Pulsations and Vibrations in the Discharge Piping of a Throttled Supercritical CO2 Turbine

Choke nozzles present an economical option to control the expander downstream conditions in high-pressure, high-temperature supercritical CO2 loops; however, the expansion and high fluid momentum exiting the choke may induce instability, causing pulsating behavior and vibration of the loop due to fluid-structure interaction. The current study investigates the performance of a conventional choke design utilizing CFD simulations to provide detailed insight into jet behavior and interaction with the wall. The analysis uses constant boundary conditions and doesn’t account for any flow instability due to fluctuations and swirl velocities at the expander outlet. The CFD outcomes reveal the whirling motion of the jet after exiting the choke sticking to the pipe wall during the initial phase of jet formation. This whirling jet interacts with the downstream pipe and was shown during testing to create large pressure pulsations in the downstream piping. The study concludes with the design, analysis, and testing of a flow conditioner-type choke that redistributes and homogenizes flow downstream of the choke, which reduces the undesirable excitations and consequently eliminates the pipe vibrations.