应用于燃气轮机余热回收系统的超临界CO2径流式透平性能探究

Performance of Supercritical CO2 Radial Turbine for Gas Turbine Waste Heat Recovery System

关键词:  燃气轮机  超临界CO₂  径流式透平  叶顶间隙  轮背密封  气动性能

Key words:  Gas turbine  Supercritical CO₂  Radial inflow turbine  Tip clearance  Impeller back seal  Aerodynamic performance

摘要:

为研究应用于燃气轮机余热回收系统的超临界CO₂（SCO₂）径流式透平整机性能，基于RANS（雷诺时均）模型，对透平整机系统内部流场进行数值模拟，获得了SCO₂径流式透平整机在起动、升速、自持及额定工况下的运行情况，并针对出口压力变化分析了透平非设计工况下的性能。模拟结果表明：透平起动工况下会发生运行失稳，产生“倒吸”现象且通道内出现大面积的低速区域，但随转速增加流动趋于稳定；叶顶间隙的泄漏流会对透平的性能产生不利影响，额定工况下0.5 mm的间隙使透平减少近47 kW的功率输出，等熵效率降低6%左右；轮背密封可以起到良好密封作用，密封引入气还提高了透平整机的功率和效率；在偏离设计点工况运行时，透平可以有效应对出口压力的变化；通过对比简化通道和全周通道的仿真数据，透平轴功率和等熵效率的误差均在0.5%以内，可知应用简化通道模型仿真能够保证计算精度。因此，在SCO₂径流式透平的仿真与应用过程中，研究人员要注重透平低速起动工况下的运行，以及叶顶间隙对这种小尺寸高转速透平机性能的影响。

Abstract:

To investigate the performance of the supercritical CO₂ （SCO₂） radial inflow turbine （RIT） for gas turbine waste heat recovery， the internal flow fields of the RITs are simulated with RANS （Reynolds Averaged Navier-Stokes） model. The operating characteristics of the SCO₂ RITs under starting， speed-rising， self-sustaining and designed condition are studied. The off-design performance of RITs is also analysed through varying outlet pressure. The simulation results show that 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. However， the flow tends to be stable as the speed increases. The leakage flow from the tip clearance reduces the machine performance （0.5mm 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. Comparison between the simulations of the simplified passage and the full-circumference passage indicates the error of shaft power and isentropic efficiency is less than 0.5%， showing that the simplified passage has sufficient accuracy for CFD simulation. Therefore， in the process of simulation and application of SCO₂ RITs， researchers should pay attention to the operation of the turbine under low-speed starting conditions， and the effect of the tip clearance on the flow of the small-size and high-speed turbines.