Abstract
In the dispersed flow film boiling regime (DFFB), which exists under post-LOCA (loss-of-coolant accident) conditions in pressurized water reactors (PWRs), there is a complex interplay between droplet dynamics and turbulence in the surrounding steam. Experiments have accredited particular significance to droplet collision with the spacer-grids and mixing vane structures and their consequent positive feedback to the heat transfer recorded in the immediate downstream vicinity. Enabled by high-performance computing (HPC) systems and a massively parallel finite element-based flow solver—PHASTA (Parallel Hierarchic Adaptive Stabilized Transient Analysis)—this work presents high fidelity interface capturing, two-phase, adiabatic simulations in a PWR sub-channel with spacer grids and mixing vanes. Selected flow conditions for the simulations are informed by the experimental data found in the literature, including the steam Reynolds number and collision Weber number (Wec={40,80}), and are characteristic of the DFFB regime. Data were collected from the simulations at an unprecedented resolution, which provides detailed insights into the continuous phase turbulence statistics, highlighting the effects of the presence of droplets and the comparative effect of different Weber numbers on turbulence in the surrounding steam. Further, axial evolution of droplet dynamics was analyzed through cross-sectionally averaged quantities, including droplet volume, surface area and Sauter mean diameter (SMD). The downstream SMD values agree well with the existing empirical correlations for the selected range of Wec. The high-resolution data repository from the simulations herein is expected to be of significance to guide model development for system-level thermal hydraulic codes.
Highlights
The mesh for the single-phase simulation was split into 16,384 partitions and run on 128 nodes, while that for the two-phase simulations was split into 131,072 partitions and run on 2048 nodes
The simulations are performed in a typical Pressurized water reactors (PWRs) sub-channel with spacer-grids and mixing vane structures, identified as the dominant structures governing the overall thermal hydraulic feedback of a PWR core in the post-LOCA, dispersed flow film boiling (DFFB) regime
Data are collected from the simulations at a high spatial and temporal resolution, which allows insight into hitherto unexplored turbulence and two-phase statistics
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The heat transfer increase across spacers has been empirically correlated with the observed decrease in the Sauter mean diameter (SMD) of the droplets [17], which provides a statistical measure to characterize their morphology and is accounted for in STH codes [18,19] It was noted by Hochreiter et al [16], that the aforementioned models for the DFFB regime have considerable implicit measurement uncertainty and model uncertainty related to their implementation in STH codes. Axial evolution of the droplet dynamics was analyzed through droplet volume, surface area and Sauter mean diameter and compared with existing empirical correlations for all Weber number cases Both instantaneous and time-averaged data from the simulations were archived and are intended to be used for training machine-learning-based turbulence or STH closure models
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