The natural convection heat transfer characteristics and mechanism for copper micro-wires in water are investigated experimentally and numerically.The wires with diameters of 39.9 μm

65.8 μm and 119.1 μm are placed horizontally in a sealed tube.Using Joule heating

the heat transfer coefficients and Nusselt numbers of natural convection for micro-wires in ultra pure water are obtained.A three dimensional incompressible numerical model is used to investigate the natural convection

and the prediction with this model is in reasonable accordance with the experimental results.With the decrease of micro-wire diameter

the heat transfer coefficient of natural convection on the surface of micro-wire increases

while the Nusselt number of natural convection becomes smaller.The change rate of Nu  decreases apparently with the increase of heat flux and the decrease of wire diameter

attributed to the thinner boundary layer.The thickness of boundary layer on the wall of micro-wire becomes thinner with the decrease of diameter

but the ratio of boundary layer thickness to the diameter increases.As a result

the proportion of conduction in total heat transfer increases

while the convective heat transfer decreases.The velocity distribution

temperature field and the boundary layer in the natural convection are compared with those of conventional tube.It is found that the boundary layer around the micro-wire is an oval-shaped film on the surface

which is different from that around the conventional tube.This apparently reduces the convectional intensity in the natural convection and the heat transfer presents a conduction characteristic.