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CHARACTERISTICS OF THE TIP LEAKAGE VORTEX IN A LOW-SPEED AXIAL COMPRESSOR WITH DIFFERENT ROTOR TIP GAPS

Zhibo Zhang National Key Laboratory of

Science & Technology on Aero-Engine Aero-Thermodynamics,

School of Jet Propulsion, Beijing University of Aeronautics and Astronautics, Beijing 100191,

China

Xianjun Yu National Key Laboratory of

Science & Technology on Aero-Engine Aero-Thermodynamics,

School of Jet Propulsion, Beijing University of Aeronautics and Astronautics, Beijing 100191,

China

Baojie Liu National Key Laboratory of

Science & Technology on Aero-Engine Aero-Thermodynamics,

School of Jet Propulsion, Beijing University of Aeronautics and Astronautics, Beijing 100191,

China

ABSTRACT The detailed evolutionary processes of the tip leakage flow

/vortex inside the rotor passage are still not very clear for the difficulties of investigating of them by both experimental and numerical methods. In this paper, the flow fields near the rotor tip region inside the blade passage with two tip gaps, 0.5% and 1.5% blade height respectively, were measured by using stereoscopic particle image velocimetry (SPIV) in a large-scale low speed axial compressor test facility. The measurements are conducted at four different operating conditions, including the design, middle, maximum static pressure rise and near stall conditions. In order to analyze the variations of the characteristics of the tip leakage vortex (TLV), the trajectory, concentration, size, streamwise velocity, and the blockage parameters are extracted from the ensemble-averaged results and compared at different compressor operating conditions and tip gaps. The results show that the formation of the TLV is delayed with large tip clearance, however, its trajectory moves much faster in an approximately linear way from the blade suction side to pressure side. In the tested compressor, the size of the tip gap has little effects on the scale of the TLV in the spanwise direction, on the contrary, its effects on the pitch-wise direction is very prominent. Breakdown of the TLV were both found at the near-stall condition with different tip gaps. The location of the initiation of the TLV breakdown moves downstream from the 60% chord to 70% chord as the tip gap increases. After the TLV breakdown occurs, the flow blockage near the rotor tip region increases abruptly. The peak value of the blockage effects caused by the TLV breakdown is doubled with the tip gap size increasing from 0.5% to 1.5% blade span.

NOMENCLATURE

Abbreviations De compressor design condition Mid compressor middle condition Max compressor maximum static pressure rise condition Ns compressor near-stall condition SPIV stereoscopic particle image velocimetry TLV tip leakage vortex

Nomenclature Ab local reduced through-flow area caused by the TLV A, T, R compressor axial, tangential and radial directions Bm mass-flow-based blockage coefficient C blade chord length of rotor tip Cp static pressure rise coefficient, (Pout - Pin)/(½ ρVtip

2) H rotor blade height Kp the slope of the linear least squares fit line of Yp and L/C Kx the slope of the linear least squares fit line of Lx and L/C Ky the slope of the linear least squares fit line of Ly and L/C Kp the slope of the linear least squares fit line of Yp and L/C L distance away from blade leading edge along the chord-

wise direction Lx the TLV blockage scale along the X direction Ly the TLV blockage scale along the Y direction mb local reduced mass flow rate caused by the TLV mt total mass flow rate of the compressor Pin compressor inlet static pressure Pout compressor outlet static pressure PW the width of blade passage at the rotor tip Rec blade chord based Reynolds number u,v,w velocity in X, Y, Z directions, respectively

Proceedings of ASME Turbo Expo 2012 GT2012

June 11-15, 2012, Copenhagen, Denmark

GT2012-69148

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Vtip rotor tip speed Vaix axial flow velocity vm streamwise velocity vm,avg the averaged streamwise velocity at 80% blade height Wext local mean streamwise velocity at the outer edge of the

core of the TLV Xp distance between the core of the TLV and the casing X,Y in-plane coordinates of SPIV measurement planes Yp distance between the core of the TLV and the blade

suction surface Z out-of-plane coordinates of SPIV measurement planes r, θ radial and circumferential direction ρ air density λ2 criteria for identifying the vortex core τ tip gap of 1% blade height φ mass flow coefficient, Vaix/Vtip

zω out-of-plane vorticity Ω rotor rotational speed

INTRODUCTION The flow field in the tip region is extremely complex due

to interactions of tip leakage flow with the main stream, annulus wall boundary and other secondary flows, which can lead to decreased efficiency [1-3], decreased reliability, off-design operation [3-5], and enhanced rotor-stator interaction. Typically, the flow losses caused by the tip leakage dominated annulus wall flows may account for 30-50% inefficiency in these blade rows, and a clearance gap equal to 1% of the blade height is associated with an about 2% penalty in efficiency [6]. Therefore, there have been continuous studies on TLV over the past decades.

