  A series of numerical simulations of Rayleigh-B{\'e}nard convection in a
cubic cavity are conducted in order to examine the structure of the thermal
boundary layer in case of mixed boundary conditions. The main goal of the study
is the physical mechanism which provides increasing of heat flux with spatial
frequency of conducting-adiabatic pattern. Different spatial configuration of
conducting plates, including the fractal one, are considered for Rayleigh
numbers from $\Ray=10^{7}$ to $\Ray=2.0\times 10^{9}$. We have shown that the
temperature boundary layer in case of mixed boundary conditions at the bottom
is strongly non-uniform. This non-homogeneity is a result of several factors
such as conducting-adiabatic pattern, large-scale circulation and small-scale
motions over conducting plates. The thickness of the thermal boundary layer
strongly depends on the size of the conducting plates and can be substantially
smaller than for a classical Rayleigh-B{\'e}nard convection. This effect
increases the heat flux with decreasing the size of hot plates, which
corresponds to the increasing of spatial frequency of conducting-adiabatic
pattern.
