Accuracy of Centrifugal Compressor Stages Performance Prediction by Means of High Fidelity CFD and Validation Using Advanced Aerodynamic Probe

Abstract

During the design of modern high efficiency, wide operating range centrifugal compressor stages, Computational Fluid Dynamics (CFD) plays an increasing role in the assessment of the performance prediction. Nevertheless experimental data are valuable and necessary to assess the performance of the stages and to better understand the flow features in detail. A big effort is currently being made to increase the fidelity of the numerical models and the probe measurement accuracy during both the design and validation phases of centrifugal compressor stages. This study presents the flow analysis of centrifugal compressor stages using high fidelity computational fluid dynamics with a particular attention to the cavity flow modeling and comparison with experimental data, using an advanced fast response aerodynamic pressure probe. Different flow coefficient centrifugal compressor stages were used for the validation of the numerical models with a particular attention to the effects of cavity flow on the flow phenomena. The computational domain faithfully reproduced the geometry of the stages including secondary flow cavities. The availability of a new in-house automated tool for cavity meshing allowed to accurately resolve leakage flows with a reasonable increase in computational and user time. Time averaged data from CFD analysis were compared with advanced experimental ones coming from the unsteady pressure probe, for both overall performance and detailed two-dimensional maps of the main flow quantities at design and off design conditions. It was found that the increase in computational accuracy with the complete geometry modeling including leakage flows was substantial and the results of the computational model were in good agreement with the experimental data. Moreover the combination of both advanced computational and experimental techniques enabled deeper insights in the flow field features. The comparison showed that only with advanced high fidelity CFD including leakage flows modeling did the numerical predictions meet the requirements for efficiency, head and operating margin, otherwise not achievable with simplified models (CFD without cavities).

Simulation of a centrifugal compressor to obtain the characteristic map through computational effort

“…ANSYS CFX 15.0 was chosen due to its robustness, which is essential when dealing with the complex flow field developed within a centrifugal compressor [12]. The steady state simulation was set to solve the Reynolds Average Navier-Stokes (RANS) equations using the Shear-Stress Transport (SST) turbulence model [13] that has been proven to be the most proper model for this machine [14,15]. Interface model between frames of reference were set to frozen rotor since regions where the circumferential variation of the flow is large, such as the entry of the volute, benefit from this setting that preserves the relative orientation of the components across the interface.…”

“…The incompressible nature of the flow hinders numerical convergence while using pressurebased solvers. Therefore, as the discrepancy increases beyond 5%, the change from static pressure to the typical boundary condition for incompressible flow-mass flow rate-provided means to simulate operation conditions close to surge [5,6,[9][10][11]15]. Although the most common option when dealing with this situation is leaving the inlet boundary condition as total pressure while the outlet becomes mass flow rate (robust), the former set (most robust) resulted in more accurate predictions close to surge, as seen in Fig.…”

Analysis of Vaneless Diffuser Stall Instability in a Centrifugal Compressor

“…In a real diffuser design the channel width is narrow where viscous effects in the boundary layer cannot be neglected. This is due to the growth of the displacement thickness that may be different on the wall boundaries, depending on the quality of the flow delivered by the impeller, see e.g., [10]. Additionally, the rotation of the impeller with high blade tip speed results in a high Reynolds number flow that enters the diffuser.…”