Yanhui Wu

Investigation of Pre-Stall Behavior in an Axial Compressor Rotor—Part I: Unsteadiness of Tip Clearance Flow

To investigate the pre-stall behavior of an axial flow compressor rotor, which was experimentally observed with spike-type stall inception, systematic experimental and whole-passage simulations were laid out to analyze the internal flow fields in the test rotor. In this part, emphases were put on the analyses of experimental results and the predicted results from steady simulations and unsteady simulations, which converged to equilibrium solutions with nearly periodic fluctuations of efficiency. The objective was to uncover the unsteady behavior of tip clearance flow and its associated flow mechanism at near-stall conditions. To validate the steady simulation results, the predicted total characteristics and spanwise distributions of aerodynamic parameters were first compared with the measured steady data, and a good agreement was achieved. Then, the numerically obtained unsteady flow fields during one period of efficiency fluctuations were analyzed in detail. The instantaneous flow structure near casing showed that tip secondary vortex (TSV), which appeared in the previous unsteady single-passage simulations, did exist in tip flow fields of whole-passage simulations. The cyclical motion of this vortex was the main source of the nearly periodic variation of efficiency. The simulated active period of TSV increased when the mass flow rate decreased. The simulated frequency of TSV at flow condition very close to the measured stall point equaled the frequency of the characteristic hump identified from the instantaneous casing pressure measurements. This coincidence implied that the occurrence of this hump was most probably a result of the movement of TSV. Further flow field analyses indicated that the interaction of the low-energy leakage fluid from adjacent passages with the broken-down tip leakage vortex (TLV) was the flow mechanism for the formation of TSV. Once TSV appeared in tip flow fields, its rearward movement would lead to a periodic variation in near-tip blade loading, which in turn altered the strength of TLV and TSV, accordingly, the low-energy regions associated with the breakdown of TLV and the motion of TSV, thus establishing a self-sustained unsteady flow oscillation in tip flow fields.

Experimental and Numerical Investigation of Flow Characteristics Near Casing in an Axial Flow Compressor Rotor at Stable and Stall Inception Conditions

Casing instantaneous pressure measurements and full-annulus unsteady simulations were undertaken to analyze flow characteristics near casing at stable and stall inception conditions in an axial flow compressor rotor, and the objective was to establish its linkage with the stall inception process. The measured flow characteristic at near-stall stable operating conditions was the appearance of rotating instability (RI), which attributed to the activity of an unsteady flow with varying frequency. A similar flow characteristic was found in the simulated near-stall stable flow conditions, and detailed analyses of instantaneous flow field indicated the formation and activity of tip secondary vortex could be flow mechanism for the appearance of RI as far as nonuniform tip loading distribution in measurements was concerned. The measured flow characteristic before spike emergence was still the activity of RI. However, it was submerged into flow field accompany by the emergence of spike. The simulated stall inception process was similar to that from measurement, and further analyses of instantaneous flow field established the causal linkage between RI and stall inception process for the test rotor.

Numerical Investigation on the Effects of Circumferential Coverage of Injection in a Transonic Compressor With Discrete Tip Injection

Discrete tip injection upstream of the rotor tip is an effective technique to extend stability margin for a compressor system in an aeroengine. The current study investigates the effects of injectors’ circumferential coverage on compressor performance and stability using time-accurate three-dimensional numerical simulations for multi passages in a transonic compressor. The percentage of circumferential coverage for all the six injectors ranges from 6% to 87% for the five investigated configurations. Results indicate that circumferential coverage of tip injection can greatly affect compressor stability and total pressure ratio, but has little influence on adiabatic efficiency. The improvement of compressor total pressure ratio is linearly related with the increasing circumferential coverage. The unsteady flow fields show that there exists a non-ignorable time lag of the injection effects between the passage inlet and outlet, and blade tip loading will not decline until the injected flow reaches the passage outlet. Stability improves sharply with the increasing circumferential coverage when the coverage is less than 27%, but increases flatly for the rest. It is proven that the injection efficiency which is a measurement of averaged blockage decrement in the injected region is an effective guideline to predict the stability improvement.Copyright

Experimental study of self-recirculating casing treatment in a subsonic axial flow compressor

Parametric studies of recirculating casing treatment were experimentally performed in a subsonic axial flow compressor. The recirculating casing treatment was parameterized with injector throat height, injection position, and circumferential coverage percentage. Eighteen recirculating casing treatments were tested to study the effects on compressor stability and on the compressor overall performance at three blade speeds. The profiles of recirculating casing treatment were optimized to minimize the losses generated by air recirculation. In the experiment, the stalling mass flow rate, total pressure ratio, and adiabatic efficiency of the compressor were measured to study the steady-state effects on the compressor performance of recirculating casing treatments, and static pressure disturbances on the casing wall were monitored to study the influence on stall dynamics. Results indicate that both the compressor stability and overall performance can be improved through recirculating casing treatment with appropriate geometrical parameters for all the test speeds. The influence on stall margin of one geometric parameter often depends on the choice of others, i.e. the interaction effects exist. In general, the recirculating casing treatment with a moderate injector throat and a large circumferential coverage is the optimal choice to enhance compressor stability. The injector of recirculating casing treatment should be placed upstream of the blade tip leading edge and the injector throat height should be lower than four times the rotor tip gap for the benefits of compressor efficiency. At 71% speed, the blade tip loading is decreased through recirculating casing treatment at the operating condition of near peak efficiency and increased near stall. Moreover, the outlet absolute flow angle is reduced in the tip region and enhanced at lower blade spans for both operating conditions. The stall inceptions are not changed with the implementation of recirculating casing treatment for all the test speeds, but the stall patterns are altered at 33% and 53% speeds, i.e. the stall with two cells is detected in the recirculating casing treatment compared with the solid casing with only one stall cell.

