Numerical Investigation of Supercritical CO2 in Hydrocyclone for Enhanced Thermodynamic Cycle Efficiency

Belzung, Anthony
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In power cycles, such as Brayton cycles, the use of Supercritical carbon dioxide (sCO2) as a working fluid enhances the cycle efficiency for applications such as Geothermal power generation. In this research study, the role of sCO2 in enhancing the efficiency of a cycle is demonstrated using a Hydrocyclone, a device used to condition the flow characteristics of sCO2 before entering the compressor or turbine of the cycle. Hydrocyclones are vortex generator devices that use swirling flows to enhance mixing through physical instabilities, vorticity and turbulence effects, and can work to separate any dispersed fluid phase particles through centrifugal flow. This study simulated the flow behavior of sCO2 with computational fluid dynamics numerical modeling using a commercial compressible flow solver COMSOL Multiphysics. The objectives of the study were: 1. To conduct a grid convergence study to quantitatively measure the spatial discretization error and to optimize the mesh to capture the flow physics, 2. To conduct a parametric study of inlet temperature effects on flow characteristics and performance characteristics as a comparison between isothermal and nonisothermal flow models, and 3. To quantify the compressibility effects, and determine mixing efficiency enhancement as a comparative study of velocity, pressure, and temperature ratios between outs and inlets. The grid convergence study determined the optimal mesh grid sizes required to get an established and consistent solution. A comparative analysis was also presented between isothermal and nonisothermal flow simulations. The isothermal simulations illustrated that increasing the inlet temperature has no effect on the velocity magnitude or pressure plots, though it does decrease the Mach number due to the temperature dependence of material speed of sound. The nonisothermal flow demonstrates that increasing the inlet temperature decreases the magnitude of flow velocity for fluid outside of the vortex core and within the core centerline. Likewise, the nonisothermal flow demonstrate the Mach number decreases along the centerline and outside the vortex core with increasing inlet temperatures. Parametric studies were also carried out for the effect of inlet temperature on the flowfield. Simulated results showed that the rotational nature induces a negative pressure gradient at the center of the vortex finder, which increases the upward component of the flow velocity magnitude; increasing the inlet temperature decreases the absolute pressure outside the core, and has a slight increase inside the core. The ratios of velocity magnitude at the outlets to the inlets show an increased backflow into the overflow as temperature increases. Outlet to inlet pressure ratios indicate that increasing the temperature decreases the pressure loss. Finally, the outlet to inlet temperature ratios show that increasing the temperature decreases the temperature loss. These parameters show that increasing the temperature enhances the mixing behavior. A qualitative comparison with experimental data was also made which showed that the basic flow and thermal features are well captured by the COMSOL simulations.

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Hydrocyclone, Cycle efficiency, Supercritical carbon dioxide, sCO2
Mechanical Engineering