Pool stratification and mixing induced by steam injection through spargers: CFD modelling of the PPOOLEX and PANDA experiments

Abstract

Spargers are multi-hole injection pipes used in Boiling Water Reactors (BWR) and Generation III/III+ Pressurized Water Reactors (PWR) to condense steam in large water pools. During the steam injection, high pool surface temperatures induced by thermal stratification can lead to higher containment pressures compared with completely mixed pool conditions, the former posing a threat for plant safety. The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were previously developed and validated for the modelling of a steam injection through blowdown pipes. The goal of this paper is to extend the EHS/EMS model capabilities towards steam injection through multi-hole spargers. The models are implemented in ANSYS Fluent 17.0 Computational Fluid Dynamics (CFD) code and calibrated against the spargers experiments performed in the PPOOLEX and PANDA facilities, analysed by the authors in [1] (Gallego-Marcos, I., et al., 2018). CFD modelling guidelines are established for the adequate simulation of the pool behaviour. A new correlation is proposed to model the turbulent production and dissipation caused by buoyancy. Sensitivity studies addressing the effect of different assumptions on the effective momentum magnitude, profile, angle and turbulence are presented. Calibration of the effective momentum showed an inverse proportionality to the sub-cooling. Differences between the effective momentum calibrated for PPOOLEX and PANDA are discussed. Analysis of the calculated flow above the cold stratified layer showed that the erosion of the layer is induced by the action of turbulence rather than mean shear flow.

Effective momentum induced by steam condensation in the oscillatory bubble regime

“…The EMS for the oscillatory bubble regime was defined in [11] as a function of the steam injection parameters and the non-dimensional coefficient given by…”

“…where is the frequency of collapse, the number of injection holes (implicit assumption of no changes in the momentum due to interaction between injection holes), and the sphere equivalent bubble radius. Sections 4.1-4.3 will be dedicated to the analysis of the SEF experiments to find closure correlations for equation (11).…”

“…It should be noted that the collapse phase at large subcoolings of some steam bubbles led to a steam-water cloud with no clear boundary between vapor and liquid (Figure 6). This behavior is not captured in equations ( 10) or (11), where it is assumed that the gas-liquid interface is a well-defined boundary.…”

“…Estimation of the Kelvin Impulse using equation (11) requires that the correlation for the time-dependent radius reaches zero upon complete collapse. In this work, the and coefficients could not be calibrated for the last stages of the collapse phase due to the large uncertainty of the bubble radius as a result of the steam-water cloud formed during the collapse (see Figure 5).…”

“…A set of experiments was carried out in the PPOOLEX and PANDA facilities to analyze the pool behaviour induced by this regime [10]. Simulation of the pool transients using the CFD code of ANSYS Fluent was later done in [11] to infer the effective heat and momentum sources, which could not be directly measured in the experiments. Parametric studies done in [11] showed that the uncertainty in other injection parameters specific to the sparger (such as the injection angle, azimuthal profile of the momentum, and induced turbulence source) could affect the estimated momentum sources.…”