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Flow instability suppression and deep surge delay by non-axisymmetric vaned diffuser in a centrifugal compressor
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a
Zhenzhong Sun , Xinqian Zheng
a,b,∗
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, Hideaki Tamaki , Yuta Kaneko
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Department of Aerodynamics and Thermodynamics, Institute for Aero Engines, Tsinghua University, Beijing, 100084, China b Turbomachinery Laboratory, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, 100084, China c Corporate Research & Development, IHI Corporation Yokohama, 235-8501, Japan
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Article history: Received 31 March 2019 Received in revised form 17 September 2019 Accepted 16 October 2019 Available online xxxx Keywords: Centrifugal compressor Flow instability Surge Non-axisymmetric diffuser Dynamic measurement
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The performance of centrifugal compressors is significantly limited by the deep surge with strong flow oscillations, and the occurrence of deep surge is found to be related to flow instabilities. A novel in-house designed non-axisymmetric vaned diffuser (NAVD) is applied in a centrifugal compressor, and this paper experimentally analyzes influences of the NAVD on suppressing the flow instability and delaying the deep surge. Results show that the flow instability at the diffuser inlet region are distinctly suppressed by the NAVD, meanwhile the deep surge is delayed at 90% speed which extends the stable flow range (SFR) by 25.0%. Nevertheless, the NAVD neither delays the deep surge at other rotating speeds, nor does it suppress the flow instability at the impeller inlet region. According to a previous research, only at 90% speed, the deep surge onset initiates from the diffuser inlet flow instability, and this explains why the NAVD fails to delay the deep surge occurrence at other rotating speeds except for 90% speed. This paper experimentally proves that the NAVD has a positive influence on the flow instability suppression and deep surge delay in some situations, and it provides a new method to improve the compressor stability by non-axisymmetric vaned diffuser. © 2019 Published by Elsevier Masson SAS.
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1. Introduction
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Centrifugal compressors are widely used as core components in many field, such as the aerospace engineering, the turbocharging, the chemical industry, and gas delivery. However, flow instability in a compressor significantly limits the performance of the compressor. This problem draws extensive attention when designing or selecting centrifugal compressors. Therefore, investigating the flow instability of compressors and developing effective flow control methods to extend the flow range are vital for the development of compressors [1]. Surge and stall are two common forms of instability that occur in compressors. The former involves the whole compression system with oscillating annulus-averaged flow [2] and is classified as mild surge when there is no reverse flow and deep surge when reverse flow occurs [3]. In contrast, the latter is characterized by a constant time-averaged mass flow rate and is believed to be related to flow separations within a compressor [4]. Many researchers have focused on stall inceptions. Two types of stall in-
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*
Corresponding author at: Department of Aerodynamics and Thermodynamics, Institute for Aero Engines, Tsinghua University, Beijing, 100084, China. E-mail address:
[email protected] (X. Zheng). https://doi.org/10.1016/j.ast.2019.105494 1270-9638/© 2019 Published by Elsevier Masson SAS.
ception are recognized: modal wave type and spike type. Paduano et al. [5] reviewed modal wave type stall inception, and Tan et al. [6] discussed spike type stall inception in detail. The flow instability inside a compressor has been studied by many researchers with experimental methods [7] and numerical methods [8,9], and many flow control methods have been developed for changing the instability behavior and improving the compressor stability [10–12]. A representative passive flow control method is the self-recirculation casing treatment (SRCT), which can change the flow field and the flow instability at the impeller inlet. The SRCT was first studied experimentally by Fisher et al. [13] and demonstrated to have positive effects on flow range extension [14,15], and recently, Zheng et al. [16] and Tamaki et al. [17] developed asymmetric SRCT to adapt to the non-uniformity of the flow field and achieved satisfying results on flow range extension. Yang et al. [18] and Sun et al. [19] also investigated the influence and mechanism of SRCT in stability enhancement. As an active flow control method, air jet is another widely used method that often aims at controlling the flow in the tip region [6]. Examples of elaborate implementations of air jet methods include tip injection [20], stator shroud air injection [21], end-wall recirculation [22] and micro-air injection [23]. These methods could extend the compressor flow range by 5% to 10%. Additional research focused on the influence of the compres-
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Nomenclature
η ϕ π ρ σ τ ξ ϕ ξ c m pr r s u BPF FFT ME
