CFD ANANLYSIS OF THE MATIS-H EXPERIMENTS ON THE TURBULENT FLOW STRUCTURES IN A 5x5 ROD BUNDLE WITH MIXING DEVICES

The measurements of a turbulent swirl flow in the subchannels of a 5 x 5 rod array having typical mixing devices at the MATIS-H (Measurements and Analysis of Turbulence In SubchannelsHorizontal) facility have been analysed using a commercial CFD code of STAR-CCM+ 4.02 with a unit grid model having no unmatched grid interface. The main objectives of this analysis is to establish a proper CFD analysis methodology to this kind of very complicated turbulent swirling flow in a subchannel since previous results show that a predicted turbulent swirl flow within 5 times of hydraulic diameter length from the mixing devices tip shows different results, which are dependent on the models chosen, such as the turbulent model, wall function model, the y+ value, and the numerical model for a convection term. In the MATIS-H experiment, the intrinsic features of turbulent flow along the elevation, which shows distinct characteristics very sensitively by depending on the types of the mixing devices (split and swirl types), were measured by 2-D LDA (Laser Doppler Anemometry) at the condition of Re=48,000 in the water loop being operated at 35C and 1.5bar. A series of sensitivity analyses shows that a very dense mesh distribution along the stream-wise direction around the mixing devices is recommended to use for resolving a drastic change of the turbulent intensity due to a strong swirl flow induced by the mixing devices. And it is also shown that a non-linear or secondorder closure model is required to adopt for properly predicting the anisotropic characteristics of the turbulent flow structures caused by the mixing devices in the sub-channel.