The present paper has developed a mathematical separated flow model for annular upward flow in vertical narrow rectangular channels to predict the critical heat flux. The theoretical model is based on fundamental conservation principles: the mass, momentum, and energy conservation equation of liquid film and the momentum conservation equation of vapor core together with a set of closure relationships (such as entrainment rate, deposition rate, interfacial shear stress and initial entrainment fraction at the onset of annular flow). The predicted results are compared with the experimental data and fairly good agreement between them is achieved. By numerically solving the equations, liquid film thickness, liquid film velocity in the liquid film and heat transfer coefficient are obtained. With the applications of the present model, the critical heat flux in the rectangular channel is calculated and analyzed.

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