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Experimental investigation on unsteady pressure fluctuation of rotor tip region in high pressure stage of a vaneless counter-rotating turbine ZHAO QingJun†, LIU XiYang, WANG HuiShe, ZHAO XiaoLu & XU JianZhong Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China
An experimental investigation has been performed to study the unsteady pressure fluctuation of rotor tip region in high pressure stage of a vaneless counter-rotating turbine. The experiment is carried out on a blow-down short duration turbine facility. The investigation indicates that the blow-down short duration turbine facility is capable of substituting continuous turbine facilities in most turbine testing. Through this experimental investigation, a distinct blade-to-blade variation is observed. The results indicate that the combined effects of vane wake, tip leakage flow, complicated wave systems and rotor wake induce the remarkable blade-to-blade variations. The results also show that the unsteady effect is intensified along the flow direction. counter-rotating turbine, pressure fluctuation, unsteady effect, short duration experiment
In the recent few years, more and more attentions have been paid to counter-rotating turbine (CRT) because it can offer some significant benefits compared with conventional two-stage turbine, such as the elevated thrust-to-weight ratio of aero-engine, the improved performance of aircraft, and so on. At present, 1-1 stage counter-rotating turbine has been used in some advanced active duty aero-engines, and vaneless counter-rotating turbine (VCRT, 1-1/2 stage counter-rotating turbine), which is composed of a highly loaded single stage high pressure turbine (HPT) and a vaneless counter-rotating single stage low pressure turbine (LPT), has been also applied in some up-to-date test engines in Euro-American developed countries. It is obvious that the VCRT will be widely adopted in real aero-engines in the future. From the 1950s, counter-rotating turbines have been ― carefully investigated[1 8]. Researchers focus on aerodynamic design of CRT, flow field analysis in CRT, and so on. These conclusions drawn by predecessors drive the development of the CRT technology. Some of approaches to increase efficiency or decrease
loss need to be applied to enhancing specific work and reducing fuel consumption of gas turbine engines. In modern high load turbines, the loss induced by tip clearance leakage flow is remarkable. A number of investigations on tip clearance leakage flow have been ― performed by many researchers[9 22] to decrease the flow [9] loss. Farokhi analyzed the tip clearance loss in an axial flow turbine and proposed a model that accounts for tip pressure loading, relative wall motion and stage characteristics. A simple two-dimensional model for the calculation of the leakage flow over the blade tips of axial turbine was described in ref. [10]. In the model, the importance of pressure gradients along the blade chord was emphasized as a major factor influencing the tip leakage flow. Liu and Bozzola[11] investigated the tip clearance flow structures, leakage vortex, flow underturning, and the effect of the tip clearance on blade loading and losses by means of a three-dimensional code. The predicted Received February 2, 2009; accepted March 5, 2009 doi: 10.1007/s11431-009-0169-2 † Corresponding author (email:
[email protected]) Supported by the Award Fund of the President of CAS
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results were in good agreement with the experimental data. This investigation showed that this code could be used as an analysis tool to help designers understand the tip clearance flow features. Prasad and Wagner[12] made time-resolved measurements of the unsteady pressure on the blade outer air seal in a low-speed turbine rig. The measurements indicated the existence of a separation zone on the blade tip, which causes a vena contracta to form at the entrance of the tip gap. In addition, a comparison between the ensemble-averaged pressure measurement and the corresponding result from steady computation illustrated that the pressure on the blade outer air seal could largely be described as being due to a steady flow sweeping past a stationary probe. The comparison also indicated that the bulk of unsteadiness was confined to the tip gap. The effects of tip clearance on the transient and time-averaged flow fields in a supersonic turbine were investigated by Dorney et al.[13] using a parallelized unsteady three-dimensional Navier-Stokes code. The numerical results showed that the turbine performance with tip clearance configuration is better than the configuration without tip clearance. The improved turbine performance was attributed to a reduced secondary flow loss and a weakened shock system in the configuration with tip clearance. It also indicated that the reductions of the shock losses with tip clearance were greater than the losses introduced by tip clearance. Azad et al.[14,15] carried out a series of experimental investigations to measure the heat transfer coefficient and static pressure distributions on a gas turbine blade tip and squealer tip in a five-bladed stationary linear cascade. The pressure measurements in the near tip and on the shroud surface provided complementary information of the tip leakage flow pattern. And the pressure data provided a basis for determining the tip leakage flow and also explained the heat transfer results. The results indicated that tip clearance has a significant influence on local tip heat transfer coefficient distribution. The heat transfer coefficient increased about 15%―20% along the leakage flow path at higher turbulence intensity level of 9.7 over 6.1%. The heat transfer coefficient on the cavity surface and rim increased with an increase in tip clearance. A pressurecorrection based, 3D Navier-Stokes CFD code was used by Tallman and Lakshminarayana[16,17] to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade. The results indicated that the reduced tip clearance resulted in less mass flow
