Heat and Mass Transfer in Strong Stable Stratification.

Abstract

Heat and mass transfer mechanism in strong stable stratification was experimentally investigated in unsheared water flows downstream of turbulence-generation grids. Instantaneous velocity, temperature and concentration were simultaneously measured using a two-component laser doppler velocimeter (LDV), a resistance thermometer and a laser induced fluorescence technique. The results show that the difference between turbulent diffusions of active heat and passive mass in strongly stratified water flow appears in the high-frequency region, and that it generates a turbulent mass flux slightly larger than the heat flux. The countergradient heat and mass fluxes occur under strong stable stratification conditions, and the countergradient scalar transfer mechanism can be explained in terms of the balance between potential and turbulent kinetic energies. The countergradient scalar flux becomes larger with increasing Prandtl or Schmidt number of the active scalar.