Citation: Li G, Xu Y J, Lin B, et al. Control of endwall secondary flow in a compressor cascade with dielectric barrier discharge plasma actuation. Sci China Ser E-Tech Sci, 2009, 52(12): 3715—3721, doi: 10.1007/s11431-009-0187-0
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Control of endwall secondary flow in a compressor cascade with dielectric barrier discharge plasma actuation
LI Gang1†, XU YanJi1, LIN Bin1, ZHU JunQiang1, NIE ChaoQun1, MA HongWei2 & WANG ZhaoFeng2 1 Laboratory of Advanced Energy and Power, Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China;
2 Beijing University of Aeronautics and Astronautics, Beijing 100083, China
Three dielectric barrier discharge plasma actuators were mounted at the positions of 20%, 40% and 60% of chord length on the endwall in a compressor cascade. The downstream flow field of the cascade has been measured with a mini five-hole pressure probe with and without the plasma actuation. The measured results show that the plasma actuation most effectively reduces total pressure loss and flow blockage when the actuators are operated simultaneously. As each of the actuators is operated independently, the actuator at the position of 20% of chord length most effectively reduces flow blockage, and the actuator at the position of 60% of chord length fairly reduces total pressure loss. However, negative pressure loss reduction occurs with the plasma actuator at the position of 40% of chord length. In brief, the plasma actuation placed on the endwall in the cascade apparently influences the endwall secondary flow, and the optimal locations and strength of actuation are critical for the control of endwall secondary flow in a compressor cascade with the plasma actuators.
plasma actuation, compressor cascade, secondary flow, flow control
1 Introduction
The secondary flows in a compressor cascade are very complicated. In the zones of endwall, especially the corner zones formed by the suction surface and the endwall, there are endwall boundary layer, blade bound-ary layer, many kinds of vortexes and interactions be-tween each other. They cause severe flow losses, and are the main sources of loss in axial compressors. Previous studies adopted nonaxisymmetry endwall[1,2], distortion endwall[3], mount of endwall fences[4—10] and a non-uni- form clearance[11] to reduce flow losses in a cascade and improve the mechanical efficiency of turbomachinary.
This work utilizes dielectric barrier discharge (DBD) plasma actuation to reduce negative impacts of secon-dary flows close to endwall on the aerodynamic per-formance of a compressor cascade. The previous results of experiment and numerical simulation by Nie et al.[12]
showed the control of boundary layer separation with DBD plasma actuation was effective. The speed of endwall secondary flows is low and the region is limited, so it is suitable to use DBD plasma actuation for the control. No such result has been presented yet. This work will introduce a new way to control secondary flows close to the endwall in a compressor cascade.
2 Experimental setup and measurement system
The experiment was performed on a planar cascade in a low speed wind tunnel. The sketch of the wind tunnel is shown in Figure 1. The maximum flow in the wind tun-nel is about 1.5 kg/s. The size of exit of the wind-tunnel Received November 17, 2008; accepted February 4, 2009 doi: 10.1007/s11431-009-0187-0 †Corresponding author (email: [email protected] ) Supported by the National Natural Science Foundation of China (Grant No. 50776086)
3716 Li G et al. Sci China Ser E-Tech Sci | Dec. 2009 | vol. 52 | no. 12 | 3715-3721
is 250 mm×120 mm. The speed of incoming flow is ap-proximately 29 m/s at temperature of 20℃. The thick-ness of boundary layer is 5 mm. The turbulence intensity in the measured flow area is about 0.34%. The meas-urement system and the experimental errors were intro-duced and analyzed in details in ref. [11].
We adopted the NACA65-0010 planar cascade as shown in Figure 2. The turning angle of blade is a 34°. The blade height is 118 mm. The blade chord length is 200 mm. The blade spacing is 94 mm. The tip clearance is 2 mm. The stagger angle is 70°. The experiment was performed at a chord Reynolds number of 1.2×105. In the experiment the width of plasma actuator was 12.5 mm, about 6.25% of the chord length. The first pair of electrodes of the plasma actuator was placed with the gap center of electrodes at the position of 40 mm (about 20% of the chord length) on the endwall. The second pair of electrodes of the plasma actuator was placed with the gap center of electrodes at the position of 80 mm, about 40% of the chord length on the endwall. The third pair of electrodes of the plasma actuator was placed with the gap center of electrodes at the position of 120 mm, about 60% of the chord length on the endwall. The sin-
gle dielectric barrier was a quartz sheet with a thickness of 3 mm. The voltage applied cross the quartz sheet was 20 kV and the frequency was 37 kHz. The equipment used to generate plasma was given in Figure 2(a). Plasma induced flow can accelerate boundary layer on the endwall as showed by Figure 2(b).
