Simulations of flow and scalar transport in stirred tanks operated in transitional and mildly turbulent regimes (Re = 3000-12000) are presented. The moderate Reynolds numbers allow the flow to be simulated directly, without the use of turbulence closure or subgrid-scale models. The Newtonian liquids that are blended have different densities and/or viscosities, and the emphasis is on how these differences affect mixing times. The density difference is characterized by a Richardson number (Ri) that varies in the range of 0-0.5. The kinematic viscosity ratio is between 1 and 4. The results show that mixing times increase steeply with increasing Ri and that changing the tank layout can partly mitigate this effect. The viscosity ratio has a much weaker influence on the mixing time.