Flow behavior of laval nozzle sets in steam turbine governing stage at low loads

Abstract

To achieve carbon neutrality, thermal power units (TPU) will be deeply involved in peak regulation, which is crucial for the development of renewable energy generation. However, the efficiency of TPU is significantly reduced at low loads. The main reason for this is that the convergent nozzles in conventional TPU are difficult to efficiently expand steam with large pressure differentials (i.e., pressure ratios below a critical value), and thus the unit is designed to operate in sliding-pressure mode, resulting in a low Rankine cycle efficiency. To reveal the mechanism of energy loss in the governing stage under peak shaving conditions, the flowing behavior of governing stage nozzle sets of a 330 MW steam turbine is investigated by CFD simulation in this study. The results indicated that the shock waves generated by the supercritical streamflow and the angle deviation at the exit of nozzle sets are the main factors leading to energy efficiency degradation. Based on this, two supersonic Laval nozzles were designed and analyzed in the 10 %∼40 % load range. Compared with conventional nozzles, the designed supersonic nozzles I and II showed average efficiency improvements of 4.9 % and 5.8 %, respectively, and maximum efficiencies of 6.6 % and 9.5 %, respectively. The research in this paper can provide theoretical support for the design and development of nozzle sets under deep peaking conditions, which can help achieve carbon neutrality.