Numerous and extensive experimental/numerical/ theoretical studies have been conducted to investigate tip leakage flow/vortex related compressor aerodynamics [7-21]. The formation of the TLV has been reasonably described by Rains et al. [22] and Chen et al. [23], which can be concluded as the result of the pressure-driven from the pressure surface to suction surface of the blade and the interacting between the tip leakage flow and the main stream. Storer and Cumpsty et al. [11] studied the tip leakage flow behavior in a compressor cascade by both experimental and computational investigations, and discovered that the location where the TLV initiated was affected significantly by the pressure distribution along the blade chord. It usually formed at the maximum loading point and moved downstream as the tip gap sizes increased. Kang and Hirsch et al. [14-17] analyzed a large number of compressor cascade measurement results, depicted detailed 3D flow pictures of the TLV and evaluated its aerodynamic losses. The studies of Khalid et al. [18] and Suder et al. [19] provided modeling methods to evaluate mixing loss and blockage and supposed the mechanisms of the loss was the mixing between the TLV and the main stream. Obviously, for the reason of measurement difficulties in real engines, most of the previous experimental results were obtained in cascade test facilities or

just at the exits of the rotor passage. Hence, the studies based on the experimental investigations to describe flow details, particularly the variations of the kinematic and dynamic characteristics, of tip leakage flows inside the rotor passage are very rare.

Since 1990s, the CFD method began to be used as a routine method gradually for studying the very complicated flow phenomena difficult to be measured in turbomachinery. Vo et al [12] and Hah and Rabe [24] using CFD methods simulated the tip leakage flow behaviors from the near stall condition to the stall condition in the subsonic and transonic compressors and found the well-known criterion for compressor stall, i.e. the spillage of the tip leakage flow over the leading edge of the adjacent blade. Because of the inherent problems of turbulence modeling and numerical schemes, the simulated results should also be carefully validated by detailed experimental results [13, 20].

In this paper, based on the previous studies of Yu et al. [21], the characteristics of the tip leakage vortex in a large scale low speed axial compressor with different rotor tip gaps were studied in detail. SPIV measurements were conducted to investigate the flow field near the rotor tip region at different compressor operating conditions and the measurements sections covered the whole rotor blade passage. In order to reveal the variations of the characteristics of the TLV in different tip clearances, both the kinematics and dynamics related quantities of the TLV were extracted and analyzed. Based on these analyses, it is hoped that the results will be helpful in understanding the flow mechanisms of TLV and providing some supports for CFD codes validation and flow approximate modeling.

EXPERIMENTAL SETUP

Compressor facility

Fig. 1 The static pressure rise coefficient characteristics of the test facility

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The experiment in this paper was carried out in the Low-speed Large-scale Axial compressor Facility (LLACF) of Beijing University of Aeronautics & Astronautics (BUAA). The test facility is a typical single-stage axial compressor including inlet guide vane (IGV), rotor and stator. The rotor and stator blades with C4-series airfoil are designed in terms of the free vortex law. Two rotor tip gap configurations, 0.5% (0.5τ) and 1.5% (1.5τ) blade height were investigated in the experiments and the corresponding compressor characteristic lines are shown in Fig. 1. More detailed design parameters of the test facility are summarized in Table 1 for further comparison and analysis.

Table 1 Design parameters of the compressor test facility

Outer diameter (m) 1.0 Hub-to-tip ratio 0.6 Design Speed (rpm) 1200 Design mass flow rate (kg/s) 22.4 Flow coefficient 0.58 Pressure rise coefficient 0.48 Rec 7.5×105 Configuration IGV+Rotor+Stator Number of blades 36+17+20 Blade camber angle at mid-span (degree)

17.4+26.5+49.1

Blade stagger angle at mid-span (degree)

10.4+33.4+12.3

Blade tip chord (mm) 100+180+180 Rotor tip clearance (mm) 1.0/3.0

Fig.2 Schematic of the SPIV measurement configuration

SPIV setup and data processing A commercial SPIV system, developed by TSI

Incorporation, was employed in the measurement, and a schematic diagram of its installation is shown in Fig. 2. The light source is a dual cavity Nd: YAG laser and the maximum illumination energy is 150 mJ/pulse at a 15 Hz repetition rate. However, the data collection frequency is about 1.7Hz for the limitation of the data transfer from the camera to computer. A pair of 1,280×1,024 pixels and 12-bit frame-straddling-based

CCD cameras (PIVCAM 13-8) were used and, after detailed analyses and comparing, configured in different sides of the laser light sheet in Scheimpflüg condition [25].

