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García Rosa Nicolás, Dufour Guillaume, Barènes Roger, et al. Experimental Analysis of the Global Performance and the Flow Through a High-Bypass Turbofan in Windmilling Conditions J. Turbomach. 137, 051001 (2015) (8 pages);   Paper No: TURBO-14-1024;   doi:10.1115/1.4028647

A detailed study of the air flow through the fan stage of a high-bypass, geared turbofan in windmilling conditions is proposed, to address the key performance issues of this severe case of off-design operation. Experiments are conducted in the turbofan test rig of ISAE, specifically suited to reproduce windmilling operation in an ambient ground setup. The engine is equipped with conventional measurements and radial profiles of flow quantities are measured using directional five-hole probes to characterize the flow across the fan stage and derive windmilling performance parameters. These results bring experimental evidence of the findings of the literature that both the fan rotor and stator operate under severe off-design angle-of-attack, leading to flow separation and stagnation pressure loss. The fan rotor operates in a mixed fashion: spanwise, the inner sections of the rotor blades add work to the flow while the outer sections extract work and generate a pressure loss. The overall work is negative, revealing the resistive loads on the fan, caused by the bearing friction and work exchange in the different components of the fan shaft. The parametric study shows that the fan rotational speed is proportional to the mass flow rate, but the fan rotor inlet and outlet relative flow angles, as well as the fan load profile, remain constant, for different values of mass flow rate. Estimations of engine bypass ratio have been done, yielding values higher than six times the design value. The comprehensive database that was built will allow the validation of 3D Reynolds-averaged Navier–Stokes (RANS) simulations to provide a better understanding of the internal losses in windmilling conditions.

Dodds J., Vahdati M. Rotating Stall Observations in a High Speed Compressor—Part I: Experimental Study J. Turbomach. 137, 051002 (2015) (9 pages);   Paper No: TURBO-14-1123;   doi:10.1115/1.4028557

In this two-part paper, the phenomenon of part span rotating stall is studied. The objective is to improve understanding of the physics by which stable and persistent rotating stall occurs within high speed axial flow compressors. This phenomenon is studied both experimentally (Part I) and through the use of unsteady RANS simulations (Part II). In this paper, the behavior of an eight stage high speed compressor is studied during slow acceleration maneuvres along a fixed working line. Casing mounted pressure transducers and rotor mounted strain gages are used to examine the spectral content of any unsteadiness in the flow and its behavior across the operating range. By deliberate aerodynamic mismatching of the front stages through adjustment of three rows of variable stator vanes (VSVs), stable rotating stall is initiated. The observed behavior falls into two “families” of high and low frequency when tracked on the instrumentation. Further analysis based on the Doppler shift between the static and rotating measurements confirms that these respective phenomena are due to rotating stall of high and low cell count. Acoustic modes resulting from stall/rotor interaction are also identified. Strong correlation of the stall intensity with simple 1D meanline predicted loading parameters suggests that these families of behavior are independently linked to the stalling of different regions within the compressor.

Dodds J., Vahdati M. Rotating Stall Observations in a High Speed Compressor—Part II: Numerical Study J. Turbomach. 137, 051003 (2015) (10 pages);   Paper No: TURBO-14-1124;   doi:10.1115/1.4028558

In this two-part paper the phenomenon of part span rotating stall is studied. The objective is to improve understanding of the physics by which stable and persistent rotating stall occurs within high speed axial flow compressors. This phenomenon is studied both experimentally (Part I) and numerically (Part II). The experimental observations reported in Part I are now explored through the use of 3D unsteady Reynolds-averaged Navier–Stokes (RANS) simulation. The objective is to both validate the computational model and, where possible, explore some physical aspects of the phenomena. Unsteady simulations are presented, performed at a fixed speed with the three rows of variable stator vanes adjusted to deliberately mismatch the front stages and provoke stall. Two families of rotating stall are identified by the model, consistent with experimental observations from Part I. The first family of rotating stall originates from hub corner separations developing on the stage 1 stator vanes. These gradually coalesce into a multicell rotating stall pattern confined to the hub region of the stator and its downstream rotor. The second family originates from regions of blockage associated with tip clearance flow over the stage 1 rotor blade. These also coalesce into a multicell rotating stall pattern of shorter length scale confined to the leading edge tip region. Some features of each of these two patterns are then explored as the variable stator vanes (VSVs) are mismatched further, pushing each region deeper into stall. The numerical predictions show a credible match with the experimental findings of Part I. This suggests that a RANS modeling approach is sufficient to capture some important aspects of part span rotating stall behavior.

