Kazutoyo Yamada

The Role of Tip Leakage Vortex Breakdown in Compressor Rotor Aerodynamics

The breakdown of tip leakage vortex has been investigated on a low-speed axial compressor rotor with moderate blade loading. Effects of the breakdown on the rotor aerodynamics are elucidated by Navier-Stokes flow simulations and visualization techniques for identifying the breakdown. The simulations show that the leakage vortex breakdown occurs inside the rotor at a lower flow rate than the peak pressure rise operating condition. The breakdown is characterized by the existence of the stagnation point followed by a bubblelike recirculation region. The onset of breakdown causes significant changes in the nature of the tip leakage vortex: large expansion of the vortex and disappearance of the streamwise vorticity concentrated in the vortex. The expansion has an extremely large blockage effect extending upstream of the leading edge. The disappearance of the concentrated vorticity results in no rolling-up of the vortex downstream of the rotor and the disappearance of the pressure trough on the casing. The leakage flow field downstream of the rotor is dominated by the outward radial flow, resulting from the contraction of the bubblelike structure of the breakdown region. It is found that the leakage vortex breakdown plays a major role in characteristic of rotor performance at near-stall conditions. As the flow rate is decreased from the peak pressure rise operating condition, the breakdown region grows rapidly in the streamwise, spanwise, and pitchwise directions. The growth of the breakdown causes the blockage and the loss to increase drastically. Then, the interaction of the breakdown region with the blade suction surface gives rise to the three-dimensional separation of the suction surface boundary layer, thus leading to a sudden drop in the total pressure rise across the rotor.

Unsteady flow behavior due to breakdown of tip leakage vortex in an axial compressor rotor at near-stall condition

The unsteady flow nature caused by the breakdown of the tip leakage vortex in an axial compressor rotor at near-stall conditions has been investigated by unsteady three-dimensional Navier-Stokes flow simulations. The simulations show that the spiral-type breakdown of the tip leakage vortex occurs inside the rotor passage at the near-stall conditions. Downstream of the breakdown onset, the tip leakage vortex twists and turns violently with time, thus interacting with the pressure surface of the adjacent blade. The motion of the vortex and its interaction with the pressure surface are cyclic. The vortex breakdown causes significant changes in the nature of the tip leakage vortex, which result in the anomalous phenomena in the time-averaged flow fields near the tip at the near-stall conditions: no rolling-up of the leakage vortex downstream of the rotor, disappearance of the casing wall pressure trough corresponding to the leakage vortex, large spread of the low-energy fluid accumulating on the pressure side, and large pressure fluctuation on the pressure side. As the flow rate is decreased, the movement of the tip leakage vortex due to its breakdown becomes so large that the leakage vortex interacts with the suction surface as well as the pressure one. The interaction with the suction surface gives rise to the three-dimensional separation of the suction surface boundary layer.© 2000 ASME

Unsteady Three-Dimensional Flow Phenomena Due to Breakdown of Tip Leakage Vortex in a Transonic Axial Compressor Rotor

Unsteady three-dimensional flow fields in a transonic axial compressor rotor (NASA Rotor 37) have been investigated by unsteady Reynolds-averaged Navier-Stokes simulations. The simulations show that the breakdown of the tip leakage vortex occurs in the compressor rotor because of the interaction of the vortex with the shock wave. At near-peak efficiency condition small bubble-type breakdown of the tip leakage vortex happens periodically and causes the loading of the adjacent blade to fluctuate periodically near the leading edge. Since the blade loading near the leading edge is closely linked to the swirl intensity of the tip leakage vortex, the periodic fluctuation of the blade loading leads to the periodic breakdown of the tip leakage vortex, resulting in self-sustained flow oscillation in the tip leakage flow field. However, the tip leakage vortex breakdown is so weak and small that it is not observed in the time-averaged flow field at near-peak efficiency condition. On the other hand, spiral-type breakdown of the tip leakage vortex is caused by the interaction between the vortex and the shock wave at near-stall operating condition. The vortex breakdown is found continuously since the swirl intensity of tip leakage vortex keeps strong at near-stall condition. The spiral-type vortex breakdown has the nature of self-sustained flow oscillation and gives rise to the large fluctuation of the tip leakage flow field, in terms of shock wave location, blockage near the rotor tip and three-dimensional separation structure on the suction surface. It is found that the breakdown of the tip leakage vortex leads to the unsteady flow phenomena near the rotor tip, accompanying large blockage effect in the transonic compressor rotor at the near-stall condition.Copyright

