Subsonic Axial Flow Compressor Rotor Research Articles

Influence of Active Flow Control of Synthetic Jet at Suction Surface on the Performance of a Subsonic Axial Compressor Rotor

In order to investigate the effect of a synthetic jet arranged at the suction surface on the aerodynamic performance of an axial flow compressor, a high-speed subsonic axial flow compressor rotor was numerically simulated in this paper. The results showed that after being excited by the synthetic jet, the flow margin of the compressor decreased by 1.3%, but the peak efficiency increased by 0.47%. After analyzing the internal flow field, it was found that although the alternating blowing and suction effect of the synthetic jet can reduce the separation loss of the suction surface, the radial velocity of the flow is higher when stimulated and converges towards the blade tip, resulting in more severe blockage at the blade tip than in the prototype compressor, leading to a slight decrease in stability margin. To balance the stability margin of the compressor, a coupled flow control numerical simulation combining passive control of the casing treatment and active control of the suction surface synthetic jet was subsequently carried out. The results showed that the coupled flow control could exert their respective advantages, resulting in an increase of 17.84% in the compressor flow margin and a decrease of only 0.2% in peak efficiency.

Tip leakage flow, tip aerodynamic loading and rotating instability in a subsonic high-speed axial flow compressor rotor

Rotating instability as an unsteady flow phenomenon is closely related to rotating stall in the compressor. In this paper, dynamic pressure measurements and full-annular URANS simulations were conducted in a subsonic axial flow compressor rotor to enhance the understanding of the flow mechanism of RI. RI characterized by a frequency band in the spectrum is detected at a narrow stable operating range in the experiments. The monitoring results from simulations show that RI characterized by a frequency hump in the spectrum appears near stall condition which is consistent with the observation in the experiments. RI shifts to lower frequency band in the rotating frame with the decrease of mass flow rate. Details of the numerical flow field indicate that RI develops synchronously with the oscillation of the tip leakage flow (TLF). The tip leakage vortex (TLV) forms a low static pressure near the pressure surface side of neighboring blade through an induced vortex. It varies the tip aerodynamic loading. Both the tip aerodynamic loading and TLF get strong with the decrease of mass flow rate. The stronger TLV will form a stronger induced vortex near the pressure surface side of the neighboring blade and propel the induced vortex toward the leading edge of blade. When a high tip aerodynamic loading region is influenced by the induced vortex near the pressure surface side, the intensity of TLF in the next passage will be weakened. Then, TLF fluctuates and leads to the oscillation of neighboring blade tip aerodynamic loading as well as the oscillation of TLF issuing from the neighboring blade. The circumferential propagation of this influence process results in RI.

Investigation into the flow mechanism of rotating instability in a subsonic axial flow compressor rotor

Experimental and numerical investigations on a subsonic axial flow compressor rotor are carried out to clarify the flow mechanism of rotating instability (RI). RI is observed at the narrow stable operating range near the stability limit of the test rotor for the experiments. The simulation results show that RI appears near stall condition, which is similar to that in the experiments. An unsteadiness of tip leakage flow (TLF) is detected in a passage for the numerical results. Its characteristic frequency agrees well with the dominant mode frequency of RI in experiment. Details of the simulated flow fields indicate that the unsteadiness of TLF is induced by the interaction between TLF and the loading of blade tip in a single passage. This process propagates circumferentially against the rotor and forms a traveling wave in the circumferential annulus, which is responsible for RI. Since the blockage effect increases with decreasing mass flow rate, the fluctuation region of TLF gets larger, the period of flow unsteadiness increases and the parameters of RI vary.