Banjac Milan, Petrovic Milan V., Wiedermann Alexander. Secondary Flows, Endwall Effects, and Stall Detection in Axial Compressor Design J. Turbomach. 137, 051004 (2015) (12 pages);   Paper No: TURBO-14-1179;   doi:10.1115/1.4028648

This paper describes a methodology and a fully tested and calibrated mathematical model for the treatment of endwall effects in axial compressor aerodynamic calculations. Additional losses and deviations caused by the clearance and secondary flows are analyzed. These effects are coupled with endwall boundary layer losses (EWBL) and blockage development. Stall/surge detection is included, and mutual interaction of different loss mechanisms is considered. Individual mathematical correlations for different effects have been created or adopted from earlier papers with the aim of forming one integral model that is completely described in this paper. Separate mathematical correlations and calibration measures are discussed in detail in the first part of the paper. The developed overall model is suitable for application in two-dimensional (2D) or mean-line compressor flow calculations. During the development, it was tested, calibrated, and validated using throughflow calculations comparing numerical results with experimental data for a large number of test cases. These test cases include compressors with very different configurations and operating ranges. The data on the compressors were taken from the open literature or obtained from industrial partners.

Grosvenor Allan D., Rixon Gregory S., Sailer Logan M., et al. High Resolution RANS Nonlinear Harmonic Study of Stage 67 Tip Injection Physics J. Turbomach. 137, 051005 (2015) (13 pages);   Paper No: TURBO-14-1188;   doi:10.1115/1.4028550

Numerical prediction of the Stage 67 transonic fan stage employing wall jet tip injection flow control and study of the physical mechanisms leading to stall suppression and stability enhancement afforded by endwall recirculation/injection is the focus of this paper. Reynolds averaged Navier–Stokes (RANS) computations were used to perform detailed analysis of the Stage 67 configuration experimentally tested at NASA's Glenn Research Center in 2004. Time varying predictions of the stage plus recirculation and injection flowpath were executed utilizing the nonlinear harmonic (NLH) approach. Significantly higher grid resolution per passage was achieved than what has been generally employed in prior reported numerical studies of spike stall phenomena in transonic compressors. This paper focuses on characterizing the physics of spike stall embryonic stage phenomena and the influence of tip injection, resulting in experimentally and numerically demonstrated stall suppression.

Amirante Dario, Hills Nicholas J. Large-Eddy Simulations of Wall Bounded Turbulent Flows Using Unstructured Linear Reconstruction Techniques J. Turbomach. 137, 051006 (2015) (11 pages);   Paper No: TURBO-14-1196;   doi:10.1115/1.4028549

Large-eddy simulations (LES) of wall bounded, low Mach number turbulent flows are conducted using an unstructured finite-volume solver of the compressible flow equations. The numerical method employs linear reconstructions of the primitive variables based on the least-squares approach of Barth. The standard Smagorinsky model is adopted as the subgrid term. The artificial viscosity inherent to the spatial discretization is maintained as low as possible reducing the dissipative contribution embedded in the approximate Riemann solver to the minimum necessary. Comparisons are also discussed with the results obtained using the implicit LES (ILES) procedure. Two canonical test-cases are described: a fully developed pipe flow at a bulk Reynolds number Reb = 44 × 103 based on the pipe diameter, and a confined rotor–stator flow at the rotational Reynolds number ReΩ = 4 × 105 based on the outer radius. In both cases, the mean flow and the turbulent statistics agree well with existing direct numerical simulations (DNS) or experimental data.

