This paper describes a procedure for assessing the efficiency of a turbine operating under unsteady periodic flow at the inlet. Time-resolved flow simulations of the modified Garrett turbine case A3K7 are conducted with time-varying inlet conditions upstream of the turbine. A mass-weighted moving average is applied to the instantaneous data over a period corresponding to the pulsing frequency of the time-varying inlet conditions. The resulting time averaged data is then used to assess the convergence of the numerical solution and evaluate the turbine performance. Three methods of computing the time-averaged turbine efficiency are presented. The first method, referred to as the TP method, is based upon the use of mass-weighted average of the total temperature (Tt) and total pressure (Pt). The second method, the TS method, uses the mass-weighted average of total temperature (Tt) and entropy (S). The third method, the WS method, employs the moving average of specific work output (Ws) and work lost due to entropy (S) increase. A comparison of time-averaged specific work output, stage efficiency, and rotor efficiency is made for three turbine operating configurations: partial admission, synchronized pulsation, and sequential pulsing flow conditions of the sector inlets. Results show that for the tested rotor speed, the efficiency increases as the inlet pulsing frequency is reduced, and that when inlet sectors are not opened simultaneously, a large drop in efficiency occurs due to spillage of high energy fluid between the open and closed sectors. Additionally, it is shown that due to the non-convective nature of total pressure, the TP method for efficiency is not reliable while the TS and WS methods are able to produce consistent results.
Numerical analysis of the transonic NASA CC3 centrifugal compressor stage with vaned diffuser is undertaken and the mechanism for the onset of stall investigated. The analysis methodology employs standard practice numerical methods, the mixing-plane approximation at the rotor-stator interface and periodic boundary conditions within a given row. Simulation results were then compared against full annulus time accurate simulations and the methodologies evaluated for estimation for the onset of stall. In the study, steady state and full annulus time accurate simulations were conducted at the 100% design speed, from choke to stall. Many time-resolved simulations “stalled” at pressure ratios where steady state mixing-plane solutions converged. One observation from the time-resolved simulations is that the number of revolutions required for the flow to stall was over 5 revolutions and increased to 9 revolutions closer to the last stable point. Integrations of the time-resolved solutions at the onset of stall reveal that the flow field of both the impellers and diffusers are not circumferentially uniform, and a stall “cell” is rotating in the opposite direction of the rotor. However, the “cell” is stationary in the stationary frame of reference thus, it is not a conventional rotating stall. There is a notable difference in choke flow between experiment and CFD results, which has been seen by others for this configuration. The reason for the miss is not a focus of this paper, and the numerically predicted unsteady stall point should not be viewed as the exact mechanism for stall within the NASA CC3 experiment, instead this work is to provide a review of standard practice numerical approximations and how cost effective full wheel unsteady analysis can be used to improve transonic compressor design.
This paper presents a numerical analysis of oscillating airfoils in turbomachinery cascades using the unsteady nonlinear Reynold’s Averaged Navier-Stokes (URANS) equations. The periodic unsteady flow solutions are determined using a conventional time marching method (DTS) and the Nonlinear Harmonic Balance method (NHB). Mesh motions, using a weighted distortion procedure and a linear elastic method, are described. Comparison of computed results are made with the Eleventh Standard Test Configuration (STC11) experimental data for subsonic and transonic exit flow conditions. The solutions for the NHB and DTS methods exhibit excellent correlation with each other and good correlation with the experimental data on the pressure surface. The numerical solutions deviate from the experimental data on the suction surface especially in the vicinity of the shock wave for the transonic exit flow case. A numerical influence coefficient modeling method is shown for airfoil cascades that can be used to calculate unsteady aerodynamic loading over a range of interblade phase angles. Application to the STC11 illustrates that a cascade of five airfoils is sufficient to provide accurate unsteady aerodynamic loading predictions for the modeled flow conditions.
In the drive for lower fuel consumption, engine designs for the next generation of single-aisle aircraft will require core sizes below 3 lb/s and OPRs above 50. Traditionally, these core sizes are the domain of centrifugal compressors, but materials limit OPR in these machines. An all-axial 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 larger clearance-to-span ratio. To enable small-core, high-OPR, all-axial compressors, mitigating technologies need to be developed and implemented to reduce this penalty. 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 determine whether different modeling standards are required for large clearance-to-span ratios, and if so, identify criteria for an appropriate solver and/or mesh. Multiple models are run and results compared with data collected in the NASA-GRC Low-Speed Axial Compressor. These comparisons show that steady RANS solvers can predict the pressure-rise characteristic to an acceptable level of accuracy, if careful attention is paid to mesh topology in the tip region. However, unsteady tools are necessary to accurately capture radial profiles of blockage and total pressure.
In the drive for lower fuel consumption through increased bypass ratio and increased overall pressure ratio (OPR), engine designs for the next generation of single-aisle aircraft will require core sizes below 3 lb/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 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 determine whether different modeling standards are required for a large clearance-to-span ratio, and if so, identify criteria for an appropriate solver and/or mesh. Multiple models are run and results compared with data collected in the NASA Glenn Research Center’s Low-Speed Axial Compressor. These comparisons show that steady Reynolds-Averaged Navier-Stokes (RANS) solvers can predict the pressure-rise characteristic to an acceptable level of accuracy, if careful attention is paid to mesh topology in the tip region. However, unsteady tools are necessary to accurately capture radial profiles of blockage and total pressure. The Delayed-Detached Eddy Simulation model was also used to run this geometry, but did not resolve any additional features not captured by the unsteady RANS simulation near stall.
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