The difference in turbulent diffusion between active scalar (heat) and passive scalar (mass) in a stable thermally stratified flow is investigated. The experiments are conducted in an unsheared thermally stratified water flow downstream of turbulence-generating grid. Passive mass is released into the thermally stratified flow from a point source located at 60mm downstream from the grid. Instantaneous streamwise and vertical velocities, temperature of active scalar and concentration of passive scalar are simultaneously measured using a combined technique with a two-component laser-Doppler velocimeter (LDV), a resistance thermometer and a laser induced fluorescence (LIF) method. Turbulent heat and mass fluxes and eddy diffusivities for both active heat and passive mass are estimated.
The results show that stable stratification causes the large difference in eddy diffusivities between active heat and passive mass. The difference suggests that an assumption of identical eddy diffusivity for active heat and passive mass, used in conventional turbulence models, gives significant errors in estimating heat and mass transfer in a plume under stably stratified conditions.