Experimental assessment of the internal flow behavior of supercritical carbon dioxide

Abstract

This thesis presents an experimental assessment of the internal flow behavior of supercritical carbon dioxide. The investigation focused mainly on assessing condensation onset during rapid expansion of CO₂ into the two-phase region. An experimental blowdown test-rig with a modular test section was developed with the capability of operating a converging-diverging nozzle with a wide range of charge conditions. The test-rig demonstrated repeatable results with relative errors of less than 1 percent. An innovative method to measure the speed of sound through the use of Helmholtz resonators was developed and investigated. Shock tube experiments and static high pressure air tests in the blowdown facility were conducted to determine the viscous damping in the resonators. The results indicate that a Helmholtz resonator neck Reynolds number of 106 is required for underdamped response. The speed of sound measurement technique was demonstrated at relevent Reynolds numbers and at static conditions in air, showing promise for blowdown testing in S-CO₂ Blowdown experiments were conducted in supercritical CO₂ from charge conditions both away and near the critical point to sonic conditions in the two-phase region. To determine whether there was condensation onset, static pressure measurements were compared to theoretical and numerical models. Numerical models utilized the Span-Wagner equation of state extrapolated into the two-phase region to characterize the metastable state. Away from the critical point, for operating conditions typically encountered in S-CO₂ compressor stages, condensation was not observed. Near the critical point, results were inconclusive and future work is proposed to provide more conclusive assessment of condensation near the critical region.