热能与动力工程专业英语翻译Ch01教案

1.3 传热学基础传热学是一门研究在存在温差的物体间发生能量传递的科学。热力学中将这种方式传递的能量定义为热量。传热学不仅可以解释热量传递是如何传递的，而且可以计算在特定条件下的传热速率。事实上，传热速率正是一个分析所期望的目标，它指明了传热学和热力学间的差别。热力学处理的是平衡状态下的系统，它可计算当系统从一个平衡状态过渡到另一个平衡状态时所需要的能量，但不能解决系统处于过渡过程的非平衡状态时能量变化的快慢程度。传热学提供了可用于计算传热速率的实验关联式，从而对热力学第一定律和第二定律进行补充。这里，我们介绍热量传递的三种方式和不同型式的换热器。1.3.1 Conduction heat transferWhen a temperature gradient exists in a body, experience has shown that there is an energy transfer from the high-temperature region to the low-temperature region. We say that the energy is transferred by conduction and that the heat transfer rate per unit area is proportional to the normal temperature gradient: q/AT/x. When the proportionality constant is inserted (1-3)Where q is the heat transfer rate and T/x is the temperature gradient in the direction of heat flow. The positive constant is called the thermal conductivity of the material, and the minus sign is inserted so that the second principle of thermodynamics will be satisfied; i.e., heat must flow downhill on the temperature scale. Equation (1-3) is called Fouriers law of heat conduction after the French mathematical physicist Joseph Fourier, who made very significant contributions to the analytical treatment of conduction heat transfer. It is important to note that Equation (1-3) is the defining equation for the thermal conductivity and that has the units of watts per meter per Celsius degree in a typical system of units in which the heat flow is expressed in watts.1.3.1 热传导当物体内部存在温度梯度时，经验表明，就有能量从高温区向低温区传递。我们说，此时的能量通过传导进行传递，单位面积上的传热速率与法向温度梯度成正比，即q/AT/x。引入比例系数，则有 (1-3)其中q是热流量，T/x是热流方向上的温度梯度，正常数l称为材料的导热系数。方程中插入的负号表示热传导过程应满足热力学第二定律，即热量必须沿温度降低的方向传递。式(1-3)称为傅立叶导热定律，以法国数理学家约瑟夫傅立叶的名字命名，傅立叶在导热的分析处理方面做出了极其重大的贡献。值得注意的是，式(1-3)也是导热系数的定义式，在典型的单位体系中，当热流量q的单位为W时，l的单位为W/(m)。1.3.2 Convection heat transfer It is well known that a hot plate of metal will cool faster when placed in front of a fan then when exposed to still air. We say that heat is convected away; and we call the process convection heat transfer. The term convection provides the reader with an intuitive notion concerning the heat-transfer process; however, this intuitive notion must be expanded to enable one to arrive at anything like an adequate analytical treatment of the problem. For example, we know that the velocity at which the air blows over the hot plate obviously influences the heat transfer rate. But does it influence the cooling in a linear way; i.e., if the velocity is doubled, will the heat transfer double? We should suspect that the heat transfer rate must be different if we cooled the plate with water instead of air, but, again, how much difference would there be? These questions may be answered with the aid of some rather basic analyses. For now, we sketch the physical mechanism of convection heat transfer and show its relation to the conduction process.图1-8 对流换热1.3.2 对流换热众所周知，与热金属板放置在静止的空气中相比，放置在转动的风扇前的热金属板会更快地冷却。我们说热量通过对流进行传递，称此类换热过程为对流换热。对流这个术语给读者提供了有关传热过程的直观概念，然而，必须扩展这种直观概念，使我们可以达到对某一问题进行充分的分析和处理。例如，我们知道流过热平板的空气速度会明显影响其传热量，但它是以线性方式影响冷却的吗？即如果速度增加一倍，传热量也会增加一倍吗？我们猜想，如果用水代替空气冷却热平板，传热量可能有所不同，但是，二者的差异会有多少呢？这些问题在了解一些非常基本的分析后，可得以回答。现在，我们来简要描述对流换热的物理机理，并且说明它和传导过程的联系。图1-8 Consider the heat transfer plate shown in Fig.1-8. The temperature of the plate is Tw and the temperature of the fluid is T. The velocity of the flow will appear as shown, being reduced to zero at the plate as a result of viscous action. Since the velocity of the fluid layer at the wall will be zero, the heat must be transferred only by conduction at that point. Thus we might compute the 教材12页heat transfer, using Equation (1-3), with the thermal conductivity of the fluid and the fluid temperature gradient at the wall. Why, then, if the heat flows by conduction in this layer, do we speak of convection heat transfer and need to consider the velocity of the fluid? The answer is that the temperature gradient is dependent on the rate at which the fluid carries the heat away; a high velocity produces a large temperature gradient, and so on. Thus the temperature gradient at the wall depends on the flow field, and we must develop in our later analysis an expression relating the two quantities. Nevertheless, it must be remembered that the physical mechanism of heat transfer at the wall is a conduction process. 被加热的平板如图1-8所示，平板的温度为Tw，流体的温度为T。速度分布如图所示，受黏性作用，平板上的速度减小为零。因为壁面处流动薄层的速度为零，因此，在该点上热量只能以导热方式传递。因此，可以利用式(1-3)，以及壁面上的流体导热系数和温度梯度来计算传热量。如果热量在该层经导热传递，那么，为什么我们要谈及对流换热以及需要考虑流体速度的影响呢？答案是，温度梯度依赖于流体带走热量的速度，较高的流速将产生较大的温度梯度。因此，壁面上的温度梯度依赖于流场的变化，在以后的分析中，我们将建立这二者间的关系。然而，必须记住，壁面上传热的物理机理是一导热过程。 To express the overall effect of convection. We use Newtons law of cooling: (1-4)Here the heat-transfer rate is related to the overall temperature difference between the wall and fluid and the surface area A. T