In this paper, two sets of SPIV experiments were conducted, corresponding to the two tip gap configurations. The experiments for the 0.5τ tip configuration were introduced by Liu et al in detail [25]. As for the 1.5τ configuration, the experimental methods and setups are nearly the same as the former one, except for some detailed parameters introduced as follows. The field of view is set as 150 × 65 mm. The number of the measurements cross sections is 17 and it was set in nearly the way shown in Fig. 3. At least 300 instantaneous realizations are recorded at each cross section. The commercial software PIVview 3.1 and an in-house developed program are employed for data processing. Images are analyzed with a 50% overlap multi-pass process with the first pass of 64×64 pixels interrogation window and the second pass of 24×24 pixels interrogation window, resulting the final velocity vector grid spatial resolution of about 0.9 mm. The inter-frame time, dt, is set as 8 μs at all the measured compressor operating conditions. Additional details about the SPIV setup, data acquisition, data processing, and uncertainty estimates can be found in [25].

Experiment layout Considering that the tip leakage vortex (TLV) is mainly a

streamwise vortex, in order to investigate the evolutionary processes of the TLV, the layout of the measurement cross sections were chosen as that shown in Fig. 3. The measured cross sections are nearly perpendicular to the rotor tip chord-wise direction with an interval of 10% chord length. The three components of velocity in directions of X, Y, and Z shown in Fig. 3 correspond to u, v, and w, respectively.

The tested compressor operating conditions are all marked as solid stars or triangles on the compressor operating lines shown in Fig. 1. With the tip clearance of 0.5τ, the measurements were conducted at both the design (De, flow coefficient of 0.58) and near stall (Ns, flow coefficient of 0.38) conditions. However, with the tip clearance of 1.5τ, additional two compressor operating conditions were investigated, i.e. the middle (Mid) condition and the maximum static-pressure-rise (Max) condition, with the flow coefficient of 0.51 and 0.42 respectively.

Fig. 3 Layout of SPIV measurement cross sections

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MEASUREMENT RESULTS AND COMPARATIVE ANALYSIS

The streamwise velocity and vorticity fields Figures 4-9 shows the normalized streamwise velocity and

vorticity results at the six investigated compressor operating conditions. A, T and R represent the axial, tangential and radial direction, respectively. The figures on the left side show the streamwise velocity fields and the figures on the right side show the streamwise vorticity fields. The streamwise velocity is normalized by the rotor tip rotating speed, and the streamwise vorticity is normalized by the rotor angular velocity of rotation. It can be seen clearly that the tip clearance flow caused the deficit of the streamwise velocity at the tip region and the low momentum flow region spread gradually as the compressor mass flow rate decreases. The results also show TLV has distinct concentrate negative vorticity. According to the low velocity region, as well as the distribution of concentrate negative vorticity, the trajectory can be discriminated approximately and marked by the dashed lines shown in figures 4-9.

Because the deficit of the streamwise velocity caused by the TLV is very weak during its formation phase and the scale of the TLV is very small, with a relative insufficient spatial resolution of the SPIV measurements, the initial position of the TLV can hardly be observed from the velocity fields. However, according to the vorticity fields, it can be seen that the formation of the TLV should be before 30% /50% suction side cross section at the 0.5τ /1.5τ tip clearance configuration at the De condition. As for the Ns condition, the TLV should be formed before 20%/30% suction side cross section for the corresponding two tip clearance configurations, which means as the tip clearance increases, the formation location of the TLV moves downstream.

