Submitted to Prof. Dr. Wahid Ghaly

Contents

Abstract.......................................................................................................................................................4

Introduction.................................................................................................................................................4

Diffuser.......................................................................................................................................................5

Vortex generators........................................................................................................................................6

A typical vortex generator system...........................................................................................................9

Instrumentation........................................................................................................................................11

Result and Discussion................................................................................................................................13

Discussion..............................................................................................................................................17

Conclusion.................................................................................................................................................19

References.................................................................................................................................................20

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Figure 1 Typical section of S-shaped duct...................................................................................................5Figure 2 A passive vortex generators on a turbine blade............................................................................7Figure 3 Supersonic VG integration in a rectangular nozzle lip for the purpose of jet................................7Figure 4 VG inside a C-D supersonic nozzle...............................................................................................8Figure 5 (a) Counter-rotating low-profile VG arrangement (b) realized section of the VG.........................8Figure 6 2D-duct with suggested position for VG at outside duct...............................................................9Figure 7 Triad of a VG system....................................................................................................................9Figure 8 Meridional section of the 3D-duct with probe measurement planes............................................10Figure 9 Instrumentation of the 2D-duct with total pressure rake at the (a) duct inlet and (b) exit (c) total temperature rake........................................................................................................................................12Figure 10 Distribution of the static pressure rise coefficient along the outer contour of 2D-duct..............13Figure 11 Oil flow visualization without VG............................................................................................14Figure 12 Oil flow visualization with VG.................................................................................................14Figure 13 Distribution of the static pressure rise coefficient along the outer casing, inner duct contour of the annular S-shaped duct with and without VG........................................................................................15Figure 14 Oil flow visualization at the casing for the ITD without VG.....................................................16Figure 15 Oil flow visualization at the casing for the ITD with VG..........................................................16Figure 16 Total pressure loss.....................................................................................................................17

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The Application of Low-Profile Vortex Generators

Abstract

Flow separation limits the efficiency of low-pressure turbines (LPT) in aircraft engines. With the trend toward higher bypass ratios in turbofans, demands are increasing for the low pressure turbines (LPT) that drive the large fan assemblies. Modern compressors make use of variable stators to successfully negotiate changes in operating conditions. Due to the elevated temperatures and rapidly expanding through flow area in the turbine, adaptive structures have yet to be incorporated into the “hot section”. However, several researchers have been exploring effective means of controlling LPT flow fields with aerodynamic flow control such as passive trips, plasma actuators, synthetic jets, and vortex generating jets. To minimize weight, fuel, and costs, particularly in the turbine which represents nearly 30% of the engine weight and 40% of the life cycle cost for parts replacement and servicing, this component has to be designed as short as possible. For this reason Low profile vortex generators are applied within the first bend of this S-shaped intermediate turbine diffuser in order to energize the boundary layer and further reduce or even suppress the occurring separation. In this project I am going to do in depth study of how this vortex generator affect the flow in turbine.

Introduction

During high-altitude cruise, the operating Reynolds number for the low-pressure turbine (LPT) in an aircraft gas-turbine engine can drop well below 2.5*10^4. This low Reynolds number condition is particularly acute in the class of small gas-turbine engines typically used or planned for use in many high-altitude uninhabited air vehicles. At these low Reynolds numbers, the boundary layers on the LPT blades are largely laminar, even in the presence of high free stream turbulence. This makes the blades very susceptible to low separation near the aft portion of the blade suction surface. Such separation causes a significant increase in losses through the turbine stage, with an associated system-level performance drop. Because of these large energy losses associated with boundary layer separation, flow-separation control remains extremely important for many technological applications of fluid mechanics. Controlling flow separation can result in an increase in system performance with consequent energy conservation as well as weight and space savings. Competitive pressures in the civil transport aircraft industry drive aircraft designers toward low-cost solutions, whereas combat aircraft have to operate efficiently over a wide range of conditions. This means compromises have to be made in aerodynamic design thus, considerations must given for certain aircraft system configurations featuring flows that are either separated or close to separation. Vortex generators are highly efficient aerodynamic devices used widely in both external and internal aerodynamics as means of flow control. They are local geometrical imperfections that cause the formation of longitudinal vortices giving rise

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to local mixing of the flow, energizing the boundary layer and consequently delaying or preventing separation or inducing secondary flow motion, which restructures the entire flow field. The vortex generators are located immediately downstream of the airfoil's pressure peak, and are contained completely within the boundary layer.

