In the drive for lower fuel consumption through increased bypass ratio and increased overall pressure ratio (OPR), future engine designs for the next generation of single-aisle aircraft will require core sizes below 3 lbm/s and OPRs above 50. Traditionally, these core sizes are the domain of centrifugal compressors, but materials limit pressure ratio in these machines to well below 50. An all-axial high pressure compressor (HPC) at this core size, however, comes with limitations associated with the small blade spans at the back of the HPC, as clearances, fillets and leading edges do not scale with the core size. The result is a substantial efficiency penalty, driven primarily by the tip leakage flow produced by the relatively larger clearance-to-span ratio, which negates the cycle efficiency benefits of the high OPR. In order to enable small-core, high-OPR, all-axial compressors mitigating technologies need to be developed and implemented to reduce the large clearance-to-span efficiency penalty. However, for this technology development to be successful, it is imperative that predictive design tools accurately model the overall flow physics and trends of the technologies developed.

In this paper we describe an effort to improve the modeling of an axial compressor with a large clearance-to-span ratio. Results are compared with data collected in NASA Glenn’s low-speed axial compressor, with four stages redesigned for this program. Stages are repeating, but only the 3^rd^ stage is run with clearance-to-span ratios representative of a small-core compressor. Comparisons between data and CFD results show that steady Reynolds-Averaged Navier-Stokes (RANS) solvers can predict the pressure-rise characteristic to an acceptable level of accuracy, both in terms of choke flow and rollover point if careful attention is paid to grid topology in the tip region. However, the steady tools do not capture radial profiles of blockage and total pressure correctly. Unsteady RANS (uRANS) was found to be more successful in capturing the dominant features of the radial profiles. However, it was found that it was only necessary to run the stage(s) with the open clearance in an unsteady fashion. In addition to the RANS and uRANS computations, results from a Delayed-Detached Eddy Simulation (DDES) with grid refinement in the rotor tip region were also analyzed. The DDES simulation captured the radial profiles with greater accuracy than the uRANS simulations, but no additional features were resolved. Hence it was concluded that the incremental improvements provided by DDES could not justify its substantially greater computational cost in this case.


Modeling of Axial Compressor With Large Tip Clearances

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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