Abstract: Based on the theory of bubble coalescence in the wall bubbly layer, for a deeper understanding of departure from nucleate boiling (DNB) in two-phase flow boiling under near-critical pressures, critical heat flux (CHF) of DNB type in a vertical upward rifled tube was numerically studied. Some improvements of the existing bubble coalescence model were proposed for the rifled tube. Meanwhile, the limiting transverse interchange of mass flux crossing the interface of the bubbly layer and core region, the turbulence velocity distribution, critical void fraction of the bubbly layer and other constitutive relations were determined. The influence of the bubble contact angle was taken into account in the detached bubble diameter calculation. The new α_b empirical correlation was proposed through fitting a large number of CHF experimental data. Finally, based on Fortran language, the theoretical prediction numerical model of CHF was developed. The effects of pressure, mass flux, equilibrium quality and inlet subcooling on CHF were analyzed. The model was evaluated by the values of CHF Lookup Table. And the comparison between the predictions by Bowring model, Katto model, Shah model as well as the present CHF model and the experimental CHF data shows the best agreement of the proposed model, indicating that the proposed model can predict CHF of DNB type well for the vertical upward rifled tube.