See the velocity fields in figures 6-9, it is clear that the TLV grows up continuously and monotonously from the De condition to the Max condition, however expands rapidly at the Ns condition. At the Ns condition, in the core of the TLV, flow stagnation and flow reverse can be observed in some instantaneous results after the 70% suction side measurement cross section, as shown in Fig. 10. Figure 9 shows that TLV core expands abruptly and losses the concentration feature after 70% chord cross section, where rapid dissipation of negative vorticity occurs. This phenomenon indicates the occurrence of vortex breakdown. The similar phenomenon can be observed for the 0.5τ configuration at the Ns condition. As can be seen in Figs. 5 and 9, the TLV breakdown occurs at about the 60% and 70% chord cross sections near the blade suction surface for the 0.5τ and 1.5τ configurations, respectively.

As can be seen in Figs. 4 and 6, the strength of the TLV is weak at the De condition for both the 0.5τ and 1.5τ configurations. Although the tip clearance has tripled in height, the low momentum flow caused by the TLV seems not increasing so much. However, as for the Ns condition, comparing Figs. 5 and 9, the scale of low energy fluid is much larger for the 1.5τ configuration than that at the 0.5τ configuration. Moreover, as indicated in these two figures, as the breakdown of the TLV occurs in the 1.5τ configuration, the low momentum flow of the TLV reaches the pressure side corner of the rotor passage, which may indicate the appearance of double leakage flow phenomena.

In this section, we discussed the qualitative features of the TLV based on the ensemble-averaged streamwise velocity and vorticity fields. In order to quantify these features, some critical quantities should be extracted from the measured results. In the next three sections, the kinetic and kinematic related quantities of the streamwise vortex will be extracted and analyzed.

Fig. 4 The distribution maps of ensemble-averaged streamwise velocity (left) and streamwise vorticity (right) with 0.5τ clearance at the design condition

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Fig. 5 The distribution maps of ensemble-averaged streamwise velocity (left) and streamwise vorticity (right) with 0.5τ clearance at the near stall condition

Fig. 6 The distribution maps of ensemble-averaged streamwise velocity (left) and streamwise vorticity (right) with 1.5τ clearance at the design condition

Fig. 7 The distribution maps of ensemble-averaged streamwise velocity (left) and streamwise vorticity (right) with 1.5τ clearance at the middle condition

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Fig. 10 The instantaneous streamwise velocity field at the 70% suction side cross section (left) and 16% pressure side cross section (right) with the tip gap of 1.5τ at the near stall condition (for 0.5τ, similar results shown in Liu et al. [25])

The trajectory of the tip leakage vortex

Criteria for identifying the vortex core In order to extract the flow parameters of the TLV from the

measured flow fields, the core of vortex should be identified correctly at first. It is well known that there are several popular used criteria for identifying the vortex core, such as Q criterion, λ2 criterion and Δ criterion; most of the criteria are based on local flow kinematics (the velocity gradient tensor), which have nearly the same capability to distinguish the vortex filaments from the vortex sheets [26]. Hence, the particular criteria chosen for the classification of the TLV may have little effect. In this paper, we will use the criterion mentioned in the paper [27] and [21], which is a simplified 2D form of λ2 criterion for using in SPIV results. The criteria is

Fig. 8 The distribution maps of ensemble-averaged streamwise velocity (left) and streamwise vorticity (right) with 1.5τ clearance at the maximum static pressure rise condition

Fig. 9 The distribution maps of ensemble-averaged streamwise velocity (left) and streamwise vorticity (right) with 1.5τ clearance at the near stall condition

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2 22 =( ) ( ) 2 0∂ ∂ ∂ ∂

+ + ⋅ <∂ ∂ ∂ ∂u v u vx y y x

λ (1)

Figure 11 shows the vorticity field at the 40% suction side cross section with 1.5τ configuration at the Ns condition. The dished line indicates the boundary of the core of the TLV identified by the λ2 criterion, which coincides with concentrated negative vorticity of the TLV very well.

Fig. 11 The vorticity field at the 40% suction side cross section with 1.5τ configuration at the near stall condition and the dashed line indicates the vortex core identified by -λ2 criterion (for 0.5τ, similar results shown in Yu et al. [21])

The distribution of TLV trajectory Based on the above-mentioned criterion, the core of the

TLV can be extracted from the measured results. The distributions of the trajectory of the TLV along the streamwise direction at different compressor operating conditions are shown in Figs. 12 and 13. It is clear that the core of the TLV moves away gradually from the blade suction surface (increase of Yp, normalized by passage width, PW) and the casing wall (increase of Xp, normalized by blade height, H). As can be seen in Figs. 4-9, the concentrated negative vorticity state of the core of the TLV will disappear at a certain location. This is because of the inherent unsteady flow features of the TLV, which has been discussed clearly by Yu et al [21]. As the TLV moves downstream, it will destabilize appearing as the vortex core wandering, splitting or even breakdown. After the core of the TLV destabilizes seriously, it cannot be identified well based on the above mentioned λ2 criterion. Hence, the trajectory lines of the TLV are terminated at different positions at different compressor operating conditions, as shown in Figs. 12 and 13.

