化工专业英语-U.ppt

学 时： 30 授课班级：1014091/2 Specialized English for Chemical Engineering and Technology 授课教师：李 霞 联系电话Exercise Translate the following sentences into English. 1.热是能量的一种形式,不能创造也不能消灭。热可以从 一个物体传递到另一个物体。 译文：Heat, as a form of energy, cannot be created or destroyed. Heat can be transferred from one substance to another. 2.传热的三种方式分别是传导，对流，辐射。传导是两 个不同温度的物体直接接触而在两者之间发生的热传 递，或者是从同一个物体的一个部分传递到另一个部 分，不发生物质微粒的移动或者混合。 译文： Three methods of heat transfer are conduction, convection, and radiation. Conduction is the transfer of heat that occurs by direct contact between two objects at different temperatures, or the transfer from one part of an object to another part of the same object, without movement or mixing of particles. After completing this unit, you should be able to: 1. What are the three research areas of transport phenomena? 2. Why should engineers study transport phenomena? 3. Do you think that all problems can be solved by the methods of transport phenomena? 4. Can you describe briefly the famous Reynolds experiment? Unit 12 What do we mean by transport phenomena? Transport phenomena is the collective name(总称，统称) given to the systematic and integrated study of three classical areas of engineering science: (i) energy or heat transport, (ii) mass transport or diffusion, and (iii) momentum transport or fluid dynamics. 译： 传递现象是工程科学中三类传统领域的系统研究 和整体研究的总称：（1）能量或热量传递，（2）质 量传递或（质量）扩散，和（3）动量传递或流体动力 学。 Of course, heat and mass transport occur frequently in fluids, and for this reason some engineering educators prefer to include these processes in their treatment of fluid mechanics(流体力学). 译： 当然，热量和质量传递经常在流体内发生，由于 该原因，一些工程教育工作者偏向于（prefer to）把 热量和质量传递过程放在在流体力学中处理。 Since transport phenomena also includes heat conduction and diffusion in solids, however, the subject is actually of wider scope than fluid mechanics. 译： 然而，因为传递现象也包括固体中的热传递和热 扩散，所以该学科实际上较流体力学的研究范畴更广 。 It is also distinguished from(区别于，与 区别) fluid mechanics in that the study of transport phenomena make use of the similarities between the equations used to describe the processes of heat, mass, and momentum transport. 译： 传递现象研究利用了用于描述热量、质量和动 量传递过程的等式（方程式）间的相似性，这也区 别于流体力学。 These analogies, as they are usually called, can often be related to similarities in the physical mechanisms whereby the transport takes place. 译： 这些类似性(它们通常被这么叫)，经常与那些发 生传递现象的物理机制的相似性联系起来。 As a consequence, an understanding of one transport process can readily lead to an understanding of other processes. 译： 因此，一种传递过程的理解有助于理解其他过 程。 Moreover, if the differential equations and boundary conditions(界面条件) are the same, a solution need be obtained for only one of the processes since by changing the nomenclature(命名法，术语) that solution can be used to obtain the solution for any other transport process. 译： 而且，如果微分方程式和边界条件相同，那么只 要得到其中一个过程的解决方法即可，因为通过改变 专业术语，该方法可用于得到其他任何传递过程的解 决方法。 It must be emphasized, however, that while there are similarities between the transport processes, there are also important differences, especially between the transport of momentum (a vector矢量) and that of heat or mass (scalars标量 ). 译： 然而，必须强调的是，尽管传递过程中存在相似 性，但是它们也存在重要的差别，尤其在动量（矢量） 传递和热量或质量（标量）的传递之间。 Nevertheless, a systematic study of the similarities between the transport processes makes it easier to identify and understand the differences between them. 译： 然而,系统地研究传递过程之间的相似性,会使得 确定和理解它们之间的差别更容易。 1How We Approach the Subject In order to demonstrate the analogies between the transport processes, we will study each of the process in parallel- instead of studying momentum transport first, then energy transport, and finally mass transport. 译： 为了找出传递过程间的相似性，我们将同时研究 每一种传递过程取代先研究动量传递，再传热，最 后传质的方法。 Besides promoting understanding, there is another pedagogical(教育学的) reason for not using the serial approach that is used in other textbooks: of the three processes, the concepts and equations involved in the study of momentum transport are the most difficult for the beginner to understand and to use. 译： 除了促进理解之外，不采用其他课本中采用的连续 研究方法，有另外一种教学方面的原因：在这三种传递 过程中，研究动量传递时所涉及到的概念和方程式对于 初学者来说最难理解，最难使用。 