Numerical Modeling of the Flow in a Vaneless Diffuser of Centrifugal Compressor Stage

This paper presents the results of numerical investigation of the flow in a vaneless diffuser of centrifugal compressor stage. Simulations were performed using both a commercial CFD package ANSYS CFX and the own-designed computer program. Steady conditions involving SST turbulence model were used for the calculations using CFX. To consider the interaction between impeller and diffuser, not just a diffuser but the whole stage was calculated. The own-designed methodology is based on solving of conservation equations with assumptions that flow in a diffuser is steady state and axisymmetric. The flow area is divided into the flow core and boundary layers. Results of calculation were compared with experimental data. 1. Introduction The behavior of vaneless diffusers of centrifugal compressors has been widely studied theoretically, experimentally, and numerically. Theoretical analyses have been carried out by Jansen [1], Senoo and Kinoshita [2], Tsujimoto et al. [3], Ljevar et al. [4], and so forth. Experimental measurements have been performed by Nuzhdin [5], Abdelhamid [6], Kinoshita and Senoo [7], Jaatinen-Varri et al. [8], and so forth. It was shown that stages with vaneless diffusers have a wide operating range and high polytropic efficiency at high flow rates; however, at low flow rates their efficiency extremely decreases because of flow separation and rotating stall inception. The last one results in dramatically loss of compressor performance and instability and even can cause damage of the machine. Numerical simulations of vaneless diffusers have been performed by Gao et al. [9], Khalfallah and Ghenaiet [10], Izmailov et al. [11], Tamaki [12], and others. Obtained numerical results show good agreement with the experimental ones, but all the authors have faced the problem of rotor/stator interaction. This paper presents the results of both experimental and numerical investigation of a centrifugal compressor vaneless diffuser. Simulations were performed using both commercial CFD package Ansys CFX and the own-designed computer program. Investigation has been performed to predict the overall performance of a diffuser and to obtain flow patterns. Finally, the flow structure was analyzed to identify and quantify the sources of losses. 2. Experimental Procedure First of all an experimental investigation of the flow has been performed. The test vaneless diffuser is an element of the Sumy Frunze NPO gas compressor end stage. Cross-section of the diffuser is shown in Figure 1(a). Diffuser width ratio is . Figure 1: Cross-section of the test diffuser (a) and