Single-Phase Crossflow Mixing in a Vertical Tube Bundle Geometry

An Experimental Study

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The vertical rod/tube bundle geometry has a wide variety of industrial applications. Typical examples are the core of light water nuclear reactors (LWR) and vertical tube steam generators. In the core of a LWR, primarily coolant flows upward but their also exist a flow in lateral direction, called crossflow, in the gaps in between the sub-channels. The radial transport of scalars and vapors (i.e., crossflow mixing) due to crossflow has an important bearing on the assessment of the probable fuel rods damage. Motivated by this safety aspect of a LWR, the aim of the present study is to enhance the current understanding of crossflow mixing in a vertical tube and to develop a mixing model. An experimental study, supported by a few numerical simulations, was performed to identify different fluid flow patterns in a vertical tube bundle/similar geometry and the contribution of these patterns towards the lateral inter channel transport of a passive scalar. The experiments were performed in water at isothermal, single-phase flow and ambient operating conditions for Reynolds numbers ranging from approximately 900 to 22,000. The experimental results show the existence of counter rotating coherent vortices adjacent to the channel-gap interface for all flow regimes, i.e., laminar, transitional and turbulent flows. These coherent vortices are identified as a major contributor towards the crossflow. Experiments on the radial mixing of a passive scalar induced by crossflow show that the coherent structures impart a significant contribution towards crossflow mixing for all Reynolds numbers. Based on the experimental results a continuous stirred tank (CST) based crossflow mixing model has been proposed for the full range of channel Reynolds numbers. The model predictions are found to be within a band of ±20% of the experimental values for the full range of channel Reynolds numbers.