CFD analysis of flank clearance sizes on micro-scale transcritical CO2 scroll expander

The scroll-type expander can be the promising candidate for micro-scale (<10 kW) transcritical CO2 waste heat recovery power system. The performance of the expander significantly depends on the flank clearance design. The current paper provides a transient CFD analysis for transcritical CO2 scroll expander, which includes 19 flank clearance sizes from 20 to 200 & mu;m. The results showed that increasing the flank clearance led to a significant exergy efficiency drop from 89.13% to 46.72%. Although large flank clearance is normally perceived as negative, it indeed eliminates the possibility of over-expansion. The "turning point"for the selected expander geometry is roughly around 40 & mu;m. Excessive flank clearance enlargement resulted in a severe under-expansion issue, and the average expansion ratio was reduced from 3.19 to 1.8. The velocity of leakages also increased from 90 to 370 m/s due to the increased pressure differences. The pressure imbalance issue between two symmetrical working processes got continuously optimised by increasing the flank clearance. The optimal flank clearance range of 100 to 150 & mu;m has been established with appropriate pressure balance and efficiency. This paper provides useful insights and design guidelines in the sizing of operational flank clearances for transcritical CO2 scroll expander. The micro-scale (grid) power plant has the potential to change the energy structure considering grid stability and load balancing, and the performance of micro-scale expanders is the key to achieving the objective.