Experimental investigation on the pressure drop characteristics of subcritical and supercritical CO2 heated in a horizontal tube

Abstract

For achieving high-efficiency power generation and refrigeration, CO2 cycles are receiving great attention and have been widely studied. In these cycles, the in-tube pressure drop of CO2 determines the required pumping power to maintain tube wall temperature and consequently affects the system efficiency, stability, and safety, and is thus of significant importance. In this work, we experimentally studied the pressure drop characteristics of CO2 in a horizontal heated tube with an inner diameter of 8 mm under both subcritical and supercritical pressures. The effects of mass flux, heat flux and working pressure on the pressure drop characteristics are investigated and discussed. The pressure drop increases with mass flux and heat flux, but shows complex behavior with varying pressure. The variation of pressure drop with working pressure is attributed to the two-phase effect in both subcritical and supercritical pressures. Moreover, the pressure drop characteristics are compared with existing correlations, and new correlations are developed. For subcritical pressure, a modified Blasius correlation is used to yield a modified Chisholm correlation, which can accurately predict the frictional pressure drop of the experiments. For supercritical pressure, a new correlation is proposed for predicting the friction factors by incorporating the supercritical pseudo-vapor quality, which shows greatly improved prediction accuracy. Our work demonstrates the similarities of the flow characteristics of CO2 between subcritical and supercritical conditions, and provides an alternative pathway to understand supercritical fluid flow using the three-regime-model by analogy with subcritical two-phase flow.