Experimental investigation of supercritical carbon dioxide non-equilibrium condensation in a highly unsteady environment

The current work presents a study of qualitative relationships between the flow unsteadiness, induced by nozzle geometry variation, and non-equilibrium condensation behavior at high temporal scales for supercritical carbon dioxide (sCO₂) flow. A closed sCO₂ loop facility is used to drive a supercritical carbon dioxide flow through a rectangular converging–diverging channel. As the flow traverses through the channel, it experiences a local pressure reduction in a stalled region, eventually leading to non-equilibrium condensation of the working fluid. The flow is visualized using high-speed shadowgraphy and schlieren techniques to characterize the unsteady flow dynamics at the diverging section of the nozzle. Spectral information of the condensation behavior is retrieved from the optical diagnostics using power spectral density calculations, revealing multiple dominant frequencies in the diverging section of the nozzle. Three different nozzles are studied to understand how these dominant frequencies of condensation phenomenon change with respect to the nozzle geometry. Each nozzle design is distinguished with a unique dominant frequency, which is significantly impacted by the effect of flow throat velocity and the degree of adverse pressure gradient. Nozzles with divergence of 15° shows a dominant frequency in the range of 5 – 6.25 kHz in condensing behavior while nozzles with 6° divergence exhibited half the frequency of the phase change. These effects resulted from the extent of adverse pressure gradient imposed by the divergence angle of the nozzle. Additionally, these dominant frequencies are also compared to the wall-pressure signals acquired from high-frequency pressure transducers. Spectral analysis of wall-pressure signals, which are directly linked to nozzle designs, reveals a strong coupling between the condensation behavior and the wall-pressures.