Single phase turbulent mixing in simulated rod bundle geometries

Single phase air and water turbulent mixing rates, between two square array subchannels simulating parallel flow areas in a reactor rod bundle array, were measured employing tracer techniques. Mixing rates increased with Reynolds number over the range 1,300 to 38,000. Variation of the interconnection gap spacing from 0.015 to 0.080 in. indicated that the mixing mechanism was both gap spacing and Reynolds number dependent. At low Reynolds number and low gap spacings, molecular diffusion plays an important role in the mixing mechanism. However generation and penetration of secondary flows are believed to be an important mode of mixing in the intermediate Reynolds number range – the largest secondary flows and minimum penetration being encountered for the lowest pitch to rod diameter ratios.