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1、【精品文档】如有侵权，请联系网站删除，仅供学习与交流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
2、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
3、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,
4、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 contribu
5、tions 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.
6、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
7、 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 c
8、oncerning 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.
9、 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 b
10、e 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，
11、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
12、 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 transfe
13、r, 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 gra
14、dient 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. Neverth
15、eless, 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
16、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. The
17、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 sq
18、uare 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 (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,
19、 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 convection, which is experienced in the case of the fan blowing air over a plate. Boiling and condensation phenomena are also grouped under
20、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 transferIn contrast to the mechanisms of conduction and convection, where energy transfer through a material medium is involved, heat may al
21、so 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
22、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
23、 (1-5Where is the proportionality constant and is called the Stefan-Boltzmann constant with the value of 5.66910-8W/(m2K4). 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 therma
24、l 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 (1-5)式中，s为比例系数，称为斯忒藩玻耳兹曼常数，其值为5.66910-8 W/(m2K4)。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。 Equation (1-5) governs only relation emitted by a b
25、lackbody. The net radiant exchange between two surfaces will be proportional to the difference in absolute temperatures to the fourth power; i.e.教材13页式(1-5)只能用于确定单个黑体的辐射能。两个表面间的净辐射换热量与其绝对温度四次方的差成正比，即 (1-6)We have mentioned that a blackbody is a radiate energy according to the T4 law. We call such a
26、body black because black surface, such as a piece of metal converted with carbon black, approximate this type of behavior. Other types of surfaces, such as a glossy painted surface or a polished metal plate, do not radiate as much energy as the blackbody; however, the total radiation emitted by thes
27、e bodies will generally follow the T41 proportionality. To take account of the “gray” nature of such surfaces we introduce another factor into Equation (1-5), called the emissivity , which relates the radiation of the “gray” surface to that of an ideal black surface. In addition, we must take into a
28、ccount the fact that not all the radiation leaving one surface will reach the other surface since electromagnetic radiation travels in straight lines and some will be lost to the surroundings. We therefore introduce two new factors in Equation (1-5) to take into account both situations, so that (1-7
29、)Where F is an emissivity function and FG is a geometric “view factor” function. It is important to alter the reader at this time, however, to the fact that these functions usually are not independent of one another as indicated in equation (1-7).我们已经提到，黑体是按四次方定律辐射能量的物体。因其黑色的表面我们称之为黑体，如覆盖炭黑的金属片，就近似具
30、有这种辐射特性。其它类型的表面，如有光泽的漆面或抛光的金属板，并不具有黑体那样大的辐射力，然而，这些物体的辐射力仍大致与成正比。为了考虑这些表面的“灰”特性，在式(1-5)引入另一个参数，称为发射率，发射率将这些“灰”表面的辐射与理想黑体的表面辐射联系起来。此外，我们必须考虑这样一个事实，并非一个表面发出的所有辐射都可以到达到另一个表面，因为电磁辐射是沿直线传播的，将有部分能量散失到周围环境中。因此，考虑到这两种情况，式(1-5)引入另外两个新的参数，则有 (1-7)式中，F是发射率函数，FG是几何角系数。此时，值得提醒读者的是，式(1-7)中的这两个函数通常并不是相互独立的。1.3.4 Ty
31、pes of heat exchangers The simplest type of heat exchanger consists of two concentric pipes of different diameters, called the double-pipe heat exchanger. One fluid in a double pipe heat exchanger flows through the smaller pipe while the other fluid flows through the annular 环形 space between the two
32、 pipes. Two types of flow arrangement are possible in a double pipe heat exchanger; in parallel flow, both the hot and cold fluid enters the heat exchanger at the same end and move in the same direction. In counter flow, on the other hand, the hot and cold fluids enter the heat exchanger at opposite
