Analytical prediction of Reynolds-number effects on miniaturized centrifugal compressors under off-design conditions

Abstract

The use of micro-scale centrifugal compressors in closed-loop thermodynamic cycles working with fluids different than air is gaining in importance in novel miniaturized energy systems. Miniaturization implies a decrease in the Reynolds number, which usually entails efficiency drops. The aim of this paper is to study the low-Reynolds-number effects on the performance of micro-scale centrifugal compressors under off-design conditions. Consequently, an analytical prediction method is formulated to characterize the efficiency variations using different working fluids and pressurization factors. A numerical model of a reference micro-scale compressor is developed to assess the degree of agreement between the results obtained by these two methods. The results show the Reynolds-number effects on the whole characteristic map of the reference compressor for air, carbon dioxide, propane and isobutane. These fluids were chosen according to their efficiency increase potential, which is measured by a proposed Reynolds-number-based estimator. Although some divergences have been identified, the agreement between analytical and numerical results can be considered to be globally satisfactory. Efficiency increases in the range of 2–6% points are obtained with slight pressurization factors using air or carbon dioxide. These variations could even achieve a rise of 9 pp in the case of propane and isobutane.