The characteristics of the flow and convective heat transfer on the conditions of both developed laminar and turbulent flow in bilaterally heat narrow annuli were investigated. Investigations show that the ratio of heating flux at the inner wall to that at the outer wall, and the gap size of narrow annuli as well, have great influences on the heat transfer characteristics during flowing in bilaterally heated narrow annuli. Under laminar flow condition, the decreases of the gap size will lead to the heat transfer deterioration both on the inner and outer walls. However with respect to the case of the turbulent flow conditions, it is quite different. The decrease of the gap size will yield heat transfer deterioration on the inner wall, but it will enhance the heat transfer coefficient on the outer wall. In addition, whether the convective heat transfer is enhanced or reduced depends on the combination effect of the heat-flux ratio and the Reynolds number. The decrease of the gap size will reduce the convective heat transfer when the heat-flux ratio is smaller, and enhance the heat transfer coefficient when the heat-flux ratio becomes greater under a certain Reynolds number. Predicted friction coefficients and heat transfer coefficients were compared to experimental data as well, and they were found to be in good agreement. (c) 2006 Elsevier Ltd. All rights reserved.