Abstract: 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.

Key words: micro-wire, natural convection, Nusselt number, boundary layer

摘要： 采用实验与数值模拟相结合的方法分别研究了封闭空间内水平放置的直径为39.9、65.8、119.1 μm的微细铜丝（微丝）在水中的对流换热，分析了微丝表面自然对流换热特性及机理。实验通过焦耳加热的方法测量了不同直径微丝在水中自然对流的传热系数及Nusselt数。同时建立三维不可压数学模型对微丝在水中的自然对流进行数值模拟，并将计算结果与实验值进行了对比。研究表明，数值模拟结果与实验值基本吻合，微丝在水中自然对流的传热系数随直径减小而显著增大，Nu则明显减小，且Nu随热通量增加的变化率也随直径减小而明显降低；微丝表面边界层厚度随直径减小而变薄，但边界层厚度与微丝直径的比值则逐渐增大；另外，对比微丝与常规尺度圆管表面自然对流的流场、温度场以及边界层分布，发现相同温差下微丝表面自然对流换热的边界层与常规尺度下沿壁面由底部向上发展的形状不同，而是沿微细丝表面呈椭圆形包裹于其上，因此削弱了表面对流换热强度，导致温度场呈现出较明显的导热特征。

关键词: 微丝, 自然对流, Nusselt数, 边界层