Experimental investigation of flow characteristic of tip leakage flow in an axial flow compressor rotor

Tip leakage flow fields in a small-scale high-speed subsonic axial flow compressor rotor were investigated by virtue of the steady and casing instantaneous pressure measurements over wide stable operating range. Fast Fourier transformation and short time Fourier transformation analyses were conducted to elucidate the flow characteristics at different operating conditions, with an emphasis on extending experimental database to validate the previous and subsequent numerical investigations, which would be crucial to the understanding of the origin of tip flow unsteadiness. The measured quasi-steady flow characteristics agreed well with the flow features of tip leakage flow from previous steady simulations. The unsteady flow characteristic of tip leakage flow was the appearance of rotating instability, which was of varying frequency closely related to tip loading variation. An agreement between the unsteady flow characteristics of tip leakage flow from the current measurements and from the previous unsteady simulations to some extent indicated that further systematical unsteady simulations and a deeper quantitative validation of simulation results should be made so that the origin of rotating instability could be uncovered precisely.

Numerical and experimental research of stall inception on subsonic axial-flow compressor rotor

Three-dimensional flow simulation and blade tip high-response static pressure measurements were performed on an isolated subsonic compressor rotor to gain more insight into the stall inception mechanism of the compressors. The Navier—Stokes solver, EURANUS, was used for computation. The steady-state flow solution was achieved at the convergence of a four-stage explicit Runge—Kutta integration scheme. The time-dependent calculation was implemented in the implicit dual time stepping scheme, which allowed for the solution of a steady-state problem at each physical time step. The high-response static pressure was measured using five Kulite sensors installed on the casing. The data acquisition frequency was 100 kHz. The recorded data were later analysed using wavelet analysis method. Correlating the simulation result and the measurement result, it was shown that as the compressor was approaching the near stall (NS) condition, the tip leakage vortex dissipation and shedding became violent. The shedding or dissipated flow structures would consequently scatter around the blade tip passage, which formed the initial onset of stall disturbances. These scattered vortexes would finally lead to compressor stall as they hit and merge with each other into low-frequency disturbance of significant size and energy. A stall pre-alarming method based on the monitoring of the low-frequency spectrum power at the NS condition was also suggested by this study.

Numerical Investigations of the Coupled Flow Through a Subsonic Compressor Rotor and Axial Skewed Slot

In order to advance the understanding of the fundamental mechanisms of axial skewed slot casing treatment and their effects on the subsonic axial-flow compressor flow field, the coupled unsteady flow through a subsonic compressor rotor and the axial skewed slot was simulated with a state-of-the-art multiblock flow solver. The computational results were first compared with available measured data, that showed the numerical procedure calculates the overall effect of the axial skewed slot correctly. Then, the numerically obtained flow fields were interrogated to identify the physical mechanism responsible for improvement in stall margin of a modern subsonic axial-flow compressor rotor due to the discrete skewed slots. It was found that the axial skewed slot casing treatment can increase the stall margin of subsonic compressor by repositioning of the tip clearance flow trajectory further toward the trailing of the blade passage and retarding the movement of the incoming∕tip clearance flow interface toward the rotor leading edge plane.

Numerical investigations of the flow mechanisms in a subsonic compressor rotor with axial-skewed slot

By applying a concept similar to ‘domain scaling’ treatment often used in multi-stage turbomachinery flow field to the interface between the rotor blade passage and end-wall treatments, a series of computational studies were carried out to understand the physical mechanism responsible for improvement in stall margin of a modern subsonic axial-flow compressor rotor due to the axial-skewed slot casing treatment. Particular attention was given to examining the interaction between the rotor tip flow and the axial-skewed slot. In order to validate the multi-block model applied in the rotor blade end-wall region, the computational results for the modern subsonic compressor rotor both with and without axial-skewed slot casing treatment were correlated with available experimental test data. The computed results agree fairly well with the experimental data, and the computations are then used to describe the interaction between the rotor tip flow and axial-skewed slot. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases with axial-skewed slot and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by the axial-skewed slot casing treatment is a result of the tip flow manipulation. The repositioning of the tip vortex trajectory further towards the trailing edge of the blade passage and delaying the movement of incoming/tip clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.

