Single-phase turbulent mixing in rod bundles

In the subchannel analysis of rod bundles, the mass exchange between adjacent subchannels is quantified by mixing coefficients, or mixing factors. This process of turbulent cross-flow mixing was found to be related to the phenomenon of quasi-periodic “flow pulsations” across the gaps between adjacent subchannels. By means of hot-wire measurements of spectra of the components of the turbulent fluctuating velocities in the gap regions and a phenomenological model for the flow pulsations, mixing factors were determined for several four-rod bundles with different aspect ratios (P/D, W/D). Comparison of the results with the literature shows good agreement. A correlation for the mixing factor as a function of the dimensionless gap width, obtained by linear regression of the new data, is proposed.