Haoguang Zhang

The effect of injector size on compressor performance in a transonic axial compressor with discrete tip injection

The present study investigates the effects of injector size, which covers both circumferential extent and radial extent on compressor stability and radial work redistribution using time-accurate three-dimensional numerical simulations for multi-passages in a transonic compressor with discrete tip injection. Injector width ranges from 1% to 14.5% of casing perimeter and injector throat height varies from 2 to 6 times the height of rotor tip clearance. Six identical injectors are uniformly mounted around the annulus. Results indicate that, with sufficient covering of injection, tip injection can significantly improve compressor stability. Stability margin is improved by 3.5% employing 0.65% of the annulus mass flow. With injector width increased, stability improves sharply when the width is less than 4.5% of casing perimeter, but increases flatly for the rest coverage. Expanding injector throat height to an intermediate level can further enhance compressor stability, but excessive throat height is detrimental on account of the deterioration of the aerodynamic characteristic of Coanda injector. The average decrement of the blockage in injection region, which is defined as injection efficiency, is a key factor that takes primary responsibility for the capacity of tip injection in stability improvement. Injection efficiency derived from the same operating condition for all the configurations with tip injection is well-qualified to estimate the trend of stability improvement. The unsteady influence on radial work redistribution is much more significant than the time-averaged one.

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

Numerical Investigation of the Circumferential Grooved Casing Treatment as well as Analyzing the Mechanism of Improve Stall Margin

The Circumferential Grooved Casing Treatment is known as a powerful method to enhance the flow stability in compressors. Numerical investigation was conducted to study the influence on a subsonic compressor performance with the circumferential grooved configuration. The calculated overall performance lines showed good agreement with the experiments. The fundamental flow mechanism was obtained by detailed analyzing the flow-filed at the blade tip, which was the circumferential grooved casing treatment affects the compressor performance and flow field.

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.

Numerical investigation on the effect of a plenum chamber with slot-type casing treatment on the performance of an axial transonic compressor

Casing treatment is an effective approach for improving compressor stall margin in axial compressors. A plenum chamber, which connects slots in the circumferential direction, can occasionally be found in slot-type casing treatments. This study investigates the effect of a plenum chamber on the compressor overall performance and stall margin via time-accurate three-dimensional numerical simulations in a transonic compressor NASA Rotor 37 with a slot-type casing treatment. Four configurations with variable height of plenum chamber are numerically investigated. The unsteady influence of the casing treatment on compressor flow fields is studied using fast Fourier transforms to gain a deeper insight into the function of the plenum chamber. The results indicate that the plenum chamber can reduce the penalty on the compressor’s total pressure ratio and adiabatic efficiency induced by the casing treatment effect at the design operating condition. A detuning effect of the plenum chamber is found and proven to be responsible for the decreased compressor performance penalty through a detailed analysis in both the blade passage and casing treatments. The detuning effect dampens the unsteady influence of the casing treatment on the compressor passage flow in terms of shock oscillations and mass flow fluctuations. Thus, the radial scope and degree of the influence induced by the casing treatment is distinctly restrained by the plenum chamber. Slot circulation, which is composed of a bleed flow from the passage into the slots and injection flow from the slots into the passage, is closely related to the stall margin improvement as the casing treatment is implemented. The plenum chamber has no significant impact on the slot circulation and thus on compressor stall margin. However, shallow slots decrease the slot circulation efficiency, which leads to a much lower stall margin enhancement.

Mechanism study of performance enhancement in a subsonic axial flow compressor with recirculating casing treatment

The improvements in both compressor efficiency and stability have been observed in a subsonic axial flow compressor with recirculating casing treatment. The study aims to understand the underlying flow mechanisms of the beneficial effect on the compressor efficiency of recirculating casing treatment. The recirculating casing treatment was investigated experimentally and numerically. In the experiment, static pressure fluctuations over the rotor tip were measured with fast-response pressure transducers. Whole-passage time-accurate simulations were also implemented to help in understanding the flow details. The unsteadiness of double-leakage flow and its effect on the loss generated at the rotor tip were discussed in detail for the solid casing. The effect of recirculating casing treatment on the double-leakage flow related loss was subsequently investigated. The results indicate that the double leakage flow is a main loss source in the rotor tip region. The double-leakage flow is completely unsteady, which induces a cyclical fluctuation of the amount of loss generated in the rotor tip region for the solid casing. With the recirculating casing treatment installed, the high-energy jet created in recirculating loops changes the unsteady characteristics of double-leakage flow by getting it diffused over the rotor tip gap along the chordwise direction, which results in the loss fluctuation that is dominated by the high-energy jet rather than the unsteadiness of double-leakage flow. The double-leakage flow is also pushed downstream by the jet and the amount is reduced when it passes through the recirculating loops. The effect of recirculating casing treatment on the double-leakage flow is primarily responsible for the improvement in the compressor efficiency.

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.

Effects of modified micro-vortex generators on aerodynamic performance in a high-load compressor cascade

In the current study, the effects of micro-vortex generators on the flow characteristics of a high-load compressor cascade are investigated, and four types of micro-vortex generators including “rectangular,” “curved rectangular,” “trapezoidal,” and “curved trapezoidal” are considered and named VGR, VGCR, VGT, and VGCT separately. The calculated results show that a rising reverse flow region, which is considered a main reason for occurring stall at +8° incidence, collapses rapidly from the leading edge in the cascade. Therefore, the micro-vortex generators are all mounted on the end-wall in front of the passage to suppress the development of the secondary flow, and the stall occurrence is delayed from +8° to +11° incidence by applying VGCT. At the design condition, the VGT can make the total pressure loss decrease by 0.54%. The modified micro-vortex generators show an obvious superiority when the range of incidence is between +3° and +8°. At the stall condition, the VGCT can make the total pressure loss decrease by 9.36%. Moreover, the reduction of the secondary flow loss is considered a main goal of the adoption of micro-vortex generators which is an achievement for decreasing the total pressure loss, and the highest reduction of the secondary flow loss reaches 34.6% at the stall condition in the cascade with VGCT.

Effective end wall profiling rules for a highly loaded compressor cascade

This paper presents a numerical study of a linear compressor cascade to investigate the effective end wall profiling rules for highly loaded axial compressors. The first step in the research applies a correlation analysis for the different flow field parameters by a data mining over 600 profiling samples to quantify how variations of loss, secondary flow and passage vortex interact with each other under the influence of a profiled end wall. The result identifies the dominant role of corner separation for control of total pressure loss, providing a principle that only in the flow field with serious corner separation does the does the profiled end wall change total pressure loss, secondary flow and passage vortex in the same direction. Then in the second step, a multiobjective optimization of a profiled end wall is performed to reduce loss at design point and near stall point. The development of effective end wall profiling rules is based on the manner of secondary flow control rather than the geometry features of the end wall. Using the optimum end wall cases from the Pareto front, a quantitative tool for analyzing secondary flow control is employed. The driving force induced by a profiled end wall on different regions of end wall flow are subjected to a detailed analysis and identified for their positive/negative influences in relieving corner separation, from which the effective profiling rules are further confirmed. It is found that the profiling rules on a cascade show distinct differences at design point and near-stall point, thus loss control of different operating points is generally independent.

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