Journal of Thermal Science Vol.23, No.2 (2014) 120126
DOI: 10.1007/s11630-014-0685-7
Article ID: 1003-2169(2014)02-0120-07
Effects of Probe Support on the Flow Field of a Low-Speed Axial Compressor MA Hongwei, LI Shaohui, WEI Wei National Key Laboratory of Science and Technology on Aero-engines, School of Energy and Power Engineering, Beihang University, Beijing, 100191, China © Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2014
This paper presents an investigation on the effect of probe support on the flow field of an axial compressor. The experiment is carried out in a large-scale low-speed research compressor. A cylindrical probe support intruding to 50% blade span was installed at 50% chord upstream from the rotor leading edge. The region from 5° to 32° off the probe support in the direction of rotation at the rotor outlet was measured with a 5-hole probe and a high-response total pressure probe. The experiment is performed at both near-design and near-stall points. The measuring results of 5-hole probe and high-response total pressure probe indicate that the probe blockage effect is different at different blade spans. The wake of the probe support weakens the leakage vortex intensity at the tip region, leading to greater total pressure rise. At near-design condition, the presence of probe support has a negative effect on the region from 75% to 92% span, while improves the flow field below 75% span. At near stall condition, the probe support has a negative effect on the region from 70% to 90% span, and almost has no influence on the flow field below 70% span.
Keywords: probe support, compressor, 5-hole probe, high-response total pressure probe, tip leakage flow
Introduction Aerodynamic probes are widely used to measure the aerodynamic performance and flow parameters of a compressor [1]. Placing probes in the compressor, however, would cause flow blockage, which affects the flow structure in the vicinity of the probe as well as downstream, even upstream [2]. The blockage effect is more severe in micro compressors, which will even trigger stall in some cases. Some relative researches have been carried out to investigate the effects of probes on the flow field of compressors. Simon Coldrick [3] performed a 3D numerical simulation of a cylindrical probe fixed at the leading-edge of a stator at mid passage. The results indicate
that the probe changes the velocity and pressure distribution around the probe and causes mass flow reduction in the blocked passage. Lepicovsky [4] carried out an experimental investigation of distortions of the rotor exit flow field caused by a rotating aerodynamic probe mounted in the rotor. The results validate that the presence of probe has a negative effect on the rotor exit flow field, leading to a depression in pressure and axial velocity component. Wyler [5] identified the blockage effects in closed channels by rotating a cylindrical probe with one pressure tapping through a complete revolution in closed channels. He concluded that the presence of the probe caused an increase in speed and a corresponding decrease in pressure indicated at the tapping. Moreover, Xiang, et al [6-8] conducted both experi-
Received: July 15, 2013 MA Hongwei: Professor This work was funded by the National Natural Science Foundation of China, Grant No. 51161130525, 51136003, and the 111 Project, No. B07009. www.springerlink.com
Hongwei Ma et al. Effects of Probe Support on the Flow Field of a Low-Speed Axial Compressor
Nomenclature Ps0 inlet static pressure on the endwall P0 atmosphere pressure Pin the averaged pressure before the rotor Pout the averaged pressure after the rotor Ut rotor blade tip tangential velocity Dp 0.5·ρ·Ut2, dynamic pressure relative total pressure at inlet of the Ptr0 compressor Cp pressure rise coefficient of the rotor mental and numerical research on the effects of airfoilprobes on the compressor performance. They concluded that the airfoil-probes have a negative influence on the compressor aerodynamic performance at both design and off-design operating points. Until now, few researches of the effects of stationary probe placed upstream of the rotor on the compressor performance have been published. This paper presents an experimental investigation of effects of the probe installed upstream of the rotor on the flow field of an axial compressor.