Pullan G., Young A. M., Day I. J., et al. Origins and Structure of Spike-Type Rotating Stall J. Turbomach. 137, 051007 (2015) (11 pages);   Paper No: TURBO-14-1202;   doi:10.1115/1.4028494

In this paper, we describe the structures that produce a spike-type route to rotating stall and explain the physical mechanism for their formation. The descriptions and explanations are based on numerical simulations, complemented and corroborated by experiments. It is found that spikes are caused by a separation at the leading edge due to high incidence. The separation gives rise to shedding of vorticity from the leading edge and the consequent formation of vortices that span between the suction surface and the casing. As seen in the rotor frame of reference, near the casing the vortex convects toward the pressure surface of the adjacent blade. The approach of the vortex to the adjacent blade triggers a separation on that blade so the structure propagates. The above sequence of events constitutes a spike. The computed structure of the spike is shown to be consistent with rotor leading edge pressure measurements from the casing of several compressors: the centre of the vortex is responsible for a pressure drop and the partially blocked passages associated with leading edge separations produce a pressure rise. The simulations show leading edge separation and shed vortices over a range of tip clearances including zero. The implication, in accord with recent experimental findings, is that they are not part of the tip clearance vortex. Although the computations always show high incidence to be the cause of the spike, the conditions that give rise to this incidence (e.g., blockage from a corner separation or the tip leakage jet from the adjacent blade) do depend on the details of the compressor.

Hiradate Kiyotaka, Kobayashi Hiromi, Sugimura Kazuyuki, et al. Proposal and Experimental Verification of Design Guidelines for Centrifugal Compressor Impellers With Curvilinear Element Blades to Improve Compressor Performance J. Turbomach. 137, 051008 (2015) (11 pages);   Paper No: TURBO-14-1218;   doi:10.1115/1.4028650

This study numerically and experimentally examines the effects of applying curvilinear element blades to fully shrouded centrifugal impellers on the performance of the centrifugal compressor stages. The curvilinear element blades we developed for centrifugal turbomachinery were defined by the coordinate transformations between a revolutionary flow-coordinate system and a cylindrical coordinate system. All the blade sections in the transferred cylindrical coordinate system were moved and stacked spanwise in accordance with the given “lean profile,” which meant the spanwise distribution profile of movement of the blade sections, to form a new leaned blade surface. The effects of the curvilinear element blades on the impeller flowfield were investigated using numerical simulations, and the optimum design guidelines for impellers with curvilinear element blades were considered. Then, a new impeller using these design guidelines was designed and the performance improvement of a new compressor stage was evaluated by numerical simulations. As mentioned in several papers, we numerically confirmed that curvilinear element blades with a negative tangential lean (TGL) profile improved the velocity distribution and stage efficiency because they help to suppress the secondary flows in the impeller. The negative TGL mentioned in this paper represents the lean profile in which the blade hub end leans forward in the direction of the impeller rotation compared to the blade shroud end. At the same time, we also found that the stall margin of these impellers deteriorated due to the increase in relative velocity deceleration near the suction surface of the shroud in the forward part of the impeller. Therefore, we propose new design guidelines for impellers with the curvilinear element blades by applying a negative TGL to line element blades in which the blade loading of the shroud side in the forward part of the impeller is reduced. We confirmed from the numerical simulation results that the performance of the new compressor stage improved compared to that in the corresponding conventional one. The new design guidelines for the curvilinear element blades were experimentally verified by comparing the performance of the new compressor stage with the corresponding conventional one. The measured efficiency of the new compressor stage was 2.4% higher than that of the conventional stage with the stall margin kept comparable. A comparison of the measured velocity distributions at the impeller exit showed that the velocity distribution of the new impeller was much more uniform than that of the conventional one.

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