Heat and mass transfer in a stable thermally stratified flow

Abstract

Heat and mass transfer mechanism in strong stable thermal-stratification is experimentally investigated in unsheared water flows downstream of turbulence-generation grids, where both active scalar (heat) with a Prandtl number of about six and passive scalar (mass) with a Schmidt number of about 600 are diffused. Instantaneous velocity, temperature and concentration are simultaneously measured using a laser-Doppler velocimeter, a resistance thermometer and a laser-induced fluorescence technique, and the turbulence quantities such as turbulent scalar fluxes, joint probability density functions and cospectra are calculated. The results show that the difference of turbulent diffusion between heat (active scalar) and mass (passive scalar) with different molecular diffusivities in thermally stratified water flows appears in the high-frequency region, and it results in a slightly larger turbulent mass flux than heat flux in strong stratification. The dissipation rate is rather different between heat and mass, and therefore the temperature fluctuation decays more rapidly than the concentration fluctuation. The counter-gradient scalar transfer occurs in strongly stably stratified conditions, and the counter-gradient transfer mechanism is explained from the relationship between buoyancy and turbulent motions. The counter-gradient scalar transfer is initiated by buoyancy-induced small-scale finger-like motions, and then the contribution of large-scale motions pushed back by buoyancy to the counter-gradient scalar transfer becomes dominant. The contributions of small- and large-scale motions in the present thermally stratified water flows are in contrast to the measurements in previously investigated thermally stratified air flows, where the counter-gradient heat transfer is generated mainly by large-scale motions.