A numerical investigation of the transient natural convection from two vertically aligned horizontal cylinders

Numerical solutions to the equations governing transient natural convection from two vertically aligned horizontal cylinders are presented for three cylinder spacing-to-diameter ratios. For small spacing-to-diameter ratios, the upper cylinder heat transfer is degraded by the thermal plume from the bottom cylinder. As the spacing increases, however, the increasing velocity of the buoyant plume causes a mixed convection effect and the result is an increase in heat transfer for the upper cylinder. Results for the spacing effect on steady-state heat transfer are compared to experimental data, where good agreement is seen at low spacings, while some error occurs at the higher spacing. This is apparently due to higher gradients occurring in a courser grid. The development of the flowfield for each spacing was explored. A qualitative comparison to physical observations shows that real fluids exhibit stability due to viscous forces not seen in these numerical solutions.