Flow mechanism of affecting an axial flow compressor performance and stability with cross-blade slot casing treatments

The objective of this study is to evaluate the effect of cross-blade slot casing treatment on the stability and performance of an axial flow compressor rotor. The experimental and unsteady calculated results both show that cross-blade slot casing treatment can generate about 22% stall margin improvement, and the compressor peak efficiency is reduced by about 13%. The detailed flow-field analyses indicate that the sucked and injected flow caused by the slots of cross-blade slot casing treatment can restrain the rotor tip passage blockage, which is made by the low energy tip clearance leakage vortex. When cross-blade slot casing treatment is applied, not only the rotor wheel flange work becomes lower in most of the rotor blade span, but also the flow loss in the blade tip passage becomes fairly large due to the strong interaction between the mainstream and the injected flows made by the slots. As a result, the compressor total pressure ratio and efficiency for cross-blade slot casing treatment are reduced obviously. Three kinds of new cross-blade slot casing treatment were designed according to the previous successful experience and investigated in this paper. The numerical results show that the new three cross-blade slot casing treatments both generate about 54% stall margin improvement at the cost of minor peak efficiency. For one new cross-blade slot casing treatment (CSCT2), the compressor peak efficiency is reduced by about 0.3%. The low energy TLV, which is present for cross-blade slot casing treatment, is removed by the strong sucked flow made by CSCT2. Moreover, the interaction between the mainstream and the injected flows caused by CSCT2 becomes weak obviously, and the corresponding flow loss is reduced greatly. Hence, the compressor stability and performance with CSCT2 are higher than those with cross-blade slot casing treatment.

Numerical Investigation of Pressure Fluctuation on the Volute of a Backward Curved Blade Centrifugal Fan

The pressure fluctuations on the volute surface induced by internal unsteady flow are the important sources of fan casing vibration and noise generation. In this paper, a three-dimensional and unsteady flow of a whole impeller-volute structure have been carried out by using commercial CFX code in order to obtain the wall pressure fluctuations on the volute of a large scale centrifugal fan (especially in the vicinity of volute tongue). The two important different flow rates have been simulated, the best efficiency point (BEP) and 130 percent of the BEP (1.3 × BEP), multi-domain structure grids have been applied in all the domains of current simulations. The pressure fluctuations of setting locations on the volute have been obtained by this method. Characteristics of these fluctuations in time and frequency domains were mainly analyzed. The results showed that the amplitudes of these pressure fluctuations over the volute changed with the flow rates variation. The blade passing frequency and their second harmonic frequency were observed clearly, and an important peak presented at the blade passing frequency (BPF). The amplitude of BPF has related with the position of the volute. On the circumferential direction of the volute, the highest values appeared in the vicinity of volute tongue; on the axial position, the peak value was discovered near to impeller shrouds. All the calculation results have been compared to the experimental results showing a good agreement.Copyright

Experimental and Numerical Investigation of Near-Tip Flow Field in an Axial Flow Compressor Rotor: Part II — Flow Characteristics at Stall Inception Condition

Systematical casing pressure measurements were undertaken to supplement instantaneous experiment data to available database of a high-speed small-scale compressor rotor, which was crucial for understanding the flow mechanism of short-length scale stall inception. At the same time, improved full-annulus simulations were conducted to assist in interpretation of experimental observations. In Part I of current investigation, FFT (fast Fourier transformation) and STFT (short time Fourier transformation) analyses of instantaneous casing pressure signals were conducted to conclude flow characteristics near casing at stable operating conditions, and reasonable explanation of experimental observations was given in combination with the current and previous numerical results.FFT analyses of casing pressure signals showed a characteristic hump with varying band lower than blade passing frequency (BPF) appeared at near-stall stable conditions. This indicated that an unsteady phenomenon emerged from the near-tip flow field for the test rotor. The variation in the amplitude of characteristic hump implied that underlying flow mechanism leading to the emergence of unsteady phenomenon originated from a location near leading edge and within passage. Further STFT analyses showed that the active frequency of this unsteady phenomenon varied with time, thus leading to the appearance of excitation band in FFT analysis results. FFT and STFT analyses of monitoring results of numerical probes arranged in absolute frame showed a similar unsteady phenomenon appeared in the simulated near-tip flow field. Detailed analyses of simulated instantaneous flow fields and comparison with measured flow characteristics indicated that the unsteady flow phenomenon observed in experiments was equivalent to rotating instability (RI) as far as non-uniform tip loading distribution was concerned, and the formation and activity of tip secondary vortex (TSV) was the flow mechanism of emergence of RI.Copyright