Research Of Three-dimensional Vortex Structure In A Transonic Compressor Stator

Advanced gas turbine technology marks a nation’s industrial level, military strength, and even an important symbol of comprehensive national strength. The development of gas turbine technology has become a worldwide focus of research in the field of sicence, technology and military industry. The compressor, one of the three core parts of a gas turbine plays a key role for the overall performance of it. In the compressor, there exist three dimensional flow characteristics, such as viscous, compressible and unsteady. There exist some separation flow and vortex movement in the flow field, which complex fluid movement affect highly the efficient and stable operation of the compressor. Thus, there is a great significance to understand that the mechanism of generation and development of the separation flow and vortex structures, to improve the compressor aerodynamic performance, reduce the flow loss, and improve the efficiency and stability. The research for compressor vortex structures at home and abroad is mostly based on the vortex structure in the time-averaged flow field, or focused on the unsteady flow in local region of the compressor, such as the rotor blade tip leakage flow. However, there was less research work on three-dimensional unsteady vortex structure in the whole flow passage of the compressor. A single stage axial transonic compressor was chosen as the research object in this paper, and the evolution mechanism of space and time of three-dimensional unsteady vortex structures of compressor stator was carried out.First, the experimental data of NASA Rotor37 was used to verify the numerical simulation method in detail, to verify the credibility of the numerical simulation software ANSYS CFX. Meanwhile, a low-speed compressor experiment was used to analyse the secondary flow formula in the annular cascade Cartesian coordinate system.Then, the detail aerodynamic analysis and topological analysis of the flow flied on the stator surface and hub endwall in the design condition, near stall condition and choke condition at the design rotating speed with a steady calculation method are given out. And, the method of Q-criterion was used to supply the evolution rule of the vortex structures in the stator passage. Thus, the three-dimension steady vortex structures of the compressor stator was established. Results show that there is a closed separation zone near the suction side of the stator tip in the design condition. Most part of the flow on the stator suction surface is in the separation state under near stall condition, which may leads to the rotation stall of the compressor. Due to the transonic flow and shock, there exist separation state at both surfaces of the stator blades in the choke condition, which lead to the blocking state in the passage. There are several three-dimensional steady vortex structures in the design condition, such as the horseshoe vortices formed near the leading edge of the stator, the pressure side separation vortices formed from the pressure surface of the stator blades on the action of crosswise pressure gradient, thus under the influence of the horseshoe vortices and the pressure side separation vortices, the passage vortices are formed, and the concentrated shedding vortices formed from the suction surface of the stator blades, and so on. There are complex blade separation vortices formed by the separation flow near the suction side of the stator blades in the near stall condition. There are the passage derivative vortices formed by the interaction action of the vortices at the pressure side of the stator blades in the choke condition.Third, based on the three-dimensional steady vortex structures, the vortex structures during the stator stalling process are established with topological analysis method in the design speed and the near stall condition under different rotation speeds, in order to reveal the mechanism of the compressor dynamic stall through topological characterization method. Results show that the stall process under the design rotation speed is mainly manifested in the rising degree of separation flow on the suction surface of the stator blades, and the closed separation region develops to a three-dimensional closed bubble. There is a radial separation vortex, which transforms to be the complex blade separation vortices near the stator blade suction side with the development of separation degree. Thus, on one hand, other vortices are weaken by them; on the other hand, there are a large amount of low-energy fluid clusters convoluted by them, which accumulating in the passage to form to be a closed separation region. Thus, the effective flow area is decreased, and then the mass flow of the compressor is decreased. Meanwhile, due to the twisting, absorbing and breaking down movement among the vortices, there generate more energy losses, thus the pressurization capacity of the compressor is weaken, and rotation stall of compressor might occured. Under different rotation speeds, the separation area continuously extends with the increase of the rotation speed, but their vortex structures are the same.Finally, the detail unsteady calculation of the flow filed of the compressor in the design condition are obtained through the distribution of entropy, static pressure, Mach number and the streamlines on the stator surface and endwall. The influence mechanism of the unsteady factors to the stator flow filed is revealed, such as rotor wake, rotor blade tip leakage flow, the interaction between shock in the stator passage and boundary layers. The three-dimensional unsteady vortex structures of the compressor stator are established, and the influence mechanism of the unsteady factors to the vortex structures is revealed. Results show that there are basically the same number and type of the vortex structures between the steady and unsteady methods, especially there is neither obviously change about the vortex structures at the near half height of the stator passage. Meanwhile, the unsteady factors mainly affect the generation position, the scale and intensity of the vortices, which perform as an unsteady periodic fluctuation. The sweeping of rotor wake to downstream stator leads to the periodic fluctuation of the flow angle at the interface between rotor and stator, which affects the incidence angle of the stator. The intensity of shock around the suction side of the stator is also affected, as the same as the distribution of static pressure in the stator passage, thus the intensity and generation location of the passage vortices and the pressure side separation vortices are influenced. In addition, the separation degree and the form of the concentrated shedding vortex are influenced, too. The intensity change of shock in the stator passage has the effect on the interaction the boundary layers on the stator surface, which leads to the periodic change of the closed separation bubble at the leading edge of the stator. Meanwhile, the entropy increases volatility eventually lead to the periodic fluctuation of efficiency of the compressor stage. The rotor blade tip leakage flowchanges mainly the distribution of incidence angle of the stator, and inhibits the generation of the horseshoe vortices.