Effects of the Leakage Flow Tangential Velocity in Shrouded Axial Compressor Cascades *

TSINGHUA SCIENCE AND TECHNOLOGY ISSNll1007-0214ll21/21llpp105-110 Volume 14, Number S2, December 2009 Effects of the Leakage Flow Tangential Velocity in Shrouded Axial Compressor Cascades * KIM Jinwook (O) **, KIM Tongbeum (), SONG Seungjin () Turbomachinery Laboratory, School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Korea; Key Laboratory for Strength and Vibration of Ministry of Education, School of Aerospace, Xi an Jiaotong University, Xi an 710049, China Abstract: Although compressor blades have long been shrouded for aerodynamic and structural reasons, the importance of the leakage flow in the shrouded axial compressor has been investigated recently. However, the effects of the leakage tangential velocity variation on the blade passage flow are unknown. Therefore, this paper presents an experimental investigation of the loss and flow turning in the blade passage in shrouded axial compressor cascades subject to the variation of the leakage tangential velocity. The newly found results are as follows. First, increasing the leakage tangential velocity reduces overall loss up to 32.6% compared to the reference case. Second, increasing the leakage tangential velocity spreads loss core in the pitch-wise direction so loss core becomes more two-dimensional. Third, increasing the leakage tangential velocity makes the near hub passage flow more radially uniform. Key words: shrouded axial compressor; leakage flow tangential velocity; total pressure loss; passage vortex Introduction Gaps between stationary and rotating components cannot be avoided in turbo-machines, and leakage flows will impair the compressors aerodynamic performance. Shrouding is one way to reduce the leakage and improve the blades structural integrity. When stators are shrouded, the hub ends (tips) of stator blades are connected to an annular ring, i.e., the so-called inner-band. Between the inner-band and shrouded cavity end-wall, a single or multiple seal-tooth assembly is used to reduce leakage through the shrouded cavity. Figure 1 shows schematically a stator blade shrouded Received: 2009-05-08; revised: 2009-06-20 * Supported by the 2nd BK21 Program, the Micro Thermal System Research Center at Seoul National University, Korea, and the SRC/ ERC Program of MOST/KOSEF (No. R11-2001-095-02002-0) ** To whom correspondence should be addressed. E-mail: kjw7604@snu.ac.kr; Tel: 82-2-8801701 at the hub with a single tooth labyrinth seal. In such seals, due to the adverse axial pressure gradient, the leakage flow enters the seal cavity downstream of the blades and rejoins the main flow upstream of the blades. Fig. 1 Meridional view of the shrouded cavity and stator blade In multi-stage axial compressors, the leakage flow through stator hub seals increases theblockage and secondary flow mixing in the main flow, resulting in the increased loss [1,2]. Typically one percent of the total

106 Tsinghua Science and Technology, December 2009, 14(S2): 105-110 mainstream mass flow leaks through for each percent of seal-clearance/blade span ratio [1]. Wellborn and Okiishi [3,4] found that the leakage flow rate spoiled the near hub performance of the stator and altered stator exit flow conditions. They also found an efficiency degradation of one percent and a three percent penalty in the pressure rise for every one percent increase in the seal-tooth clearance to blade height ratio. Similar trends have been predicted numerically by Wellborn and Okiishi [3-5] and Heidegger et al. [6] Heidegger et al. [6] and Demargne and Longley [7,8] also examined the influence of the tangential velocity of the leakage flow on the flow field. The tangential velocity of the leakage flow is set by the blade loading and the relative motion between rotating and stationary surfaces. Thus, it depends on the compressor s operating point. They found reductions in the overall blockage and loss with increasing leakage tangential velocity. Typically, the leakage flow has a lower momentum than the mainstream flow. The mass fraction and the tangential velocity of the leakage relative to the mainstream flow determine the extent of the inlet boundary layer distortion that influences secondary flows in the blade passage. Demargne and Longley [7] argued that the increase in the leakage tangential velocity reduced the tangential momentum thickness and the overall loss. Despite such efforts, the influence of the leakage flow tangential velocity variation on the blade passage flows remains unknown. Therefore, this paper aims to understand how the leakage flow tangential velocity affects passage flows by examining the flow downstream of the blade passages. The scope of current research is limited to shrouded axial compressor cascades. 1 Experiment 1.1 Test rigs A compressor stator cascade employing air was used for experiments. This facility has full seal cavities and secondary flow loops for the seal flow. Thus, these simulate the effects of the seal and the relative motion and facilitate detailed flow investigation in a stationary environment. In comparison, the cascade facility used by Demargne and Longley [7,8] did not have a seal but an upstream slot (and a separate downstream slot) to focus on the upstream (and separately down-stream) seal cavity trench effect. A secondary flow loop was used to circulate the leakage flow in the seal cavity. Inside the secondary loop, a controllable fan was used to adjust the tangential velocity in the cavity [9,10]. The test sections in both test rigs contained six stator blades and a shrouded cavity. The upper ends of the blades were attached to an end-wall plate (casing) while the hub ends were mounted on an inner-band. A single-tooth seal (Fig. 1) was placed under the innerband. The blade geometry at the mid-span of GE s LSRC was used to fabricate two-dimensional blades [1,2]. Parameters of the blades and the shrouded cavity are listed in Table 1. Table 1 Parameters of compressor stator cascade and operating conditions. c is the incoming flow velocity. Parameter True chord, C Span, H Pitch, S Inlet and exit flow angles, 1 and 2 Value 200 mm 196 mm 126.6 mm 47.6 o and 21 o Solidity, 1.58 Cavity depth, h 17.03 mm Seal-clearance/Cavity depth, /h 0.11 Reynolds number based on the true blade chord v y /c (Estimated flow coefficient, ) Note: *, values from Eq. (1) 1.2 Instrumentation 2.6 10 5 0.09 (2.31 * ) 0.20 (1.04 * ) 0.25 (0.83 * ) 0.35 (0.59 * ) 0.45 (0.42 * ) A 5-hole probe (United Sensor TM ) was traversed to measure stagnation pressure and flow angle at x/c x =1.30 from the blade leading edge. The pressure signals were acquired via a Scanivalve TM. In measuring plane, loss and flow angles were obtained from a grid of 34 (over the entire span) and 21 (over the one pitch) points in one blade passage located at the center of the test section. A Pitot-tube was positioned at the mid-span 0.5 axial chord upstream of the blade leading edge to monitor the incoming mainstream flow velocity.

