Very low flow coefficient centrifugal compressors are often applied as the last stages of multistage compressors. Due to the lower volume flow rate, the flow channels in the impeller and diffuser are so narrow that friction loss becomes the main factor, which leads to lower efficiency than that of other stages in the same compressors. In addition, most of design methods are generally based on medium flow coefficient centrifugal compressors. Taking on researches on the low flow coefficient centrifugal compressors is significant and necessary. One-dimensional (1D) code, consisting of design and analysis parts, is developed in this study to provide basic geometric data and predict the entire performance of centrifugal compressor. Three-dimensional geometry of the impeller is built. CFD simulation is carried out as well to be compared with 1D prediction. With the continuous geometry adjustment, the final performance of the centrifugal compressor will be fixed once the performance discrepancy between CFD and one-dimensional code is acceptable. The details on the flow field within impeller will be presented through CFD. 1. Introduction Centrifugal compressors are usually designed in the flow coefficient range = 0.01 to 0.16. Often the need arises to design very low or high flow coefficient, , radial stages, but the aerodynamic challenges are not easy to satisfy; therefore, a designer usually has to make certain critical decisions. Figure 1 shows the typical application area of various compressor impellers and the typical areas where most industrial radial impellers are designed. Narrow radial impellers (smaller and smaller eye diameter) have nontwisted blades. Wider impellers (larger and larger eye diameter) by contrast have twisted blades for better adaption to local flow conditions. Figure 1: Application areas of high flow coefficient radial impellers and typical industrial compressors [ 1]. In very low flow coefficients stage design, the major challenges are mechanical configuration and diffuser stability, and most significantly the aerodynamic and parasitic losses are substantial as seen in Figure 1. Low flow coefficient compressors are used for many applications in industry and generally found as the last stages of multistage compressors where the volume rate is lower. Two-dimensional radial impeller is usually applied. Compared to the centrifugal compressors with considerably higher flow coefficient, the range of very low flow coefficient centrifugal compressors is between 0.007 and 0.02. It means that a low volume flow rate passes through the impeller
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