Turbomachinery

The Application of Low-Profile Vortex Generators Gaurav (Student ID-6244653)

Concordia University Montreal CANADA +438-392-5350 gaurav_maniyar2250@yah oo.co.in 12/5/2012

Flow separation limits the efficiency of low-pressure turbines in aircraft engines. 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. 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. 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 .................................................................................................... 6 Figure 2 A passive vortex generators on a turbine blade ............................................................................. 8 Figure 3 Supersonic VG integration in a rectangular nozzle lip for the purpose of jet................................. 8 Figure 4 VG inside a C-D supersonic nozzle ................................................................................................ 8 Figure 5 (a) Counter-rotating low-profile VG arrangement (b) realized section of the VG ......................... 9 Figure 6 2D-duct with suggested position for VG at outside duct ................. Error! Bookmark not defined. Figure 7 Triad of a VG system.................................................................................................................... 10 Figure 8 Meridional section of the 3D-duct with probe measurement planes ............................................ 11 Figure 9 Instrumentation of the 2D-duct with total pressure rake at the (a) duct inlet and (b) exit (c) total temperature rake.......................................................................................................................................... 12 Figure 10 Distribution of the static pressure rise coefficient along the outer contour of 2D-duct .............. 14 Figure 11 Oil flow visualization without VG ............................................................................................. 14 Figure 12 Oil flow visualization with VG .................................................................................................. 15 Figure 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 ........................................................................................ 16 Figure 14 Oil flow visualization at the casing for the ITD without VG ..................................................... 17 Figure 15 Oil flow visualization at the casing for the ITD with VG .......................................................... 17 Figure 16 Total pressure loss ...................................................................................................................... 18

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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 4

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 welldesigned 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. 5

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