The oxidation of cyclohexane is an important route in the chemical industries for producing cyclohexanol, cyclohexanone, plasticizers, surfactants and so on. However, it will be in danger of explosion once the ratio of cyclohexane to oxygen reaches the explosion limits. Therefore, it is necessary to research the distribution of temperature and density in Rayleigh-Bénard convection of cyclohexane/oxygen mixture to avoid the potential hazard of deflagration. In present work, a series of three-dimensional numerical simulations on Rayleigh-Bénard convection of cyclohexane/oxygen mixture near the maximum density in a cubic cavity were carried out. The cubic cavity with an aspect ratio of 2 was heated from the bottom wall and cooled at the top wall. The side walls are assumed to be perfectly adiabatic. The Prandtl number of the cyclohexane/oxygen mixture is 0.71 and Rayleigh number ranges from 2×103 to 6×105.Results show that the flow transits from static heat conduction to three-dimensional steady convection, and then to oscillatory convection with the increase of the Rayleigh number. The density inversion phenomenon has a significant effect on the Rayleigh-Bénard convection of cyclohexane/oxygen mixture. With the increase of the density inversion parameter, the location of the maximum density gradually moves to the lower region of the cavity, which leads to a thinner fluid layer below the density maximum and a thicker fluid layer restraining convection above the maximum density. The critical Rayleigh number for the onset of steady convection and for the oscillatory convection shows an upward tendency with increasing density inversion parameter. When the density inversion parameter is relatively small, the flow occurs in the whole cavity and the deformation of isotherms is violent. When the density inversion parameter is large, the isotherms are almost parallel and the fluid layer is static in the region near the top wall; induced by Rayleigh-Bénard convection rolls in the fluid layer below the maximum density, the secondary flow rolls with opposite rotation direction are formed in the fluid layer above the maximum density. The bifurcation sequences and stability ranges of the flow patterns are obtained at various density inversion parameters, which are totally different from those of common fluids. It is found that the multiple flow patterns from the Rayleigh-Bénard convection of cyclohexane/oxygen mixture can coexist at the same Rayleigh number. Except for the Rayleigh number and density inversion parameter, the flow patterns and their evolutions depend on the initial condition and flow history as well. Furthermore, the transition hysteresis phenomenon is also discovered for Rayleigh-Bénard convection of cyclohexane/oxygen mixture near the maximum density. The heat transfer ability is enhanced with the increase of Rayleigh number, while it weakens along with the density inversion parameter. Different flow patterns suggest different heat transfer abilities at each density inversion parameter. Besides, the local maximum heat transfer rates appear where the cold fluid descends toward the bottom wall, while the local minimum value exists where the hot fluid ascends from the bottom wall. When the Rayleigh number exceeds a certain critical value, the flow becomes unsteady and behaves in a periodic oscillatory behavior. With the increase of the Rayleigh number, the oscillation gets more apparent and the oscillation frequency is increased.

Three-Dimensional Numerical Simulation of Rayleigh-Bénard Convection of Cyclohexane-Oxygen Mixture Near Its Maximum Density in a Cubic Cavity

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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