Generation Of Longitudinal Vortices Research Articles

Experimental Investigation on Vortex Generator's Distance From Film Cooling Hole

Abstract The effects of vortex generator distance from film cooling hole on increasing gas turbine blade film cooling efficiency have been studied experimentally. The technique of infrared (IR) thermography has been implemented to study the temperature field. The diameter of the film cooling hole and the cross-flow velocity define the Reynolds number, which is fixed at 2369. There are now three different jet to cross-flow blowing ratio adjustments: 0.5, 1.0, and 1.5. Three different vortex generator distance from film cooling hole have been studied experimentally to increase the gas turbine blade film cooling efficiency. It has been shown that lowest distance among vortex generator distance and film cooling hole, yields better film cooling efficiency. Overall, the delta winglet pair with lowest distance between vortex generator distance and film cooling hole outperforms the higher distance because of the generation of secondary longitudinal vortices that completely destroy counter rotating vortex structures because their rotational tendency is opposite to that of the counter rotating vortex pair (CRVP).

Experimental and numerical study on configuration, shape, distance and angle of attack in film cooling implementing vortex generator

The impact of vortex generator configuration, shape, angle of attack and distance between vortex generator and film cooling hole has been studied using both experimental and numerical methods for enhancing the gas turbine blade’s film cooling efficiency. Infrared (IR) thermography technique has been used for investigating the temperature field. In order to obtain the velocity field ANSYS FLUENT has been implemented. The diameter of the film cooling hole and the cross-flow velocity are used to calculate the Reynolds number, which is set at 2369. The blowing ratio of the jet to the cross-flow has been changed to 0.5, 1.0, and 1.5. Effect of CFU and common flow down configuration has been investigated. The angle of attack has been varied as 35° and 45°. It is observed that common flow down configuration of vortex generator performs better than Common Flow Up configuration. Common flow down configuration as well as lower distance between vortex generator and film cooling hole enhance the film cooling effectiveness due to generation of secondary longitudinal vortices which decrease the strength of the counter rotating vortex structures. Among all the shape (triangular, rectangular and trapezoidal) rectangular vortex generator performs better. Growths of shear layer vortices are modified due to the inclusion of vortex generator. Overall, Vortex Generator with common flow down configuration and 35° angle of attack performs better than Common Flow Up configuration due to generation of secondary longitudinal vortices which annihilates the counter rotating vortex structures due to having rotational tendency opposite to Counter rotating vortex pair.

Numerical analysis of microchannel heat sink performance using delta vortex generator

The design of unique microsinks arises from the desire to provide efficient cooling at low pressure drop for a more compact miniaturized electronic equipment. Three-dimensional numerical investigations of heat transfer and fluid flow have been carried out with delta vortex generator (DVG) structures in the rectangular microchannel. A pair of delta vortex generators (DVGs) are placed symmetrically about the horizontal mid plane in rectangular microchannels. Performance variances are assessed on the basis of the average Nusselt number (Nu), friction factor (f), and thermal performance (TP) along with the thermal resistance (RT) and hydraulic resistance (RH) for the Reynolds number in the range of 93 to 746. The parametric variation has been carried out by altering geometric parameters such as the angle of DVGs with centerline (θ), the number of pair of DVGs (n), the length of DVGs (l), the distance of DVGs from the centerline (B), the position of DVGs from the inlet for first pair (L1), and the distance between the vortex generators (S). The objective is kept to maximize the TP, to minimize the value of thermal resistance, and to decrease the increase in the hydraulic resistance for the same geometrical variation. The highest thermal performance equal to 1.256 has been achieved with the combination of θ, n, l, B, S, and L1 equal to 45°, 3, 0.6 mm, 0.1 mm, 4.5 mm, and 5.334 mm, respectively. For the same geometrical parameters, a minimum thermal resistance equal to 0.98 K/W and a hydraulic resistance equal to 3.234 kPa-min/ml have been attained at Re about 745.66. By minimizing the thermal and hydraulic resistance of the microchannel with DVGs, it becomes possible that heat can be more effectively transferred across the interface and the desired cooling can be achieved with less power input. The implementation of DVGs enhances intermixing through the generation of longitudinal and transverse vortices, leading to a more evenly distributed temperature across the channel. This enhancement in the heat transfer reduces the thermal resistance. Additionally, the vortices generated by DVGs modify the velocity profile, thereby decreasing the pressure drop and the hydraulic resistance. The specific impact of longitudinal vortices on the enhancement of the heat transfer through adjustments in geometric parameters is thoroughly elucidated.

