  Turbulent thermal diffusion is a combined effect of the temperature
stratified turbulence and inertia of small particles. It causes the appearance
of a non-diffusive turbulent flux of particles in the direction of the
turbulent heat flux. This non-diffusive turbulent flux of particles is
proportional to the product of the mean particle number density and the
effective velocity of inertial particles. The theory of this effect has been
previously developed only for small temperature gradients and small Stokes
numbers (Phys. Rev. Lett. {\bf 76}, 224, 1996). In this study a generalized
theory of turbulent thermal diffusion for arbitrary temperature gradients and
Stokes numbers has been developed. The laboratory experiments in the
oscillating grid turbulence and in the multi-fan produced turbulence have been
performed to validate the theory of turbulent thermal diffusion in strongly
stratified turbulent flows. It has been shown that the ratio of the effective
velocity of inertial particles to the characteristic vertical turbulent
velocity for large Reynolds numbers is less than 1. The effective velocity of
inertial particles as well as the effective coefficient of turbulent thermal
diffusion increase with Stokes numbers reaching the maximum at small Stokes
numbers and decreases for larger Stokes numbers. The effective coefficient of
turbulent thermal diffusion also decreases with the mean temperature gradient.
It has been demonstrated that the developed theory is in a good agreement with
the results of the laboratory experiments.
