mmHg; 4 far-spaced VGs 13.84 ± 0.09mmHg; and 4 closely-spaced VGs 15.37 ± 0.16mmHg). Co-rotating VGs for this application induce a more delayed flow separation and a more homogenized and streamlined transition of flow compared with the counter-rotating VGs. Passive flow control techniques deployed on BHMVs is potentially beneficial as significant control of flow at small length scales without inducing large-scale design modifications of the valve.

The optimal arrangement of vortex generators for best heat transfer enhancement in flat-tube-fin heat exchanger

The interaction of longitudinal vortices decreases the intensity of longitudinal vortices and inevitably affects the heat transfer performance of heat exchangers. In this paper, the effects of transverse distance of vortex generators (VGs) on the interaction of longitudinal vortices and the heat transfer performance are quantitatively studied. The increments of longitudinal vortex intensity, Nusselt number and friction factor resulted from the application of VGs are discussed in detail. The results show that the transverse distance of VGs obviously affects the interaction of longitudinal vortices, the heat transfer enhancement and the pressure loss characteristics of the heat exchanger. The interaction of co-rotating longitudinal vortices generated by VGs around the same tube is less affected by the transverse distance of VGs. While the interaction of counter-rotating longitudinal vortices generated by VGs around different tubes is closely related to the transverse distance of VGs. The interaction between counter-rotating longitudinal vortices plays a dominant role in the interaction process of longitudinal vortices. Optimal transverse distance of VGs exists for best heat transfer performance of the heat exchanger. Meanwhile, the transverse distance which leads to the worst heat transfer performance is also reported. For the largest Reynolds number studied in this paper, the maximum differences in the increments of intensity of longitudinal vortices, Nusselt number and friction factor for different transverse distance of VGs are 34.0%, 33.9% and 18.5%, respectively.

Experimental study of thermal performance and flow behaviour with winglet vortex generators in a circular tube

Vortex generators (VGs) are among the various technologies that have been developed to enhance heat transfer and ultimately increase the efficiency of compact heat exchanging devices. VGs create longitudinal vortices which do not decay until further downstream and consequently increase heat transfer rate with comparatively lower pressure drop. In this research, thermal performance and flow behavior with VGs insert in a circular tube is investigated experimentally. The effects of attack angles, blockage ratios, pitch ratios and arrangements of VGs on the thermal performance were studied. Experimental measurements with and without VGs insert in a tube were conducted for airflows within Reynolds numbers ranging from 6000 to 33,000, with for a constant heat flux on the tube surface. Four Delta winglets VGs were inserted in a circular pattern on the inner surface of a tube with the help of a narrow rod. The different sets of delta winglets were characterized by four attack angles β (0°, 15°, 30° and 45°), three blockage ratios B (0.1, 0.2, 0.3), three different row values, N (4, 8 and 12) and three relative pitch ratios PR (4.8, 2.4 and 1.6).The experimental results indicate that the Nusselt number decreases with pitch ratio, but increases with Reynolds number, attack angle and blockage ratio. Nusselt number increment (Nu/Nu0) decreases with Reynolds number and pitch ratio, but increases with blockage ratio and attack angle. Maximum Nusselt number increment (Nu/Nu0) with the VGs was observed as being almost 2 times larger than that of smooth tube, while the maximum friction factor increment (f/f0) was 4.8 times larger. Thermal performance enhancement (TPE) decreases with blockage ratio, attack angle, Reynolds number and pitch ratio. The largest TPE obtained was 1.45. In order to understand the mechanism of heat transfer enhancement, details of the flow behavior were also studied with a flow visualization experiment employing a high speed camera and smoke generator.

Flow control on the NREL S809 wind turbine airfoil using vortex generators

Vortex generators are widely used passive flow-control devices to improve the aerodynamic performance of large wind turbines. In this paper, the aerodynamic performance of the airfoil S809 without and with vortex generators was investigated using a computational fluid dynamic method of simulation. In this paper, the vortex generators in order to control the flow within the boundary layer of the S809 airfoil with the momentum transferring and the vortex trajectory was discussed. The results were obtained with three-dimensional incompressible Reynolds-averaged Navier-Stokes equations, and the turbulence was simulated with the shear stress transport k–ω turbulence model. It is shown that the vortex generators can effectively improve the aerodynamic performance of the airfoil S809 and decrease the thickness of the boundary layer. The stall phenomenon is delayed. Double vortex generator arrangements show better performance in the control of flow separation and further improve the aerodynamic performance of the airfoil S809. According to the results of the vortex generators, the vortex generators can effectively enhance the lift coefficient of the airfoil and control the flow separation. It can provide some instructional meaning to the application of vortex generators on the wind turbine blade, which benefits to the utilization of wind power.

Efficient Global Optimization of Vortex Generators on a Supercritical Infinite Wing

Multi-objective optimization of vortex generators on a transonic infinite wing is performed using computational fluid dynamics and a multi-objective genetic algorithm coupled with surrogate models. Vortex generator arrangements are defined by five design variables: height, length, incidence angle, chord location, and spacing. The objective functions are to maximize the lift-to-drag ratio at low angle of attack, to maximize lift coefficient at high angle of attack, and to the shift chordwise separation location downstream at high angle of attack. To evaluate these objective functions of each individual in the multi-objective genetic algorithm, the ordinary kriging surrogate model and the radial-basis-function/kriging hybrid surrogate model are employed because numerical analysis of the wing with vortex generators requires a large amount of computational time. Nondominated solutions are classified into five clusters with different aerodynamic characteristics. Comparison of the five clusters revealed that the balance among three objective functions is controlled mainly by vortex generator height, spacing, and their ratio. The solutions in each cluster have specific values of these three parameters, which identify the aerodynamic characteristics. In addition, appropriate values of design variables for generating the vortex most effectively are investigated.

