Flow Field Measurements in an Annular Gas Turbine Exhaust Diffuser ...

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FLOW FIELD MEASUREMENTS IN AN ANNULAR GAS TURBINE EXHAUST DIFFUSER WITH STRUTS

11111 11 111111111111111111 Umberto Desideri and Stefano Ubertini Dipartimento di Ingegneria Industriale, Universita di Perugia Via G. Duranti 1N4 -06125 Perugia, Italy Phone+Fax +39 0755852736 e-mail: [email protected]

ABSTRACT This paper presents the velocity and turbulence characteristics of the flow in an annular diffuser, which is a model of a gas turbine

exhaust diffuser with six struts. The diffuser where the measurements were made is a scaled down model of a 10 MW gas turbine, built by GE/Nuovo Pignone. In a previous paper (Desideri and Manfrida, 1995) 2-D turbulence and velocity measurements were presented with axial inlet velocity conditions. In this paper a more detailed 3-D analysis of the design and off design behavior of the diffuser is presented. Turbulence characteristics were determined by means of two hot split-film probes, which allowed measuring axial, radial and tangential components of the mean velocity and their fluctuating components. The measuring point is moved inside the diffuser by means of two step-motors, which allow the rotation of the hub and the radial displacement of the probe. Off-design behavior of the annular diffuser was determined by changing the inlet velocity angle of 10° from axial direction. The effect of swirl on the performance of the diffuser will be presented. Turbulence rnicroscales were also calculated in regions of interest inside the diffuser, with particular attention to the strut wake.

NOMENCLATURE lateral correlation coefficient axial velocity component = Um+u tangential velocity component = Vm+v axial velocity component = Wm+w Temperature coefficient Dissipation rate Kolmogorov length scale Taylor's microscale Cinematic viscosity

INTRODUCTION Global parameters and equations, which correlate static and total pressure at inlet and outlet, are generally used to determine diffusers' performance. The design of diffusers is based on performance maps, such as pressure recovery and pressure loss maps (Japikse, 1986). In the cases, where the length of the diffuser is not a primary constraint, diffuser angles are determined from performance maps with ample margins to avoid separation phenomena. Design based on performance maps is easy, but maps do not consider the presence of obstacles inside the diffusers and the distortion of the flow at inlet. The performance of diffusers located after a turbomachine (axial compressors and turbines) is influenced by the highly turbulent flow at inlet. Therefore the knowledge on the inner flow behavior is required, in order to predict the flow field of the diffuser (Pfeil and Going, 1987). It is no longer appropriate to treat the diffuser, as an isolated component in the design process, but the whole diffusing system including the turbomachine has to be taken in account (Raab, et al., 1996). In fact, the increased level of turbulence caused by the blades of the turbine upstream of an exhaust diffuser, can sometimes improve the pressure recovery (Zierer, 1995). All the above reasons push towards a more detailed analysis of diffusers and mainly of their interaction with the flow coming from the turbine or the compressor. The exhaust diffuser of a gas turbine often features structural elements (struts), which are necessary to support the shaft. The presence of these objects influences the flow field and increases pressure losses, even though they are generally designed using aerodynamic profiles. Putting some attention to the design of the struts can effectively modify the structure of the vortexes shedding from the strut, and improve diffuser perfrornance (Fric, et al., 1996). Experimental investigations made by Norris, et al., (1998) within a diffusing S-shaped duct show that the presence of a row of struts has two detrimental effects:

Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Indianapolis, Indiana — June 7-June 10, 1999

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1). To keep geometric similarity of the model with the OT diffuser, the area ratio is maintained the same along the axis in both the model and the OT diffuser. The model was designed to operate in geometric and Reynolds number similarity with the OT diffuser. The OT diffuser operates with 41.3 kg/s at 462 °C and has a Reynolds number higher than 10 6. The Reynolds number in the model is high enough (Re > 6x10 5) to assume similar flow conditions in the OT diffuser and in the 35% scale model. The inner hub is a stainless steel cylinder rotatable over 360°. Four automated carriages for probe radial movements are mounted inside the inner hub, which has four holes to position the probes into the flow field. The carriages motion is automated by small geared stepping motors (Fig. 2). Another stepping motor has the function of rotating the hub in order to realize the circumferential movement of the probes. A 2-channels MSTEP-5 motor management board permits the control of the stepping motors by personal computer

They cause a blockage and consequently the flow accelerates, reducing the diffusion; They increase the size of the separation bubble. This paper presents the 3D description of the flow field of a model of an exhaust diffuser for gas turbines, including 24 blades at inlet that simulate the effect of the turbulence induced by the last stage of the upstream turbine. DIFFUSER MODEL

The aim of this paper is the characterization of the flow in a diffuser model built by Nuovo Pignone S.p.A. reducing in 0.35:1 scale the exhaust diffuser of a POT 10 gas turbine. The inlet of the model has been shaped in order to prevent flow separation in the first part of the diffuser. The shell is made up of transparent Plexiglas to check the position and the movements of the probes. At the inlet section there are 24 guide vanes which generate a wake similar to that produced by the blades of the last turbine rotor. The guide vanes can be rotated about their axis to simulate non-Tidal off-design flow at turbine outlet. The interesting part in the characterization of the diffuser is the distortion of the flow produced by the presence of six high-solidity struts, which support one of the shaft bearings and have piping oil supply inside for bearings lubrication of the gas turbine. 60 cm

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The inner and outer diameters of the inlet section are 190 mm and 320 mm. Axial length of the diffuser is 340 mm with an outer diameter at outlet of 420 mm. The resulting conicity of the shell is 1:0.28. The area ratio of the diffuser is 1.53 and the length/wide ratio is 2.01 (Fig.