isentropic efficiency passage angle of diffuser flow passage total pressure ratio density solidity of diffuser passage tangential direction stagger of diffuser vane passage angle difference stagger difference length of the vane chord line corrected mass flow rate reference pressure radius pitch between adjacent diffuser vanes tangential velocity of the impeller blade blade passing frequency Fast Fourier Transform measurement errors
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MR N NAVD SFR SRCT
measurement ranges compressor rotating speed non-axisymmetric vaned diffuser stable flow range self-recirculation casing treatment
Subscripts 2 atm choke k LE surge
outlet of the impeller atmospheric condition choke condition diffuser passage number or diffuser vane number leading edge of the diffuser vane condition at the surge line
Superscripts M O
the in house designed vaned diffuser the original vaned diffuser
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2. Motivation and structure
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sion system on the flow instability. Galindo et al. [24] investigated the influence of the duct length in the compression system and found that the compressor could switch from deep surge to mild surge when the duct length is changed. Riess et al. [25] recorded the pressure ahead of a three-stage axial compressor with various volumes on the downstream side, and the results showed that surge occurred when the downstream volume was large whereas only rotating stall was produced when the downstream volume was small. Much research has focused on flow instabilities of centrifugal compressors, and many attempts have been made to improve compressor stability. In this paper, the flow instability within a centrifugal compressor is suppressed by a novel in-house designed non-axisymmetric vaned diffuser (NAVD), and experiments are carried out to assess influences. Performances of two compressors, one with the NAVD, the other with original vaned diffuser, are compared in detail in terms of the steady-state characteristics and the dynamic instability behavior, and changes in deep surge and flow instabilities are analyzed.
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The work that is described in this paper is part of a series of investigations whose objective is to examine flow instabilities in centrifugal compressors and develop effective flow control methods. Previous works have been discussed by Sun et al. [26]. They indicated that the vaned diffuser had a significant influence on the compressor stability, and the occurrence of deep surge is closely related to the flow instability at the diffuser inlet region at intermediate rotating speed. Therefore, if this flow instability could be suppressed, it could be possible to delay the occurrence of deep surge and extend the flow range at intermediate rotating speed. Different from the work of reference [26], in which the instability evolution process (operating points varies from one to another) of the compressor with a conventional axisymmetric vaned diffuser is considered, this paper mainly focuses on the influences of a NAVD on flow instabilities at fixed points and the deep surge occurrence. This paper is organized as follows: First, the design methodology of the NAVD and the experimental setup are introduced. Then, experiments are carried out on both the NAVD and an original axisymmetric vaned diffuser using the same strategy. Next, the surge line of the compressor with the NAVD is compared with that of the
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Fig. 1. Diffuser passages ( P ) and vanes (V ) & definition of passage angle and stagger.
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compressor with the original axisymmetric vaned diffuser to assess the influence on the compressor stability. Dynamic instability behavior near the surge line is also compared, which demonstrates the suppression of the diffuser inlet flow instability. In the final part of this paper, the influence of the NAVD on the flow range extension of the compressor are discussed. 3. Design methodology of non-axisymmetric vaned diffuser and experimental setup
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The NAVD is designed by modifying the geometric parameters of an original axisymmetric vaned diffuser, and it is termed as “Modified-VD” for convenience, in contrast, the original axisymmetric vaned diffuser is termed as “Origin-VD”. Fig. 1 shows the naming rule of diffuser passages and diffuser vanes and illustrates the definition of the passage angle and the vane stagger, and the subscript k is used as a counter for diffuser vanes or diffuser pas-
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Table 1 Difference of vane stagger and passage angle between Modified-VD and Origin-VD.
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Passage/vane number [k]
ξk
ϕk
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[degree]
[degree]
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0 0 0
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+1 +2 +2 0 0 0 0
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sages. The stagger (ξk ) of a diffuser vane k is defined as the angle of the chord line from the radial direction. A larger stagger indicates that the vane is more inclined to the tangential direction. The passage angle (ϕk ) is the central angle between the leading edges of two adjacent diffuser vanes, and it is used to represent the solidity (σ ) that is defined as the ratio of the vane chord length (c) to the pitch (s), as expressed in equation (1). Assuming that the profile and the radius of the leading edge (rLE ) of all vanes are identical, for a certain diffuser flow passage k, the solidity can be expressed by equation (2). Thus, the solidity is only depended on the passage angle.
