热能与动力工程专业英语翻译2.5

2.5Natural Convection 自然对流Heat transfer involving motion in a fluid caused by the difference in density and the action ofgravity 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 radiationin 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) gravitycoils 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 expansion β, the kinematic viscosity υ(=μ/ρ), and the thermal diffusivity α=(k /ρc p ). These variables can be expressed in terms of dimensionless numbers: the Nusselt number, Nu, is afunction of the product of the Prandtl number, Pr, and Grashof number, Gr, which, when combined, depend on the fluid properties, the temperature difference between the surface and the fluid, △t, and the characteristic length of the surface, L. The constant c and exponent n depend on the physical configuration and nature of flow. 现在我们来考虑冷流体和热表面间由自然对流引起的热传递。

与表面直接接触的流体由于导热而被加热、变轻，并且由于与相邻流体的密度差而上升。

这种运动由于流体粘性而受到阻碍。

热传递受下列因素影响：（1）因热膨胀而引起的重力作用；（2）粘性阻滞；（3）热扩散。

这种热传递被认为取决于重力加速度g、热膨胀系数β、运动粘滞系数υ(=μ/ρ)，以及导热系数α=( k /ρc p )。

这些变量可以用无因次数的形式给出：努谢尔特数Nu，它是普朗特数Pr和格拉晓夫数Gr乘积的函数，Pr与Gr的乘积，取决于流体特性、表面与流体间的温差△t和表面特征长度L。

常数c和指数n取决于物体外形和流动性质。

The entire process of natural convection cannot be represented by a single value of exponent n, but can be divided into three regions: (1) turbulent natural convection for which n equals 0.33, (2) laminar natural convection, for which n equals 0.25 and (3) a region that has (Gr·Pr) less than for laminar natural convection, for which the exponent n gradually diminishes from 0.25 to lower values. Note that, for wires, the (Gr·Pr) is likely to be very small, so that the exponent n is 0.1.自然对流的全部过程不能由单一的指数n表示，但可以分成三个区域：（1）紊流自然对流，n等于0.33；（2）层流自然对流，n等于0.25；（3）（Gr·Pr）小于层流自然对流的区域，指数n从0.25逐渐减小到更低的值。

注意，对金属线，（Gr·Pr）可能会很小，所以n是0.1。

To calculate the natural convection heat transfer coefficient, determine (Gr·Pr) to find whetherthe boundary layer is laminar of turbulent, then apply the appropriate equation. The correct characteristic length indicated must be used. Since the exponent n is 0.33 for a turbulent boundary layer, the characteristic length cancels out, and the heat transfer coefficient is independent of the characteristic length. Turbulence occurs when length or temperature difference is large. Since thelength of a pipe is generally greater than its diameter, the heat transfer coefficient for verticalpipes is larger than for horizontal pipes. 为了计算自然对流传热系数，先算出（Gr·Pr），确定边界是层流还是紊流，然后应用合适的方程。

必须使用已指出的正确的特征长度。

由于紊流边界层的指数n是0.33，特征长度消去，传热系数与特征长度无关。

当长度或温差大时就产生紊流。

由于管道的长度一般大于它的直径，竖管的传热系数大于横管的传热系数。

Convection from horizontal plates facing downward when heated (or upward when cooled ) is a special case. Since the hot air is above the colder air, there is no theoretical reason for convection. Some convection is caused, however, by secondary influences such as temperature differences on the edges ofthe plate. As an approximation, a coefficient of somewhat less than half of the coefficient for a heated horizontal plate facing upward can be used. 热面朝下（冷面朝上）的水平板对流是个特例。