Higher Order Finite Element Meshes for Centrifugal Impeller Blade Analyses Using Pointwise

Abstract

Finite Element Methods (FEM) have become a key tool to predict stresses, deformations, eigenfrequencies and eigenmodes in centrifugal impellers under centrifugal loads. Accurate prediction of values of principal stresses and natural frequencies is critical to avoiding field failures of impellers. Accurate prediction of maximum principal stresses in centrifugal impeller blades is a straightforward task using linear finite elements with a moderate mesh resolution. Centrifugal impeller failures due to high stresses have been virtually eliminated as a failure mode because of this level of analysis. However, the same level of analysis of natural frequencies does not always result in accurate predictions of higher frequency modes. It has been Cameron’s experience that the most common reason for impeller field failure has been a high frequency resonance resulting in a high cycle fatigue failure. Although there are a few commercially available tools for stress and modal analyses of centrifugal impellers, we are not aware of any that offers the flexibility, generality and, most importantly, accuracy in predicting high modal frequencies. The impeller designer has to find a solution that allows the impeller to survive with the exciters caused by diffuser vane pass frequencies and/or inlet guide vane pass frequencies and still meet the performance guarantee. Historically, design practice focused on stress levels and avoiding one time vane pass frequencies. In general, this approach has been very successful at eliminating stress and one-time vane pass frequency failures. However, it must be emphasized that current tools can incorrectly predict excitation modes due to twice the diffuser vane or inlet guide vane pass frequencies. It has been Cameron’s experience that most field failures are in this two times vane pass frequency range. The development of a custom in-house tool for accurate stress and modal analyses of centrifugal impellers has been a major task at Cameron in the past few years. This thrust resulted in Cameron’s cooperation with Pointwise, Inc. to develop a set of automated tools to utilize higher order finite element meshes for such analyses. Pointwise has established itself as the world-leading mesh generation software for Computational Fluid Dynamics (CFD) analysis. However, the flexibility and versatility of this meshing software also makes it a very attractive and powerful tool for generating Finite Element Analysis (FEA) meshes. This work focuses on a utilization of Pointwise for automated generation of higher order finite element meshes for centrifugal impeller blades. Pointwise-based automated tools generate a fully structured mesh of higher order elements that involves every detailed feature of the centrifugal impeller. The mesh retains geometric validity of the centrifugal impeller and maintains accurate geometrical resolution of the higher order surfaces associated with the impeller blade and hub. The structural and modal analyses are then automatically performed in ANSYS. We show that the Pointwise-based meshing tools are both general and robust. They allow for high quality, higher order finite element meshes for an entire range of applications that we encounter at Cameron Compression Systems on a daily basis. This work demonstrates the attractiveness of higher order finite element meshes for accurately predicting the natural frequencies for centrifugal impellers for an entire range of desired frequencies. We present finite element mesh convergence studies, and utilize a Pointwise based meshing tool together with ANSYS to accurately predict failure modes for impellers that have experienced field failures in the last decade.