Effect of tip clearance on stall evolution process in a low-speed axial compressor stage

Effect of the tip clearance on the transient process of rotating stall evolution has been studied experimentally in a low-speed axial compressor stage with various stator-rotor gaps. In the previous authors’ experiments for the small tip clearance, the stall evolution process of the rotor was sensitive to the gaps between the blade rows. For the large tip clearance, however, little difference is observed in the evolution processes independently of the blade row gap. In the first half process, it is characterized by gradual reduction of overall pressure-rise with flow rate decreasing, and the number of short length-scale disturbances is increasing with their amplitude increasing. In the latter half a long length-scale disturbance develops rapidly to result in deep stall. Just before the stall inception the spectral power density of the casing wall pressure reveals the existence of rotating disturbances with broadband high frequency near a quarter of the blade passing frequency. This is caused by the short length-scale disturbances occurring intermittently. A flow model is presented to explain mechanisms of the rotating short length-scale disturbance, which includes a tornado-like separation vortex and tip-leakage vortex breakdown. The model is supported by a result of a numerical unsteady flow simulation.Copyright

The Behavior of Tip Clearance Flow at Near-Stall Condition in a Transonic Axial Compressor Rotor

It is known that the tip clearance flow is dominant and very important flow phenomena in axial compressor aerodynamics because the tip clearance flow has a great influence on the stability as well as aerodynamic loss of compressors. Our goal is to clarify the behavior of tip clearance flow at near-stall condition in a transonic axial compressor rotor (NASA Rotor 37). In the present work, steady and unsteady RANS simulations were performed to investigate vortical flow structures and separated flow field near the tip for several different clearance cases. Boundary layer separation on the casing wall and blade suction surface was investigated in detail for near-stall and stall condition. In order to understand such complicated flow field, vortex cores were identified using the critical point theory and a topology of the three-dimensional separated and vortical flows was analyzed. In the nominal clearance case, the breakdown of tip leakage vortex has occurred at a near-stall operating condition because of the interaction of the vortex with the shock wave, leading to a large blockage and unsteadiness in the rotor tip. On the other hand, the calculation with no clearance suggested that the separation on the suction surface was different from that with the nominal clearance. Since the shock wave induced the boundary layer separation on the blade suction surface in the transonic axial compressor rotor, focal-type critical points appeared on the suction surface near the tip at near-stall condition.Copyright

Experimental and Numerical Investigations of Wake Passing Effects upon Aerodynamic Performance of a LP Turbine Linear Cascade With Variable Solidity

This paper deals with experimental and numerical studies on the flow field around a low-pressure linear turbine cascade whose solidity is changeable. The purpose of them is to clarify the effect of incoming wakes upon the aerodynamic loss of the cascade that is accompanied with separation on the airfoil suction surface, in particular for low Reynolds number conditions and/or low solidity conditions. Cylindrical bars on the timing belts work as wake generator to emulate wakes that impact the cascade. Pneumatic probe measurement is made to obtain total pressure loss distributions downstream of the cascade. Hot-wire probe measurement is also conducted over the airfoil suction surface. Besides, LES-based numerical simulation is executed to deepen the understanding of the interaction of the incoming wakes with the boundary layer containing separation bubble.

Effects of tip clearance on the stall inception process in an axial compressor rotor

