Experimental and Numerical Investigations on S-CO2 Critical Flow with Implications in Turbomachinery Seal Design

Abstract

In order to eliminate sodium-water reaction (SWR) when the current conventional steam Rankine cycle is utilized with Sodium-cooled Fast Reactor (SFR) as a power conversion system, a concept of coupling the Supercritical CO2 (S-CO2) cycle with SFR has been proposed. From the many past studies of S-CO2 cycle, it was identified that the S-CO2 cycle technology has a big potential to outperform the existing steam cycle and eventually replacing them. The major reasons are relatively high efficiency under moderate turbine inlet temperature (450~750ºC), simple layout, and physically compact power plant size due to small turbo-machinery and heat exchangers which reduces the total footprint of the power plant significantly. It is known that for a closed system controlling the inventory is important for stable operation and achieving high efficiency. Since the S-CO2 power cycle is a highly pressurized system, certain amount of leakage flow is inevitable in the rotating turbo-machinery via seals. The parasitic loss caused by the leakage flow should be minimized since this greatly influences the cycle efficiency. Thus, a simple model for estimating the critical flow in a turbo-machinery seal is essential to predict the leakage flow rate and calculate the required total mass of working fluid in a S-CO2 power system to minimize the parasitic loss. This paper presents both numerical and experimental investigations on S-CO2 critical flow while special attention is given to the turbo-machinery seal design. A simple computational model is described and experiments were conducted to validate it. Various conditions have been tested to study the flow characteristic and provide validation data for the model.