In application, flexible material such as Teflon can be used as the dielectric barrier. Figure 3 shows the com-pressor case equipped with the plasma actuators[13].
Figure 4 shows the measured area. A five-hole pres-sure probe was placed at the position of 110% of the chord length downstream from the trailing edge. The minimum distance between the five-hole pressure probe and the endwall was 7 mm. A high voltage applied for too long a time may heat the dielectric barrier and may cause potential safety problems. Therefore, we only measured aerodynamic parameters in the neighboring area of the vortex in the experiment.
3 Results and analysis
In the experiment, to prevent errors introduced by loca tion changes of the five-hole probe, we collected a set of
Figure 1 Experimental setup of the low speed wind tunnel.
Figure 2 Setup of plasma actuation equipment and arrangement of plasma actuators. (a) Experimental setup of plasma actuation equipment; (b) arrange-ment of electrodes and direction of plasma induced flow.
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Figure 3 Compressor case equipped with the plasma actuators. (a) Powered off ; (b) powered on.
Figure 4 The measured area and the plasma generation. (a) With no actuation; (b) with the plasma actuation.
data with the five-hole probe with no actuation and sev-eral sets of data with the five-hole probe with plasma actuators at different locations, then moved the five-hole probe to the next position. The elaborate process assured the consistence and credibility of the experimental data and the conditions. The wind tunnel was kept running while the experimental data were collected to prevent the start of the fan motor from affecting the quality of wind.
3.1 Flows in a compressor cascade
Figure 5 provides distributions of the measured axial velocity coefficients with no actuation and with plasma actuation. The axial velocity coefficient is defined as ω = va/v0, where va is the axial velocity of outgoing flow, and v0 is the axial velocity of incoming flow.
Figure 5 shows that the axial velocity coefficient was improved most obviously with the first pair of electrodes powered on.
Figure 6 shows the mean axial velocity coefficients over those at the same height of the blade.
In Figure 6 the axial velocity coefficient close to the endwall was improved most obviously, by about 14% at the position of 8% of span, with the first plasma actuator on. It dropped rapidly, by decrease 4% at the height of 22% of span. The impact on axial velocity coefficient
was negative with the second plasma actuator on. The impact on axial velocity coefficient was positive with the third plasma actuator close to the trailing edge. With the three plasma actuators on simultaneously, the maximum increase of velocity coefficient was similar to that with the first plasma actuator on. However, the de-crease of velocity coefficient was smaller than that with the first plasma actuator on.
3.2 Total pressure loss coefficient in a compressor cascade
Figure 7 shows distributions of the measured total pres-sure loss coefficients with no actuation and with plasma actuation. The total pressure loss coefficient is defined as t t1 t s( ) /( )w p p p p= − − , where Pt is the total pressure of the incoming flow, Pt1 is the total pressure at the out-let, and Ps is the static pressure of the incoming flow.
Figure 7 shows that the total pressure loss coeffi-cient was improved most obviously with three pairs of electrodes powered on simultaneously.
Figure 8 shows the mean total pressure loss coeffi-cients over those at the same height of the blade.
The above experimental results indicate that the axial velocity coefficient and the total pressure loss co-efficient are interrelated. The total pressure loss coeffi-cient is small where the axial velocity coefficient is big.
(b)
(a) (b)
(a)
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Figure 5 Distributions of the measured axial velocity coefficients without and with the plasma actuation at different positions. (a) With no actuation; (b) with the first plasma actuator on; (c) with the second plasma actuator on; (d) with the third plasma actuator on; (e) with the three plasma actuators on simulta-neously.
Figure 6 The measured mean axial velocity coefficients over those at the same height of the blade with plasma actuation at different positions. (a) Axial velocity coefficients without and with plasma actuation; (b) axial velocity coefficients increase with plasma actuation.
The fluctuations of the increase of total pressure loss coefficients shown in Figure 8 are in a larger range than those of the decrease of axial velocity coefficients shown in Figure 6. At the height of 35% of the blade, the fluctuation of axial velocity coefficient is very small. However, the fluctuation of the increase of total pressure loss coefficient is still obvious. The above results indi-
cate that the plasma actuations at different positions slightly affect the axial velocity coefficient in the area (at the height of over 35% of blade) far away from the endwall, and apparently influence the total pressure loss coefficient in that area.
