Heat sinks appearing as one vertical cylinder with longitudinal fins are widely used in heat dissipation of
LED bulbs and other electric devices. Numerical models of natural convection heat transfer are built
based on CFD method. Thermal boundary layers are discussed along with the air flow patterns. Numerical
results disclose that the edge of thermal boundary layer can be fitted into a sinusoidal function in
2D polar coordinate system. The local Nu distributions on both cylinder and fin surfaces are presented
and analyzed. It is found that average Nusselt number along the fin length is approximately proportional
to an exponential function of z. A new correlation is proposed by introducing finned ratio, achieving a
deviation within ±10%.