σ = c /s
(1)
−1 −1 σk = c · rLE ϕk
(2)
For the axisymmetric vaned diffuser Origin-VD, all sixteen vanes are distributed uniformly along the circumferential direction, the passage angle of each flow passage is 22.5 degrees, and the stagger of each diffuser vane is 77.0 degrees, as expressed by equations (3) and (4). Previous work [27] introduced the design of the ModifiedVD, the guiding ideology is adapting the distorted flow field by changing the local passage angle and the local vane stagger based on a axisymmetric vaned diffuser (the Origin-VD), and thus the modified vaned diffuser becomes non-axisymmetric. Meanwhile, the diffuser width, the vane profile and the diffuser inlet radius keeps to be unchanged. In reference [27] many NAVDs have been designed, and finally a best design is selected according to that it obtains the most uniform flow field in the diffuser region, and this NAVD is the “Modified-VD” used in this paper. The geometry of Modified-VD could be decided by the stagger difference and the flow passage angle difference, as defined in equations (5) and (6), where the superscript “M” and “O ” denote the “Modified-VD” and the “Origin-VD”, respectively. The diffuser vane V 7, which is close to the volute tongue, is set to the same circumferential position between two vaned diffusers. Starting with vane V 7, the circumferential position of the leading edge of each remaining vane can be fixed in sequence with the value of ϕk . Meanwhile, the stagger of each vane is determined by the value of ξk . Therefore, each diffuser vane of the Modified-VD is determined. According to the distributions of ϕk and ξk , which are presented in Table 1 and Fig. 2, the Modified-VD is summarized as follows: compared with the Origin-VD, vanes V 4, V 5 and V 6 are closed by 1.0, 2.0 and 2.0 degrees, respectively; in addition, passages P 1 ∼ P 4 and P 15 are narrower, whereas passages P 5, P 6, P 12, P 14 and P 16 are wider compared with the Origin-VD. Besides, Fig. 2 clearly shows that the Modified-VD has variable stagger and variable passage angle (or solidity) along circumferential direction. Hence, the Modified-VD shows non-axisymmetric geometric feature and it is absolutely a NAVD. Different from works
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Fig. 2. Distributions of the passage angle and vane stagger of the Modified-VD.
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Fig. 3. Schematic diagram of the test rig.
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in reference [27] which focuses on the uniformity of the flow field in diffuser regions, this paper concerns more about the influence of the NAVD on the flow instability suppression and the deep surge occurrence by comparing dynamic pressure behaviors at the diffuser inlet.
ξkO
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ϕkO = 22.5◦ (k = 1, 2, . . . , 16)
(4)
ξk = ξkM − ξkO
(5)
ϕk = ϕkM − ϕkO
(6)
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The test rig and the experimental setups in this paper are similar to those that were introduced by Sun et al. [26] and are briefly introduced below. Fig. 3 shows a schematic diagram of the test rig. The high-pressure air supply and the fuel injection are adjusted with the feedback control system so that the turbine can drive the compressor with the required rotating speed, and the speed variation range can be controlled within ±0.5%. Total pressure and total temperature are measured upstream (location “S1”) and downstream (location “S2”) of the compressor, and the mass flow rate is measured with a lemniscate flowmeter at “S1”. In addition, the atmospheric pressure and atmospheric temperature, together with the rotating speed, are measured during experiments. All the measurement equipment is connected with the data acquisition unit,
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Parameters [unit]
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Mass flow rate [kg/s] Temperature [K] Pressure [kPa]
0.08∼0.50 273∼473 S1: −30∼30 S2: 0∼700
≤1% ±0.25 ≤0.05% FS ≤0.05% FS
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Table 2 Measurement ranges (MR) and measurement errors (ME) of parameters.
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*FS - full scale.
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Table 3 Compressor specifications.
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Parameters
Value [unit]
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Impeller blade number Impeller outlet diameter Impeller backsweep angle Diffuser type Diffuser vane number Diffuser inlet diameter Diffuser outlet diameter Diffuser width
16 100.00 [mm] 34.50 [◦ ] Vaned diffuser 16 117.60 [mm] 144.40 [mm] 3.98 [mm]
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Fig. 5. Compressor performance comparison between the Origin-VD and the Modified-VD.
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4.1. Influence on the steady performance
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Fig. 4. Geometry and installation of the dynamic transducers.