Because it is impossible to cover heat and mass transport thoroughly without prior(预先的) knowledge of momentum transport, one is forced under the serial approach to take up(从 事，占据) the most difficult subject (momentum transport) first. 译： 因为在不具备有关动量传递的知识前提下一个 人不可能完全理解传热和传质，所以在连续研究法的 情况下他就被迫先研究最难的课程即动量传递。 On the other hand, if the subjects are studied in parallel, momentum transport becomes more understandable by reference (参考，涉 及) to the familiar subject of heat transport. 译： 另一方面，如果同时研究这些问题，那么可参照 与热量传递的相似性，动量传递就变得容易理解。 Furthermore, the parallel treatment makes it possible to study the simpler concepts first and proceed later to more difficult and more abstract ideas. 译： 而且，同时研究使得这种做法（先研究较简单的 概念，然后在深入到较难和较深奥的概念的观点）成 为可能。 Initially we can emphasize the physical processes that are occurring rather than(而不 是) the mathematical procedures and representations. 译： 一开始，我们可以强调正在发生的物理过程，而 不是数学过程和数学描述。 For example，we will study one-dimensional transport phenomena first because it can be treated without requiring vector notation(向量 符号) and we can often use ordinary differential equations (常微分方程) instead of partial differential equations(偏微分方程), which are harder to solve. 译： 例如，我们将首先研究一维传递现象，因为一维传 递在处理时不需要矢量符号，而且我们通常可使用常微 分方程而不是使用难以解答的偏微分方程。 This procedure is also justified by the fact that many of the practical problems of transport phenomena can be solved by one-dimensional models. 译： 这种解决方案也可由该事实得到证实：传递现象 的许多的实际问题能够用一维模型加以解决。 2. Why Should Engineers Study Transport Phenomena? Since the discipline of transport phenomena deals with(涉及，处理) certain laws of nature, some people classify it as a branch of engineering. 译：因为传递现象这个学科牵扯到自然界定则，一些人 就把它划分为工程的一个分支。 For this reason the engineer, who is concerned with(参与，干涉，关心) the economical design and operation of plants and equipment, quite properly should ask how transport phenomena will be of value in practice . 译：正因如此，对于那些关心工厂和设备设计和操作经 济性的工程师而言，十分应该探知在实际中传递现象如 何起到价值作用。 There are two general types of answers to those questions. The first requires one to recognize that heat, mass, and momentum transport occur in many kinds of engineering, e.g., heat exchangers, compressors, nuclear and chemical reactors, humidifiers, air coolers, driers, fractionators(分馏器), and absorbers. 译： 对于那些问题有两种普遍的答案。第一种答案要 求大家认识到传热，传质和动量传递发生在多种工程设 备中，如热交换器，压缩机，核化反应器，增湿器，空 气冷却器，干燥器，分离器和吸收器。 These transport processes are also involved in the human body as well as in the complex processes whereby pollutants react and diffuse in the atmosphere. 译：这些传递过程也发生在人体内以及大气中污染物 反应和扩散的一些复杂过程中。 It is important that engineers have an understanding of the physical laws governing these transport processes if they are to understand what is taking place in engineering equipment and to make wise decisions with regard to (关于,至于) its economical operation. 译： 如果工程师要知道工程设备中正在发生什么并要 做出能达到经济性操作的决策，对主导这些传递过程 的物理定律有一个认识很重要。 The second answer is that engineers need to be able to use their understanding of natural laws to design process equipment in which these processes are occurring. To do so they must be able to predict rates of heat, mass, or momentum transport. 译：第二种答案是工程师需要能够运用他们对自然定 律的理解来设计那些正在发生的过程的工艺设备。要 做到这点，他们必须能够预测传热，传质，或动量传 递速率。 For example, consider a simple heat exchanger, i.e., a pipe used to heat a fluid by maintaining its wall at a higher temperature than that of the fluid flowing through it. 译：例如，设计一个简单的热交换器，也就是一根管道 通过维持壁温高于流经管道的流体温度来加热流体 。 The rate at which heat passes from the wall of the pipe to the fluid depends upon a parameter, called the heat-transfer coefficient which in turn depends on pipe size, fluid flow rate, fluid properties, etc. 译：热量从管壁传递到流体的速率取决于传热系数，传热 系数反过来取决于管的大小，流体流速，流体性质等。 Traditionally heat-transfer coefficients are obtained after expensive and time-consuming laboratory or pilot-plant measurements and are correlated through the use of dimensionless empirical equations. 译：传统上传热系数是在耗费和耗时的实验室或模范工厂 的测量之后获得并且通过使用一维经验方程关联起来。 Empirical equations are equations that fit the data over a certain range; they are not based upon theory and cannot be used accurately outside the range for which the data have been taken. 