Journal of Thermal Science Vol.20, No.1 (2011)
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DOI: 10.1007/s11630-011-0426-0
Article ID: 1003-2169(2011)01-0001-05
A Study of the Unsteady Tip Flow Field of a Transonic Compressor JingXuan Zhang1, Chengqin Li2,3, Hongwu Zhang2 and Weiguang Huang1 1. Shanghai Advanced Research Institute, Chinese Academy of Sciences 2. Institute of Engineering Thermophysics, Chinese Academy of Sciences 3. Graduate School of the Chinese Academy of Sciences © Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2010
An experimental investigation on the unsteady tip flow field of a transonic compressor rotor has been performed. The casing-mounted high frequency response pressure transducers were arranged along both the blade chord and the blade pitch. The chord-wise ones were used to indicate both the ensemble averaged and time varying flow structure of the tip region of the rotor at different operating points under 95% design speed and 60% design speed. The pitch-wise circumferential transducers were mainly used to analyze the unsteadiness frequency of the tip leakage flow in the rotor frame at the near stall condition. The contours of casing wall pressure show that there were two clear low pressure regions in blade passages, one along the chord direction, caused by the leakage flow and the other along the tangential direction, maybe caused by the forward swept leading edge. Both low pressure regions were originated from the leading edge and formed a scissor-like flow pattern. At 95% design speed condition, the shock wave interacted with the low pressure region and made the flow field unsteady. With the mass flow reduced, the two low pressure regions gradually contracted to the leading edge and then a spike disturbance emerged.
Keywords: transonic, compressor, unsteady flow
Introduction It is well known that the tip clearance flow plays an important role in the generation of overall loss and virtually governs the compressor’s operability. Hoying [1] ever demonstrated that the stall would occur once the tip leakage flow is flush with the leading edge in a low speed axial compressor. Vo et al. [2] further verified this phenomenon and proposed two criteria of the occurrence of spike inception, which is one of the two basic modes of stall inception. In recent years, the unsteadiness of tip clearance flow attracted many authors’ attention. One of the popular topics is the flow mechanisms of such unsteadiness, and another topic is the relation between the
unsteadiness and the occurrence of stall inception. So far, there are three different views about the occurrence of unsteadiness of tip clearance flow. Mailach et al. [3] has shown that the investigated low speed compressor develops a two passage periodic tip leakage vortex pattern, which is called ‘rotating instabilities’ .Zhang et al. [4] and Tong et al. [5] found that prior to the initiation of spike inception, the tip leakage flow shows obvious self-induced unsteadiness caused by the fluctuation of the pressure difference across the tip clearance in their computational and experimental work, respectively. Such unsteadiness has been proved to have some relations with the initiation of a spike inception by Lin et al [6]. When studying a high-speed compressor rotor, Yamada et al. [7]
Received: September 2010 Jingxuan Zhang: Associate Professor www.springerlink.com
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found that breakdown of tip leakage vortex occurring in the compressor rotor resulted in self-sustained flow oscillation. Hah et al. [8] also showed a fluctuating tip leakage vortex in a transonic compressor close to the stability limit, but no tip vortex breakdown by numerical simulation. It is no doubt that for the current technical means it is impossible to make the highly unsteady flow features before and during stall clear in short time. This paper aims at presenting the detailed unsteady flow field in the tip region of a transonic rotor and explores the flow mechanisms of the formation of stall inception. A novel arrangement of the pressure transducers and unique data post-process method make it possible to analyze the unsteadiness frequency in rotor frame.
Experimental Facility and Measurement Transonic Compressor Test Rig The experimental program was carried out in the transonic axial compressor test rig in BUAA (Beijing University of Aeronautics and Astronautics) in a single rotor arrangement. The rotor is driven by a free turbine with the gas supplied from a gas generator. Fig. 1 shows the layout of the experimental facility. Table 1 summarizes some of the aerodynamic design and geometric parameters of the tested compressor rotor. Three group of transducers were used in the experiment, as shown in Fig.2. A group which is spaced along the chord-wise is mainly for the phase-locked contour of
Fig. 2
The arrangement of transducers.
the static pressure of casing. B group is used for static pressure observation of some fixed point in rotor frame. C group is used for the detection of propagation of stall inception.