Enhancement in Film Cooling Effectiveness Using Delta Winglet Pair

Abstract A simulation study has been carried out to investigate the influence of delta winglet pair (DWP) for improving the film cooling effectiveness of a gas turbine blade. Incompressible continuity, momentum, energy, and two equations k − ω SST model have been used for investigating the nature of flow field, temperature field, and turbulent statistics. The Reynolds number based on the jet velocity and the diameter of the film cooling hole is set at a constant value equal to 4232. The jet to the crossflow blowing ratio has been varied as 0.5, 1.0, and 1.5. The corresponding Reynolds numbers based on the crossflow inlet velocity and film hole diameter are equal to 6462, 4229, and 3231, respectively. It is observed that common-flow-down (CFD) DWP configuration augments the film cooling effectiveness due to generation of secondary longitudinal vortices, which annihilates the counter-rotating vortex structures present in the baseline flow. The generation of hairpin vortices and the growth of shear layer vortices are modified due to the implementation of DWP. The overall turbulence intensity and vorticity get reduced due to the presence of DWP. A maximum of 97.46% and a minimum of 61.50% enhancement in film cooling effectiveness have been observed at blowing ratio of M = 1.5 and M = 0.5, respectively. The wake region of the film cooling jet is modified due to DWP leading to formation of stagnation region and lower mixing resulting in higher film cooling effectiveness. The strength of horseshoe vortices is reduced due to the presence of DWP, resulting in lower mixing between the jet with the cross flows and stable film formation. Lower thermal boundary layer thickness is observed in DWP configuration compared to that of the baseline case confirming lower mixing due to the implementation of DWP. Overall, the present study explains the effectiveness of DWP configuration for enhancement of film cooling effectiveness of a DWP.

Generation of Arbitrary Longitudinal Polarization Vortices by Pupil Function Manipulation

Optical Vortices In article number 2000087, Xiang Hao and co-workers show that by manipulating phase and amplitude at the back pupil of a high numerical-aperture (NA) objective, arbitrary longitudinal polarization vortices within the tightly focused light field are realized.

Generation of Arbitrary Longitudinal Polarization Vortices by Pupil Function Manipulation

The properties of optical vortex provide new insights into a wide range of optical and physical phenomena, giving rise to numerous applications in multiple disciplines. Notably, as much as the associated technologies have become increasingly flexible, it is still challenging to simultaneously generate multiple longitudinal polarization optical vortices with random topological charges (l) and tunable intensities at arbitrary positions. Herein, a novel approach that, by manipulating phase and amplitude at the back pupil of a high numerical‐aperture (NA) objective, realizes arbitrary longitudinal polarization vortices within the tightly focused light field is developed. Building upon the same model, the impacts of polarization on the optical vortex field and its potential of tuning the ratio of the longitudinal polarization component are further investigated.

Flow Symmetry and Heat Transfer Characteristics of Winglet Vortex Generators Arranged in Common Flow up Configuration

The generation of longitudinal vortices is an effective method for promoting thermal performance with a relative low-pressure penalty in heat exchangers. The winglet pair can generate symmetrical longitudinal vortices on the cross-section of the channel. The heat transfer and pressure-loss characteristics of a pair of winglet vortex generators with different transverse pitches are numerically studied in this paper. The winglet pair arranged in a common flow up configuration generates a pair of symmetrical longitudinal main vortices with counter-rotating directions. The symmetrical flow structure induces fluid to flow from the bottom towards the top of the channel in the common flow region between the longitudinal vortices. The flow symmetry of the longitudinal vortices and the heat transfer performance are strongly affected by the transverse pitch of the winglet pair owing to the interaction between the longitudinal vortices. The optimal transverse pitch of the studied winglet pair with the best thermal performance is reported. The increments in the vortex intensity and the Nusselt number for the optimal pitch are increased by up to 21.4% and 29.2%, respectively.

Turbulization of a wake behind the double roughness elements in a hypersonic boundary layer

The study of the effect of close placing of two individual elements of roughness with the cylindrical shape on the nature of laminar-turbulent transition were performed. Experiments were performed for a blunt conical model with the radius 9 mm at Mach number M = 5. These double elements of roughness with different heights and divergence angles between elements were allocated at the blunted nose of the tested model. Measurements with a hot-wire anemometer provided information about the averaged and unsteady parameters of the boundary layer in the wake behind the roughness elements. For all types of roughness, we confirmed existence of an effective height of roughness; for the heights above the effective one, we observe deformation of the boundary layer margin and the growth of non-equilibrium in the wake. Process of turbulization behind a double roughness element (similar to the case of single roughness) is accompanied by the generation of longitudinal vortices and by the deformation of the velocity profile. Depending on this deformation, pulsations in the wake either enhance or decline. In contrast to the single roughness configuration, the double element roughness decreases the mass flowrate for a narrow range of angles (and fullness of the boundary layer profile is more significant). Meanwhile, flow turbulization occurs right behind the single element of turbulization, for the case of double element turbulization, the main gain in the mass flowrate occurs with the wake (developing from the boundary between twin elements). Roughness has significant influence on unsteady characteristics of the boundary layer (when the height of roughness element is lower than the effective height). The wake downstream the double elements roughness exhibits the interaction between vortices, and this reduces the effective Reynolds number (compared to the case of single roughness).