Experimental and numerical investigation of a subsonic compressor with bend-skewed slot-casing treatment

On the basis of the test results of discrete axial and blade angle slot casing treatment, a new type of casing treatment was designed for a subsonic axial flow compressor rotor by optimizing various geometry parameters. To obtain a wide operating range and to minimize penalties in terms of isentropic efficiency, seven compressor configurations incorporating casing treatments of 0, 16.6, 33.3, 50, 66.6, 83.3, and 100 per cent rotor exposures were experimentally investigated. The results showed that significant improvements in stall margin are possible in all exposures and insignificant isentropic efficiency sacrifices are recorded in some exposures. Nearly 21.43 per cent stall margin improvement in terms of the corrected mass flow-rate was achieved with 33.3 per cent rotor blade tip axial chord exposure. The compressor built with 16.6 per cent rotor exposure was the best configuration in terms of maximum isentropic efficiency gain. The second issue of the paper was to offer a contribution to the understanding of the physical mechanism by which bend-skewed slot-casing treatment improves stall margin under subsonic conditions. By applying a concept similar to ‘Domain scaling’ approach (as often used in multistage turbomachinery flow-fields) to the interface between the rotor blade passage and end-wall treatments, a time-dependent three-dimensional numerical simulation was performed for the subsonic axial-flow compressor rotor with bend-skewed slot-casing treatment. The numerical results agreed well with the available experimental results. Detailed analyses of the coupled flow through bend-skewed slot-casing treatment and rotor blade passage under subsonic conditions led to some preliminary conclusions as to the flow physics involved in the stall margin improvements afforded by the use of bend-skewed slot-casing treatment.

Numerical and Experimental Investigations of Steady Micro-Tip Injection on a Subsonic Axial-Flow Compressor Rotor

Steady tip injection has been demonstrated to be an effective means of extending the stable operating range of a tip-critical compressor. This study presents a state-of-the-art design for the tip injection through the casing with flush-mounted inclined holes and the effectiveness of steady micro-air injection to enhance stability in a subsonic axial-flow compressor rotor using an external-air supply. For the tested rotor, experimental results demonstrate that at 53% design speed, the stalling mass flow can be reduced by 7.69% using an injected mass flow equivalent to 0.064% of the annulus flow. Time-dependent CFD simulations were conducted to identify the physical mechanic that accounts for the beneficial effects of the steady micro-air injection on the performance and stability of the compressor. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases without tip injection and with tip injection. It was found that the primary stall margin enhancement afforded by the steady micro-air injection is a result of the tip-clearance flow manipulation. The repositioning of the tip-clearance vortex further towards the trailing edge of the blade passage and delaying the movement of incoming/tip-clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.

Numerical and experimental investigation of stepped tip gap effects on a subsonic axial-flow compressor rotor

This article investigates the flow field at the tip region of compressor rotor. In particular, the effect of stepped tip gaps on the performance and flow field of axial compressor was reviewed using experimental and computational methods. An axial compressor rotor with no inlet guide vanes was tested under subsonic conditions. A parametric study of clearance levels and step profiles was performed using eight different casing geometries. This study was aimed at comparing compressor performance in specified configurations. The experimental results showed that the inclusion of stepped tip gaps with the small clearance level gave increased pressure ratio, efficiency, and stall margin throughout the mass flow range at both speeds. However, when using medium and large clearance levels, the benefits of stepped tip gaps were not noticed for all rotor operating conditions when compared with the baseline case. Steady-state Navier-Stokes analyses were performed for cases involving small clearance level and stepped tip gap geometries. They highlighted the mechanisms associated with performance improvement. The numerical procedure correctly predicted the overall effects of stepped tip gaps. Detailed numerical simulation results showed that the interaction between the stepped groove flow and the blade passage flow could entrain the blockage produced by upstream tip leakage flow into the tip gap of adjacent blades of the compressor rotor. It is through this process that stepped tip gaps can help in dissipating blockage that was caused by upstream tip leakage flow. Thus, the path and extent of the blockage in the tip region are altered to increase the passage through-flow area, and so, the rotor performance can be improved.