Ptr Pt0 Pt Cvz Cvr Cpt Ptr-loss Cva
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local relative total pressure total pressure at the inlet of the compressor local total pressure Vz/Ut, axial velocity coefficient Vr/Ut, radial velocity coefficient (Pt-Pt0)/Dp, total pressure rise coefficient (Ptr0-Ptr)/Dp, relative total pressure loss coefficient mass flow coefficient
tion of the test facility is shown in Fig. 1. A 6-mm-dia five-hole probe and an 8-mm-dia high response total pressure probe were used to measure the flow parameters of the flow field 10% chord downstream from the rotor trailing edge. The measurement flow angle error of the 5-hole probe is less than 1°, and total pressure error less than 1%.
Experiment Facility and Measurement Layout The experiment is carried out on the low-speed large-scale axial compressor (LLAC) in Beijing University of Aeronautics and Astronautics. The compressor consists of 17 rotor blades, and the profiles of the rotor blades are of C4 with a circular camber line. Details of the design parameters are given in Table 1. Table 1
Parameters of the compressor rotor
Parameters Outer diameter (m) Hub to tip ratio
Value
0.6
Design speed (r/min)
1200
Design mass flow rate (kg/s)
22.4
Blade chord (mm)
180
Blade height (mm)
200
Rotor tip clearance (mm)
Fig. 1 The test compressor and measuring layout
1
3
As this paper aims at investigating the effect of probe on the flow field of micro compressors, so the geometrical size of the probe support should be set according to the geometrical similarity principle. Here, a cylindrical probe support intruding to 50% blade span was installed at 50% chord upstream from the rotor leading edge. The diameter of the cylindrical probe support is 40mm (length 100mm), and it occupies a projected area ratio of 13% to the single rotor passage. The overall configura-
Fig. 2 Measuring stations in the measured plane
The region from 5° to 32° off the probe support in the direction of rotation at the rotor outlet (Fig. 1) was measured according to the 3D unsteady numerical simulation. The numerical simulation results shows that the region
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from 5° to 32° off the probe support in the direction of rotation is most affected. This region is named ‘measured plane’ in the following section of the paper. The measured plane contains 10 span-wise stations and 45 pitch-wise stations as shown in Fig. 2. The 5-hole probe is used to measure the steady flow parameter of the measured plane, while the dynamic flow parameters are obtained by rotating the high-response total pressure probe at three angular positions (Fig. 3), which is based on the measuring principle of 3-hole probe. The initial probe setting angle is in correspondence with the absolute flow angle obtained by the 5-hole probe, then rotating the total pressure probe by +30°and -30° respectively. The flow parameters can be acquired based on the aerodynamic performance of the total pressure probe (calibrated in a wind tunnel). More details about this measuring method refer to [9].
supposes that the circumferential variation due to the presence of probe support can be ignored at 250% chord downstream from the rotor trailing edge. Cp and Cva are defined as follows: Cp = (Pout - Pin) / Dp (1) Cva = 2(P0 - Ps0) / / Ut (2) It can be seen that static pressure rise after installing the probe support is lower than that of the normal case (before installing probe support). At near-design point (Cva=0.65), the static pressure rise coefficient is lowered by 1.3%, and it is 1.1% smaller at the near-stall point (Cva=0.55). It can be concluded that the presence of probe support causes a negative effect to the overall performance of the compressor.
Fig. 4 Overall static pressure rise characteristic
Fig. 3 Measuring method of the total pressure probe
Four on-casing static pressure taps are distributed uniformly at both 150% chord upstream from the rotor leading edge and 250% chord downstream from the rotor trailing edge to measure the overall compressor performance. The measurement static pressure error of the static pressure taps are less than 0.1%. The NI-PXI dynamic data acquisition system is used to obtain the pressure signals. A once-per-revolution mark (a black stripe) on one of the rotor tip is used in conjunction with an optical pickup to generate a single voltage pulse per rotor revolution.
