Abstract: Based on the separated flow model, a mathematical model for annular upward flow in vertical narrow annuli with bilateral heating was developed. The theoretical model is based on the mass, momentum, and energy conservation equations of liquid films and the momentum conservation equation of the vapor core. The predicted results agree with the experimental data very well. By numerically solving the equations, the influences of curvature on the characteristics of annular upward flow, including liquid film thickness, temperature and velocity distribution in liquid films and critical heat flux, were obtained. The thickness of inner liquid film decreases with an increase in curvature, while the thickness of outer liquid film increases with curvature. When the dryout point occurs on the inner tube, the critical heat flux will increase with the decrease of curvature. And the case reverses when the dryout point occurs on the outer tube.