Numerical investigation of laminar natural convection flows in obstructed vertical channels

Abstract

Natural convection heat transfer is numerically studied in a vertical parallel-walled, convergent and divergent channels with a single square obstruction. The channel walls are maintained at a uniform temperature which exceeds the ambient temperature. The working fluid is air. It can be seen that the average Nusselt number curve for the obstructed channel represents the three flow regimes (fully developed limit, transition and isolated plate limit). The presence of an obstruction reduces the average Nusselt number compared to the unobstructed channel. This reduction increases as the Rayleigh number decreases. The average Nusselt number depends on the Rayleigh number, the aspect ratio, the obstruction size, the obstruction location and the angle of convergence or divergence. The effects of all these parameters are studied and reported. It is found that moving the obstruction location away from the entrance towards the exit of the channel will decrease the average Nusselt number. Increasing the angle of convergence, increases the average Nusselt (Nu )or Rayleigh number (Ra )<700>700. Basing the average Nusselt number and the Rayleigh number on the entrance width of the convergent channel (S ), average Nusselt number curves for the parallel-walled channel can be used for convergent channel with a maximum error of 20%. Increasing the angle of divergence increases the average Nusselt number at low Rayleigh numbers. Finally, a comparison between convergent and divergent channels having the same angle of convergence and divergence, shows that the average Nusselt number for the convergent channel is higher than that for the divergent one.