Numerical study on the effect of grid mixing span in 2 × 1 spacer grid

Spacer grid or Mixing Vane Grid (MVG) is recognized as an important component that could greatly improve Critical Heat Flux (CHF) performance in the fuel assembly. The temperature around the rod would be more uniform, bubbles around the rod wall would be broken off or driven to the center of the subchannel by the mixing effects produced by spacer grids. This would potentially result in a higher CHF value. However, as the grid span increases, the mixing effects will decay as the coolant travels further away from the mixing vane grid. The local thermal hydraulic conditions become severe as the heated length increases. As a result, from the decay of the mixing function, CHF usually occurs at immediate upstream of spacer grid, or the furthest distance from the MVG upstream. On the other hand, with the narrowing of grid span, the pressure drop increases with increasing hydraulic resistance. Therefore, a proper grid span is quite important in the fuel assembly design. In this paper, the effects of grid span were studied by applying Computational Fluid Dynamics (CFD) simulation. The flow field with 2 × 1 spacer grids were simulated instead of prototype full scale spacer grid. CFD was validated by the experimental data from XJTU-ANFR low pressure and low temperature test loop. Flow field with different length of grid span were modeled to investigate the fundamental mechanism on how key factors, such as “mixing performance”, are impacted by the length of grid span. In order to evaluate the mixing performance, several popular mixing indexes, including Secondary Flow (SF), and thermal non-uniformity, were introduced to evaluate the effects of grid span on the mixing performance. Effective Mixing Length (EML) was defined by considering the effect of mixing and temperature distributions. Grid Span (GS) function was proposed to take various thermal hydraulic indexes into consideration in order to address the issue of local condition dependency and to obtain proper grid span. Different grid design thermal hydraulic indexes have different weighting factors, in such, the optimal grid span functions may not be the same for different fuel assemblies.