through the gap, a smaller leakage vortex, and less aerothermal losses in both the gap and the vortex. And the casing relative motion caused less mass flow through the gap and a smaller leakage vortex. The structure of the aerothermal losses in the passage changed dramatically when the outer casing motion was incorporated, but the total losses in the passage remained very similar. You et al.[18] carried out a study on the temporal and spatial dynamics of an incompressible rotor tip clearance flow by means of a large-eddy-simulation-based flow solver. The computational approach was shown to be capable of capturing the evolution of the highly complicated flow field characterized by the interaction of distinct blade-associated vortical structures with the turbulent end-wall boundary layer. The local measurements of the heat transfer coefficient and pressure coefficient on the tip and near tip region of a generic turbine blade made by Newton et al.[19] revealed that the flow through the plain gap was dominated by flow separation at the pressure-side edge and that the highest levels of heat transfer were located where the flow reattaches on the tip surface. Particle image velocimetry (PIV) was used by Palafox et al.[20] to obtain flow field maps of several planes parallel to the tip surface within the tip gap, and adjacent passage flow. The dominant effect of the tip leakage flow on the tip end-wall secondary flow was revealed. And the reduction in the magnitude of the undertip flow near the end wall due to the moving wall was observed. Mischo et al.[21] carried out an investigation on the flow field near the tip of the blade for different shapes of the recess cavities using a three-dimensional computational fluid dynamics code. The results indicated that the total tip heat transfer Nusselt number was significantly reduced by an appropriate profiling of the recess shape. A numerical study was made by Krishnababu et al.[22] to investigate the effect of tip geometry on the tip leakage flow and heat transfer characteristics in unshrouded axial flow turbines. The results indicated that the cavity tip compared to the flat tip and suction-side squealer tip was advantageous both from the aerodynamic and from the heat transfer perspectives by providing a decrease in the amount of leakage, and hence losses, and average heat transfer to the tip. In this paper, an experimental investigation was carried out to study the unsteady pressure fluctuation of rotor tip region in high pressure stage of a vaneless counter-rotating turbine. The experiment was performed
ZHAO QingJun et al. Sci China Ser E-Tech Sci | Jun. 2009 | vol. 52 | no. 6 | 1478-1483
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on a blow-down short duration turbine facility, which was located at Institute of Engineering Thermophysics (IET), Chinese Academy of Sciences (CAS).
1 Experimental details The IET blow-down turbine facility is a transient wind tunnel, which can be used to simulate flow conditions for most modern high pressure axial turbines. The blow-down short duration turbine facility is able to substitute a continuous turbine facility in a majority of turbine testing on flow and heat transfer measurements. The valuable test time in the IET facility is about 300― 500 milliseconds. This test time is sufficiently long compared to the flow and heat transfer characteristic time in a high speed turbine stage. So, the turbine operates in a quasi-steady state during test process. The blow-down short duration turbine facility has some advantages compared to the continuous long duration facility. One advantage of the short duration facility is the lower cost of construction, operation and maintenance. The other is more convenient measurement of heat transfer characteristic. The metal temperature is the same as room temperature because the test time is short compared to the thermal transient of the turbine blade. An almost constant gas-to-metal temperature ratio is maintained during the test time. The relatively favorable environment is suitable for the application of the heat flux gauges. Then, the heat transfer data are readily acquired in the short duration test. The schematic of the blow-down short duration turbine facility is given in Figure 1. Major components shown from upstream to downstream are the supply tank
(12 m3), fast response valve, test section, tail cone and vacuum tank (20 m3). Cross-sectional view of the VCRT internal flow path is shown in Figure 2. The VCRT studied in this paper is composed of a highly loaded single stage HPT coupled with a vaneless counter-rotating LPT. It has high expansion ratio and operates in transonic regimes. The VCRT has some unique characteristics, which are different from the conventional two-stage turbine, even 1+1 counter-rotating turbine. These characteristics include: ● The HPT rotor and LPT rotor are counter-rotating; ● The LPT is vaneless; ● There are high relative Mach number (~1.5) and relative flow angle (~70°) at the outlet of the HPT. The test condition of the VCRT is shown in Table 1. Table 1
The test condition of the VCRT Inlet total temperature
320 K
Inlet total pressure
2.0×105 Pa
Mass flow
15.3 kg/s
Rotational speed of HPT rotor
5500 r/min
Rotational speed of LPT rotor
−5250 r/min
Expansion ratio of VCRT
5.4
In this paper, five high-response pressure transducers installed on the curved surface of the HPT rotor casing wall (see Figure 3) are used to measure the unsteady pressure fluctuation of rotor tip region. The pressure signal acquisition is conducted at 200 kHz (about 61 times blade-passing frequency). Five axial locations including leading edge, 25% axial chord, 50% axial chord, 75% axial chord and trailing edge of the HPT rotor are selected in this test (see Figure 4).
