We model the effects of a dense boundary layer at the base of the mantle on the thermal structure of D'' and the dynamics of convection in the lower mantle. Using a finite element model of convection in 2-D with a Rayleigh number of 10^7, we include both thermal and compositional effects on the density, and investigate the effect of increasing thermal diffusivity in the dense layer to simulate enrichment in metals. A dense boundary layer tends to stabilize upwellings and decreases the heat flow across the lower boundary. Increasing the thermal diffusivity increases the temperature of upwelling plumes and also reduces their tendency to drift laterally. Convection within the dense layer can create short wavelength variations in temperature, whereas above the dense layer, temperature variations show much longer wavelength variations (1500 - 2000 km) which reflects the spacing of upwellings and downwellings. Increasing the thermal diffusivity in the layer tends to suppress the small scale convection.