The impact of weak shock waves on the flow in the boundary layer of a flat plate with a variable sweep angle of the leading edge

The impact of weak shock waves on the boundary-layer flow of a flat blunt plate installed along the stream was experimentally studied for different sweep angles of the leading edge of the plate at Mach number 2. The shock waves in the incoming flow in the form of an N-wave were generated by 150×7×0.13-mm two-dimensional roughness provided on the side wall of the T-325 wind tunnel. It was confirmed that, when an N-wave was incident onto the plate leading edge with zero sweep angle, generation of longitudinal vortices accompanied by an increase in the pulsation level and by a change in the spectral composition of perturbations, was observed in the boundary layer. Anemometric data showed that, when the sweep angle of the leading edge changed from 0 to 25 degrees under the impact of the “catching-up” incident weak shock wave, the widening of the vortex in the supersonic boundary layer occurred.

Development of parametric space for the vortex generator location for improving thermal compactness of an existing inline fin and tube heat exchanger

Heat transfer augmentation due to the intentional generation of longitudinal vortices in a fin-and-tube heat exchanger strongly depends on the location of the generators. This combined experimental and numerical study in the Reynolds number range from 1415 to 7075 is a step forward in the direction of product development. It is aimed at first determining the parametric space for the locations of delta winglet type vortex generators (DVG) which are effective for heat transfer augmentation. Thereafter the locations suitable for maximum augmentation are found. This comprehensive investigation considers the delta winglets attacking the oncoming flow over a wide range of angles ranging from 15° to 60°. It is recognized that the winglet location suitable for maximum augmentation can be defined in two ways, one that delivers maximum enhancement while ignoring the increase in flow loss and the other that incurs justifiable increase in the flow loss. A study of the flow structure reveals that the wake of each tube of an inline fin and tube heat exchanger occupies almost the entire intervening space between the two consecutive tubes. This makes the wetted fin area thermally under-productive due to poor heat transfer coefficients. Augmenting heat transfer in the tube wake is very important for the compact design of such heat exchangers. It is observed that well positioned winglets are effective in reducing the ovality of the isotherms. Such a change is a manifestation of more effective and homogeneous heat transfer from the fin.

Influence of the louver and delta winglet geometry on the thermal hydraulic performance of a compound heat exchanger

Louvered fin heat exchangers are frequently used in heat transfer applications where air is one of the working fluids. By punching delta winglet vortex generators in the louvered fins, the size of the tube wakes can be reduced. The objective of this paper is to study the influence of the louver and delta winglet geometry on the thermal and hydraulic performance of such a compound heat exchanger. To this end, three-dimensional numerical simulations were performed. The effect of the louver and delta winglet geometry on the heat exchanger’s performance results in conflicting design requirements. A small fin pitch and large louver angle cause a strong flow deflection and thus a large contribution of the louvers. But in this case the generation of longitudinal vortices is suppressed and thus the effect of the delta winglets is small. Also the delta winglet geometry itself has an important influence. A well-considered geometry and location of the delta winglets is essential for an improved performance. Delta winglets completely located in the tube wakes should be avoided, because in this case they do not cause any enhancement effect.

On Longitudinal Vortices in Turbulent Jets at Aeroacoustic Interactions

Experimental results are presented on the generation of longitudinal vortices in turbulent jets using sawtoothed high-intensity sound waves excitation. The flow and sound waves were visualized by the direct shadowgraph method. It is shown that at external and internal acoustic excitation the streamline curvature and velocity growth direction in the generated vortices are favourable for Görtler instability appearance, and the formed longitudinal vortices can by identified as Taylor-Görtler vortices

