The thermally developing turbulent lead bismuth ((PbBi55)-Bi-45) flow within a vertical annulus with the inner surface uniformly heated is experimentally investigated. The ratio between internal and external diameter is 0.136. The investigated Reynolds numbers vary between 1.48 x 10(4) and 2.37 x 10(5) and the applied heat flux from 135 kW/m(2) to 905 kW/m(2), covering both the forced and mixed convection regime. For forced convection, the local heat transfer convective coefficient is evaluated and compared with literature correlations for low Prandtl number fluids. Additionally, a new empirical relation for the local Nusselt number in the thermal entry region is proposed. The applicability of criteria addressing the forced to mixed convection transition in medium to high Prandtl number fluids is reviewed and extended to account also for low Prandtl number fluids such as heavy liquid metals. Here, the transition from turbulent-diffusion to molecular-diffusion dominated mixed convection is clearly shown. (C) 2012 Elsevier Ltd. All rights reserved.