Impact of skin friction, tip clearance and trailing edge losses on small scale cryogenic axial turbine performance

Cryogenic Energy Storage (CES) technology which uses liquid air/nitrogen as energy carrier has attracted considerable attention recently due to its high exergy density (762kJ/kg) compared to other energy storage technologies. Liquid air/nitrogen occupies about 1/700 of the volume of its gaseous phase making it easier to store and transport. The stored energy can be recovered through a direct expansion process where the expander design and performance have a major effect on the efficiency of the energy conversion process. In this work the effects of surface roughness, tip clearance and trailing edge thickness on the performance of a small scale (tip diameter 40mm, mass flow rate 0.3 kg/s) axial cryogenic turbine have been investigated using mean line 1D analysis and ANSYS CFX 3D modelling where limited data available in the literature. Results showed that stator surface roughness has the highest impact on the turbine performance, where power output and turbine efficiency were significantly reduced as the roughness increased. For example at 20000RPM (design point) with stator roughness value of 0.5mm the efficiency and power output were 87.2% and 1197.7 W while for the same roughness on rotor blade the efficiency and power output were 89.34% and 1198.59 W. Regarding the effect of tip clearance, the efficiency decreases by 2% as the tip clearance increases from 0.35mm to 1mm.