See in Figs. 12 and 13, the TLV moves downstream in an approximately linear way. Due to the limitation of the spatial resolution, it is difficult to detect the initial location of the TLV. However, according to the evolutionary tendency of the trajectory of the TLV, the initial locations of the TLV should be at about L/C=0.15 and L/C=0.05 at the De and Ns conditions for the 0.5τ tip clearance configuration. As for the 1.5τ configuration, the initiation point of the TLV trajectory moves

upstream from about L/C=0.4 at the De condition to about L/C=0.3, 0.25 and 0.15 at the Mid, Max and Ns conditions, respectively.

We define Kp as the slope of linear least squares fit line of Yp and L/C, which can represent the speed of the TLV moving from the suction side to the pressure side of the blade passage. Figure 14 shows the change of the Kp with the mass flow coefficient. It is clear that as the mass flow rate decreases, the Kp increases, indicating that the TLV moves faster to the pressure side as the mass flow rate decreases. And this phenomenon seems a little more sensitive as the tip clearance increases from 0.5τ to 1.5τ. Moreover, it also can be seen that, although the initial point of the TLV moves downstream as the tip clearance increases, the circumferential moving velocity of the TLV increases.

Fig. 12 The distribution of tip leakage vortex core position at Y direction along streamwise

Fig. 13 The distribution of tip leakage vortex core position at X direction along streamwise

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Fig. 14 The variation of the slope Kp with different mass flow coefficients

The variation of the scale of the tip leakage vortex In the above section, we analyzed the vortex trajectory of

the TLV extracted by using the vortex identification method. However, see the Figs. 4-9, the concentrated vortex cores with negative vorticity are much smaller than the low momentum flow regions occupied by the TLV. Hence, in order to evaluate the compressor performance degradation caused by TLV, a flow blockage based scale of the TLV will firstly be defined.

The definition of flow blockage based scale of the TLV In order to extract the blockage caused by the tip leakage

flow, Khalid [18] found that using the gradient of the streamwise velocity to define region of low momentum flow was appropriate, i.e.

[ ]( ) [ ]( )2 2r m mv vθr r δ∇ + ∇ ≥ (2)

where vm is streamwise velocity and r, θ represent radial and circumferential direction, respectively. Suder et al. [19] also gave another velocity gradient based definition to extract the blockage region of the tip leakage flow from the experimental results they obtained by using LDV,

[ ] [ ]r m mv vθr r δ∇ + ∇ ≥ (3)

Yu compared the blockage area detected by using these two criteria from his SPIV measurement results [28]. The result showed there was no distinct difference by using these two methods. However, by using Suder’s criteria the gradient of the value at the border of the low momentum region was much larger, thus the chosen of δ was little sensitive. Hence, Suder’s criteria will be used in this paper. However, we found that it is still difficult to choose an appropriate value of δ. As we know that the flow in the compressor rotor passage diverges and the average streamwise velocity decreases gradually from the passage inlet to the outlet, as shown in figures 4-9, resulting the

decreasing of the velocity gradient from blade passage inlet to outlet. Hence, it is not appropriate to choose a single value of δ for all the results obtained at any measurement cross sections. In order to overcome this problem, we defined a non-dimensional criterion as follows,

δrr θ ≥

∇+

∇

avgm

m

avgm

mρ

v

v

v

v

,,

(4)

where vm,avg is the averaged streamwise velocity at 80% blade height (just below the TLV region) of the local measurement cross section. Figure 15 shows the streamwise velocity distribution and blockage area extracted by the newly defined criterion in eq. (4) at the 1.5τ configuration at the Ns condition. It can be seen that border of the low momentum flow area can be distinguished clearly. In this figure, Lx and Ly represent the TLV blockage scales along the X and Y directions, respectively.

