Abstract
With limitations of traditional power cycles on their operating temperature ranges and max efficiencies, the research community has shifted the attention to usage of supercritical fluids as the main working fluid of power cycles. Among the supercritical fluids, supercritical carbon dioxide (sCO2) is considered as the most promising candidate for its relatively low critical point, low cost, and availability. The main merit of using sCO2 is its high density near the fluid’s critical point, which reduces the required compression power. This leads to decrease in the volume of turbomachinery and increase in the overall cycle efficiency. However, non-linear property variations and mixing of different supercritical regimes in close proximity to the critical point pose great risks for cycles taking the full advantage of these benefits. The non-linear property variations close to the critical point provide a challenge in developing a multiphysics model, which can serve as a backbone for simulating more advanced geometries of sCO2 power cycle processes.
To understand the physical phenomena of property variations and mixing of supercritical fluids at differing conditions, a classical shear layer experiment is performed in a high-pressure test chamber with supporting closed-loop cycle facilities. Experiments are conducted under various fluid density and velocity ratios at multiple isobars to understand if classical mixing theories and hydrodynamics follow the same trend in supercritical conditions. Specifically, turbulent flow characteristics and the growth parameters are captured through density gradients between two channels using Schlieren visualization. The qualitative results show that the mixing interfacial features follow the classical mixing growth rate theories despite nonlinear fluid property variations. Dissipative nature of the supercritical fluids contributes to resemblance of gas-gas mixing and presents decrease in mixing intensity as inlet conditions deviate away from the critical point.
