Design Method for Channel Diffusers of Centrifugal Compressors

The design method for channel diffusers of centrifugal compressors, which is based on the solving of the inverse problem of gas dynamics, is presented in the paper. The concept of the design is to provide high pressure recovery of the diffuser by assuming the preseparation condition of the boundary layer along one of the channel surfaces. The channel diffuser was designed with the use of developed method to replace the vaned diffuser of the centrifugal compressor model stage. The numerical simulation of the diffusers was implemented by means of CFD software. Obtained gas dynamic characteristics of the designed diffuser were compared to the base vaned diffuser of the compressor stage. 1. Introduction The operating conditions and purpose of centrifugal compressor should be taken into account when choosing the type of its diffuser. Channel diffusers (CD) could be more preferable as compared to other types in the following cases: (1) at small flow angles at the diffuser inlet; (2) when the gas passes from the diffuser channels to the separated cameras; (3) if the diffuser channels turn to the channels of the return element of compressor. Traditional geometry of channel diffusers includes the initial section shaped as a logarithmic spiral along one of the vane surfaces and the main section with straight walls (Figure 1(a)). The channel diffusers with wedge vanes are also widely used (Figure 1(b)). Figure 1: Channel diffusers of traditional geometry: (a) channel diffuser with initial section shaped as a logarithmic spiral, (b) channel diffuser with wedge vanes. Some authors recommend using the Reneau et al. [1] and Runstadler Jr. and Dean Jr. [2] database for the flat plane diffusers to design wedge-shaped channel diffusers. Kano et al. [3] and Clements and Artt [4] indicated that such data cannot be used to design the high-performance diffuser of centrifugal compressor because of the distorted three-dimensional swirled flow at the inlet. In addition, such a database does not cover the wide range of geometries and inlet flow conditions. Generally, the most effective vane geometry may be obtained by solving the inverse problem of gas dynamics. The maximum pressure rise in diffuser may be achieved when the flow is close to separation. Stratford [5] obtained experimentally the flow with stable preseparation condition of the boundary layer along one of the surfaces of the two-dimensional diffuser. It was shown that, by specifying the preseparation pressure distribution, the required pressure rise may be attained in the shortest possible distance and with the