Figure 3: Measurement sections arrangement.

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The model is connected to a wind tunnel with a 22 kW centrifugal-flow industrial fan. The flow rate is controlled by a sluice gate, positioned 3.40 m before the exhaust.

MEASUREMENT SECTIONS

The measurement sections, shown in Fig. 3, are named as section A, B, C and D. Section A is just behind the guide vanes; sections B and C allows the characterization of the flow in front and behind the strut. Section D is the section at outlet. Since the inner hub diameter is constant over the entire length, it is also possible to move the measurement sections in the axial direction. Figure 3 also shows the position of the measurement points in each section. The first radial point of acquisition is 15 min far from the inner hub and it corresponds to 0 mm radial position in all the plotted graphs. The radial movement of the probe is made up of 10 mm steps, to a safety distance from the shell. 31 angular positions correspond to every radial one, covering a sector of 60° containing one strut in the middle. Results have been produced for a 60°sector because the flow field repeats itself every 60 0 . The 60° sector was chosen in the upper part to avoid any interaction with the step-motor and its support, which disturb the flow behind in the lower part of the model.

Figure 4: Reference measurement system.

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In section B the downstream strut at 30° causes a strong deviation of the fluid with a significant reduction of the axial velocity. The presence of the guide vanes at 15° and 45° is still traceable. The ycomponent velocity graph, shown in Fig. 6, confirms the deviation caused by the presence of the strut. The plot of section C shows a wide area of low axial velocity, caused by the wake of the strut. Approaching the inner hub, axial velocity slows down and this can be mainly attributed to the growth of the boundary layer. The presence of the guide vanes is still detectable. It seems that the interaction between the guide vanes and the strut wakes causes the onset of the flow separation at the shroud. The Ycomponent of velocity graph (Fig. 6), shows how the flow reattaches behind the strut. In section D, the growth of the boundary layer is more evident, since the axial velocity decreases from 55 m/s, at mid-span, to 20 m/s near the hub. A clear zone of separation can be observed between the strut and the guide vanes close to the shroud. The Vm and Wm components of the velocity vector are shown in Figs. 7 to 10. Vm and Wm components are very small in Section A (Fig. 7). Span-wise deviations from the axial velocity are due to the presence of guide vanes. Since the thickness of the guide vanes is 5 mm and they present a rounded trailing edge, the V m component is always in the interval t4 m/s. The W m component is generally zero, except along the guide vanes, where a slight upward direction from the hub to the shroud can be observed. Before the strut there is a significant deviation in the V m direction, with a left-wise direction on the left of the strut and a rightwise direction on the other side (Fig. 8). Near the hub the W m component is negative (upward to the center of the section), whereas near the shroud it is positive, i.e. directed downward. This characteristics is the same along all the hub and the shroud, but the highest Wm component can be found along the sides of the strut (at 30°) and along the wakes generated by the guide vanes at 15° and 45°. In section C a secondary flow is quite evident on the sides of the main wake generated by the strut. The V m component is positive on both sides of the strut in the region near the hub from 0 to 35 mm, and negative near the shroud (Fig. 9). If we combine this component with the W, two vortexes with counterclockwise direction can be observed.

MEASUREMENT TECHNIQUES

The 3D characterization of the diffuser flow was made by using two types of split-film probes: DANTEC 55R56 split-film probe with R20 = 4.5 0 for one film and 4.80 for the other and a20 = 0.0039 for one film and 0.0041 for the other DANTEC 55R57 split film probe with R2o = 5.27 0 and 4.57 0, and a20 = 0.0042 and 0.0041. The 55R.57 probe allows the measurement of the x and y components of the velocity vector, whereas the 55R56 measures the components on the x-z plane (Fig. 4). The measurements with the two probes are made with the same flow velocity in the tunnel at two different times, so it is not possible to correlate turbulence quantities obtained on the x-y plane to those on the x-z one. The probes were connected to a dual-channel anemometric system by A.A. Labsystems, which includes amplification and filtering for each channel. The anemometer output was then transmitted to a sampling board and a PC elaborated the results. The sampling board used is a Metrabyte DAS-I 600, which features 16 channel and a 12digit AJD converter. The maximum acquisition rate is 100 kHz. The similarity of flow conditions among measurements made at different times is guaranteed by keeping the wind tunnel flow rate constant. This assumption also allows making measurements with only one probe at a time thus avoiding flow interaction between probes simultaneously located in the flow field. EXPERIMENTAL RESULTS

The flow field of the diffuser model was studied in two different conditions: with straight inlet blades; with 10° inclined inlet blades. Figure 5 shows the contour plots of the axial component of velocity at the four sections with straight inlet blades. The presence of the guide vanes can be easily detected in the plot of section A. A flow deviation and a reduction of the axial velocity, Um, is evident at 15°, 30° and 45°, which are the angular positions of the guide vanes.


In section D (Fig. 10), the flow is mainly redirected along the axis and Vm and Wn, components have almost disappeared. There still remains aslight Vm component due to the reattachment of the flow

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