| dc.description.abstract | Adopting natural eco-friendly refrigerant carbon dioxide (CO2) in energy conversion devices is gaining prominence globally. Supersonic ejectors are passive devices that energize and compress a secondary stream using a motive flow through complex gas dynamics interactions. CO2 ejectors operating in the supercritical region can reduce energy consumption in combined refrigeration and power cycles and enhance the system’s performance.
The operating regions of s-CO2 ejector is far from that of an ideal gas. Thus, the mathematical modelling of ejectors, including real gas effects present in CO2, is challenging but crucial for predicting performance. A comprehensive physics-based hybrid model of the ejector operating in the critical flow regime where both the primary and secondary mass flow rates are choked is proposed. The method of characteristics is used to model the primary supersonic flow and is concurrently solved with a discrete quasi-1d model for the secondary flow with appropriate pressure-matching interface conditions and boundary conditions. The compressible turbulent mixing layer growth between the primary and secondary flow is modelled, and the location of choking is evaluated without any prior assumptions. The 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 an entropy minimization principle. The overall model exhibits remarkable fidelity and robustness in the prediction of previous experimental results of air ejectors. Comparisons between CFD and the physics-based model with s-CO2 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).
The ejector performance is primarily dependent on the aerodynamic shaping of the ducts. Thus, the s-CO2 ejector is optimized by generating a surrogate model from a comprehensive mapping of various geometrical parameters and operating conditions on the performance of supercritical CO2 (s-CO2) ejector from a sufficiently widespread data set computed using CFD. The dominant influence of area ratio, nozzle exit point, and length of mixing duct on the ejector performance is inferred from sensitivity analysis. A Multi-objective evolutionary algorithm (MOEA) is used on the surrogate model to optimise the ejector. The objective functions considered for the optimisation are maximisation of the entrainment ratio (𝜔) and compression ratio (CR) and minimisation of the rate of entropy generation (S_gen). 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 the ejector performance, and experimental data of s-CO2 nozzles using optical diagnostics and point measurement 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 flow visualisation and pressure measurements along the nozzle wall. The experiment reveals that the increase in back pressure leads to the transition of the shock structure from 'X' shock with the asymmetric flow to 'λ' shock and finally to normal shock with the symmetric flow. Flow patterns shift from restricted to free shock separation on both sides of the wall, accompanied by a series of weak curved shocks indicating successive expansions and compressions within the separated flow. Numerical modelling is carried out in the same flow conditions on finer grid with different combination of various equations of state and RANS turbulence model. The location of separation, separation pressure ratio, wall static pressures and shadowgraphs from experiments and numerical simulations are analyzed side-by-side. The combination of Span and Wagner equation of state and the k-ω-SST turbulence model best predicts the overexpanded nozzle flow through a CO2 nozzle. It is also found that the one equation Spalart-Allmaras (SA) turbulence model is close enough and a good alternative to the more computationally expensive k-ω-SST model. The c_p/c_v variation has a strong influence on the shock and boundary layer interactions based on which early separation or delayed separation can happen. A modified Schmucker criteria based on these studies predicts the onset of separated overexpanded nozzle flow in the real gas CO2 flows to a greater accuracy than the original Schmucker criteria which was proposed for ideal gas flow.
A detailed study of the feasibility analysis of the s-CO2 ejector in the cogeneration cycle by using thermodynamic analysis. Dairy plants, which are energy-intensive due to the combined heating, cooling, and supplementary equipment demands, making the cogeneration cycle suitable. A new solar based cogeneration cycle with CO2 as a working fluid is proposed, followed by improvements with a supercritical CO2 (s-CO2) ejector. Performance maps for the proposed cycles are generated considering various operating parameters, ensuring a net positive power output. The implementation of the s-CO2 ejector enhances the system COP by up to 35%, increases the milk handling capacity by 20%, and reduces the irreversibility losses by 24%.
Finally, the insights and findings from this research are used to design and fabricate an s-CO2 ejector and then assemble it at the Supercritical CO2 jet facility in the Laboratory for Hypersonic and Shock Wave Research (LHSR) at the Indian Institute of Science (IISc) to carry out the further work. | en_US |
