A model of Boussinesq convection in a fast rotating spherical layer with the free-rotating inner core is considered. To solve the problem, two sets of equations are used. First, we solve the original Navier-Stokes equation and thermal-flux equation on the coarse grid. The small-scale flow features are described in terms of the shell model technique. The influence of small scales on large-scales is taken into account by means of effective viscosity, which is calculated from the spectral energy flux defined in the shell model. The properties of the turbulent spectrum is considered.