Reynolds-number-dependent efficiency characterization of a micro-scale centrifugal compressor using non-conventional working fluids

The selection of working fluids other than air is a key issue in improving the efficiency of new thermodynamic cycles intended for low-to-moderate temperature small power plants. The aim of this paper is to study whether the low efficiency typical of small turbomachinery is still a problem when using alternative fluids. Based on a new design of power cycles named balanced hybrid Rankine-Brayton cycles, five different fluids were selected as potential working fluids: carbon dioxide, propane, isobutane, pentafluoroethane and sulfur hexafluoride. Dimensional analysis was used to compare the performances of a micro-scale centrifugal compressor working in homologous points where the efficiency variation depends only on the Reynolds-number (Re). The influence of Re on efficiency was calculated by means of four different methods for comparative purposes. Numerical simulations were also carried out in order to validate the methodological approach proposed. The results show the efficiency variations as a function of Re for increasing fluid densities. All the non-conventional fluids studied provide better performance in terms of efficiency than air. Particularly, isobutane and propane have been identified as potential working fluids candidates for the aforementioned innovative power cycle.