A. Serizawa

S." Satake

T. Kunugi

Y. "Yamamoto

National Institute for Fusion Science (NIFS-Repository)

§38. Buoyancy Effect of High Prandtl Free-surface Turbulent Flow Yamamoto, Y., Kunugi, T., Serizawa, A. (Kyoto Univ.), Satake, S. (Toyama Univ.) In this study, Direct Numerical Simulations (DNS) were employed to investigate the buoyancy effects on the heat transfer and turbulent structure in high Prandtl number turbulent flows simulated the liquid wall concept in nuclear fusion applications. Numerical simulations were conducted for two cases in the stable or neutral stratified open-channel flow. Governing equations in this study were Navier-Stoke equations used the Boussinesq approximation, continuity equation and the energy equation and numerical procedure was base on Yamamoto et al1. Computational domain is 6.4 h, h, 3 .2h for stream, vertical and spanwise directions respectively, where h denotes flow depth. The computations were carried out with 8585216 grid points (256 X 131 X 256, in stream, vertical and spanwise directions ) for the Reynolds number: u, hi v is 200 in both cases and Richardson number: Ri= 8 g L1 Thl u2 , is 27.6 in the stable case, where u, is the friction velocity, vis the kinetic viscosity, 8 is the thermal coefficient of volumetric expansion, g is the gravitational force, L1 Tis the temperature difference between the free-surface temperature ( T surface) and the bottom wall temperature (Twau), receptively. Boundary conditions for velocity fields is periodic condition in stream and spanwise directions, no-slip condition at the bottom wall, free-slip condition at the free-surface, respectively. For the temperature fields, periodic condition in stream and spanwise directions and constant temperature at the free-surface and the bottom wall (in this study, Tsurface > Twall) is imposed, respectively. Figure I shows the mean temperature: 8 + and turbulent intensity: e + profiles, where r ( =h +- y) is the distance from the free-surface. Near free-surface, the conductive sublayer, where mean scalar profile is follows to the formula; 8 +=Pr r, exists and mean temperature profile in Pr=5, stable case is laminarized compared with neutral cases, because the stable stratification constrained the near free-surface turbulent motion. On the other hands, the scale and peak of the turbulent intensity in stable case is larger and wider than the neutral cases. This is why that high temperature fragments which scale 1:1 +is from 5 to 30 are existed in high Pr fluid (Fig.3) and buoyancy acts on these fragments. But the turbulent motion by the buoyancy is opposite to the heat transfer direction. As the results, heat transfer was constrained and turbulent intensity was increased near free-surface. Figure 2 shows this buoyancy effects on the velocity fields. In stable case, vertical turbulent intensity is bigger than neutral case at the high wave number region corresponded to the high temperature fragments scale. 370 References 1) Yamamoto, Y. et al., 8th European Turbulence Conference (2000) 231. + 0 qc)- 10 10 let r+=h+-y+ Figure 1. Mean scalar and fluctuation profiles near free-surface --Euu --Evv - - Eww --Euu Fig.2-2 lD energy spectra (Near surface, Spanwise,Stable) Fig.3 Instantaneous temperature field Side view O.l(Blue)< 6 *<0.9(Red) 

Buoyancy Effect of High Prandtl Freesurface Turbulent Flow

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)