河南科技大学热能与动力工程专业英语考试.docxVIP

2.6 Natural Convection 自然对流Heat transfer involving motion in a fluid caused by the difference in density and the action of gravity is called natural or free convection. Heat transfer coefficients for natural convection are generally much lower than for forced convection, and it is therefore important not to ignore radiation in calculating the total heat loss or gain. Radiant transfer may be of the same order of magnitude as natural convection, even at room temperatures, since wall temperatures in a room can affect human comfort. 由于密度差和重力作用引起流体运动而产生的热传递称为自然对流或自由对流。自然对流的传热系数一般远低于强迫对流，因此在计算总的吸热量或放热量时，主要的一点就是不要忽略辐射热。辐射热传递与自然对流可以有相同的数量级，甚至在室温下也如此，因为室内墙体温度影响人体的舒适感。Natural convection is important in a variety of heating and refrigeration equipment: (1) gravity coils used in high humidity cold storage rooms and in roof-mounted refrigerant condensers, (2) the evaporator and condenser of household refrigerators, (3) baseboard radiators and convectors for space heating and (4) cooling panels for air conditioning. Natural convection is also involved in heat loss or gain to equipment casings and interconnecting ducts and pipes. 在各种供热和制冷设备中自然对流是非常重要的：（1）在高湿度冷藏室内及室内安装的制冷剂冷凝器内使用的重力盘管；（2）家用冰箱的蒸发器和冷凝器；（3）空间采暖用的踢脚板散热器和对流器；（4）空调用辐射对流护壁板。设备外壳及其连接风道和管道的放热量和吸热量中也包含自然对流。Consider heat transfer by natural convection between a cold fluid and a hot surface. The fluid in immediate contact with the surface is heated by conduction, becomes lighter and rises because of the difference in density of the adjacent fluid. The motion is resisted by the viscosity of the fluid. The heat transfer is influenced by: (1) gravitational force due to thermal expansion, (2) viscous drag and (3) thermal diffusion. It may be expected to depend on the gravitational acceleration g, the coefficient of thermal expansionβ,the kinematic viscosity υ(=μ/ρ), and the thermal diffusivity α=(k /ρcp ). These variables can be expressed in terms of dimensionless numbers: the Nusselt number, Nu, is a function of the product of the