In the three plasma actuators, the first one is at the upstream of other two and the direction of the induced
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Figure 7 Distributions of the measured total pressure loss coefficients with the plasma actuation at different positions. (a) With no actuation; (b) with the first plasma actuator on; (c) with the second plasma actuator on; (d) with the third plasma actuator on; (e) with the three plasma actuators on simultane-ously.
Figure 8 The measured mean total pressure loss coefficients over those at the same height to the endwall with plasma actuation at different posi-tions. (a) Total pressure loss coefficients with and without plasma actuation; (b) total pressure loss coefficients decrease with plasma actuation.
flow by the first plasma actuator is most different from the main flow. Therefore, the impacts of the first plasma actuator on the axial velocity coefficient and the total
pressure loss coefficient are most notable. In the region close to the endwall of blade the axial velocity coeffi-cient and the total pressure loss coefficient by the first
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plasma actuator are improved much more than those by the second or the third plasma actuator. With the in-crease of the height of blade the negative impacts of the first plasma actuator on the axial velocity coefficient and the total pressure loss coefficient are more notable than those of the second and the third plasma actuators. The impacts of the second plasma actuator on the axial ve-locity coefficient and the total pressure loss coefficient are basically negative. The impacts of the third plasma actuator on the axial velocity coefficient and the total pressure loss coefficient decrease with the increase of the height of blade. Because the directions of the in-duced flows by the plasma actuation in the experiment are different from the main flow, the plasma actuation produces negative impact on flows locally. Therefore, the locations of plasma actuators are very important and should be considered carefully and properly.
3.3 Radial flow in a compressor cascade
We utilized a five-hole probe to measure the three-di-
mensional velocity field at the position of 110% of chord downstream from the trailing edge in the com-pressor cascade. In the measured plane the perpendicular velocity vector was removed. The velocity vectors par-allel to the measured plane, namely the secondary flows were analyzed.
Figure 9 shows the vectograms of the measured sec-ondary flows at the position of 110% of chord down-stream from the trailing edge in the compressor cascade with the plasma actuation at different locations. We can see that the secondary flows concentrated in the corner region with interaction between suction layers and formed obvious vortexes. The impacts of the plasma actuation at different positions slightly influenced the position and intensity of the vortex. The impacts on the position and intensity of the vortex with the first plasma actuator on were moving the center of the vortex up, from the position of 90% of pitch to the position of 92% of pitch. The results were similar to that of the three
Figure 9 Distributions of the measured secondary velocity vectors with plasma actuation at different positions. (a) Without actuation; (b) with the first
plasma actuator on; (c) with the second plasma actuator on; (d) with the third plasma actuator on; (e) with the three plasma actuators simultaneously on.
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plasma actuators on simultaneously. The position of the center of vortex barely moved with the second or the third plasma actuator on. One of the reasons is that the pitchwise component of the velocity vector is directed from the suction surface to the pressure surface, which is quite different from the evolution direction of the leak-age flow. Meanwhile, the position of the first plasma actuator is around the position of the formation of leak-age flows vortex. Therefore, the first plasma actuator suppressed the leakage flows. However, the strength of the plasma actuators was so weak that the suppression was not apparent. 3.4 The changes of total pressure loss coefficients and axial velocity coefficients in the measured area
Figure 10 provides the changes of the measured total pressure loss coefficients and axial velocity coefficients in the measured area with no actuation and with the plasma actuators at different positions. The abscissa numbered 1, 2 and 3 indicate the first, second and third plasma actuator on and 4 indicates the three plasma ac-tuators on simultaneously. Therefore, the plasma actua-tion improves the axial velocity coefficient and the total pressure loss coefficient by up to about 4% and 2.5%, respectively.
4 Conclusions
When the plasma actuators are mounted on the endwall of blade in the compressor cascade as above, the actua-tion with the three plasma actuators on simultaneously is
Figure 10 The changes of total pressure loss coefficients and axial ve-locity coefficients in the measured area.
most effective. The plasma actuation at the position of 20% of chord is very strong. The plasma actuation at the position of 40% of chord is very weak. The fluctuation is steady with the plasma actuation at the position of 60% of chord length. The carefully chosen positions, strength or combinations of the plasma actuators are critical to improve the flow close to the endwall.
With the voltage endurance limitation of the dielectric barrier, the width of the produced plasma in the chord direction in the experiments was no more than 10 mm, which was less than 1/20 of chord length. If blades with shorter chords length are used, the plasma actuation will be more obvious. Further research will be carried out with a small blade and at the same time numerical simu-lation will be used for a better understanding.
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