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This section compares the steady performance and the transient instability behavior between the Origin-VD and the ModifiedVD via detailed analysis of experimental results. The ability of the Modified-VD to suppress flow instabilities is assessed. Additionally, this section also discussed the deep surge occurrence.
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4. Experimental results and analysis
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and measurement ranges and measurement errors of these tested parameters are listed in Table 2. During the experiment, two centrifugal compressors are tested with the same strategy. The only difference between them is that one uses the Origin-VD whereas the other uses the Modified-VD; detailed specifications of the compressors are shown in Table 3. Additionally, to investigate the transient instability characteristic within the compressor, dynamic pressure transducers are installed at the impeller inlet (named “A3”) and the diffuser inlet (named “B5”), and they are the XTE-140 (M) SERIES PRESSURE TRANSDUCER (detailed introduction could be seen in reference [28]) produced by Kulite Semiconductor Products, Inc. Information about their geometry and installation is illustrated in Fig. 4. The sensing part of the transducer is surrounded by a cylinder-shaped protective cover with a diameter of 2.60 millimeters, and the sensing principle is “Fully Active Four Arm Wheatstone Bridge Dielectrically Isolated Silicon on Silicon”. The transducer obtains ultra-high natural frequency (240 kHz for “A3” and 300 kHz for “B5”) by using a standard miniature silicon diaphragm, which guarantees its ability to capture quick changes in transient pressure signals. The measurement error of the dynamic pressure transducer is within ±0.10% full scale, and the full scale is 25.0 psi for “A3” and 50.5 psi for “B5”.
The steady performance comparison between the Modified-VD and the Origin-VD is presented in Fig. 5. The lines with circle marks indicate the performance of the Origin-VD, while the lines with square marks indicate the performance of the Modified-VD. The results show that the use of NAVD significantly influences the total pressure ratio and the choke mass flow rate of centrifugal compressors, and the difference becomes more obvious at high rotating speeds. Fig. 5 also illustrates the surge lines of two compressors, which is the last operating points without the occurrence of deep surge. Obviously the two surge lines are different only at intermediate speed (90%). Additionally, the enlarged view (top of Fig. 5) of the near-surge region at 90% speed shows that the Modified-VD changes the flow instability evolution path from stable to unstable conditions: for the Origin-VD, stable states, mild surge and deep surge (not shown in the figure) successively appear when decreasing the mass flow rate; however, for the Modified-VD, mild surge is followed not by deep surge, but stall points when further decreasing the mass flow rate, after which deep surge finally occurs. This is very similar to the “two-regime-surge” phenomenon introduced by Zheng et al. [29]. As a result, the occurrence of deep surge is delayed and the surge mass flow rate significantly decreases. At lower rotating speeds, the surge mass flow rates of the two compressors are almost identical; hence, the two surge lines overlap with each other. At higher rotating speeds, surge mass flow rates of the two compressors are almost the same, but surge lines are a little different because the NAVD decreases the total pressure ratio.
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4.2. Influence on the dynamic instability behavior
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The remarkable differences in the deep surge occurrence and the flow instability evolution path between the Modified-VD and the Origin-VD at 90% speed are attributed to the suppressed flow instability at the diffuser inlet region, and this can be proved by comparing the dynamic instability behavior between the ModifiedVD and the Origin-VD. First, the dynamic instability behaviors at 90% speed are compared to analyze the remarkable differences in the surge line and the flow instability evolution path. Shown in Fig. 6 are pressure
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Fig. 7. Comparison of the frequency spectrum at point MP2 of 90% speed between Origin-VD and Modified-VD (signals recorded at the diffuser inlet).
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Fig. 6. Pressure oscillations at various mass flow rates at 90% speed (solid-line box: Origin-VD; dash-line box: Modified-VD).
