The effects of thermal instability on a liminar thermally developing entrance flow between two horizontal plates heated from below are studied experimentally. The experiments cover a range of Rayleigh numbers between 2.2 × 104 and 2.1 × 105, and Reynolds numbers between 50 and 300 using air. Results for the heat transfer rate and entrance length show that they are influenced not only by the Rayleigh number but also by the ratio Re2/Gr. The heat transfer rate is increased as much as 4.4 times due to thermal instability. The flow visualization experiments show that the critical wavelength is determined by the channel height, not by the thermal boundary layer thickness.

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