Counter-rotating Axial Compressor Research Articles

Experimental and numerical investigation on rotating stall flow pattern in a counter-rotating axial compressor

Rotational stall is a common flow instability phenomenon in axial and centrifugal compressors. The experimental and numerical results of the three-dimensional flow field in a low-speed counter-rotating axial compressor operating in deep rotating stall are presented in this paper. The obtained results provide a detailed description of the flow inside and outside the stall cell. The circumferential flow of the reversed flow upstream and downstream is dominated, respectively, by the counter-rotating front and rear row rotors. The reverse flow downstream reenters the rear row rotor and the front rotor at higher blade heights, undergoing repeated work by the rotors. This leads to the formation of a high-pressure region at the leading edge of stall cell, which is essential for maintaining stall conditions upstream of rotors. Stalls primarily occur in the middle and tip regions with elevated pressure, while the downstream stall cell exhibits significant spreading from hub to tip with some circumferential bias.

Design of a Compressor Test Rig for Immobilization of the Stall Cell

Capturing internal flow experimentally presents significant challenges due to the asymmetric propagation and strong unsteadiness of a stall cell in the circumferential direction. In this study, a low-speed counter-rotating axial compressor test rig was designed based on a counter-rotating compressor to immobilize a stall cell and measure its internal flow characteristics. Determining an appropriate speed ratio that is capable of stabilizing the circumferential position of the stall cell in the counter-rotating compressor enabled visualization measurements to be conducted successfully. The preliminary results demonstrated the successful immobilization of the stall cell using an appropriate rotating speed ratio. Furthermore, the oil flow visualization measurements confirmed the presence of a distinct stall cell structure on the casing wall. This work represents an innovative approach towards immobilizing stall cells in axial compression systems.

Dynamic mode decomposition for the tip unsteady flow analysis in a counter-rotating axial compressor

Counter-rotating axial compressor (CRAC) is a promising potential technology to improve the thrust-to-weight ratio of aero-engines, but its special aerodynamic layout usually causes more pronounced flow unsteadiness. Understanding the unsteady flow features and mechanism in the CRAC contributes to the aerodynamic optimization design and flow control strategy organization. A data-driven dynamic mode decomposition method is introduced to investigate the tip flow unsteadiness in a CRAC, and the unsteady features of the tip flow at the design point (DP) and near-stall point (NSP) conditions are revealed. The results show that the 1.0 times blade passage frequency (BPF) and its multi-order harmonic frequency are the dominant frequencies for both rotors at the DP condition. At the NSP condition, the 1.0 BPF is no longer the dominant frequency causing the tip flow unsteadiness, and the low frequency fluctuation of the tip leakage flow becomes the dominant frequency to induce the flow unsteadiness. In the front rotor R1, the unsteady dominant frequency is 1.0 BPF, whereas in the rear rotor R2, the frequency (0.801 BPF and 0.803 BPF) of the tip leakage flow is the dominant frequency. By reconstructing the flow field under the NSP condition, the spatiotemporal evolution of the tip flow during the unsteady stable manifests that the interference effect between the rotors is an important source of the tip flow unsteadiness. The increase in flow unsteadiness leads to an increase in the reconstruction error, indicating that more modes are required to obtain a more accurate reconstruction flow field.

Numerical investigation on the cross-stage self-recirculating casing treatment for a counter-rotating axial compressor

Although the counter-rotating axial compressor (CRAC) can promote the thrust-to-weight ratio of aero-engines, the “stall transposition” phenomenon poses a huge technology challenge in controlling the flow due to the speed ratio effect. Traditional flow control technology hardly operates under off-design conditions, posing an urgent need to develop a new flow control approach suitable for CRAC's variable speed ratio conditions. Hence, based on the conventional rear-jet self-recirculating casing treatment (SRCT), this work designs two cross-stage SRCT cases (front-jet SRCT and dual-jet SRCT) to resolve the “stall transposition” issue in a two-stage CRAC. The results demonstrate that although all SRCT cases improve the stall margin of the CRAC at the design condition, both front-jet SRCT and dual-jet SRCT cases have a significant stabilization capacity without changing the first stall stage, opposing the rear-jet SRCT case, which changes the first stall stage. The dual-jet and front-jet SRCT cases have a significant stabilization capacity under the off-design speed ratio conditions; since they can simultaneously act on the tip clearance flow of both the upstream and the downstream rotors and increase the stall margin by about 6.23%∼10.78% and 6.66%∼10.42%, respectively. Variations in speed ratio may shift the location of the first stall stage, forcing the rear-jet SRCT case to fail in expanding the stall margin at the partial speed ratio conditions. Moreover, the suppression of SRCT on the flow instability of the tip leakage flow decreases the unsteady pressure fluctuation intensity near the rotor tip.

Stage reduced counter-rotating axial compressor for jet engine

Variation in the states of system, mass flow and pressure are some of the disturbances which are experienced by the compressors in the jet engine under working condition. One of the main factors that influence the efficiency of a jet engine is the pressure ratio. In order to achieve the required pressure ratio, we should have relatively a greater number of stages in the compressor that leads to an increase in the weight of the engine. The stator and rotor are the essential parts of an aircraft's axial compressor. CFD is used in order to evaluate the pressure ratio. In this paper, we are going to analyze a three-stage compressor instead of an actual six-stage compressor. The mass flow rate inside the control system can be used to maintain the stability of the system. Compressor weight and pressure ratio at each stage can be reduced if we have a clockwise and anti-clockwise rotating rotor. With the use of a universal gear system, the two clockwise rotors and one anti-clockwise rotor were analyzed. The main outlook of this work is to show the maximum pressure ratio of the compressor at the outlet with our desired configurations. In conclusion, it was shown that the weight of the aircraft engine can be effectively reduced.

