UDC 519.46:533.375
I. I. RyzhkovThe group properties of the thermal-diffusion equations for a binary mixture in plane flow are studied. Optimal systems of first-and second-order subalgebras are constructed for the admissible Lie operator algebra, which is infinite-dimensional. Examples of the exact invariant solutions are given, which are found by solving ordinary differential equations. Exact solutions are found that describe thermal diffusion in an inclined layer with a free boundary and in a vertical layer in the presence of longitudinal temperature and concentration gradients. The effect of the thermal-diffusion parameter on the flow regime is studied.Introduction. Thermal diffusion is molecular transfer of material due to the presence of a temperature gradient in the medium (liquid solution or gas mixture). In the case of thermal diffusion, the components have different concentrations in the regions of elevated and decreased temperature. The presence of concentration gradients results in ordinary diffusion. A steady state is established when the diffusion and thermal-diffusion processes compensate for each other (i.e., the process of separation of the mixture components is compensated by the process of their mixing). In practice, a frequently occurring case is normal thermal diffusion, in which the heavier components move to the colder regions and the lighter components pass to the more heated regions. In some cases, there may be anomalous thermal diffusion, in which the direction of motion of the components is opposite.The present paper considers a model for the convective motion of a binary mixture taking into account thermal diffusion. The model is based on the Navier-Stokes equations supplemented by diffusion and heat-transfer equations. The Oberbeck-Boussinesq approximation, intended to describe convective flows under natural earth conditions, is used. It is assumed that the density of the mixture depends linearly on the temperature and concentration of the light component: ρ = ρ 0 (1 − β 1 T − β 2 C). Here ρ 0 is the density of the mixture for the average values of the temperature and concentration, T and C are small deviations from the average values, β 1 is the thermal-expansion coefficient of the mixture, and β 2 is the density concentration coefficient (β 2 > 0 since C is the concentration of the light component). The motion of the mixture is described by the system [1]where u is the velocity, p is the pressure deviation from the hydrostatic value, ν are χ are the kinematic viscosity and thermal diffusivity of the mixture, respectively, d is the diffusion coefficient, α is the thermal-diffusion parameter, and g is the free-fall acceleration. It is assumed that all characteristics of the medium are constant and correspond