Study On The Effect Of CO₂-based Mixtures On The Performance Of Supercritical CO₂ Radial-inflow Turbine

Recently,the supercritical CO2(sCO2)power cycle has been foreseen as one of the hot spots in the field of energy conversion due to its Simple configuration,compactness,and high efficiency.How to realize the high efficiency of sCO2 Brayton cycle in engineering application has become the focus of research.It can improve the performance of a single cycle component such as turbine,compressor,etc.,to improve the overall cycle efficiency.Moreover,from the thermodynamic point of view,the sCO2 power cycle can improve the efficiency by changing the working fluid properties.For example,we can change the physical properties of sCO2 by mixing other working fluids,which leads to the idea of the mixture Brayton cycle.This study focuses on the research of the sCO2 Brayton cycle component,and CFD simulation method was used to simulate and verify the performance of sCO2 radial-inflow turbine(RIT).Then a 500kW RIT was designed and simulated to study the aerodynamic characteristics when it applies different CO2-based mixtures.In order to explore the turbine performance in the low-power sCO2 power generation cycle applied to industrial waste heat recovery,the small-size and high-speed RIT system is numerically simulated by using CO2 working fluid with real physical properties.For the RIT simulation results:the "inverse suction " phenomenon and a large area of slow-motion in the channel will occur under the starting conditions of the turbine,resulting in unstable operation,but the flow tends to be stable as the speed increases;the leakage flow from the tip clearance reduces the machine performance(0.5 mm clearance under designed condition reduces 47 kW turbine output power,and the isentropic efficiency is reduced by about 6%);the impeller back seal can play a good sealing role,and the intaken seal gas also improves the power-output and the efficiency of RIT to a certain level;when the RITs running on off-design conditions,the given turbine can effectively cope with changes in outlet pressure.Therefore,in the process of simulation and application of sCO2 RIT,we should pay attention to the operation of the turbine under lowspeed starting conditions,and the effect of the tip clearance on the flow of the small-size and high-speed turbomachinery.The compressor can be close to the critical point under any working conditions by changing the critical point of the working fluid so that the sCO2 power cycle can improve the operating efficiency.The utilization of additives to change the working fluid’s properties has been proven to be an effective way to improve the sCO2 power cycle efficiency.As one of the core components of the system,the influence of CO2-based mixtures on turbine performance needs to be further explored.In this study,the preliminary design and three-dimensional numerical simulation of a 500 kW radial-inflow turbine(RIT)for small-scale sCO2 power systems were carried out.Furthermore,the performance of high Reynolds number and small size turbine under the change of the CO2-based binary mixture compositions and mixing ratios were studied.Increasing the amount of nitrogen,oxygen,or helium into CO2 has a negative effect on the RIT performance,and the appropriate amount of xenon or krypton can improve the turbine efficiency.Moreover,mixtures with higher krypton additions adapt to higher heat source conditions.The loss of the turbine stage passage shows that a large amount of helium greatly reduces the working fluid density,and the high amount of xenon has a great influence on the dynamic viscosity,which all makes the RIT operation deviate from the steady state.Therefore,the CFD model simulation fails indicating that RIT designed based on pure CO2 may not run smoothly and continuously.The losses in the stage with pure CO2 and CO2-Kr mixture were investigated.The results indicate that the losses originated from the stator cannot be ignored and that the improvement of efficiency is mainly owed to the reduction in clearance losses.There is no doubt that the viewpoints proposed in this paper have significant reference value for the practical application of the sCO2 power cycle and the mixture power cycle.