Density variations due to changes in bulk chemistry in the lowermost mantle play an important role in the dynamics and chemistry of plumes. In this study we perform a series of high-resolution numerical experiments in an axisymmetric spherical shell to systematically investigate the formation of plumes from a thermochemical boundary layer and the entrainment of the dense material by plumes. In particular, we investigate the effects of the temperature dependence of viscosity, the chemical density contrast, and thickness of the compositionally distinct layer. Our results show that all three factors influence the nature of plume formation and entrainment of dense material. The complexity of the evolution of the plumes and of the entrainment process increases profoundly with increasing viscosity contrast. The entrainment commonly becomes a transient process for models with variable viscosity. It is generally stronger in the isoviscous models, but the development of plumes is easier in the models with variable viscosity, which is mainly determined by the mechanical coupling between the dense material and the surrounding mantle. Our model results indicate that the compositionally distinct region in D″ can survive in the Earth's history if the intrinsic density contrast of the compositionally distinct layer is greater than about 2%. We also present a series of benchmark tests for thermochemical convection problems in an axisymmetric spherical shell. Our tests indicate that high-resolution models are necessary for the thermochemical convection systems in which strongly temperature-dependent viscosity is considered.