Effects of Surface Roughness on Windage Loss and Flow Characteristics in Shaft-Type Gap with Critical CO2

To investigate the effects of surface roughness on windage loss and flow characteristics in a shaft-type gap, the skin friction coefficient (Cf) and flow versus Reynolds number (Re) at different surface roughness (Ra) and radius ratio (η) values were investigated. The results showed that Cf decreased as Re increased, and the rate of decrease was constant at low Re but reduced at high Re. The growing relative deviations between the coefficients of smooth and rough walls with Ra indicated that Cf was influenced by rough walls when Re > 102. Moreover, Cf and the variation rate increased with η and were easily influenced by Ra for larger η at low Re, since the interaction between wall roughness and fluid influences windage loss. In addition, the flow field implied the flow had transitioned to Taylor-Couette flow, Taylor vortexes occurred when Re > 102, and the number of vortexes increased with increasing Ra and were reduced with increasing η. The velocity was divided into three regions and the pressure rose from the rotational to stationary walls, but decreased with growing η as a whole. This paper improves the research exploring windage loss and will help design smaller supercritical CO2 power devices.