Experimental and Numerical Investigations of the Scale Effect of the Nonequilibrium Steam in the Flow Patterns

Abstract

Abstract. This research examines the influence of nonequilibrium steam, causing thermodynamic losses due to supercooling and condensation, the major source of wetness losses in low-pressure steam turbines. The nonequilibrium state arises from rapid expansion, deviating from equilibrium phase change behavior, particularly in low-pressure stages with significant expansion and cooling. During development, verification tests using scaled-down model turbines are often conducted. However, few cases investigate the scale effect on performance and flow patterns, especially regarding nonequilibrium phase change phenomena. This research compares measurement results from a full-scale verification test turbine with equilibrium and nonequilibrium steam computational fluid dynamics (CFD) results for full-scale and scaled-down models. It aims to understand the scale effect of phase change around the low-pressure end stage on flow patterns. While scaling down dimensions, rotational speed is increased to maintain the same Mach number and pressure as the full-scale turbine. However, the steam expansion rate, influencing supercooling and nucleation, differs between full-scale and scaled-down models, leading to differences in thermodynamic losses and flow patterns where phase changes occur. Accurate prediction of nonequilibrium phase change is crucial for designing and optimizing low-pressure steam turbines. Understanding scale effects allows designers to account for differences between model and full-scale turbines, enabling more accurate performance predictions and improved designs. Insights from this research can contribute to developing advanced computational models and numerical techniques for simulating nonequilibrium phase change processes in turbomachinery.