KIM Jinwook (O) et al.effects of the Leakage Flow Tangential Velocity in Shrouded Axial 107 1.3 Determination of the leakage flow tangential velocity In a linear cascade, no rotating motion exists. In real machines, however, the motion of the hub relative to both the stator blades and the inner-band is expected to yield additional velocity mismatch between the mainstream and leakage flows. To the author s knowledge, Demargne and Longley s work [7,8] was the first attempt to use a linear cascade with upstream and downstream slots to study the effect of relative motion (i.e., leakage flow tangential velocity) in a linear cascade. However, the presence of an actual labyrinth seal differentiates the present work from theirs (Fig. 1). Also, a separate flow loop for the cavity regions was used to connect both sidewalls of the linear cascade, and the leakage flow tangential velocity (momentum) was imposed by a controllable fan (Fig. 2). value for both upstream and downstream cavities [3] U hub /U tip ~0.8 for the blade span used in this study and c=c x /cos( 1 ) where U hub is the rotating hub speed. In validating this expression for a flow coefficient of 0.44 [2], Eq. (1) yields v y /c 0.4, which shows good agreement to within 4.5% with numerical simulation results [11]. The use of the above correlation allows us to relate the operating conditions in the linear cascade experiments to the flow coefficient in rotating machines. An annular shrouded compressor cascade (with a stationary cavity wall) was selected as the reference case. In this case, the tangential motion of the leakage flow in the shrouded cavity is determined by the tangential component of the mainstream and shear stress on cavity surfaces. Computational fluid dynamics (CFD) analyses of this case were conducted in Ref. [12], and the value of v y /c=0.09 was obtained. Thus, v y /c=0.09 was selected to represent the reference case of annular cascade with a stationary cavity wall. 1.4 Data reduction Fig. 2 Schematics of the wind-tunnel with full shrouded cavity In this paper, a new parameter is introduced to characterize the leakage flow tangential velocity (v y ) relative to the mainstream velocity vy / c ( c ( cx cy ) ). 2 2 1/2 The use of this parameter facilitates determination of v y to be imposed for a given Reynolds number (or mainstream velocity). A simplified expression of v y /c can be obtained as a function of flow coefficient ( c x /U tip ) as vy U tip 1 0.308cos( 1) 0.208 (1) c cx where U tip is the rotor tip speed in rotating machines and 1 is the inlet flow angle. This expression was obtained by adopting v y /U hub ~0.385, which is the average Loss was measured using a 5-hole probe at the downstream. Here, the loss coefficient (Y P ) is defined as Pt P YP (2) P P t where P is the stagnation pressure. Reynolds number (Re) is based on the blade true chord C and upstream mean velocity c. Measurement uncertainty associated with the loss coefficient (Y P ) was found to be within 3.2% in a 95% confidence interval [13]. 2 Discussion 2.1 Overall loss Figure 3 shows the pitch-wise mass-averaged loss measured at x /C x 1.30 downstream from the blade leading edge. In all the five cases, the loss is concentrated near the casing and hub end-walls. Increasing v y /c from 0.09 to 0.45 reduces the span-wise extent of the blade hub region under the influence of the secondary flow from z/h 0.35 to 0.20. On the other hand, the loss in the outboard span region (z/h > 0.4) is largely unaffected. Also, for v y /c 0.45 (the operating point), the value of the loss coefficient is about 0.04 at the mid-span. This value matches the LSRC s s

