Abstract: Reactor cavity cooling system (RCCS) is a competitive residual heat removal scheme for small modular reactors. In this paper, the RCCS has been simplified to a scenario that ascending air flow is got heated in vertical annular channels and driven by mixed convection. The mixed convection is a combination condition of forced convection (external-applied pressure difference) and natural convection (internal-generated buoyancy force). The scenario has been analyzed by numerical approach using SST (shear stress transport) k-ω turbulent model, which can obtain good accuracy at near-wall low Reynolds number region. Buoyancy number, which is defined as Gr/Re^3, increases with heat loading, and it represents the relative intensity of natural convection and forced convection. The result shows that, heat transfer in ascending turbulent mixed convection is impaired with respect to forced convection at moderate heat loadings, and enhanced at high heat loadings. Thus, mixed convection can be divided into forced convection dominant region, mixed convection impairment region and natural convection enhancement region. According to different aspect ratios of vertical channels, the most serious impairment occurs when buoyancy number equals to 1×10^-5. Additionally, the impairment in narrow channel is more serious than wide channel. The mixed convection heat transfer correlations are fitted based on Symolon’s correlation, which shows the best agreement with numerical analysis results in predicting the heat transfer. The mechanism of turbulent mixed convection phenomenon can be explained as turbulent laminarization and intensify, which is caused by non-uniformity of density and variation of gravitational body force. With increasing heat loading, non-uniform buoyancy effects cause marked distortion of the flow structure, with the velocity maximum moving from the core of flow to the near-wall region. Velocity profiles switch from inverted U-profile to M-profile, which is a typical profile of mixed convection. At the same time, Reynolds stress at near-wall region reduces, suppresses the production of turbulent kinetic energy, leads to laminarization and heat transfer reduction. The negative Reynolds stress generates from the velocity gradient of the M-shape profile in the core region, and increases with heat loading, provides extra turbulent kinetic energy. There are two peaks in the profile of turbulent kinetic energy, one is associated with the near-wall region and the other is associated with the inner flank of the M-profile, due to the negative turbulent shear stress. Turbulent kinetic energy and heat transfer coefficient are enhanced and increase with heat loading. Flow regime develop towards natural convection.