Dai Kato

Numerical Analysis of Flow in a Transonic Compressor With a Single Circumferential Casing Groove: Influence of Groove Location and Depth on Flow Instability

The effect of circumferential single grooved casing treatment on the stability enhancement of NASA Rotor 37 has been examined with computational fluid dynamics analysis. Stall inception mechanism of Rotor 37 is presented first with principal focus on the tip leakage flow behavior, passage blockage, and the vortical flow structures. Detailed observation showed that the combined interaction of the stagnated flow of tip leakage vortex breakdown and the jetlike leakage flow from the midchord region leads to the blade tip-initiated stall inception. The result of numerical parametric study is then demonstrated to show the effect of varying the axial location and the depth of a circumferential single groove. The evaluation based on stall margin improvement showed a higher potential of deeper grooves in stability enhancement, and the optimal position for the groove to be located was indicated to exist near the leading edge of the blade.

CFD Modeling Effects on Unsteady Multistage Simulation for a Transonic Axial Compressor

Unsteady three-dimensional multistage calculations are performed for a highly loaded, high-speed axial compressor to investigate the impact of real geometry modeling and different numerical approaches on the accuracy of the performance prediction. First, two features of the real geometries are separately compared with the calculation which consists of a pure flow path model except that rotor tip clearances are considered. One treats leakage generated by part gaps between variable stator vanes and the annulus lines. Another incorporates seal cavities to model leakage underneath the shrouded stators. Then, the influence of different numerical approach with different turbulence models is also investigated. Discussion on the impact of the CFD modeling on the performance prediction focuses on the prediction accuracies of stage operating points and spanwise mixing. It is suggested that a realistic simulation of turbulent-type flow unsteadiness in a multistage machine is important for an accurate prediction of spanwise mixing phenomena.Copyright

Unsteady flow structure in an axial compressor at windmill condition

This study investigates experimentally and numerically unsteady flow fields in an axial compressor operating at high-flow rate in order to understand the flow structure in the stator row operating at windmill condition. The experimental and numerical data are compared by time- and phase-lock-averaged techniques. Additionally, unsteady vortex structure is investigated by numerical technique. At windmill condition, the incidence angle to the stator row becomes extremely negative. Therefore, large separation occurs near pressure surface in the stator passage. The experimental and numerical results indicate that a large vortex is generated in the separation area. According to the numerical results, part of the vortex migrates downstream, and the vortex produces blockage of the main stream of the stator passage. Therefore, net flow area in the stator passage becomes small so that the flow between the vortex and suction side of stator vane is accelerated. As a result, the total pressure deterioration is generated in the stator passage because the high speed flow and the vortex cause high shear.Copyright

Numerical Analysis of Flow in a Transonic Compressor With a Single Circumferential Casing Groove: Application to Two Different Compressor Rotors

The effect of a single circumferential casing groove on the stability enhancement of two different transonic compressors has been examined with CFD analysis. The differences in flow field and stall inception mechanism between two rotors are presented with principal focus on passage blockage and tip leakage flow behavior. Detailed observation showed that the blockage flow which leads the compressor to stall was different between each other. A parametric study conducted with respect to the axial location of the groove has clarified that the effect which groove has on the tip leakage flow behavior changes according to the blade tip loading and the design tip clearance gap at the location where the groove is applied. When the casing treatment was applied to the compressors with different instability mechanism, whether the casing treatment could enhance the stability of compressor or not was not only dependant on the extent of the influence which it had on the flow field but also on whether it could affect the original stall-initiating phenomena at the adequate location.Copyright

The Effect of the Casing Movement Relative to the Blades on the Tip Leakage Loss in Axial Flow Compressors

This paper investigates the effect of the casing movement relative to the blades on the tip leakage loss generation mechanisms by using experimental results from a linear cascade test facility, and viscous numerical results. Traverse measurements in the pitch-wise and span-wise directions are made using a five-hole Pitot tube at the inlet and exit planes of a compressor linear cascade comprising seven equally-pitched blades. The blades are two-dimensionally stacked with a cross section representing a typical rear stage rotor of a highly loaded axial-flow compressor. A moving belt, driven by a motor and a pulley system, runs linearly at constant speed under the horizontally suspended cascade to simulate the relative motion of the blade and the casing. Tip clearance can be adjusted by changing the height of the blades. The experimental results, at 2% and 4% tip clearance to blade heights, indicate that the tip leakage loss decreases when the casing is in movement. The Reynolds-averaged Navier-Stokes numerical calculations with Spalart-Almaras turbulence closure model, run with the experimental boundary conditions, agree well with the test data, especially in terms of dependencies of the leakage loss magnitude on the relative movement between the blade and the casing. It is interesting that, contrary to the tendency in the leakage loss to decrease, the computed tip leakage mass flow rate increases with moving endwall. The computations show two distinct regions of high entropy creation rate near the blade tip. The first one is located close to the blade suction surface where the leakage flow leaves the clearance gap. The second one is located further from the suction surface and the entropy creation rate in this region decreases when the casing is in movement. This paper attempts to provide a qualitative analysis of the flow mechanisms involved in the entropy generation in the second regions. Finally Computations of a high loaded rotor show that the second region identified in the static cascade may also be present in the case of rotating cascades.Copyright

The Influence of Shrouded Stator Cavity Flows on the Aerodynamic Performance of a High-Speed Multistage Axial-Flow Compressor

This paper investigates numerically the effects of shrouded stator seal cavity flows on a high-speed, six-stage, advanced axial-flow compressor performance. Two cases of fully three-dimensional unsteady Reynolds-averaged Navier-Stokes simulations are performed. The first case includes only the main flow path without cavities, while the second case takes into account the effect of cavities by fully meshing and solving the seal cavity flows under each of the stator vanes. Both simulations included rotor blade tip clearances. The latter case showed 1.7 point degradation in efficiency from the first case. Contributors to the overall performance degradation, such as windage heating, mixing loss due to seal leakage flow with the main flow, and additional loss of the rotors and stators due to alteration in velocity triangles, are identified by comparing the two simulation results. Compared to theoretical or semi-empirical leakage and windage models, higher loss production and temperature rise are found especially in mid to rear stages. Unsteady effects for such differences are discussed.Copyright

Unsteady Flow Effects in a High-Speed Multistage Axial Compressor

As a first step in practical application of virtual rig test, in present work, a full unsteady multistage simulation was performed for a 6-stage transonic axial compressor at design point. A steady multistage simulation was also performed and compared with the results of the unsteady simulation to clarify the unsteady flow effects. Comparison of overall performance, stage matching, and inter stage performance with measured test data showed that the unsteady multistage simulation could predict the performance reasonably. It was also demonstrated that, at design point, the steady multistage simulation could predict the stage matching at the same level as the unsteady multistage simulation. To clarify the effects of the unsteady flow in a multistage compressor, the pressure loss by unsteady wake interaction was investigated specifically at the inter-blade passage or down stream blade rows. Some issues in the predictive ability of the unsteady multistage simulation were newly found in the spanwise mixing phenomena.© 2009 ASME