The paper presents experimental and numerical studies on the effects of tip clearance on the stall inception process in a low-speed axial compressor rotor with a large tip clearance. It has been revealed that in the small tip clearance case, shortly after the spike disturbance which results from the leading-edge separation near the rotor tip appears, the tornado-like vortex is generated by the separation, and soon the compressor falls into stall. In the large tip clearance case, the experiment showed that the performance characteristic differs from that in the small tip clearance case at near-stall conditions. This implies that the stall inception process differs with the tip clearance size. The flow phenomenon in the stall inception leading to such difference has been investigated in this study.Pressure and velocity fields which were ensemble-averaged and phase-locked by the periodic multi-sampling technique were measured on the casing wall and downstream of the rotor, respectively. In addition, to capture the unsteady flow phenomena inside the rotor, “Instantaneous Casing Pressure Field Measurement” was carried out: instantaneous casing pressure fields in one rotor passage region were measured by 30 high response pressure transducers mounted on the casing wall. In order to investigate further details of near-stall flow field for the large tip clearance, DES (Detached Eddy Simulation) has been conducted using a computational mesh with 120 million points. The results are compared with those from previous studies for the small tip clearance.As expected, the measurement results show notable differences in the near-stall flow field between the two tip clearance cases. The results from the casing pressure measurement show that high pressure fluctuation appears on the pressure side near the rotor leading-edge in the large tip clearance case. In the result of the velocity field measurement downstream of the rotor, high turbulence intensity is found near the casing in the large tip clearance case. The numerical results reveal that the vortex breakdown occurs in the tip leakage vortex and induces the oscillation of the tip leakage vortex with its unsteady nature. The flow phenomena confirmed in the experimental results are clearly explained by considering the breakdown of the tip leakage vortex. The vortex breakdown gives rise to not only large blockage but also the rotating disturbance through the interaction of the fluctuating tip leakage vortex with the pressure surface of the adjacent blade, and governs the stall inception process.Copyright

Influences of axial gap between blade rows on secondary flows and aerodynamic performance in a turbine stage

A study on the effects of the axial gap between stator and rotor upon the stage performance and flow field of a single axial flow turbine stage is presented in this paper. Three axial gaps were tested, which were achieved by moving the stator vane in the axial direction while keeping the disk cavity constant. The effect of the axial gap was investigated at two different conditions, that is design and off-design conditions. The unsteady three-dimensional flow field was analyzed by time-accurate RANS (Reynolds-Averaged Navier-Stokes) simulations. The simulation results were compared with the experiments, in which total pressure and the time-averaged flow field upstream and downstream of the rotor were obtained by five-hole probe measurements. The effect of the axial gap was confirmed in the endwall regions, and obtained relatively at off-design condition. The turbine stage efficiency was improved almost linearly by reducing the axial gap at the off-design condition.Copyright

Numerical investigation of relation between unsteady behavior of tip leakage vortex and rotating disturbance in a transonic axial compressor rotor

The purpose of this study is to have a better understanding of the unsteady behavior of tip clearance flow at near-stall condition from a multi-passage simulation and to clarify the relation between such unsteadiness and rotating disturbance. This study is motivated by the following concern. A single passage simulation has revealed the occurrence of the tip leakage vortex breakdown at near-stall condition in a transonic axial compressor rotor, leading to the unsteadiness of the tip clearance flow field in the rotor passage. These unsteady flow phenomena were similar to those in the rotating instability, which is classified in one of the rotating disturbances. In other words it is possible that the tip leakage vortex breakdown produces a rotating disturbance such as the rotating instability. Three-dimensional unsteady RANS calculation was conducted to simulate the rotating disturbance in a transonic axial compressor rotor (NASA Rotor 37). The four-passage simulation was performed so as to capture a short length scale disturbance like the rotating instability and the spike-type stall inception. The simulation demonstrated that the unsteadiness of tip leakage vortex, which was derived from the vortex breakdown at near-stall condition, invoked the rotating disturbance in the rotor, which is similar to the rotating instability.Copyright

EFFECT OF WAKE PASSING ON UNSTEADY AERODYNAMIC PERFORMANCE IN A TURBINE STAGE

In the present work, unsteady RANS simulations were performed to clarify several interesting features of the unsteady three-dimensional flow field in a turbine stage. The unsteady effect was investigated for two cases of axial spacing between stator and rotor, i.e. large and small axial spacing. Simulation results showed that the stator wake was convected from pressure side to suction side in the rotor. As a result, another secondary flow, which counter-rotated against the passage vortices, was periodically generated by the stator wake passing through the rotor passage. It was found that turbine stage efficiency with the small axial spacing was higher than that with the large axial spacing. This was because the stator wake in the small axial spacing case entered the rotor before mixing and induced the stronger counter-rotating vortices to suppress the passage vortices more effectively, while the wake in the large axial spacing case eventually promoted the growth of the secondary flow near the hub due to the migration of the wake towards the hub.Copyright