Results and Discussion The effect of probe support on the overall compressor performance The experiment is carried out at 75% design rotor speed. The overall compressor performance in terms of the static pressure rise coefficient (Cp) and the mass flow coefficient (Cva) is shown in Fig. 4. Here the author
The effect of probe support on the mass flow rate at rotor exit According to the dynamic axial velocity obtained by high-response total pressure, the time-variation of mass flow rate in the measured plane could be calculated by taking the area integral of the product of axial velocity and density. Fig. 5(a) and Fig. 5(b) gives the change of mass flow rate of the measured plane within the period of a rotor blade passing through the measured plane at near-design point and near-stall point respectively. The unit time in Fig. 5 is defined as the time interval from a rotor blade entering into the measured plane to coming out of the measured plane. It is clear that the mass flow rate of the measured plane drops at a large scale after installing the probe support. It means the increase of the mass flow rate of the remained rotor exit region, which is not measured in the experiment. The time-averaged mass flow rate at the measured plane drops by 11.15% at near-design point and 12.89% at near-stall point. It indicates that the flow blockage effect is more severe at near-stall point than that at near-design point. The depression in mass flow rate is another manifestation that the presence of probe support has a negative effect on the flow field of the compressor.
Hongwei Ma et al. Effects of Probe Support on the Flow Field of a Low-Speed Axial Compressor
(a) Cva=0.65
(b) Cva=0.55
Fig. 5 Time-variation of mass flow rate in the measured plane
(a) Cva=0.65
(b) Cva=0.55
Fig. 6 Radial distribution of axial velocity coefficient of the measured plane
The circumferential averaged radial distribution of the axial velocity coefficient of the measured plane obtained by 5-hole probe is presented in Fig. 6. There is a notice-
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able decrease of the axial velocity coefficient between 65% and 95% span at near-design condition after placing the probe support, while the sector between 40% and 70% span shows an increase in axial velocity coefficient, and it almost stays the same with the normal case in the remained region. The situation is a little different for the near-stall condition. The axial velocity shows a large decrease between 70% and 95% span, while it is almost the same with the normal case at the rest region of the measured plane. Comparing the two figures, it can be easily concluded that the blockage effect at near-stall condition is more severe than that at near-design condition, which is in good agreement with the dynamic results. Steady analysis of the influence of the probe support on rotor exit flow field Steady flow parameters of the measured plane are obtained by 5-hole probe. Fig. 7 gives the radial distribution of the flow parameters of the measured plane at near-design point (Cva=0.65). In order to show clearly the distortions of the relative flow angle (defined as the angle between relative flow direction and the circumferential direction) of the measured plane, only upper half span of the measured plane is shown in Fig. 7(a), and the lower half span almost keeps the same with the normal case. As seen here, the distortions in relative flow angle, total pressure rise, relative total pressure loss and radial velocity are all notable. It appears that below 75% span, the total pressure rise is greater than the normal case, the relative total pressure loss is lower at the same time, and the radial velocity drops at a large scale, all these facts indicate that the presence of probe support does a positive effect on the flow field below 75% span. It is because that the probe support causes a severe flow blockage on the upper half span, which results a depression on the radial velocity. As a consequence, the flow loss caused by radial flow is lowered and the rim power is more converted to total pressure rise. At the sector between 75% and 95% span, there is a decrease in relative flow angle and total pressure rise, and the relative total pressure loss is higher than the normal case, and these facts signify the negative influence of the probe support. It is because of the wake generated from the probe support interacts with the main flow, adding another kind of secondary flow in the rotor passage, thus resulting in higher flow losses. At the tip region (above 95% span), both the relative flow angle and the total pressure rise are higher than the normal case, which indicates the positive effect of the probe support. The reason is that the wake of the probe support weakens the leakage vortex magnitude at the tip region. The Karman vortex street generated from the
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probe support is actually a radial-wise vortex, while the leakage vortex is a stream-wise vortex, thus the leakage vortex magnitude is suppressed instead of being strengthened.