PIV investigation of flow behind surface mounted permeable ribs

The flow behind surface mounted permeable rib geometries, i.e. solid, slit, split-slit and inclined split-slit ribs have been studied using flow visualization and PIV (2-C and 3-C) technique in streamwise and cross-stream measurement planes. The objective behind this study is to understand the flow structures responsible for heat transfer/mixing enhancement with simultaneous pressure penalty reduction by permeable rib geometries. The Reynolds number based on the rib height has been set equal to 5,538 and the open area ratio of permeable ribs is equal to 20%. The permeable rib geometries have shorter reattachment length in comparison to the solid rib. The maximum 41% reduction in reattachment length is observed for the inclined split-slit rib. The splitter mounted inside the slit leads to two corner vortices behind it. The corner vortices drag the flow from the primary recirculation bubble region towards the rib resulting in drop of the reattachment length. Two horseshoe vortices are present in the flow through the slit at both sides of the splitter due to the upstream flow separation. The slit inclination moves these horseshoe vortices closer to the bottom wall. A film like flow through the slit is present near the downstream corner of the inclined split-slit rib. The spanwise velocity gradient due to the splitter leads to vorticity and turbulence enhancement by vortex stretching. The inclination of the slit and the use of a splitter inside the slit are two important design parameters responsible in generation of near-wall longitudinal vortices. The flow field behind permeable ribs is dominated by vortical structures with definable critical flow patterns, i.e. node, saddle and foci. These predominant swirling flow motions contribute to the mixing enhancement behind permeable rib geometries.

A numerical study of heat transfer in fin?tube heat exchangers using winglet-type vortex generators in common-flow down configuration

Numerical investigation of flow and heat transfer in a rectangular duct with a built-in circular tube in the presence of delta-winglet type vortex generators has been carried out for moderate Reynolds numbers and three different angles of attack of the winglets (24?, 30? and 34?). The investigation is necessitated by the need to enhance heat transfer on the gas side of the fin-tube heat exchangers through generation of streamwise longitudinal vortices. The strategy for heat transfer enhancement involves introduction of a strong swirling motion in the flow field. The swirling motion can be generated by the longitudinal vortices. In the present study, the longitudinal vortices have been created by the delta-winglet type vortex generators in common-flow-down configuration. The vortex generators 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. The present numerical study is limited to isothermal fins, single row of tubes and laminar flow situations. The analysis can easily be extended to multiple rows of tubes. It has been separately verified that the flow situation retains its laminar nature for the range of Reynolds number considered in the present problem. Hence the heat transfer analysis does not use any turbulence model in the present investigation. The consequences of employing these vortex generators are a significant separation delay and removal of the zone of poor heat transfer from the near-wake of the tubes. The heat transfer in the near-wake region can be enhanced as high as 230%. Results also show a marked increase in overall heat transfer in the channel.

A thermodynamic approach to the boundary layer flow system

A thermodynamic approach to the boundary layer flow system is used to investigate the control rule underlying the flow field. Application of Pontryagin's Maximum Principle from control theory shows minimum rate of entropy production to be the control rule for the flow field, in common with other dissipative processes. This result is used to investigate possible organised motion in the turbulent boundary layer and the generation of longitudinal vortices which is known to be an intrinsic characteristic of turbulence.

Generation of longitudinal vortices in internal flows with an inclined impinging jet and enhancement of target plate heat transfer

Abstract Flow velocity measurements and heat transfer experiments were carried out for a jet obliquely discharged into a crossflow in a duct. The flow measurements were made with optical fiber laser Doppler velocimetry, and the heat transfer experiments were made by making use of a thermochromic liquid crystal. The obtained velocity data reveal that the inclined jet is effective in generating longitudinal vortices in the crossflow. It is demonstrated that the longitudinal vortices are effective in enhancing target plate jet impingement heat transfer. It is, therefore, promising as a means to effectively cool inner surfaces of high temperature gas turbine vanes. The obtained heat transfer data are discussed in detail in comparison with the measured flow field and turbulence data.

Mass transfer enhancement by longitudinal vortices

From numerical and experimental investigations it has been recently established that convective heat transfer can be dramatically enhanced by the generation of longitudinal vortices in the flow. The phenomenological similarity between heat and mass transfer suggests that longitudinal vortices should increase also mass transfer. The mixing between two parallel streams of two components in a rectangular channel with and without a pair of rectangular winglets as vortex generators has been numerically investigated. The results show that one pair of vortex generators can increase the global mixing by more than 50 for laminar flow. This global mixing has been defined as the sum of the square of the differences of concentrations.

