Title: Investigations on Flow in Supercritical CO2 Ejectors
Name of the Student: Mr. Sanjoy Paul, PhD student
Research Supervisors: Prof. Srisha Rao (AE) , Pramod Kumar (ICER)
Date & Time: October 29, 2024 at 10:00 AM
Venue: Class Room, ICER, IISc
Abstract
Globally, there is a prominent shift towards the use of carbon dioxide (sCO2) as a natural eco-friendly working fluid in energy conversion systems. In such applications, sCO2 works in a thermofluidic domain, which is far from the ideal gas domain, which makes modelling of gas-dynamic systems challenging, and experimental data on canonical systems like the nozzle is scarce. The supersonic ejector is a passive gas-dynamic device that uses a motive flow to entrain and compress a secondary flow. It has shown beneficial effects in improving the overall performance of power and refrigeration systems. A new solar-based cogeneration cycle with CO2 as a working fluid, which includes a supercritical (sCO2) ejector is proposed for a diary application. Thermodynamic analysis is conducted by using a highly simplified control volume model of the ejector. Performance maps for the proposed cycles are generated considering various operating parameters, ensuring a net positive power output. The implementation of the (sCO2) ejector enhances the system COP by up to 35%, increases the milk handling capacity by 20%, and reduces the irreversibility losses by 24%. Since system performance is highly dependent on the ejector performance, a hybrid mathematical model of the sCO2 ejector is developed, which uses the ‘Method of Characteristics’ to model the supersonic primary flow and a quasi-1D model of the secondary flow with pressure-matching boundary conditions while incorporating the real gas effects into each sub model. Empirical fits of the non-mixed length in the ejector are used to ascertain the length of the mixing duct, and the shock location in the mixed flow is estimated using the entropy minimization principle. The overall model exhibits remarkable fidelity and robustness in the prediction of experimental results of air ejectors. Comparisons between CFD and the physics-based model with sCO2 as a working fluid confirm the accuracy of prediction of the current model (<5% difference in entrainment ratio) compared with conventional modelling approaches (10-15% difference in entrainment ratio). Aerodynamic optimization of the ejector passage is carried out by developing a surrogate model that is trained on a large dataset of CFD-RANS simulations. A multi-objective evolutionary algorithm (MOEA) is used on the surrogate model to optimize the ejector. The objective functions considered for the optimization are maximization of the entrainment ratio (𝜔) and compression ratio (CR) and minimization of the entropy generation. The optimization result shows that the ejector efficiency is improved by a maximum of 25% over the non-optimized ejector. The supersonic nozzle is a key component that affects ejector performance, and experimental data of sCO2 nozzles using optical diagnostics and point measurements are scarce. An experimental CO2 nozzle test rig is designed, fabricated, and installed in a supercritical CO2 jet facility in the ‘Laboratory for Hypersonic and Shock Wave Research’ (LHSR) at the Indian Institute of Science (IISc) for shadowgraph flow visualization and pressure measurements along the nozzle wall. The shock structures within the nozzle at different operating conditions are captured crisply, providing details of transitions from different shock configurations. Complimentary numerical simulations on fine grids show that the combination of k-ω-SST and Span and Wagner’s equation of state best model the flow in sCO2 nozzle. Results show that the shock-boundary layer interaction phenomena in the nozzle are strongly affected by the ratio of specific heats, which varies dramatically in the sCO2 nozzle. The current work has laid a basis for further work on the fundamentals of sCO2 ejectors.
ABOUT THE SPEAKER
Sanjoy Paul is a Ph.D. student in the Interdisciplinary Centre for Energy Research, IISC Bengaluru. He obtained his B.E degree in Mechanical Engineering from Visvesvaraya Technological University, Karnataka in 2011, and M.Tech degree in Mechanical Engineering from NIT Silchar in 2014. Subsequently he worked as an Assistant Professor in Parul University for four and a half years. His research area is broadly the gas dynamics interactions in the ejector.