URANS simulation of the turbulent flow in a tight lattice: Effect of the pitch to diameter ratio

The turbulent flow in 37 rod bundles is investigated with CFD code FLUENT. The calculation results were validated with experimental data at first. Then the effects of diminishing rod–rod gap size on local flow, heat transfer and coherent structure in tight lattice are analyzed. The critical pitch to diameter ratio is next to 1.03. As the pitch to diameter ratio decreases from a high level to 1.03, the coherent structure and flow oscillation increase gradually. However, as the gap size continues decreasing, the coherent structure and flow oscillation decay very quickly. In the very narrow gap, the coherent structure is so weak that it could be neglected. The oscillations of velocity, wall shear stress and wall temperature are also very weak. But the spatial variation of the parameters in this gap is very significant.

► The effects of diminishing rod-rod gap size on local flow, heat transfer and coherent structure in tight lattice are analyzed.
► The critical pitch to diameter ratio is next to 1.03.
► In the very narrow gap, the coherent structure is so weak that it could be neglected.