CFD Analysis of Single Phase Counter-Current Buoyancy Driven Flows and its Applications to Passive Reactor Design

Increasingly, nuclear plants rely on natural circulation, for both fault conditions and / or normal power removal. Prediction of such buoyancy-driven flows is needed. However, their complex nature leads to 3D effects in ‘wide’ geometries, making prediction impossible with system codes. Even in slender “pipe-like” geometries countercurrent flow of hot and cold fluid makes a one-dimensional simulation totally misleading. However, simply moving to a three-dimensional CFD treatment is not sufficient. The strong anisotropy of the turbulence and the coexistence of various flow regimes make the choice of an appropriate turbulence model difficult. Countercurrent flow in a pipe might occur when the “natural” buoyant flow was of hot fluid up the pipe, but a feature such as a local heat-sink (an un-insulated valve in the pipe, perhaps) acts as a source of cold fluid, which attempts to flow down the pipe as a counter-current flow. On a different scale, counter current flow such as this would occur for example inside the secondary containment. This countercurrent flow problem captures the complexities of most buoyant flows, and this provides a challenging model problem. In this paper, we describe the design and preliminary analysis of an experimental rig being built to study this. Initial CFD and experimental results are presented.