This study investigates the applicability of an Euler-Lagrange approach for the calculation of nucleation and condensation of steam flows as they occur in turbomachinery applications, e.g. steam turbines. Compared to pure gas flows, condensing steam flows have two main effects to consider. First, the latent heat release due to condensation leads to a temperature and pressure rise. Second, droplets impacting the turbine blading can cause droplet erosion, which strongly impairs the aerodynamics and performance. In this study, supersonic nozzles are used as generic validation cases, as their high expansion rates replicate the flow conditions in real turbines. Experimental and numerical validation data for these nozzles as well as meshes for the simulations are provided by the International Wet Steam Modeling Project of Starzmann et al. (2018). Most participants of that project used an Euler-Euler approach to model the multiphase flow. In contrast, this study utilizes an Euler-Lagrange approach, which models gas as a continuum and droplets as discrete particles. The main advantage of Euler-Lagrange over Euler-Euler is the calculation of individual trajectories for the droplets. This is an important prerequisite for a planned numerical investigation of droplet erosion. A further advantage of the Euler-Lagrange approach over other methods is the full resolution of the droplet size distribution. Therefore, the classic nucleation theory with corrections and different droplet growth laws are incorporated into the Discrete Particle Model of ANSYS Fluent. The parallel computing efficiency is only slightly reduced compared to simulations without the multiphase models. However, the computationally expensive calculations of nucleation and condensation lead to an approximately sixfold longer runtime. The spatial discretization shows no higher requirements for Euler-Lagrange than for a single phase simulation. Considerable effort was put into an efficient implementation of Euler-Lagrange and according advice is presented in this study. The Euler-Lagrange results show a good agreement with the Euler-Euler results of the aforementioned International Wet Steam Modeling Project. Deviations are mostly related to the more accurate droplet size distributions of the former. Furthermore, the sensitivities of the Euler-Lagrange approach to modelling parameters are analysed. These parameters include corrections to the classical nucleation theory, different growth laws and variations of the material properties. Finally, an optimal parameter set for the calculation of nucleation and condensation is suggested. In conclusion, Euler-Lagrange is a slightly costly, but feasible approach to model nucleation and condensation of droplets in vapour flows.


The Feasibility of an Euler-Lagrange Approach for the Modelling of Wet Steam

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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