Frictional resistance investigation of sCO2 and a semi-empirical friction factor correlation based on wall-of-the-law

Abstract

The flow resistance characteristics of variable property fluids, particularly supercritical fluids, such as supercritical CO2 (sCO2), play a crucial role in the design, operation control, and simulation of energy systems. However, drastic changes in thermophysical properties near the pseudo-critical regime under various boundary conditions make it challenging to find suitable theoretical and correlation models that can accurately predict their friction factors.In this study, the experimental investigations on the frictional resistance characteristics of upward sCO2 flow under different heat flux and tube wall roughness is conducted. The results show that as the heat flux increases, sCO2′s friction factor decreases under the same reference temperature, and the decrease becomes less significant with increasing tube wall roughness. The SST k-ω simulation attributes this phenomenon to the suppressed sensitivity of turbulence velocity fluctuations to thermal boundary conditions in high roughness pipes. Additionally, the experiment shows that near the pseudo-critical point, sCO2′s friction factor abnormally increases, which can be reasonably explained by the density fluctuation theory. A new semi-empirical expression for the variable property fluid's friction factor by combining the law-of-the-wall is derived for the first time, and the correlation is regressed based on the experimental and literature results. The new correlation exhibits good predicting performance, with 94% of predictions falling within ±20% absolute deviation from the original experimental results. This result confirms consistent flow resistance characteristics between supercritical flow and subcritical single-phase flow.