33、 ends and flow in opposite directions.1.3.4 换热器的类型最简单的换热器是由两个不同直径的同心圆管组成，称为套管式换热器。套管换热器中的一种流体流经细管，另一种流体流经两管间的环形区域。套管换热器中包括两种不同类型的流动方式：一种为顺流，即冷、热流体从同一端进入换热器，并沿同一方向流动；另一种为逆流，即冷、热流体从相反的两端进入换热器，且沿相反方向流动。Another type of heat exchanger, which is specifically designed to realize a large heat transfer surfa
34、ce area per unit volume, is the compact heat exchanger. The ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density . A heat exchanger with 700m2/m3 is classified as being compact. Example of compact heat exchangers are car radiators (1000m2/m3), glass ce
35、ramic gas turbine heat exchangers (6000m2/m3), the regenerator of a Stifling engine (15,000m2/m3), and human lung (20,000m2/m3). Compact heat exchangers enable us to achieve high heat transfer rates between two fluids in a small volume, and they are commonly used in applications with strict limitati
36、ons on the weight and volume of heat exchangers.另一类换热器，被专门设计成单位体积内有很大的换热面积，称为紧凑式换热器。换热器的换热面积与其体积之比称为面积密度。700 m2/m3的换热器归为紧凑式换热器。例如汽车散热器（1000 m2/m3）、燃气轮机中的玻璃陶瓷换热器（6000 m2/m3）、斯特林机的回热器（15,000 m2/m3）以及人的肺部（20,000 m2/m3）。紧凑式换热器能实现小容积内两种流体的高换热率，通常用于换热器重量和容积受到严格限制的场合。 The large surface area in compact heat
37、 exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids. Compact heat exchangers are commonly used in gas-to-gas and gas-to-liquid (or liquid-to-gas) heat exchangers to counteract the low heat transfer coefficient associated with gas f
38、low with increased surface area. In a car radiator, which is a water-to-air compact heat exchanger, for example, it is no surprise 教材14页that fins are attached to the air side of the tube surface.图1-9紧凑式换热器通过在分离两种流体的壁面上附加间隔紧密的薄板或波纹翅片来扩展其表面。紧凑式换热器通常用于气气和气液（或液气）换热器，通过增加传热面积来抵消气侧低传热系数所带来的影响。例如，汽车散热器是水气紧
39、凑式换热器的典型例子，通常管子气侧表面装有翅片。Perhaps the most common type of heat exchanger in industrial applications is the shell-and-tube heat exchanger, shown in Fig.1-9. Shell-and-tube heat exchangers contain a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of t
40、he shell. Heat transfer takes place as one fluid flows inside the tube while the other fluid flows outside the tubes through the shell. Baffles are commonly placed in the shell to force the shell-side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the
41、 tubes. Despite their widespread use, shell-and-tube heat exchangers are not suitable for use in automotive and aircraft applications because of their relatively large size and weight. Note that the tubes in a shell-and-tube heat exchanger open to some large flow areas called headers at both ends of
42、 the shell, where the tube-side fluid accumulates before entering the tubes and after leaving them.图1-9 管-壳式换热器简图工业应用中最常见的换热器也许是管壳式换热器，如图1-9所示。管壳式换热器外壳里封装有大量的管束（有时为数百根），其轴线与外壳轴线平行。当一种流体在管内流动，另一种流体在管外流动并穿过壳体时，就进行了热交换。壳内通常布置有挡板，用于使壳侧流体沿壳流动以强化传热，并保持均匀的管间距。虽然管壳式换热器应用广泛，但因其相对较大的尺寸和重量，因而并不适用于汽车和航空器领域。注意，管
43、壳式换热器的管束两侧开口处的较大流动区域称为封头，它位于壳体两端，管侧流体流入、流出管子前后都在此汇集。Shell-and-tube heat exchangers are further classified according to the number of shell and tube passes involved. Heat exchangers in which all the tubes make one U-turn in the shell, for example, are called one-shell-pass and two-tube-passes heat ex
44、changers. Likewise, a heat exchanger that involves two passes in the shell and four passes in the tubes is called a two-shell-passes and four-tube-passes heat exchanger.管壳式换热器依据所含管程和壳程的数目可进一步分类。例如，换热器壳内的所有管束采用一个U型布置的称为单壳程双管程换热器（1-2型换热器）。同样地，含有双壳程和四管程的换热器叫做双壳程四管程型换热器（2-4型换热器）。An innovative type of he
45、at exchanger that has found widespread use is the plate and frame (or just plate) heat exchangers, which consist of a series of plates with corrugated flat flow passages. The hot and cold fluids flow in alternate passages and thus each cold fluid stream is surrounded by two hot fluid streams, result
46、ing in very effective heat transfer. Also, plate heat exchangers can grow with increasing demand for heat transfer by simply mounting more plates. They are well suited for liquid-to-liquid heat exchange application provided that the hot and cold fluid streams are at about the same pressure.一种广泛使用的新型
47、换热器是板翅式（或板式）换热器，它由一系列平板组成，并形成波纹状的流动通道。冷、热流体在间隔的每个通道中流动，每一股冷流体被两股热流体所包围，因此换热效果非常好。此外，板式换热器可通过简单添加更多的平板来满足增强换热的需求。该类型换热器非常适用于液液式换热场合，但需要冷、热液流的压强大致相等。Another type of heat exchanger that involves the alternate passage of the hot and cold fluid stream through the same flow area is the regenerative heat exchanger. The static-type regenerative heat exchanger is basically a porous mass that has a large heat storage capacity, such as a ceramic wire mesh. Hot and cold fluid to the matrix of the regenerator during the flow of the hot fluid, and from the matrix to the cold fluid during