译：经验式能在较大范围内吻合数据，它们不 是以理论为基础的，因此超出所得数据的范围 时经验式不能准确使用。 The less expensive and usually more reliable approach used in transport phenomena is to predict the heat-transfer coefficient from equations based on the laws of nature. 译：用于传递现象中比较经济且通常较为可靠的方法 是根据以自然定律为基础的方程来预测传热系数。 The predicted result would be obtained by a research engineer by solving some equations (often on a computer). A design engineer would then use the equation for the heat- transfer coefficient obtained by the research engineer . 译：预测的结果将由研究工程师通过解一些方程获得（ 常常在电脑上）。然后设计工程师再使用由研究工程师 获得的关于传热系数的方程。 Keep in mind that the job of designing the heat exchanger would be essentially the same no matter how(不管如 何) the heat-transfer coefficients were originally obtained. 译: 要牢记的是，不管传热系数最初是怎么样得到的， 热交换器的设计工作本质上是相同的。 For this reason, some courses in transport phenomena emphasize only the determination of the heat-transfer coefficient and leave the actual design procedure to a course in unit operations. 译: 因为这一原因，传递现象中的一些课程所强调的 只是传热系数的确定，而将实际设计步骤留到单元操 作的课程中。 It is of course a “practical” matter to be able to obtain the parameters, i.e., the heat -transfer coefficients that are used in design , and for that reason a transport phenomena course can be considered an engineering course as well as one in science. 译：当然，能够得到参数（如设计中所用的传递系数 ），是应用性（practical）的问题（matter），由于 那种原因，传递现象课程可视为一门工程课程，也可 视为理科中的一门课程。 In fact, there are some cases in which the design engineer might use the methods and equations of transport phenomena directly in the design of equipment. 译：实际上，在设备设计中有一些情况下设计工程师 可能直接使用传递现象的方法和方程。 An example would be a tubular(管式的) reactor, which might be illustrated as a pipe, e.g., the heat exchanger described earlier, with a homogeneous(均相的,同类 的) chemical reaction occurring in the fluid within. 译：一种情况就是设计可以被称为管道的管式反应器， 如，前面所提过的热交换器，它里面的流体发生着一个 均相化学反应。 The fluid enters with a certain concentration of reactant and leaves the tube with a decreased concentration of reactant and an increased concentration of product. 译：流体以一定浓度的反应物流进并以浓度降低的反 应物和浓度增加的产物流出反应管。 If the reaction is exothermic(放热的), the reactor wall will usually be maintained at a low temperature in order to remove the heat generated by the chemical reaction. 译： 如果反应是放热的，为了移除化学反应生 成的热量反应器壁通常维持在一个低的温度。 Therefore the temperature will decrease with radial position, i.e., with the distance from the centerline of the pipe. Then, since the reaction rate increases with temperature, it will be higher at the center, where the temperature is high, than at the wall, where the temperature is low . 译：因此沿径向方向也就是说随离管道中心线距离的 增大，温度降低。再者，因为反应速率随温度升高而 增大，在温度高的中心处的反应速率高于温度低的管 壁处的反应速率。 Accordingly, the products of the reaction will tend to accumulate at the centerline while the reactants accumulate near the wall of the reactor. Hence, concentration as well as temperature will vary both with radial position and with length. 译：结果，反应产物将倾向于在中心线处积累而反应 物在靠近管壁处积累。因此，沿径向和横向浓度和温 度都将改变。 To design the reactor we would need to know, at any given length, the mean concentration of product. Since this mean concentration is obtained from the point values averaged over the cross section, we actually need to obtain the concentration at every point in the reactor, i.e., at every radial position and at every length. 译：为了设计反应器我们需要知道在任意给定的管长下产物的平 均浓度。由于这个平均浓度是将整个反应器内每个点的浓度平均 起来得到的，实际上我们需要得到反应器内每个点的浓度，也就 是说，在每个径向和横向位置。 But to calculate the concentration at every point we need to know the reaction rate at every point, and to calculate the rate at every point we need to know both the temperature and the concentration at every point! 译：但是为了计算每个点的浓度我们需要知道每个点 处的反应速率，而为了计算每个点处的速率我们需要 知道温度和浓度！ Furthermore, to calculate the temperature we also need to know the rate and the velocity of the fluid at every point. 译：而且，为了计算温度我们也要知道每个点处的反 应速率和速度。 We will not go into the equations involved, but obviously we have a complicated set of partial differential equations that must be solved by sophisticated procedures, usually on a computer. 译：虽然我们不会深入研究所包含的方程，但是很显 然我们会得到一组必须由复杂繁琐的步骤（通常在电 脑上）来求解的复杂偏微分方程。 It should be apparent that we could not handle such a problem by the empirica