Three-dimensional numerical simulation of mixed convection of a cyclohexane/oxygen mixture in horizontal rectangular channels

A three-dimensional numerical simulation was conducted to investigate the mixed flow of the fluid with the density extremum in horizontal channels with varying aspect ratios from 2 to 10. The influences of the density inversion parameter ( Θ m = 0–0.7), Rayleigh ( Ra = 1.8 × 10 3 –8.2 × 10 4 ) and Reynolds ( Re = 15–100) numbers were systematically analyzed. The results show that density inversion enhances the stability of the mixed convection. With increasing Θ m , both the first critical Rayleigh number for the onset of secondary flow, and the second critical Rayleigh number associated with the destabilization of steady longitudinal rolls increase markedly, and the stabilizing effect becomes particularly pronounced when Θ m ≥ 0.5. At small Θ m , increasing Ra causes the primary longitudinal rolls to undergo splitting, leading to the formation of the smaller-scale rolls. In contrast, at large Θ m , density inversion induces a distinctly stratified flow structure, in which additional vortices form above the primary rolls together with the emergence of a stable stagnant layer, thereby restricting the vertical development of disturbances. Density inversion also significantly delays the onset of secondary flow and the establishment of longitudinal rolls. Additionally, an increase in Θ m thickens the upper stable fluid layer, suppressing the vertical development of thermal plumes, thereby weakening heat transfer ability and reducing the spanwise-averaged Nusselt number. Finally, from the simulation data, a heat transfer correlation in the fully developed region was derived.