Fig. 7
Fig. 8
Fig. 8 provides the same kind of results as that shown in Fig. 7, but for the near stall condition (Cva=0.55). As for relative flow angle, only the upper half span of the measured plane is shown for the same reason with near
Radial distribution of the flow parameters of the measured plane at near-design condition
Radial distribution of the flow parameters of the measured plane at near-stall condition
Hongwei Ma et al. Effects of Probe Support on the Flow Field of a Low-Speed Axial Compressor
design condition, and the lower half span almost keeps the same with the normal case. It is clear that the relative flow angle, total pressure rise coefficient, relative total pressure loss and radial velocity changes a little below 70% span, which is different from that at near-design condition. In conclusion, the probe support imposes little influence on the sector below 70% span. At the sector between 70% and 90% span, there is a depression in absolute flow angle and total pressure rise, and the relative total pressure loss is higher than the normal case, which signify the negative influence of the probe support. It is also because of the secondary flow caused by the wake of the probe support. At the tip region (above 90% span), both the relative flow angle and the total pressure rise are higher than the normal case, which is similar as that of the near-design condition. It is also because of the weakened tip leakage vortex magnitude. Dynamic analysis of the influence of the probe support on rotor exit flow field Based on the once per rotor revolution signal and high frequency data acquisition system, the time-variation of
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the flow parameters are obtained by the phase-average data reduction method. The contours of total pressure rise coefficient of the measured plane under near-design condition (Cva=0.65) at time series T1 and T2 are introduced in Fig. 9. The time interval between T1 and T2 is 20% unit time (unit time is defined as the same with that in Fig. 5). It is easy to find that the flow field of the measured plane comes to be much more irregular, which is due to the interaction between the wake of the probe support and the complex flow character (wake of the rotor, tip leakage flow, passage vortex, etc) in the compressor. It is clear that the noticeable tip leakage flow in the normal case is depressed both in scope and intensity after placing the probe support. One can also find that the disturbance caused by the wake of the probe support at upper span is more severe than that at the lower span. What’s more, the total pressure rise coefficient is lower between 80% and 95% span, and a bit higher below 80% span. These findings are in good agreement with the steady results obtained by 5-hole probe. The dynamic flow field obtained by the high-response total pressure probe at near-stall condition (Cva=0.55) is
Fig. 9 Contours of total pressure rise coefficient of the measured plane
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also in good agreement with the steady results. Due to the limitation of the length of the paper, no more descriptions about the dynamic results are presented here.
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Foundation of China, Grant No. 51161130525, 51136003, and the 111 Project, No. B07009.
References Conclusions The present paper performed an experimental investigation of the effects of the probe support on the flow field of a low speed axial compressor. Several conclusions can be made. (1) The static pressure rise after installing the probe support is lower than that of the normal case. For the static pressure rise coefficient, it is lowered by 1.3% at near-design point (Cva=0.65), and for the near-stall point (Cva=0.55) it is lowered by 1.1%. The presence of the probe support causes a negative effect to the overall performance of the compressor. (2) The mass flow rate of the measured plane drops at a large scale after installing the probe support. The time-averaged mass flow rate of the measured plane drops by 11.15% at near-design point and 12.89% at near-stall point. The blockage effect is more severe at near-stall condition than that at near-design condition. (3) At near-design condition, the presence of probe support has a negative influence on the sector between 75% and 95% span of the measured plane, while it improves the flow field below 75% span. It is because that the probe support causes a depression on the radial velocity and the rim power is more converted to total pressure rise. (4) At near-stall point, the presence of probe support has a negative influence on the sector between 70% and 90% span of the measured plane, and it does little effect on the flow field below 70% span. (5) The wake of the probe support weakens the leakage flow at the tip region, resulting in higher total pressure rise at the tip region both at near-design and nearstall condition. This paper presents the qualitative and quantitative findings about the flow field distortion due to the presence of a probe support with a specific diameter and intruding position. How much effect will the probe support impose to the flow field and how to reduce the effects are undoubtedly related to the diameter and intruding position of the probe support. We suggest that in future researches of probe blockage effect, one should consider the diameter and intruding position of the probe.
Acknowledgement This work was funded by the National Natural Science
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