Diffuser

Diffuser is a mechanical device usually made in the form of a gradual conical expander intended to convert the dynamic pressure into static pressure of the fluid flowing through it. Depending on application, they have been designed in many different shapes and sizes. They can be made axial, radial, and curved to conform to the constraints imposed by the aspects of design. A well-designed diffusing duct should efficiently decelerate the incoming flow, over a wide range of incoming conditions, without the occurrence of stream wise separation. A short duct is desired because of space constraint and aircraft weight consideration, however this results in the formation of a secondary flow to the fluid within the boundary layer. The axial development of these secondary flows, in the form of counter rotating vortices at the duct exit is responsible for flow non-uniformity and flow separation at the engine face. It shows that shortening the duct increases the losses. Reducing the length means that the duct has to be designed very steep with strong curvatures, which leads to aggressive and further super aggressive intermediate turbine ducts (ITD). If the deflection of the flow in these ITD is too strong, separation can occur predominantly at the outer duct contour after the first bend due to the resulting peak suction at this position and the following strong adverse pressure gradient. The impact of these parameters is decreased by existing beneficial effects such as radial fluid movement through the low energy HP-turbine wake toward the casing that leads to a local thinning of the boundary layer. Also the appearing swirl increases the transportation of high energy fluid in the direction of the casing. Additionally the tip leakage vortex increases the turbulence at the outer casing and further stabilizes the boundary layer flow by moving high energy fluid into the near-wall flow.

Subsonic curved diffusers, as air intakes, find wide applications in the field of aircraft design especially in military aircrafts in which the engine is frequently carried in the fuselage and the intake is located in an offset poison. The performance of such diffusers, not only in terms of the total pressure delivered but also, and more significantly, in terms of the uniformity, velocity and direction of flow at the engine face affects the response of the engine. S-shaped ducts are widely used as intake ducts for fighter aircraft engines. The flow pattern in these ducts is quiet complex as a result of both curvature and diffusion and is further complicated due to presence of inflexion in the curvature. Flow development in S-shaped diffusers is influenced by different geometrical parameters like area ratio (AR), aspect ratio (AS), total divergence angle (2θ), angle of turn of the centre line (Δβ), inflexion in the curvature, and dynamical parameters like inlet Mach number (Ma), inlet turbulence and specifically the angle of attack when mounted on a aircraft.

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In an S-shaped compressor duct the flow starts to separate at the hub where the reenergizing effect of the tip leakage vortex is not present. Therefore, the flow tends to separate earlier compared with the flow within the turbine duct but the application of flow control devices is more promising due to this missing favourable effect.

Figure 1 Typical section of S-shaped duct

Vortex generators

In the continuing quest for improved turbine performance, the addition of vortex generators to..... A series of smart subsonic and supersonic flow controllers are presented with applications to the design of aircraft gas turbine engine components. Various kinds of flow effectors, e.g. resonating cavity, acoustic wave generator, oscillating flap, distributed suction, jet blowing, compliant surface, heating, cooling and vortex generators(VG) are available.

VG are passive flow control devices. By producing vortices high energetic fluid is transported from the main flow into the flow close to the wall thus reenergizing and stabilizing the surface flow. Further this can suppress or at least reduce an occurring separation. It has to be distinguished between high-profile (h>d) and low-profile VG (h<d), whereas the first ones preclude the boundary layer and the latter are lower than the boundary layer height. The main

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parameters that have a high influence on the efficiency of these low-profile VG are the angle of attack and the position relative to the separation.

Vortex generators capable of injecting co-rotating and counter-rotating stream wise vortices in subsonic, transonic and supersonic flow they are in on demand. The strength and location of the vortex is a control variable and must be optimized via a closed-loop control algorithm. The subsonic smart VG assumes a ramp-type geometry and the smart supersonic VG is a tailored cavity with a movable flap concealing the cavity. The movable flap is actuated inward to expose the cavity to transonic or supersonic flow. The depth of the cavity is controlled via a closed-loop feedback control system which ties the strength of the vortex to the desired performance as measured by one or more sensors. A smart vortex generator (SVG) system promises to reduce compressor-face (steady-state and dynamic) distortion levels thereby enhancing the stability margin of the compressor in a manoeuvring aircraft. On blades, the passive version of subsonic and supersonic VG can be used to control local separation and flow instabilities. The primary benefits of VG is wider operating margin in off-design performance as well as higher stage loading possibilities which will reduce the number of stages in compressors and turbines. The tip clearance flow could also be reduced by locally aligned, passive vortex generators, on the blade, which can interact with the scraping vortex to relieve the pressure gradient zone near the blade tip. Active jet noise suppression using cavity vortex generators near the nozzle exit lip promises to reduce take-off and landing jet noise for high-speed civil transports. Mixing enhancement in a scramjet combustor can be promoted by integrating supersonic vortex generators in the inlet duct prior to fuel injection. Flow attachment on a vector thrust nozzle ramp can also be effected by smart supersonic VG.

