Comparison of efficiency losses due to leaks for turbine units of aviation air conditioning systems with petal-type gas-dynamic bearings and ball bearings

Abstract

BACKGROUND: Designers of turbomachines strive to increase the efficiency of expanding compressed gases by reducing all kinds of energy losses, particularly due to clearances between the impeller and the body elements of the turbomachine. AIM: This article aimed to evaluate a possible increase in efficiency with a decrease in the radial and axial clearance between the blades of a radial-axial impeller and the casing of a centrifugal expander in the designs of turbomachines with ball bearings and petal-type gas-dynamic bearings. MATERIALS AND METHODS: The radial and axial clearances between the blades of the radial-axial impeller and the centrifugal expander casing in the designs of turbomachines with ball bearings and petal-type gas-dynamic bearings were compared by analyzing the experience of Russian and international experts in developing turbomachines. Models were presented for estimating the efficiency losses of a centrifugal expander depending on the value of the radial and axial clearances. A comparative calculation of the efficiency loss for medium- and high-cooling-capacity refrigeration turbines of aircraft air conditioning systems was performed. RESULTS: Based on the calculations, a conclusion was derived about the predominance of the influence of the radial clearance. The calculations revealed that with a decrease in the clearances between the impeller and the casing in a design with petal-type bearings, a refrigeration turbine of medium cooling capacity (16 kW, 2 impellers) can be expected to experience an increase in efficiency by an average of 2.3%; this expected increase is 0.75% to 1.4% for a high-capacity refrigeration turbine (55 kW, 3 or 4 impellers). Findings indicate that performing works to reduce radial clearances in the designs of turbomachines with petal-type gas-dynamic bearings is necessary.