Polydispersed droplet spectrum and exergy analysis in wet steam flows using method of moments

In steam turbine flow, the complex droplet spectrum caused by nonequilibrium condensation is necessary to be modeled accurately to predict the droplet behavior and estimate the exergy destruction and erosion rate. This study built and validated a polydispersed model with Quadrature method of moments (QMOM), consisting of transition SST model, the moments and entropy generation. A spline-based algorithm was used to reconstruct the shape of the probability density function (PDF) of radius. It's proved that the polydispersed model has a better prediction result for Sauter radius compare with monodispersed model. Then, the distributions of moments and droplet spectra in the nozzle with effects of asymmetric lambda shock and evaporation were investigated. The shape of droplet spectrum is closer to gamma distribution in nucleation zone and log-normal distribution in growth zone when outflow is supersonic. In the turbine, because the oblique shock induces complex evaporation and secondary condensation, the reconstructed shape is closer to gamma distribution. Finally, the obtained maximum exergy destruction is 25.293 kJ/kg. The rate of exergy destruction increases from 1.04% to 4.45%. The range of Baumann factor is 0.574-1.312. Besides, the erosion rate in polydispersed model is only 58.4-64.3% of monodispersed model. The polydispersed model used in this study can predict the droplet spectrum and energy loss of the turbine systems more accurately.

Automated summary

This study utilized a polydispersed model to accurately predict droplet spectrum and energy loss, showing that it has better prediction results for Sauter radius compared to monodispersed model. The distribution characteristics of droplet spectrum and exergy destruction in nozzle and turbine under different conditions were investigated.