Natural convection inside a rectangular cavity with different temperature boundary conditions on the cold top plate was studied using a Mach-Zehnder interferometer for θ = 15, 45, and 60 deg to the horizontal. At θ = 60 deg coupling with external forced convection and non-coupled heat transfer from a cavity with an isothermal top plate was studied. In all experiments the bottom hot plate was isothermal. The Rayleigh number Ra was varied from subcritical to 6×105 and the cavity aspect ratio ARx, from 6.68 to 33.4. The Reynolds number of the external forced flow Redh was constant and approximately equal to 5.8×104. It was found that for Ra ≲ 3×104 the differing thermal boundary conditions at the top plate did not affect the local or average heat transfer rates from the cavity. For Ra ≳ 3×104 coupling at the top plate compared to the non-coupled case resulted not only in a reduction in the variation of the local heat transfer rates at the cold plate, but also in a significant reduction in the variation of the average transfer rates from hot and cold plates of the cavity. Forced convection at the top plate as compared to natural convection resulted only in a small reduction in the heat transfer coefficient at the cold plate. Correlation equations for coupled and noncoupled average heat transfer rates are presented.
