A flow regime map for determination of the boundary between forced and mixed convection in a horizontal circular straight tube with reentrant, square-edged, and bell-mouth inlets under uniform wall heat flux boundary condition is developed. The flow regime map is applicable to all flow regimes (laminar, transition, turbulent). From the flow regime map, for any forced flow represented by a given Reynolds number, the value of the parameter GrPr at a particular x D location indicates whether it is necessary to consider bouyancy effects. For the identified pure forced or mixed convection heat transfer regime, a heat transfer correlation is recommended. © 1995.