Figure 1 Schematic of the IET blow-down short duration turbine facility.
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ZHAO QingJun et al. Sci China Ser E-Tech Sci | Jun. 2009 | vol. 52 | no. 6 | 1478-1483
the VCRT, p is the measured static pressure near the rotor tip region, ρinlet is the density at the inlet of the VCRT, Vx is the midspan axial velocity at the inlet of the VCRT. Using the pressure coefficient, Cp, the unsteady pressure fluctuation near the casing wall of HPT rotor can be described. Figures 5―9 show the transient variation of the static pressure near the tip region of the HPT rotor at five axial locations. The abscissa Nb means blade-passing number. It is defined in eq. (2) below. Nb = fbt, (2)
Figure 2 Cross-sectional view of the VCRT internal flow path.
where fb is the blade-passing frequency, t is the test time. The test unsteady pressure fluctuations shown in Figures 5―9 have a well periodicity corresponding to the blade-passing frequency indicating that a distinct blade- to-blade variation is proved. The blade-to-blade variations at the leading edge and 25% axial chord attribute to the vane wake swept by rotor blade. When the vane wake into the HPT rotor sweeps the pressure
Figure 3 Pressure transducers installed on the casing wall of the HPT.
Figure 5 Unsteady pressure fluctuation near the tip region of the HPT rotor at the leading edge.
Figure 4 Installed location of the high-response pressure transducers.
2 Experimental results Nondimensional pressure coefficient, Cp, is defined as follows: Cp =
* pinlet −p
0.5 ρinletVx2
,
(1)
* is the absolute total pressure at the inlet of where pinlet
Figure 6 Unsteady pressure fluctuation near the tip region of the HPT rotor at 25% axial chord.
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Figure 7 Unsteady pressure fluctuation near the tip region of the HPT rotor at 50% axial chord.
Figure 8 Unsteady pressure fluctuation near the tip region of the HPT rotor at 75% axial chord.
higher pressure coefficient region is induced by other fluids except for the wake. Finally, the blade-to-blade variations with obvious periodicity appeared at the region near the leading edge of the HPT rotor. In Figure 7, the more apparent blade-to-blade variations compared to Figures 5 and 6 ascribed to the combined effects of vane wake, complex wave systems and tip leakage flow are given. There are two wave troughs per periodicity on the curve of pressure fluctuation in Figure 7. The larger wave trough displays the effect of vane wake, the other less wave trough indicates the action of complex wave systems and tip leakage flow. Compared to other four axial locations, the most complicated blade-to-blade variations induced by the combined effects of shock wave, rotor wake and further development tip leakage flow near the trailing edge are shown in Figure 9. Although there are still two wave troughs per blade-passing number on the curve of pressure fluctuation, the less wave trough has a more prominent characteristic. In Figure 9, the larger wave trough is caused by rotor wake, while the less wave trough shows the combined effects of shock wave and tip leakage flow. In addition to the above-mentioned content, the increase in the amplitude of the unsteady pressure fluctuation along the flow direction was also observed in this experiment indicating that the unsteady effect is strengthened along the flow direction. In ref. [12], researchers have also performed experimental investigation to measure the unsteady pressure fluctuation near the rotor tip region on a continuous low speed turbine facility. The comparison with the work in ref. [12] indicates that the blow-down short duration turbine facility can alternate continuous turbine facility considering the cost and test range of turbines. Through the experimental investigation in this paper, not only the ability of the short duration turbine facility was verified, but also the unsteady pressure fluctuation near the casing wall of HPT rotor was obtained.
3 Conclusions
Figure 9 Unsteady pressure fluctuation near the tip region of the HPT rotor at the trailing edge.
transducers, a curve with higher pressure value corresponding to lower pressure coefficient value is obtained and shown in Figures 5 and 6. On the other hand, the 1482
In this paper, an experimental investigation on unsteady pressure fluctuation of rotor tip region in high pressure stage of a VCRT is performed on IET blow-down short duration turbine facility. The major conclusions can be summarized as follows. 1) The blow-down short duration turbine facility is able to substitute a continuous turbine facility in a majority of turbine testing.
ZHAO QingJun et al. Sci China Ser E-Tech Sci | Jun. 2009 | vol. 52 | no. 6 | 1478-1483
2) An obvious blade-to-blade variation is evident through the short duration turbine test. 3) The vane wake driven by rotor blade induces the blade-to-blade variations near the rotor leading edge. In the middle axial chord region of the HPT rotor, the blade-to-blade variations are caused due to the combined effects of vane wake, complicated wave systems and tip 1
Wintucky W T, Stewart W L. Analysis of two-stage counter-rotating
leakage flow. Near the rotor trailing edge, the combined effects of shock wave, rotor wake and further development tip leakage flow result in the blade-to-blade variations. 4) The amplitude of the unsteady pressure fluctuation increases along the flow direction. The support of the WU ChungHua Award Foundation is gratefully acknowledged.
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