Design and Test of an Extremely Wide Flow Range Compressor

A fundamental theoretical study of the flow within a compressor wheel suggests that: (a) a necessary (but not sufficient) criterion for the prevention of surge can be obtained from the inviscid solution of the internal compressor flow problem, (b) the use of backward leaning blades is not the only blade passage modification which has the potential to increase the usable flow range of the compressor, and (c) a usable flow range much broader than previously thought to exist can be obtained. One alternative approach suggested by this study has been experimentally tested up to compressor pressure ratios in excess of 3.0. The new approach allowed a 50 percent reduction in the surge mass flow at design impeller speed while maintaining the inducer geometry of the machine identical to that of a conventional radial-bladed impeller used as a comparison standard. This new approach indicates the potential for a-priori prediction of surge flow characteristics of radial turbomachinery. Conversely, the design of hardware to a prespecified surge to choke flow ratio may be able to be accomplished by predefined blade geometry. It appears that the usable flow range for centrifugal compressors could extend down to 15 to 20 percent of the choke flow capability without sacrificing maximum component efficiencies.