Due to very high-frequency (i.e., of the order of blade passing frequency, ω*n, where ω is the shaft angular speed and n is the number of blades) actuation requirements of smart vortex generators on turbo machinery blading and the high cycle fatigue characteristics of these actuators, only a passive version can be incorporated in compressors and turbines. The passive boundary layer control devices (i.e., VG) are to be designed into the blades and shall, under optimum conditions, accrue the following inter-related benefits:

1) withstand higher local diffusion rates at the engine off-design operation,2)mitigate secondary flow losses in the blade tip region,3) inject a stream wise vortex of arbitrary spin to control corner flow losses,

The three-dimensional layout of the proposed flow controllers, i.e., both subsonic and supersonic VG, on the turbo machinery blades is to be guided by performing accurate 3-D viscous computational fluid dynamics analysis of a single blade passage followed by the dynamic analysis of the compressor/turbine stage, i.e., the mutual interference problem. The blade surface streak pattern and its variation with operating conditions will serve as the initial value, or the guiding light, for the initial layout of the design and distribution of the flow control devices. The

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process, i.e., the blade design integrated with the passive flow control devices, by necessity has to be repeated to achieve an acceptable (or even an optimum) solution. To withstand higher local diffusion rates and manage the vortical flow on turbo machinery blades and flow passages, in short, shall be the goal of passive subsonic and supersonic VG integration in compressors and turbines.

Figure 2 A passive vortex generators on a turbine blade

Figure 3 Supersonic VG integration in a rectangular nozzle lip for the purpose of jet

Figure 4 VG inside a C-D supersonic nozzle

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In theory the optimal VGs should be indefinitely thin to minimize losses. Indeed this is not feasible in reality but nevertheless the VGs have to be designed as slim as possible with sharp edges. The boundary layer height could not be specified within the unsteady flow field of the ITD because of strong secondary flow effects and wakes as well as shocks emanating from the transonic HP-turbine stage. Nevertheless in order to find the right geometries and number of VGs, as well as their position within the duct, steady CFD simulations have to be performed.

Figure 5 (a) Counter-rotating low-profile VG arrangement (b) realized section of the VG

A typical vortex generator system

The building blocks of typical vortex generator system are sensor(s) processor(s), actuator(s).

The requirements on the sensors and actuators are: robustness, reliability/maintainability, volume/weight, power requirements, cost.

Figure 6 Triad of a VG system

Figure 7 Triad of a VG system

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To meet these requirements, the current sensors in a modern aircraft engine can be used in the VG system, e.g., engine face rake, thermocouples, pressure transducers and shaft rpmsensors. In addition, aircraft flight dynamic data (i.e., body angles and rates, flight speed and acceleration) can be incorporated in the control strategy of the smart aircraft system. The requirement on the processor, i.e., the computer memory/storage, number of processors and speed, strongly depends on the control strategy as well. The use of nonlinear adaptive control, e.g., as offered by neural networks, seems to be a promising approach in the development of flow control in aircraft engines.

Due to the highly unsteady three-dimensional (3-D) inflow condition with secondary flows, wakes, etc., which partly even have a positive effect on the instable boundary layer, the effectiveness of the VG had to be proofed in a much simpler configuration. Therefore, they have been studied within a two-dimensional (2-D) rectangular S-shaped duct with the same curvature of the outer duct contour, diffusion rate, and similar Mach number level as in the annular duct but with steady inflow conditions. swirl could not be simulated due to the rectangular shape of the duct, which results in a shorter effective flow path length. That means on one side the boundary layer growth along the end walls is reduced but on the other side the favourable radial fluid movement generated by the swirl is not present as well. Also the effects resulting from the wakes and secondary effects could not be considered but instead there is a radial fluid movement at the sidewalls of the 2D-duct similar to the one within the wakes. After the successful application of the VGs within the 2D-duct they were applied in the highly unsteady 3D flow field of the annular super aggressive ITD downstream of a transonic HPturbine stage.

