A numerical study of horizontal convection in rectangular cavities of aspect ratio 2 with lower active wall was performed, for Rayleigh numbers from 10⁴ to 10⁷ with air as the test fluid. An in-house code for the SIMPLER method was used. Two forms of thermal condition at the active wall were used: perfectly conducting wall and step temperature change. The flow pattern in horizontal convection with both thermal conditions consists of a closed circulation, which is very intense in the lower half of the cavity. Although in convective regime a boundary layer forms on the active surface, steady state Nusselt numbers in this mode of natural convection scale as Ra^0.2, a very low exponent, giving heat transfer rates about 50% lower than other forms of confined natural convection. However, the heat transfer rates are superior with the step change in lower wall temperature (2.5 to 3 times higher). Final steady states are found for Rayleigh numbers of 10⁷ at least. There is a long transient period which is determined by the transition of the flow pattern from initial conductive symmetry about the vertical midline to a completely asymmetric, convective pattern, in which the circulation center moves to the upper corner of the hot side of the cavity.
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