Frictional Effects on a Base and a Flow-Trimmed Impeller of a Low Specific Speed Industrial Compressor

Flow-trimming base designs for lower flow applications are widely used in the turbo-machinery industry to minimize development costs. This process requires the use of the same casing as the base design, along with a decrease in the flow path width of the impeller, diffuser, crossover, and return vane. Two stages: a baseline and a flow-trimmed impeller, diffuser and return vane are studied in this paper to review impacts on performance and their underlying causes. To measure the performance changes of the two rigs, a traverse survey for flow angle and total pressure was done at the locations of diffuser inlet, return vane leading edge and stage discharge. It was found that the performance difference between the base impeller and trimmed one is very little. However, diffuser performance of the base rig is better than the flow-trimmed rig. Diffuser data shows frictional losses increased in the flow-trimmed rig. Rig data also suggests that increased friction in the trimmed stage changes the flow angle at the return vane leading edge compared to the base stage. This increases the loss further for the flow-trimmed rig because of resulting negative incidence on the return vane relative to the base rig. Data suggests that while flow-trimming may be a valid design approach for impellers, additional issues arise when downstream stationary components are trimmed.