Figure 8 2D-duct with suggested position for VG at outer duct

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Figure 9 Meridional section of the 3D-duct with probe measurement planes

Instrumentation

Above Figure.8 shows the test setup with 5HP and static pressure measurement locations. Full area traversing can be performed in seven different planes: downstream the HP-turbine (Plane C), within the duct (Planes C1, C3, C5, and C7), as well as upstream (Plane D) and downstream (Planes E and F) the LP-vane. There are 21 measurement positions over 1 HP-vane pitch and 15 over the channel height. In Planes C1–F the probes and the traverse gears were mounted in the inner liner of the ITD to protect the sensitive flow region at the outer wall from disturbances by the probe shaft. Due to the uncommon flow direction relative to the measurement location in some planes special five-hole-probes with an inclined probe were applied to extend the measurement range toward the probe shaft.

The incoming air is accelerated by the HP vanes in circumferential direction and impinges on the HP-rotor designed with a cylindrical outer contour. Then it is guided through the intermediate turbine duct to the vanes of a counter rotating LP-turbine assembled at a larger radius. There it is accelerated again and turned in the opposite circumferential direction. A downstream deswirler turns the flow back and recovers some pressure before the air leaves the facility through a diffuser and the exhaust casing. The HP vanes and LP vanes are fitted into fully rotatable casings to be able to change the relative position between the vanes so the applied measurement system can be kept fixed in space during measurement. For probe measurements only one linear and one

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rotational axis are necessary to adjust the radial position and to turn the probe into the flow. The inner wall of the duct is equipped with a removable plate, which is necessary for applying the oil film for surface flow visualization or it can be replaced with a large window insert for optical access. The nozzle guide vanes and the end walls were black passivated, and the rotor blades were covered with a high temperature flat black paint to reduce surface reflections.

The main component of the test setup is a super aggressive high diffusion intermediate turbine duct arranged downstream of the transonic turbine stage. The inlet flow of the ITD can be described as strongly transient with periodically impinging wake structures from the passing rotor blades, and highly turbulent together with shocks extending from the blade trailing edges. In order to reproduce the radial pressure distribution and the blockage effect generated by the downstream LP-vanes at the duct exit three simple straight profiles were placed at the same axial position. Furthermore, the rig is equipped with static pressure taps around the circumference of the inner and outer casing parts at the indicated measurement positions (see Figure. 8), to get the pressure distribution along the flow path. In Planes C1–C7 boundary layer rakes at the inner and outer surfaces of the ITD were used to record the total pressure profile near the walls.

The instrumentation of the rectangular S-shaped duct (see Figure. 9) consists of a removable total pressure rake at the duct inlet (a), a second total pressure rake implemented in the central straight vane at the duct exit (b), and static pressure taps at the outer contour. There are also

Figure 10 Instrumentation of the 2D-duct with total pressure rake at the (a) duct inlet and (b) exit (c) total temperature rake

static pressure taps at the inner contour at the duct inlet and exit. Instrumentation for the determination of duct flow characteristics has to meet high demands on accuracy and repeatability.

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Result and Discussion

Figure. 8 shows the distribution of the static pressure rise coefficient Cp along the outer duct wall of the 2D-duct at mean channel width for the configuration with and without vortex generators. The definition of Cp is,

Cp= pn−pinpout−pin

where,pn- is the local position pin- refer to the pressure at the duct inletpout- refer to the pressure at the duct exit

The gray rectangle in Figure. 8 indicates the position of the VG and the dotted line shows the location of the duct inlet. In both characteristics the flow is accelerated due to the strong curvature of the first bend until it reaches a maximum and then a fast deceleration follows. This slower acceleration at the outer duct contour of the setup with VG is a result of the upstream influence of these devices. The following deceleration starts about 5 mm further downstream for this configuration and the reached maximum pressure is lower than for the duct without VG. In both cases a pressure plateau occurs downstream the maximum of the static pressure. That indicates flow separation in this section. For the basic configuration the plateau has even a negative gradient of the pressure rise coefficient, which seems to be a consequence of the separation that decreases the effective area of the flow channel and furthermore increases the mean velocity.

The higher overall pressure recovery for the duct with VG is an evidence for the reduced separation.

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Figure 11 Distribution of the static pressure rise coefficient along the outer contour of 2D-duct

To point out the improvement through the VG the diffuser efficiency is given by,

Cp ,ideal=1−( 1

A R2)

and

Efficiency (nD )=( CpCp , ideal

)

These equations give a good estimation of the performance difference between the two configurations. For the configuration without VG the diffuser efficiency is about 38% but with the VG applied an efficiency of 60% can be reached. This again shows that the VGs have a very positive effect on the separated flow.

To get a qualitative insight into the flow along the outer duct wall oil flow visualization was performed. The results are shown in Figure. 11 for the duct without and in Figure. 12 for the duct with VG.