Generation of Longitudinal Streamwise Vortices—A Device for Improving Heat Exchanger Design

Laminar flow and heat transfer characteristics in a rectangular channel, containing built-in vortex generators of both the slender delta-wing and winglet-pair type, have been analyzed by means of solution of the full Navier–Stokes and energy equations. Each wing or winglet pair induces the creation of streamwise longitudinal vortices behind it. The spiraling flow of these vortices serves to entrain fluid from their outside into their core. These vortices also disrupt the growth of the thermal boundary layer and serve ultimately to bring about the enhancement of heat transfer between the fluid and the channel walls. The geometric configurations considered in the study are representative of single elements of either a compact gas-liquid fin-tube crossflow heat exchanger or a plate-fin crossflow heat exchanger. Physically, these vortex generators can be mounted on the flat surfaces of the above-mentioned heat exchangers by punching or embossing the flat surfaces. They can also act as spacers for the plate fins. Because of the favorable pressure gradient in the channel, the longitudinal vortices are stable and their influence persists over an area many times the area of the slender vortex generators. From a heat transfer point of view, the delta-wing generator is found to be more effective than the winglet-pair. However, most convective heat transfer processes encounter two types of loss, namely, losses due to fluid friction and those due to heat transfer across finite temperature gradient. Because these two phenomena are manifestations of irreversibility, an evaluation of the augmentation techniques is also made from a thermodynamic viewpoint. Conclusions that are drawn thus include discussion about the influence of vortex generators (wings/winglets) on irreversibility.

Large- and small-scale stirring of vorticity and passive scalar in a 3-D temporal mixing layer

We investigate, with the aid of a three-dimensional direct-numerical simulation at high resolution, the origin and topology of the longitudinal vortex filaments which appear in the temporally growing mixing layer. The basic velocity field is a hyperbolic-tangent profile U tanh(2y/δi), with a Reynolds number of Uδi/ν =100. The calculation uses pseudospectral methods, and is carried out at a resolution of 1283 grid points in a cubic box of size L containing four fundamental most-amplified wavelengths (L=4λa). The initial velocity field is the basic velocity, upon which is superposed a three-dimensional Gaussian perturbation of wide spectrum, peaking at ka=1/2πλa, with kinetic energy equal to 10−4U2, modulated by a Gaussian exp[−(y/δi)2] in the transverse direction. A passive-scalar transport equation is solved as well, with the same initial profile as the basic velocity profile. Isosurfaces of the passive scalar and three vorticity components are visualized, permitting the 3-D vortex structure of the flows to be revealed. Because of the initial spanwise decorrelation of the perturbation phase, the fundamental spanwise vortices that appear have strong spanwise oscillations which are not in phase, and hence cannot be interpreted in terms of the translative instability investigated by Pierrehumbert and Widnall.1 Pairings between the primary vortices exhibit the same spanwise decorrelation, and reconnections of the billows occur. Visualizations also show the generation of thin longitudinal vortices from the regions of reconnection. The vortex lines at the same moment indicate that the thin longitudinal vortices result from an intense longitudinal stretching of the spanwise vortex line when it is severely twisted in the reconnection region. Reconnected Kelvin–Helmholtz billows give rise to a three-dimensional Λ-shaped structure of the passive scalar. This behavior is in agreement with the theory developed by Lesieur et al., where it was proposed that the large-scale three-dimensionality of the temporal mixing layer perturbed randomly was governed by two-dimensional turbulence unpredictability mechanisms. It might explain the experimental findings of Breidenthal2 and Bernal and Roshko3 concerning a spatially-growing, unforced mixing layer. Notice also that the thin longitudinal filaments forming in our calculation do not seem to be explained by the mechanism proposed by Lasheras and Choi,4 where they originate from the straining by the big rollers of a spanwise vortex filament perturbed about the stagnation line.

Formation of longitudinal vortices in the sublayer due to boundary-layer turbulence

A number of experiments have indicated that the behaviour of longitudinal vortices in the sublayer of a turbulent boundary layer has a significant effect on the equilibrium of the whole flow field, but the formation of such longitudinal vortices remains unclarified. In the present paper, paying attention to the random turbulent motions normal to the wall which induce a dynamic instability causing the generation of the regular longitudinal vortices, a turbulence model is introduced to analyse the generation of the longitudinal vortices in the sublayer of a turbulent boundary layer.By integrating the results of the analysis with previous experimental results for the behaviour of developing longitudinal vortices, a feedback loop linking regular and irregular motions in the turbulent boundary layer is presented.It has been experimentally confirmed that even at low Reynolds number, regular longitudinal vortices are formed when a disturbance is applied to a boundary layer. As for the stability region described in terms of the intensity of the disturbance and the wavenumber of the longitudinal vortices, no conflict between analysis and experiment is observed. Using the thermodynamic argument of irreversible processes the formation of longitudinal vortices close to the wall by the turbulence in the boundary layer is shown to be physically reasonable.