Investigations Of The Non-axisymmetric Flow Behaviors In Centrifual Compressor And The Effects Of The Inlet Distortion On The Internal Flow

As the core component of small gas turbines and turbochargers, centrifugal compressor has been widely used in the military and civilian areas. Circumferential non-uniform pressure distribution exists in the compressor volute with a non-axisymmetric geometry structure, which results in the compressor performance decline at the non-design condition. On the other hand, due to the space limitation of the system arrangement, the straight pipe is generally replaced by different flexural-torsional pipes for compressor inlet. However, complex flow in the flexural-torsional pipe can results in both total pressure and swirl distortions at the impeller inlet. In this case, not only the compressor performance but also the reliability and stability of the compressor could be affected.In this paper, experimental and numerical studies were carried out on J90-2 centrifugal compressor to investigate the non-axisymmetric flow behaviors in the compressor and the effects of the inlet distortion on the internal flow. The mainly purposes including:1. The formation mechanism of the non-axisymmetric flow characteristics in the compressor will be revealed by the description of the reverse propagation process of the pressure fluctuation generated in the high static pressure zone of the volute through the blade channels. The effects of the non-axisymmetric flow on the changes of the internal flow field, the inlet /outlet flow parameters and the blade loading also will be clarified.2. The evolutions of the secondary flow structure, the total pressure distortion and the swirl distortion at the inlet of the flexural-torsional pipes with the change of the twist angle will be elucidated. The differences of the total pressure distortion, the intensity, direction and number of the swirl to describe the distorted flow in detail at the pipe outlet for different flexural-torsional forms will be quantized.3. The fundamental reasons of the performance variations of the centrifugal compressor in response to the swirl distortion of the impeller inlet will be explored by founding the correlation between the swirl distortion index and the inlet condition of the impeller and analyzing the coupling characteristics between the inlet distortion and the pressure distortion in the compressor volute. In addition, the variations of the internal flow of the compressor under various inlet swirl distortions will be concerned.Based on the abovementioned research purposes, numerical simulations of several typical compressor models with the straight pipe and different pre-designed flexural-torsional pipes were conducted. The homologous compressor experiments were carried out to measure the compressor performances and the static pressures at the key points on the compressor inside wall for different rotational speeds and mass flow rates. The simulations were verified by the results of the experiment in which the compressor connects with the straight inlet pipe. The flow distortions were obtained via numerical simulations, and the distortion characteristics under different mass flow rate were described by the quantization of the degree of distortion. The numerical results indicate that the different typical swirl structures could be formed by changing the torsion angle of the flexural-torsional pipe. The twin swirl structure is formed when the torsion angles are 0°and 180°, while bulk swirl is formed when the angle is 90°。The results show that the effects on the compressor performances are significant different between the twin swirl and the bulk swirl. Compared with other swirl forms, the bulk swirl causes the greatest decline of the compressor performance when the swirl has the same direction with the compressor rotation, while the compressor performance has the smallest decline for an opposite direction.For the non-axisymmetric flow characteristics of the compressor, the influences of the volute static pressure on the pressures of both the shroud and the inlet of the compressor were investigated under the off-design conditions. The spatial and temporal changes of the pressure in blade channel illustrate the reverse propagation path of high-static pressure perturbation wave in the volute. This perturbation wave can propagate to the impeller inlet for the large mass flow rate, but the weak perturbation wave almost has no effect on the impeller inlet under the small mass flow rate. Meanwhile, the volute static pressure has the relatively large influence under the small mass flow rate. The effects of the non-axisymmetric flow on the distribution and fluctuation of the blade loading were further analyzed, and the spectrum characteristics of the blade loading fluctuation also were achieved. The results indicate the fluctuation of the main blade loading is affected by the base frequency under the large mass flow rate, and the loading fluctuation is affected by the frequency-doubled.The investigations of the effects of typical flexural-torsional pipes on the performance and internal flow of the compressor show that the swirl distortion in impeller inlet is the main factor affecting the compressor performance. The relation between the relative flow angle and the swirl angle and the one between the relative flow angle and the swirl intensity at different span wise were presented respectively. It was revealed that the swirl angle and swirl intensity change the relative flow angle of the edge of blade. As a result, the incidences changes obviously, which is the essential cause of the change of the compressor performance.The increased positive incidence caused by the swirl distortion in the impeller inlet results in the shift of the position of the tip clearance leakage flow towards the leading edge of the blade, and also increases the intensity of leakage flow. Not only the decrease of the positive incidence but also the negative incidence leads to the tip clearance leakage flow move towards the trailing edge of the blade. The air flow rate through the blade channels is affected by both the internal structure of the inlet distorted and pressure distribution in volute. In this case, the twin swirl in the impeller inlet enlarges the static pressure gradient in the impeller exit, and causes the significant imbalance of the flow distributions among the impeller channels. However, the bulk swirl has a small impact on the flow distributions. In addition, the high symmetric degree of the inlet swirl structures have a small impact on the fluctuation of the blade loading, but it is when the bulk swirl exits in the impeller inlet results in the opposite effect.