Fig. 15 The streamwise velocity field (left) and blockage area (right) of 60% measurement plane near suction side in 1.5τ configuration at the near stall condition

Variation of the blockage caused by the TLV along the streamwise direction

As shown in Fig. 16, the scale Lx increases in approximate linear way along the streamwise direction and increases as the compressor loading raises (from the De condition to the Ns condition). At the De condition, the slopes of the curve of Lx in both the two tip gap configurations are near the same. The value of Lx at the rotor exit L/C=1.0 is a little less than 9% blade height. At the Ns condition, the slope of the curve of Lx in the 1.5τ tip clearance configuration is slightly larger comparing to that at the 0.5τ configuration and the values of Lx at the rotor exit L/C=1.0 are about 0.2 and 0.18 blade height for 1.5τ and 0.5τ tip clearance configurations, respectively. We define Kx and Ky as the slopes of linear least squares fit lines of the curves Lx vs L/C and Ly vs L/C before TLV breaks down shown in Figs 16 and 17, respectively. These two parameters represent the increasing rates of the blockage scales of the TLV in the X and Y directions, respectively. As can be seen in Fig. 18, Kx deceases with the increase of mass flow rate; moreover, the Kx is very small and insensitive to the tip gap sizes.

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Fig. 16 The variation of the blockage scale caused by the tip leakage flow in the X direction

Fig. 17 The variation of the blockage scale caused by the tip leakage flow in the Y direction

Fig. 18 The variation of the slopes of the curves of Lx and Ly before TLV breaks down shown in Figs. 16 and 17 at different mass flow coefficients

Figure 17 shows that Ly increases approximately linearly at De, Mid and Max condition. At the De condition, Ly is very

sensitive to the tip gap sizes. In the 1.5τ configuration Ly increases much faster than that in the 0.5τ case. Although, the initial point of TLV is around L/C=0.5 at the 1.5τ case, about 0.2 later than that at the 0.5τ case, at the rotor exit plane L/C=1.0, the blockage scale of Ly caused by the TLV are nearly the same at the two tip gap configurations, reaching the value of about 0.44PW and 0.4PW, respectively. At the Ns condition, as mentioned before, TLV breakdown occurs for both the two tip clearance configurations. The TLV breakdown begins at about L/C=0.6 at the 0.5τ configuration and at about L/C=0.7 for the 1.5τ configuration. See in Figs. 17 and 18, before the TLV breaks down, both the values of Ly and Ky are very close for the two tip gap configurations, and after the TLV breaks down, the growth of Ly slows down gradually. As a result, at the exit of the rotor passage, i.e. L/C=1.0, the blockage scale of Ly is much larger for the 1.5τ configuration (Ly≈0.7PW) than that at the 0.5τ configuration (Ly≈0.48PW). As also can be seen in Fig. 18, Ky is much larger at the small tip gap size configuration, indicating that the growing rate of Ly is sensitive to the tip gap size. Comparing the variations of Lx and Ly in Figs. 16 and 17, we can also find that the blockage scale of the TLV in the Y direction is much larger than that in the X direction. In the tested compressor stage, Ly is about 3~5 times of Lx.

Fig. 19 The variation of the blockage parameters caused by the TLV

The flow blockage caused by the tip leakage vortex

Blockage Parameter A frequently used blockage coefficient is defined as

1 effective flow area geometric flow areaB = − (4)

This parameter denotes how large an area is blocked by the low momentum flow, which is similar to the definition of the displacement thickness of the boundary layer. However it does not consider the streamwise velocity variation inside the flow passage [21]. Inside the rotor passage, there is a great difference of flow capacity between the blade tip and root regions, particularly for that at the Ns condition, as shown in Figs. 4-9.

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In this paper, the mass-flow-based blockage parameter defined by Yu et al [21] will be used, i.e.

( )extb

m b t tAB m m W w dA mr= = −∫ (5)

where mb and mt represent the reduction of mass flow rate caused by the blockage of the TLV and the total of local mass flow rate, respectively; Wext is the local mean streamwise velocity at the outer edge of the core of the TLV.

(a) (b)

Fig. 20 Typical instantaneous vorticity field near the rotor tip region with 1.5τ tip clearance at the near stall (a) and the max static pressure rise (b) conditions

The variation of blockage parameter Figure 19 shows the variation of the blockage parameters

caused by the TLV along the blade chord-wise direction. It can be seen that the blockage parameter increases very slowly and appears insensitive to the tip gap sizes at the De condition. At the rotor exit, a maximum blockage is achieved with a value of about 0.5%/0.35% of total mass flow rate at the 0.5τ/1.5τ tip clearance configuration.