Figure 12 Oil flow visualization without VG

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Figure 13 Oil flow visualization with VG

The white marks on the left side of the channel locate the position of the separation and occurring flow phenomena. The green arrow indicates the flow direction. The black line crossing the channel shows the duct inlet.

Investigations of the 2D-duct without VG reveal a large separation. Its onset is displayed by the red rectangle in Figure. 11. Further downstream two symmetrical counter-rotating vortical structures are visible. The dark green arrows show their direction of rotation. These structures arise from the influence of the channel side wall effects resulting from the pressure difference between the inner and outer duct walls due to the deflection of the flow. The flow visualization shows a backflow region reaching from the onset of the separation to the duct exit. This shows that the separation expands in the center of the channel until the duct exit.

Investigations of the 2D-duct with VG displays in principle a separation with the same flow phenomena (counter-rotating vortices and backflow region in the channel center) like in the baseline configuration but the onset of the separation starts about 10 mm further downstream in Figure. 12. This is a result of the vortices generated by the VG. It can also be seen that less oil remains at the surface of the duct end walls. This indicates a much smaller separation zone than in the configuration without VG.

2) Figure 14 gives the distribution of the static pressure rise coefficient along the outer and inner contours of the super aggressive ITD. The pressure rise coefficient is given by,

Cp= pn−pcptc−pc

where,pc- refers to measurement Plane C at the duct inlet.

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The gray rectangle in Figure 14 displays the position of the VG at the casing. The static pressure distributions along the hub inner are quite similar for the measurements with and without VG and also the static pressure along the casing shows no significant differences. The static pressure at the duct exit is even lower for the configuration with applied VG, which indicates that the devices not only have no effect on the separation but also they actually decrease the static pressure recovery through the ITD.

Figure 14 Distribution of the static pressure rise coefficient along the outer casing, inner duct contour of the annular S-shaped duct with and without VG

The results from the oil flow visualization at the casing with and without VGs, respectively, are shown in Figures. 14 and 15. The green arrows display the rather axial inflow. The onset of the separation is illustrated by red lines. Between the lines vortical structures are visible in both figures. The figures confirm the results from the static pressure measurements that there is no significant influence at the hub dedicated to the VG. The only difference between the two measurements is a slightly better behaviour of the flow along the inner duct contour, which means for the case without VG the flow is less deflected into circumferential direction.

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Figure 15 Oil flow visualization at the casing for the ITD without VG

Figure 16 Oil flow visualization at the casing for the ITD with VG

Discussion

The results shows on one side that this type of low-profile VG works in an S-shaped 2D-duct with strong curvature and steady inflow conditions. On the other side the application of these passive flow control devices within an annular super aggressive S-shaped intermediate turbine duct with highly unsteady 3-Dflow conditions has no positive effect on the separation occurring at the outer duct contour downstream the first bend. On contrary the VG even increase the losses. It is assumed that the main parameter responsible for the inefficiency of the VG has to be an unsteady effect like the wakes as well as the shocks emanating from the HP-rotor and secondary flows. To be able to predict the behaviour and influence of VG more accurately it would be necessary to perform unsteady CFD simulations. The reason behind increase in the pressure losses is assumed that the strong tip leakage vortex coming from the HP-rotor transports high energy fluid from the outer to the near-wall flow similar to the operation of VGs and thus delaying the flow separation further

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downstream. The applied VG represent a barrier for these vortices and therefore their positive influence on the onset of the separation is reduced, which results ina larger separation and increased losses, respectively.

Figure 17 Total pressure loss

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Conclusion

On one side the results studied in this paper showed that the investigated low-profile VG are able to decrease an existing separation in an S-shaped rectangular duct with strong curvature, high Mach number, and steady inflow conditions. The onset of the separation has been moved about 10 mm further downstream and the pressure recovery has been increased significantly. On the other side the VG applied within the super aggressive intermediate turbine duct downstream the transonic HP-turbine could not decrease the separation at the casing. On the contrary they even increased the losses through the duct.

It has also been found that the upstream HP turbine rotor’s tip clearance flow played a major role in the flow development within ITD. Under tip clearance flow’s influence, flow is far from well-mixed flow field within the highly curved duct. Therefore we can say that area change rate may decide the stream wise pressure gradient of the flow within ITD, so it is a vital parameter when designing a well performed ITD. Therefore it can be concluded that to further explore the underneath physical mechanisms and improve the performance of ITD, additional detailed work should be carried out in the future.

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References

1)

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