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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 transfer
When 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/A~?T/?x. When the proportionality constant is inserted
q???A?T (1-3) ?xWhere q is the heat transfer rate and ?T/?x is the temperature gradient in the direction of heat flow. The positive constant λ 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 Fourier’s 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 λhas 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/A~?T/?x。引入比例系数，则有
q???A?T (1-3) ?x其中q是热流量，?T/?x是热流方向上的温度梯度，正常数?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，?的单位为W/(m?℃)。
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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
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heat transfer, using Equation (1-3), with the thermal conductivity of the fluid and the
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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 Newton’s law of cooling:
q?hA(Tw?T?) (1-4)
Here the heat-transfer rate is related to the overall temperature difference between the wall and fluid and the surface area A. The quantity h is called the convection of heat-transfer coefficient, and Equation (1-4) is the defining equation. An analytical calculation of h may be made for some systems. For complex situations it must be determined experimentally. From Equation (1-4) we note that the units of h are in watts per square meter per Celsius degree when the heat flow is in watts.
为描述对流换热的整体效应，应用牛顿冷却定律
q?hA(Tw?T?) (1-4)
这里，热流量与壁面和流体间的整体温度差以及表面积A有关。参数h称为对流换热系数，式(1-4)是其定义式。对某些传热过程，可获得h的分析表达式，而复杂情形下的传热系数必须通过实验研究来确定。式(1-4)表明，当热流量的单位为W时，h的单位为W/(m2?℃)。
If a heat plate were exposed to ambient room air without an external source of motion, a movement of the air would be experienced as a result of the density gradients near the plate. We call this natural, or free, convection as opposed to forced
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convection, which is experienced in the case of the fan blowing air over a plate. Boiling and condensation phenomena are also grouped under the general subject of convection heat transfer.
如果将热平板置于没有外部风源的房间空气中，平板附近的密度梯度将造成空气运动。我们称此换热过程为自然对流，以区别于风扇吹扫平板表面时形成的强制对流。沸腾和凝结现象也属于对流换热的范畴。
1.3.3 Radiation heat transfer
In contrast to the mechanisms of conduction and convection, where energy transfer through a material medium is involved, heat may also be transferred through regions where a perfect vacuum exists. The mechanism in this case is electromagnetic radiation. We shall limit our discussion to electromagnetic radiation which is propagated as a result of a temperature difference; this is called thermal radiation. 1.3.3 辐射换热
对于导热和对流换热，其热量传递需要介质才得以进行，与此不同的是，热量也可以在完全真空中传递，其传热机理是电磁辐射。我们将讨论限定在由温差导致的电磁辐射，即所谓的热辐射。
Thermodynamic considerations show that an ideal thermal radiator, or blackbody, will emit energy at a rate proportional to the fourth power of the absolute temperature of the body and directly proportional to its surface area. Thus
qemitted??AT4 (1-5
Where is σ the proportionality constant and is called the Stefan-Boltzmann constant with the value of 5.669×10-8W/(m2·K4). Equation (1-5) is called the Stefan- Boltzmann law of thermal radiation, and it applied only to blackbodies. It is important to note that this equation is valid only for thermal radiation; other types of electromagnetic radiation may not be treated so simply.
热力学研究表明，对于理想的热辐射体或黑体，其辐射力正比于物体绝对温度的四次方及其表面积，因此有
qemitted??AT (1-5)
式中，?为比例系数，称为斯忒藩－玻耳兹曼常数，其值为5.669×10-8 W/(m2·K4)。式(1-5)称为热辐射的斯忒藩－玻耳兹曼定律，该式仅适用于黑体。值得注意的是，该表达式仅适用于热辐射，其它类型的电磁辐射要比该式复杂得多。
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