At the Ns condition, the blockage parameter is much larger than the one at the De condition. The variations of the blockage parameters are very similar before the TLV breaks down at the two tip gap configurations. However, after the TLV breaks down, the blockage increases first, gets the peak value, and then decreases gradually. The locations of the peak blockage caused by the TLV are at L/C=0.7 and 0.8 for 0.5τ and 1.5τ tip clearance configurations, respectively. The corresponding peak values are about 2.6% and 4.8% of the total mass flow rate. According to the analyses of Yu [21] and Mouri et al [29], after the TLV breaks down, there would be strong mixing between

the tip leakage flow and the main steam, as can be seen in Figs. 5 and 9. Therefore, the blockage scale should increase rapidly at first. However, as too much high energy flow from the main stream mixes into the tip leakage flow, the blockage will decrease gradually. Figure 20(a) shows the typical instantaneous vorticity field obtained at the Ns condition in the 1.5τ tip clearance configuration, it can be seen that there is still a distinct concentrated vortex at 60% cross section. However, after the TLV breaks down, the vortex core disappears suddenly, and breaks into a lot of small distorted vortices. As the leakage flow moves downstream, the TLV scale enlarges significantly and more positive vortices appear.

At the Mid and Max conditions, the blockage parameter has the similar distribution with the one at the Ns condition. Nevertheless, the growth of the blockage is mild and the peak value is much lower. Because no stagnation point or reversal flow region appears in instantaneous flow fields at these two conditions, which means no TLV breakdown occurs. At the Max condition, the core of the TLV splits into several concentrated small vortices at first, and then the mixing is strengthened gradually as can be seen in Fig. 20(b). The similar process occurs at the Mid condition. This phenomenon suggests that although there is no TLV breakdown at Mid and Max conditions, TLV still would destabilize in a mild way. Hence, the variation of the blockage parameter at the Max and Mid conditions appear the similar trends as that at the Ns condition.

Based on the above analyses it could be concluded that the destabilization of the TLV has significant effects on its flow blockage feature. The peak value normally occurs just after the location of TLV destabilization.

CONCLUSIONS The flow fields near the tip region of the rotor passage with

0.5% and 1.5% blade height configurations of the large-scale low-speed axial compressor stage were investigated by using SPIV. The investigations were conducted at the design, middle, maximum static pressure rise and the near stall conditions. According to the analyses of the ensemble-averaged velocity and vorticity flow fields and the flow parameters extracted from the core of the tip leakage vortex, some new features of the tip leakage vortex were observed. The main results are summarized as follows:

The formation of the TLV is delayed in the large tip clearance configuration. At the design condition, the initial point of the TLV transfers from about 15% blade chord location to about 40% blade chord location as the tip gap increases from the 0.5% blade height to 1.5% blade height. However, the trajectory of TLV moves much faster in an approximately linear way from the suction side to pressure side of the blade passage at the larger tip gap configuration.

A newly defined dimensionless criterion for extracting the border of the low momentum flow area was given. It can define the blockage area caused by the tip leakage vortex well at different measurement cross sections and compressor operating conditions. It is found that the scale of the TLV in the blade normal direction is about 3~5 times of that in the

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spanwise direction. The results also show that the spanwise scale of the TLV is insensitive to the size of the tip gaps. On the contrary, the scale of the TLV in the blade normal direction is very sensitive to the size of the tip gaps.

The flow blockages caused by the TLV were also evaluated in the paper. An interesting phenomenon was found that the destabilization of the TLV has significant influences on the flow blockage features. The flow blockage parameter achieved a peak value just after the TLV destabilized, and then because of the intensive interacting of the low momentum flow in the TLV and the high momentum main stream the blockage parameter decreased gradually.

The TLV breakdown was found at the near stall condition for both of the two measured tip gap configurations. For the tip clearance of 0.5% blade height, the TLV broke down at about 60% chord measurement cross section near the blade suction side, as the tip gap enlarged to 1.5% blade height, it moved downstream to the 70% chord cross section, however, the flow blockage caused by it was doubled.

ACKNOWLEDGMENTS The authors would like to acknowledge the support of

National Natural Science Foundation of China, Grant No. 50976009 and No. 51006007.

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