108 Tsinghua Science and Technology, December 2009, 14(S2): 105-110 experimental data and thus builds confidence in our data. As the leakage flow tangential velocity increases to v y /c 0.09, 0.20, 0.35, and 0.45, the loss is reduced by 3.72%, 14.9%, 23.3%, and 32.6% compared to the reference case, respectively. The dependence of loss on the leakage tangential velocity is consistent with previous findings [5-8]. Fig. 3 Span-wise variations of the pitch-wise massaveraged loss coefficient with varying the leakage flow tangential velocity at x/c x =1.30 2.2 Total pressure loss contour Loss contours have been measured at 1.30C x from the blade leading edge. Figure 4 shows the span-wise and pitch-wise loss distributions for v y /c 0.09, 0.20, 0.35, and 0.45, respectively. The same contour level and range have been used in the figures. For the reference case (v y /c=0.09) in Fig. 4a, the loss core has lifted off from the hub surface and a second loss core has emerged. According to Kim [12], immediate downstream of the trailing edge, lossy flow emerges from the downstream cavity trench into the mainstream towards where relatively low pressure exits. This leakage flow pushes the loss core away from the hub and forms a distinctive second loss core. Thus, the loss continuously increases from the leading edge as flow convects downstream. For v y /c 0.45 in Fig. 4d, the radial extent and the magnitude of the overall loss have been reduced relative to Fig. 4a. Also the high loss region has shifted even closer to the pressure side compared to Fig. 4a. The high loss region now only covers up to about 5% (a) v y /c 0.09 (b) v y /c 0.20 (c) v y /c 0.35 (d) v y /c 0.45 Fig. 4 Total pressure loss contour with varying the leakage flow tangential velocity at x/c x =1.30 (contour interval is 0.05).

KIM Jinwook (O) et al.effects of the Leakage Flow Tangential Velocity in Shrouded Axial 109 span height, and the overall loss contour shape has become even more two-dimensional, covering almost the entire pitch at up to 20% span. Thus, entrainment effect has further reduced loss in the passage. The newly found effects of increasing v y /c on the loss in the downstream of the blade row can be summarized as follows. Downstream of the downstream cavity trench at x/c x =1.30, a significant radially downward shift as well as reduction of loss occurs with increasing v y /c. This result is due to a weakened secondary flow effect and is discussed in more detail in the next section. 2.3 Flow turning by the blades To examine the influence of the leakage flow tangential velocity on the flow angle, measurements have been made at x/c x 1.30. Figure 5 shows plots of pitch-wise area-averaged radial distribution of deviation angle at x/c x 1.30. The deviation angle is obtained by subtracting the exit blade angle k 2 from the measured exit flow angle 2. Measurement uncertainties associated with deviation have been found to be within 2.6% with a 95% confidence interval, using the method of Coleman and Steele [13]. matches the LSRC s experimental data. Therefore, the data obtained in this research builds confidence once again. The near hub deviation pattern indicates a classic vortex structure (with counter-clockwise rotation viewed from downstream) or a passage vortex. Increasing v y /c decreases the deviation for 0.05 z/ H 0.4 and increases the deviation for z/ H 0.05. Thus, increasing v y /c weakens the secondary flow and makes the flow more uniform. Also the radially downward shift of the vortex is visible. To the authors knowledge, this result is the first experimental validation of the numerical predictions by Heidegger et al. [5] Finally, the weakening is due to a combination of the change in the upstream flow condition, the reduced secondary flow generation within the passage, and the entrainment effect of the downstream cavity trench. 3 Conclusions This paper examines the effects of the relative motion between stationary and rotating surfaces (i.e., leakage flow tangential velocity, v y /c) on the blade passage flows in shrouded axial compressor cascades. The findings of this study are as follows. (1) A new shrouded compressor cascade facility has been developed which enables testing of the relative motion effects in a non-rotating environment. (2) The upstream injection into the mainstream flow of the leakage flow with increasing tangential velocity makes the flow in the passage become more radially uniform and weakens the secondary flow. Thus, the mixing loss between leakage and passage flows is reduced, and the overall loss is reduced. (3) With increasing leakage tangential velocity, the three-dimensional shape of the loss core, concentrated in the hub suction side corner, becomes more two-dimensional. References Fig. 5 Variation of the pitch-wise area-averaged deviation angle with the leakage tangential velocity at x/c x =1.30 The exit flow shows positive deviation (underturned) along the entire span (Fig. 5). The same as the loss contour, there is no visible effect of the leakage flow on the exit flow turning for z/h>0.4. At mid-span, all of the cases show a deviation angle of +5º regardless of the leakage tangential velocity. This value also [1] Wisler D C. Loss reduction in axial-flow compressors through low-speed model testing. ASME Journal of Engineering for Gas Turbines and Power, 1985, 107: 354-363. [2] Wisler D C, Bauer R C, Okiishi T H. Secondary flow, turbulent diffusion, and mixing in axial-flow compressor. ASME Journal of Turbomachinery, 1987, 109: 455-482. [3] Wellborn S R, Okiishi T H. Effects of shrouded stator cavity flows on multistage compressor aerodynamic performance. NASA Contract Report 198536, 1996.

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