Abstract

In refrigeration systems, novel low-pressure low-Global Warming Potential (GWP) refrigerants drives the size of centrifugal compressors and heat exchangers beyond feasible product specifications, limiting their application to medium capacity chillers (200TR-1500TR-Tonnage of Refrigeration). The introduction of mixed-flow compressors has the potential to enable higher capacity (3000+TR) within compact chiller configurations. To supersede conventional two-stage back-to-back centrifugal compressors and deliver the static pressure rise necessary for the chiller cycle, a single-stage mixed-flow impeller was designed with a high-Mach number tandem stator and an inward-turning volute. This paper presents CFD-based analyses of the mixed-flow compressor, including unsteady analyses of the impeller-diffuser transonic interactions, as well as results from experimental validation of the predictions in a product-relevant chiller test facility. Refrigeration systems are also required to operate at various conditions, often driving compression sub-systems at the limit of their operating range. While actuated Inlet Guide Vanes (IGV) can alter the suction flow conditions of a compressor, several passive devises can be considered as alternatives for stability enhancement at a lower operating cost. This paper then presents CFD-based analyses of two passive devises to augment the stability margin of a mixed-flow compressor: a ported shroud for part-load stability enhancement and an aerodynamic swirler for full-load stability enhancement.