Numerical investigation of tip clearance size effect on the performance and tip leakage flow in a dual-stage counter-rotating axial compressor

Based on a validation of the numerical methods with an experiment, numerical simulations are carried out to study the effect of tip clearance size on the performance and tip leakage flow in a dual-stage counter-rotating axial compressor. The predicted results showed that the variation of the tip clearance size in rotor2 has a more significant impact on the overall performance and stall margin of the compressor. In addition, the impact of the tip clearance size effect is mainly on the rotor with the tip clearance size variation. The variation of the tip clearance size in rotor2 almost has no influence on the performance of rotor1, while the performance of rotor2 is increased about 1.37% at near-stall point when the tip clearance size of rotor1 is increased to 1.0 mm from 0.5 mm. At peak efficiency condition, the tip clearance size variation in rotor1 has remarkable influence on the tip leakage vortex intensity, onset point and trajectory in rotor1, but has little influence on those in rotor2. However, the tip clearance size variation in rotor2 has remarkable effect on those in both rotors. Different tip clearance size combination schemes can impact the stall-free characteristic in the counter-rotating axial compressor.

Numerical study of the unsteady behaviors and rotating stall inception process in a counter-rotating axial compressor

Unsteady computations of a counter-rotating axial compressor are performed and analyzed to investigate the unsteady behaviors in the compressor and the role of the tip leakage flow together with the rotating stall inception process. The results show that the oscillation on the pressure side is much stronger than that on the suction surface for both rotors, especially near the tip region where the trajectory of the tip leakage vortex (TLV) interacts with the blades most often. There exists a periodical leading edge spillage of the interface in rotor2 due to the unsteadiness of tip leakage flow (TLF) at near-stall condition. The blockage generated by the TLV increases dramatically due to the increasing strength of the TLV and the backflow phenomenon as the mass flow decreased. The appearance of the frequency components of 0.5 blade passing frequency (BPF) and 1.5BPF from 0.64BPF can be viewed as the rotating stall inception warning. The fluctuation strength of oscillation frequencies of 0.5BPF and 1.5BPF decreases rapidly from leading edge to trailing edge in rotor2, which indicates that the unsteady fluctuation of TLF at the leading edge in rotor2 is responsible for the stall inception of the compressor. Additionally, both the leading edge spillage and trailing edge backflow phenomena are observed for spike initiated rotating stall at stall point.

Numerical investigation of the unsteady flow behaviors in a counter-rotating axial compressor

Unsteady numerical simulations of a counter-rotating axial compressor are performed to investigate the unsteady effects of rotor–rotor interaction process and the tip flow unsteady behavior near the stall operating condition. The results show that potential effects from the rear rotor have a more pronounced influence on the front rotor. Because of the interaction between the two rotors and the unsteady flow behaviors, the strongest fluctuant regions are different for the two rotors. Additionally, the oscillation on the pressure side is generally much stronger than that on the suction side. Frequency analyses show that self-unsteadiness occurs with the frequency of 0.76 blade passing frequency due to the impingement of the tip leakage flow on the pressure side near leading edge of the adjacent blade. An interesting phenomenon, i.e., the emergence of a local high pressure spot, is observed in the tip region. As a whole, it is concluded that there are two key effects of the emergence of the local high pressure zone in the tip region: (a) more leading edge spillage of tip leakage flow and increasing the risk to trigger stall; and (b) a decrease in blockage associated with double-leakage of tip clearance flow within the influenced region of the local high pressure zone.

Unsteady Investigation on Tip Flow Field and Rotating Stall in Counter-Rotating Axial Compressor

Contra-rotating axial compressor/fan (CRAC) is a promising technology to meet the future goals aircraft industry. Massive time accurate simulations are performed to investigate rotating stall in CRAC containing two counter-rotating rotors. Particularly, the back pressure increasing with a very small step to avoid missing flow field transition from stability to instability. Due to the canceling of the stator, the instability of downstream rotor is more stronger. The present studies mostly focus on the downstream rotor. The tip leakage flow field is analyzed in detail under near stall condition, which indicates that a secondary leakage flow plays an important role in the unsteadiness of CRAC's unsteady flow field. The frequency analysis in the tip clearance of downstream rotor under multiple near stall conditions captured the transition of the second harmonic frequency which can be used as stall inception signal. Moreover, the rotating stall onset process in real CRAC is simulated on the numerical stall.

Design of a novel axial impeller as a part of counter-rotating axial compressor to compress water vapor as refrigerant

A systematic investigation to understand the use of a novel axial impeller as a part of counter-rotating axial compressor to compress water vapor as refrigerant was undertaken. Blade angle was investigated at first to understand this novel impeller’s geometry. A commercial CFD package, which solves the Reynolds-averaged Navier–Stokes equations, was used to compute the complex flow field of the impeller. Two hub/shroud tip ratios with different speed ratios were numerically simulated. The numerical simulation focused on the water vapor flow from compressor impeller inlet to outlet and the overall performance level and range were predicted. The numerical investigation revealed that at hub/shroud tip ratio of 0.54, maximum pressure ratio reached about 1.24 with isentropic efficiency around 75% at its design point. Detailed investigation into flow structure showed that a serious secondary flow exists between span ranges of 30–50%. An enlargement of hub/shroud tip ratio to 0.75 showed that pressure ratio had been improved significantly. By comparing different speed ratios and hub/shroud tip ratios, the study shows potential to utilize this novel axial impeller to compress water vapor as refrigerant.