The fluid temperature field in channels with conducting walls and flow obstructions is elliptic in the streamwise direction. Traditionally, the channel length is artificially extended, and natural condition (∂φ / ∂ x = 0) is used as the outlet boundary condition. However, for the fluid temperature field the use of natural condition (type a) is not appropriate, as the wall conduction and flow obstructions facilitate information propagation in the upstream direction. An alternative boundary condition (type b) at the channel exit is proposed to simulate the fluid temperature field. In the proposed approach, the temperature rise of the fluid in the channel exit region is assumed to be constant. Using both type a and type b boundary conditions heat transfer in a channel with heat-generating blocks and conducting walls was studied. The results of the developing flaw problem were compared with the results of the periodically fully developed flow problem. Such a comparison clearly established the superiority of the proposed type b condition. © 1994 Taylor & Francis Group, LLC.