Influence of spacer grids and mixing vanes on cross exchange between sub channels and heat transfer capabilities
Spacers are used to fix the position of the fuel rod. In addition, they affect the flow of the coolant. In case of an accident boiling might occur. By suitable design of the spacers, the turbulence in the channel can be fanned and the heat transfer behaviour from the fuel element surface is improved. For the setup four neighbouring channels were simulated considering periodic boundary conditions in lateral direction. In axial flow direction the section between two spacers was considered. The corresponding mesh consisted of ca. 3.8 Mio nodes.
Of particular interest in this regard is the delay of the onset of conditions for critical heat flux. Another phenomenon of interest is the cross exchange of flow between the subchannels. As a result, non-uniformities in the energy release of the reactor core can be compensated to some extent.
Fig. 2a shows the velocity field and Fig. 2b the vorticity to characterize the flow field. The calculated difference pressure over the length shows the correctness of the simulation. The cross flow can be compared to corresponding correlations. The simulations show the capabilities of actual CFD to contribute to spacer grid design.