Effect of parasitic losses on the design optimization of inward flow radial supercritical CO₂ turbines

Inward flow radial (IFR) supercritical CO2 (sCO(2)) turbines are smaller in size and operate at considerably higher speeds in comparison to similar capacity conventional gas or steam turbines. The compact size and high speed of IFR turbines result in significant parasitic (leakage and disk friction) losses at the rotor backface. This paper presents CFD investigations to understand the mechanism of parasitic losses of IFR turbines in the kW to MW power scales. The first part of the paper presents a parametric study to quantify the effect of backface flowpath dimensions, rotor inlet radius, and rotational speed on the magnitude of parasitic losses. Subsequently, the paper proposes the implementation of a radial labyrinth seal on the rotor backface to curtail the parasitic losses. The second part of the paper examines how the power scale of the turbine and its design parameters, specific speed and velocity ratio, influence the magnitude of parasitic losses. The investigation reveals that parasitic losses cause an efficiency drop of 8%-15% for 100 kW and 4%-9% for 1 MW IFR sCO(2) turbines. In addition, IFR turbines designed for low specific speeds and high velocity ratios result in notably higher parasitic losses, leading to a decline in turbine efficiency. Finally, the paper presents optimal turbine design parameters accounting for the parasitic losses to maximize turbine efficiency.