Experimental Results Compressor Performance Compressor performance is given in Fig.3 for two speed lines tested in the present paper. The characters and numbers labeled in the figure represent different throttle position of the two cases.
Fig. 3
The characteristic of the test rig
Stall Inception Fig.4 (a) gives the whole stall process of 95% rotor speed detected by the C group transducers. It is obvious that the stall inception is typical spike inception and no modal-like disturbance is found prior to the stall onset. The propagation speed of spike disturbance is about 65% rotor speed, labeled with arrow in the picture. As for the low rotor speed case, the spike inception propagation speed is about 71% rotor speed (as shown in Fig.4 (b)), which is a little higher than high speed case. Fig. 1
Schematic of the transonic compressor facility.
Table 1
Design Parameters of the Compressor
Mass flow Total pressure ratio Adiabatic efficiency Rotor tip speed Relative tip Mach number at inlet Rotational speed Outer diameter of rotor Inlet hub/tip radius ratio Number of rotor blades Tip clearance
13.4kg/s 1.60 88.02% 409m/s 1.404 22000r/min 0.3558m 0.565 17 0.8mm
Phase-Locked Averaged Pressure Contour of 95% Rotor Speed Case In this section, the tip flow structure at each operating points is presented in the form of a sequence of averaging static pressure contour plots in Figs.5 (a)-5(d). The averaging is done by first locking three blade passages arbitrarily, then averaging over 80 revolutions every 1 revolution of the same locations. At operation point A, the contour shows that there are two apparent low pressure regions in a blade passage, one is flush with leading edge and the other is attached to the suction surface. Obviously, the second one is induced by the tip leakage flow. However, what induces the first
Jingxuan Zhang et al.
A Study of the Unsteady Tip Flow Field of a Transonic Compressor
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Fig. 4
one is not very clear; one possible reason is because the rotor is a little forward swept at tip region. The two low pressure region just like a “scissors” .Because of the resolution of pressure measurement, it is difficult to explain the exact nature of the shock at the suction surface. However, it still can be seen that an oblique shock attaching at the leading edge. The shock passes through the first low pressure region and terminates in the second low pressure region, namely, the tip leakage flow. As the back pressure is increased, the second low pressure region shortens gradually. Compared to point A, at the high efficiency point B, the attaching shock doesn’t change much; however, the trajectory of tip leakage flow shrinks almost by half. At point C, both of the two pressure region shrink further and the shock begins detaching the leading edge. At the near stall point, point D, the second low pressure region almost coalesces with the first low pressure region and the shock becomes totally detached. Phase-Locked RMS of 95% Rotor Speed Case In order to present the unsteadiness of the tip region flow, phase-locked RMS is calculated in this section and the results are shown in Fig.6 (a)-Fig.6 (d). When the throttle is relatively wide open (point A and B), the high RMS region locates at both sides of the shock wave. The nearer the positions to the leading edge is, the higher
Fig. 5
value the RMS becomes. As the mass flow rate reduced further, at point C, there are two very high RMS regions appear in the blade passage which almost concentrated near the leading edge. At the near stall point, point D, the two very high RMS regions join together and form a large region, which means the tip region flow is highly unsteady at that time. Results of 60% Rotor Speed Case In this section, the experimental results of 60% rotor speed case are plotted in the same way as the 95% rotor speed case. Fig.7 (a)-Fig. 7(d) shows the phase-locked averaged static pressure and Fig. 8(a)-Fig.8 (d) shows the phase-locked RMS. Compared to the high speed case, the tip region flow of low speed case is “clean”, for the lack of shock wave. However, the stall process is similar, in other words, as the mass flow rate is decreased, the tip leakage flow (the low pressure region in blade passage) moves to the leading edge gradually and when it spills out a disturbance
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Fig.6
emerges. The high RMS value region of low speed case is still around the leading edge and it seems to change little from large mass flow rate condition to near stall condition. One possible reason is that there is no interaction between shock wave and tip leakage flow. The Unsteady Character of Tip Region Flow Fig.9 (a) and Fig.9 (b) give the PSD results of 95% speed case at point A and D. Compared these two pictures, we can see that there is a bump at near stall condition, namely, at point D. The bump covers from 4000Hz to 5000Hz, which is almost 70% the blade passing frequency. Fig.10 shows static pressure of one point fixed in the rotor frame. The data is got from the B group transducers. The abscissa is the time when some point passing a transducer and the ordinate is static pressure. It is obvi-
Fig .7 ous that the pressure fluctuate periodically at the near stall condition.
