In this work, we use the lattice Boltzmann method (LBM) to simulate High Rayleigh number natural convection flows in a cavity filled with air and heated from below. One of the vertical walls has a cold portion, while the other walls are adiabatic. This cavity simulates a living space with under-floor heating and fitted with a glass door (single glass). Computations are made in two dimensions for high values of Rayleigh number. The model uses the double populations approach to simulate hydrodynamic and thermal fields. The traditional LBM with a non-uniform grid has high grid requirements at higher Rayleigh number values. The results are presented in terms of streamlines and isotherms. Variation of the local Nusselt number on the walls is shown and discussed. Global heat transfer has been studied in terms of average Nusselt number which is correlated with Rayleigh number. Dimensionless profiles of temperature and velocity along the vertical and horizontal centerline are illustrated.

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The convective instability of a horizontal fluid layer subject to a time varying gradient of temperature is investigated. The stationary component of the temperature gradient is considered equal to zero and the oscillating components imposed on the horizontal boundaries are in phase and with the same amplitude. The aim of the present paper is to examine the effect of this type of modulation on the onset of convective instability. We show that unlike the case where the equilibrium configuration is stable in the absence of modulation, we have instability when the temperature at the horizontal boundaries is modulated in phase. Also, we observe that in the limit of low and high dimensionless frequency of modulation, ω < 0.5 and ω > 140, the basic state tends to a stable equilibrium configuration and for an intermediate dimensionless frequency, the system is potentially unstable. The results obtained from analytical asymptotic study for low and high dimensionless frequency are in good agreement with the numerical ones.

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In this numerical study, the evaporative heat and mass transfer of a turbulent falling liquid film in a finite vertical tube are investigated. The liquid film flows in the tube's inner wall, whose outer wall is partially subject to thermal flux. Here, different configurations corresponding to thermal flux imposed on different external surface wall percentages are examined. External face zones where the heat flux is not applied are maintained insulated. The nonlinear set of parabolic mass, momentum, energy, and mass fraction conservation equations combined with boundary and interfacial conditions are treated numerically using implicit finite difference procedure. For falling liquid film analysis, an adapted Van Driest turbulence model is used. For the present work, it is supposed that gas flows in a laminar regime. We examine in this paper the impact of the percentage of heated surface area on flows as well as on heat and mass transfer. Obtained results for a partially heated wall are compared with those produced for an entirely heated wall.