Experiments on natural convection in divergent vertical channels and correlation of divergent, convergent, and parallel-channel Nusselt numbers

The objectives of this investigation are to obtain definitive heat transfer data for natural convection in divergent vertical channels, to visualize the flow in the channels, and to develop a universal Nusselt number correlation for divergent, convergent, and parallel-walled channels. The experiments are performed with water as the working fluid (Pr ∼ 5) and are parameterized by the angle of divergence of the channel, the ratio of the channel length to minimum cross section, and the product of the Rayleigh number and the cross section/length ratio. Temperature uniformity is maintained at the channel walls, and the wall temperature exceeds the temperature of the fluid environment. The flow visualization reveals the presence of a recirculation zone which extends downward from the exit into the channel, but the Nusselt numbers are found to be independent of the extent of the downward penetration of the recirculation zone. The measured Nusselt numbers for the divergent channel are brought into tight correlation with those for the convergent and parallel-walled channels by using correlating parameters based on the maximum interwall spacing as the characteristic dimension. The correlation is conveyed by equation (4) (Nu_Smax = 0,740[( S_max L)Ra_Smax]^0,240).