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oscillations at different operating points of 90% speed. A reference pressure p r , which is defined by equation (7), is used to indicate the ordinate scale. Two near-surge conditions MP1 and MP2 are selected as comparing points and mass flow rates of Modified-VD and Origin-VD at each comparing point are similar. It can be noted that small disturbances with high frequency occur at both the impeller inlet and the diffuser inlet of both compressors at MP1, and mild surge cycles occur at the impeller inlet of both compressors at MP2. At the diffuser inlet, the pressure behaviors are different: a mild surge disturbance with periodic oscillations appears in the Origin-VD, whereas no mild surge disturbance is observed in the Modified-VD, thereby indicating that the NAVD suppresses the mild surge oscillation at the diffuser inlet. When the mass flow rate is further decreased, the Origin-VD suffers deep surge immediately, while the Modified-VD operates at a stall point (MP3). The pressure signals in Fig. 6 show that the mild surge disturbances at both the impeller inlet and the diffuser inlet are completely suppressed when the Modified-VD operates at MP3. Finally, the compressor operates at another stall point (MP4) with a smaller mass flow rate; likewise, no mild surge disturbance is observed at the diffuser inlet, and the pressure at the impeller inlet oscillates dramatically and this operating point is recognized as a stall point.
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pr =
1 2
× ρatm · u 22
(7)
where ρatm and u 2 are the air density at atmospheric conditions and the blade tangential velocity at impeller outlet, respectively. The suppression influence of the NAVD on the mild surge disturbance could also be assessed by frequency analysis. Fig. 7 compares the frequency spectra of the Origin-VD and the Modified-VD at point MP2 of 90% speed (see Fig. 6), and the result is obtained by applying the Fast Fourier Transform (FFT) to dynamic pressure signals that are recorded at the diffuser inlet (B5). A mild surge disturbance (9.2 Hz) appears at both compressors, thereby indicating that both compressors operate at mild surge conditions. Noting that the coordinate scale of the ordinate is identical, the NAVD remarkably decreases the amplitude of the mild surge disturbance at
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Fig. 8. Frequency spectra of the Modified-VD at various points of 90% speed (signals recorded at the diffuser inlet).
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the diffuser inlet. Meanwhile, several harmonic waves of the mild surge disturbance appear in the frequency spectrum of the OriginVD, and these harmonic waves are suppressed by the NAVD since they disappear at the frequency spectrum of the Modified-VD. In addition, a salient high-frequency disturbance with a frequency of approximately 14.8% blade passing frequency (BPF) appears for the Origin-VD, and it is also suppressed by the NAVD. Fig. 8 shows the frequency spectra of dynamic pressure signals from point MP1 to point MP4 at the Modified-VD’s diffuser inlet, which illustrates the development of dynamic disturbances as the mass flow rate decreases. First, the 14.8% BPF frequency appears (MP1). Then, it is soon suppressed at MP2, where a mild surge disturbance is detected. When the mass flow rate is further decreased (MP3 and MP4), both the mild surge disturbance and the 14.8% BPF disturbance are suppressed, elements of high-frequency disturbances fill the frequency spectrum, and the compressor operates at stall conditions. In summary, the frequency analysis provides additional evidence that the NAVD suppresses flow instabilities at the diffuser inlet region. According to previous works by Sun et al. [26], at 90% speed, these flow instabilities trigger the onset of deep surge. So the NAVD delays the occurrence of deep surge, and also changes the instability evolution path from stable to unstable conditions. Except for 90% speed, the NAVD seems ineffective in changing the surge mass flow rate at other speeds. The reason is that the NAVD does not change flow instabilities which induce the onset of deep surge at these rotating speeds. Actually, Sun et al. [26] have pointed out that at low speeds, the impeller inlet flow in-
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let pressure oscillation of the Modified-VD is smaller than that of the Origin-VD, which indicates that the NAVD can still suppress the diffuser inlet flow instability at high rotating speeds. However, it is not this flow instability, but the compression system characteristics, that decides the deep surge at high speeds, so the deep surge is not delayed and the surge mass flow rates are almost identical between the Origin-VD and the Modified-VD. Results of analysis above could be summarized as follows: The NAVD suppresses the flow instability that appears at the diffuser inlet region, but it neither influences the impeller inlet instability disturbance nor changes the characteristic of the compression system. Because the deep surge at 90% speed is dominantly induced by diffuser inlet flow instabilities, the suppression of these flow instabilities by the NAVD delays the occurrence of deep surge and changes the instability evolution path at this speed, but the deep surge occurrence is not delayed at other rotating speeds.
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Fig. 9. Pressure oscillation comparison at the impeller inlet at near-surge condition (56% speed).
