Direct numerical analysis of turbulent mixing in a thermally stratified flows; Turbulent mixing in thermally stratified flows

Doi, Yoshihiro; Muramatsu, Toshiharu

Thermal stratification phenomena are observed in an upper plenum of liquid metal fast breeder reactors (LMFBRs) under reactor scram conditions, which give rise to thermal stress on in-vessel structural components. Therefore it is important to evaluate chracteristics of the phenomena in the design of components in an LMFBR plenum. The phenomena are a stable stratified flow and shear layers are formed due to the velocity difference between upper and lower flows. Previous many experiments and numerical simulations confirmed the exist of the layer dominated by large, spanwise coherent vortex structures and the structures had important effect on mixing, In this study, to evaluate numerical models and constants used in the model for the thermal stratification phenomena, numerical analyses were carried out for a shear flow water test in rectangular duct. In the analyses, a direct numerical simultion code was used. The numerical results could indicate the large spanwise coherent structures with the growing and the pairing of vortexes. The analyses for different Richardson (Ri) number conditions were carried out and then calculated distributions of time-averaged velocities, velocity fluctuations, Reynolds stresses, time-averaged temperatures and temperature fluctuations were compared with the measured results. Through the comparisons, the calculated time-averaged velocities and velocity fluctuations in the main flow direction were agreed well with measured value and the velocity fluctuations decreased with increasing of the Ri number. Though calculated time-averaged temperature distributions and temperature fluctuations had three different temperature gradient regions, those were not found in the measured value. The region of Ri numbers observed the vortexes pairing is in good agreement with the calculated results and the vortexes pairing would cause large Reynolds stresses.