Experimental and Numerical Investigations of Flow Patterns at Low Volume Flow Rate Conditions in Low Pressure Steam Turbine

Abstract

Abstract This study investigates the flow patterns at the last stage exit of a low-pressure steam turbine under low volume flow conditions. The 0.33 scale 5-stage model steam turbine was used to measure turbine performance and flow patterns at the last stage exit. Numerical simulations were performed to verify the accuracy of flow pattern prediction under low volumetric flow conditions by using a mixing plane (MP), a multiple mixing plane (MMP), and a frozen rotor (Frozen). Equilibrium and nonequilibrium condensation analyses were also performed to investigate the effects of subcooling under low volume flow conditions. Numerical simulations showed good agreement with measured turbine efficiencies over a wide range of volume flow rates. A comparison of MP, MMP, and Frozen results under low volumetric flow conditions showed that the results were generally similar, but Frozen was the closest to the experimental results. Comparison of measured and equilibrium condensation analysis results for the flow pattern at the last stage exit under the low, middle, and high volumetric flow rates showed that the numerical results basically represent the experimental results well. However, the equilibrium condensation analysis results for the absolute swirl angle at middle and high volumetric flow conditions showed a discrepancy with the experimental results. The non-equilibrium condensation analysis confirmed that secondary nucleation occurred in the last stage rotating blade under middle and high volumetric flow conditions. On the other hand, no secondary nucleation was observed in the last stage rotating blade in the non-equilibrium condensation analysis for the low volume flow condition. The results show good agreement between the numerical predictions and the experimental data, highlighting the importance of considering non-equilibrium condensation effects under design conditions, but also emphasizing that such effects do not need to be considered under low volume flow conditions because of the presence of backflow at the last stage outlet.