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stability induces the deep surge, while at high speeds, the deep surge is decided by the compression system characteristic. Taking 56% speed as a representative of low speeds, a comparison of the dynamic pressure oscillations at the impeller inlet between the Origin-VD and the Modified-VD is presented in Fig. 9. The comparison is performed at a point near the deep surge condition. No obvious difference in the pressure behavior is exhibited, and it can be seen that high-frequency pressure oscillations appear at both compressors, and their amplitude are of the same order. (Recall that a different vaned diffuser is used for each compressor.) This result indicates that at low rotating speeds, the NAVD does not affect the impeller inlet flow instability which promotes the deep surge, and it cannot delay the occurrence of deep surge. Fig. 10 shows the pressure oscillation of the Origin-VD and the Modified-VD at various operating points near the deep surge condition at 100% speed. When operating at point HP1 (the tested points before mild surge), there is no obvious difference in the pressure oscillations between the two compressors. As mass flow rate decreases to point HP2, both compressors suffer a mild surge with periodic oscillations. Additionally, the amplitude of the diffuser in-
4.3. Additional discussion on compressor stability improvement
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Compared with the Origin-VD, the Modified-VD exhibits a different instability evolution path from stable states to deep surge at 90% speed (as shown in Fig. 5 and Fig. 6), and its surge mass flow rate is remarkably reduced, thereby indicating significant improvement in the compressor stability. A non-dimensional parameter, namely, the stable flow range (SFR), is often used to assess the compressor stability at a specific speed; it is defined by equation (8). Shown in Fig. 11 is a comparison of four performance parameters between the Origin-VD and the Modified-VD at 90% speed: the SFR, the surge mass flow rate, the total pressure ratio and the isentropic efficiency. Compared with the Origin-VD, the SFR of the Modified-VD is increased by approximately 25.0% (from 37.5% to 46.9%), meanwhile, the surge mass flow rate is decreased by approximately 16.1%. Therefore, it is evident that the SFR extension is dominant attributed to the reduction in the surge mass flow rate; in other words, the Modified-VD delays the occurrence of deep surge and achieves improved flow stability. Fig. 11 also presents differences in the total pressure ratio and the isentropic efficiency: a decrease of 1.15% and 1.32% is observed for these two parameters, respectively. In summary, due to the reduction in the surge
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Fig. 10. Pressure oscillations at various operating points at 100% speed (a: Origin-VD; b: Modified-VD).
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Fig. 11. Comparison of performance parameters between the Origin-VD and the Modified-VD at 90% speed.
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mass flow rate, the NAVD extends the SFR of the compressor by 25.0% at the cost of slight decreases (less than 1.50%) in the total pressure ratio and the isentropic efficiency. To some extent, the NAVD could be used as an effective flow control method to extend the compressor flow range and the cost is acceptable.
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SFR =
mchoke − msurge mchoke
N =const
× 100%
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5. Conclusions The main objective of this paper is to delay the occurrence of deep surge by suppressing flow instabilities. First, a novel inhouse designed non-axisymmetric vaned diffuser is developed by changing the local stagger and local solidity of a conventional axisymmetric vaned diffuser. Then, experimental investigations are carried out to assess deviations in both the deep surge occurrence and the dynamic flow instabilities. Two main conclusions are drawn: (1) the flow instability at the diffuser inlet region is obviously suppressed by the non-axisymmetric vaned diffuser. This delays the occurrence of deep surge at 90% speed and extends the stable flow range of the compressor; (2) the non-axisymmetric vaned diffuser neither affects the flow instability at the impeller inlet nor changes the characteristic of the compression system, so it hardly delays the deep surge at other rotating speeds. The reason is that only at 90% speed, the onset of deep surge initiates from the diffuser inlet flow instability, and the deep surge is triggered by the impeller inlet flow instability at lower rotating speeds and by the characteristics of the compression system at higher rotating speeds, respectively. In addition, experimental results show that at 90% speed, the non-axisymmetric vaned diffuser extends the stable flow range of the compressor by 25.0% (from 37.5% to 46.9%) at the cost of slight decreases in both the total pressure ratio (1.15%) and the isentropic efficiency (1.32%). This provides a new flow control method to improve the compressor stability by non-axisymmetric vaned diffuser.
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Declaration of competing interest
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Authors of this paper declare no potential conflicts of interest with respect to the research, authorship, and/or publication.
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Acknowledgements
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This research was supported by the National Science and Technology Major Project (2017-II-0004-0016), the National Natural Science Foundation of China (Grant No. 51176087), the Tsinghua University Initiative Scientific Research Program, and the IHI Corporation, Yokohama, Japan.
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