Design and Development of Advanced Three-Dimensional Non-Periodic Diffusers for Centrifugal Compressors

Abstract

Recent changes in the global economy have significantly impacted the air separation market. Companies traditionally focused upon initial capital expenditures, have shifted their mindset towards total life cycle costs. As a primary component in the air separation process, the compressor has been recognized as a significant contributor to life cycle costs of an air separation facility. Accordingly, market expectations have shifted towards more efficient compressor designs to reduce the overall power consumption. This paper highlights the development efforts at Cameron’s Compression Systems to design advanced 3D cascade diffusers, leading to the development of compressors with higher aerodynamic efficiencies. While the concept of matching the diffuser design towards the local flow field is not new, the implementation of this concept has been significantly hindered by a lack of understanding with regard to impeller discharge conditions. Furthermore, the tools necessary to explore these specifics either did not exist, or where prohibitive in terms of both cost and time. Over the past 20 years, great advances have been made in the area of computational engineering. The advancements in Computational Fluid Dynamics (CFD) tools have led to a better understanding of the transitional impeller flow fields. Furthermore, the dramatic progress in cheaper high-end computers has facilitated the design and development of sculpted three-dimensional diffusers within reasonable time frames and at relatively low cost. Recently, Cameron has deployed advanced sculpted 3D diffusers on limited production designs. This process involves a heavy utilization of STAR-CCM+ for performance analyses of centrifugal stages with various diffuser geometries. The CFD analyses vary from relatively simple steady state analyses with single impeller and single diffuser passages using indirect (mixing plane) interfaces to full stage unsteady simulations involving full inlet, impeller, diffuser and volute with a discharge pipe. This paper discusses a range of issues involved in performing CFD analyses for complex diffuser geometries. We show how CFD analyses are used to improve the aerodynamic performance of stages with respect to the regular low solidity cascade 2D diffusers. The paper provides results of comparative studies of the computational analyses with the aerodynamically tested data for the stages using both the regular low solidity 2D diffusers and sculpted non-periodic 3D diffusers. The test results confirm predicted improvements in compressor efficiency using sculpted non-periodic 3D diffusers over regular 2D diffusers by as much as 2% for peak efficiency. The paper discusses stage improvements due to use of 3D sculpted non-periodic diffusers.