In this study, viscous heating effect on convective flow in an unheated adiabatic duct is studied analytically and numerically. Two different geometries are considered: circular duct and plane duct between two parallel plates. Two new parameters are defined for internal convection studies: adiabatic wall temperature and adiabatic bulk temperature. Variations of these two parameters with varying intensity of viscous dissipation effect are determined. In view of the results obtained, usage of the constant wall temperature thermal boundary condition when viscous dissipation is included is discussed and questioned. (C) 2010 Elsevier Ltd. All rights reserved.