Summaries In this paper, the tip region flow of a transonic rotor is presented. Both phase-locked averaged static pressure and phase-locked RMS have been processed. It is obvious that the stall inception is linked to the flow pattern of tip leakage flow. With the interaction between shock and tip leakage flow the flow field becomes highly unsteady at near stall condition. However, the low pressure region near the leading edge as shown in Fig.5 (a)-Fig.5 (b) puzzles the authors and needs further investigation in the near future. One possible reason is the forward swept blade profile.
Jingxuan Zhang et al.
A Study of the Unsteady Tip Flow Field of a Transonic Compressor
Fig.10
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The unsteady character of static pressure of one point in rotor frame
Acknowledgement This work is supported by the National Natural Science Foundation in China under Grant No.50736007 and National Basic Research Program 2007CB210104 of China. The authors would like to thank Prof. LU, Y. J. for his contribution in carrying on the experimental program .
References
Fig. 8
Fig. 9
[1] D. A. Hoying, C. S. Tan, Huu Duc Vo, and E. M. Greitzer, “Role of Blade Passage Flow Structures in Axial Compressor Rotating Stall Inception”, ASME Journal of Turbomachinery, Vol.121, pp. 735 - 742, 1999. [2] Huu Duc Vo, C. S. Tan and E. M. Greitzer, 2008, “Criteria for Spike Initiated Rotating Stall”, ASME Journal of Turbomachinery, Vol.130, pp. 11 – 23. [3] Mailach, R., Sauer, H., and Vogeler, K., 2001, “The Periodical Interaction of the Tip leakage Flow in the Blade Rows of Axial Compressor”, ASME Paper 2001-GT0299 [4] Zhang, H. W., Deng, X. Y., Lin, F., Chen, J. Y., Huang, W. G., 2005,“Unsteady Tip leakage Flow in an Isolated Axial Compressor rotor”, Journal of Thermal Science, Sep. 2005, 14(3). [5] Tong, Z. T., Lin, F., Chen, J. Y., Nie, C. Q., 2007, “The Self-Induced Unsteadiness of Tip Leakage Vortex and its Effect on Compressor Stall Inception”, ASME Paper GT2007-27010. [6] J.X.Zhang, F.Lin, J.Y.Chen and C.Q.Nie, “The flow mechanism of how distorted flows deteriorate stability of an axial compressor” ASME Paper No. GT2007-27628 [7] Yamada, K., Furukawa, M., Inoue, M., Funazaki, K., 2004, “Unsteady Three-Dimensional Flow Phenomena due to Breakdown of Tip Leakage Vortex in a Transonic Axial Compressor Rotor”, ASME Paper GT2004-53745. [8] Hah, C., Rabe, D. C., Wadia, A. R., 2004, “Role of Tip-Leakage Vortices and Passage Shock in Stall Inception in a Swept Transonic Compressor Rotor”, ASME Paper GT2004-53867.