Tube Transverse Pitch Effect on Heat/Mass Transfer Characteristics of Flat Tube Bank Fin Mounted With Vortex Generators

For one heat exchanger model of three-row flat tube bank fin mounted with vortex generators (VGs), the effect of transversal tube pitch on heat/mass transfer performance was investigated by the experimental method of naphthalene sublimation. For the same arrangement of VGs around the tube, it is found that the larger the transversal tube pitch, the larger the heat transfer enhancement because of less interactions of vortices generated from different VGs. Interaction of vortices decreases the heat transfer enhancement on the fin surfaces with and without VGs for the case with small transversal tube pitch. For isothermal condition, two correlated equations of average Nusselt number and friction factor considering the fin spacing, the attack angle of VG, the height of VG, the ratio of transversal tube pitch, and Reynolds number are reported.

A Numerical Study of Flow and Heat Transfer Enhancement Using an Array of Delta-Winglet Vortex Generators in a Fin-and-Tube Heat Exchanger

This work is aimed at assessing the potential of winglet-type vortex generator (VG) “arrays” for multirow inline-tube heat exchangers with an emphasis on providing fundamental understanding of the relation between local flow behavior and heat transfer enhancement mechanisms. Three different winglet configurations in common-flow-up arrangement are analyzed in the seven-row compact fin-and-tube heat exchanger: (a) single–VG pair; (b) a 3VG-inline array (alternating tube row); and (c) a 3VG-staggered array. The numerical study involves three-dimensional time-dependent modeling of unsteady laminar flow (330⩽Re⩽850) and conjugate heat transfer in the computational domain, which is set up to model the entire fin length in the air flow direction. It was found that the impingement of winglet redirected flow on the downstream tube is an important heat transfer augmentation mechanism for the common-flow-up arrangement of vortex generators in the inline-tube geometry. At Re=850 with a constant tube-wall temperature, the 3VG-inline-array configuration achieves enhancements up to 32% in total heat flux and 74% in j factor over the baseline case, with an associated pressure-drop increase of about 41%. The numerical results for the integral heat transfer quantities agree well with the available experimental measurements.

Empirical Model for Vane-Type Vortex Generators in a Navier-Stokes Code

An empirical model which simulates the effects of vane-type vortex generators in ducts was incorporated into the Wind-US Navier-Stokes computational fluid dynamics code. The model enables the effects of the vortex generators to be simulated without defining the details of the geometry within the grid, and makes it practical for researchers to evaluate multiple combinations of vortex generator arrangements. The model determines the strength of each vortex based on the generator geometry and the local flow conditions. Validation results are presented for flow in a straight pipe with a counter-rotating vortex generator arrangement, and the results are compared with experimental data and computational simulations using a gridded vane generator. Results are also presented for vortex generator arrays in two S-duct diffusers, along with accompanying experimental data. The effects of grid resolution and turbulence model are also examined.

Suppression of Pressure Oscillations in High-Mach-Number, Turbulent, Cavity Flow

Fluctuating wall pressure measurements have been made in a high-Reynolds-number, Mach 5, turbulent cavity flow. The cavity length to depth ratio was varied from 3 to 5 with a fixed width to depth ratio of 3. Slotted and vented upstream and downstream walls, slanted downstream walls (inclined in either the streamwise direction only or inclined streamwise and swept spanwise), and spoiler and vortex generator arrangements were used to explore their effectiveness at suppressing the pressure oscillations. In practice, wall geometry will be dictated by many constraints; thus, in the absence of a particular application, it is not meaningful to claim that one particular geometry is the most effective. However, consistent with work at lower Mach numbers, it is evident that certain slanted and swept downstream walls are quite effective, reducing the high mean pressure and rms pressure levels near the downstream wall/floor junction by 30-35% and 40%, respectively. At this high Mach number, the coupling between the shear layer dynamics and cavity acoustics is weak. As a consequence, the cavity mode frequencies, which are essentially independent of geometry changes, are in as good an agreement with closed-box frequency predictions as they are with the predictions of Rossiter's modified formula (Rossiter, J. E., Wind Tunnel Experiments on the Flow over Rectangular Cavities at Subsonic and Transonic Speeds, Aeronautical Research Council Reports and Memoranda, ARCR & M No. 3438, London, 1964).

Effects of vortex generators on the growth of a compressible shear layer

An experimental study has been made of the effects of initial conditions on the growth rate of a high Reynolds number, compressible, turbulent shear layer bounded by Mach 5 and Mach 3 airstreams. The shear layer was generated by the turbulent boundary layer leaving the trailing edge of a thick flat plate with an internal twodimensional Mach 3 nozzle. The objective of the study was to determine if the shear-layer growth rate could be increased by placing cylindrical and wedge-shaped vortex generators in the turbulent boundary layer upstream of the shear-layer origin. Mean pitot pressure surveys were made in the initial boundary layer and in the shear layer at several stream wise stations using a rake with seven tips oriented spanwise. The results show that the shear-layer growth rate can be increased by about 30% using cylindrical vortex generator arrangements; whereas, with wedge-shaped arrangements the growth rate is unaffected or even reduced.