化学工程与工艺专业英语Unit 12_化学工程专业英语

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Unit 12 what do we mean by transport

phenomena ?

Transport phenomena is the collective name given to the systematic and integrated study of three claical areas of engineering science :(i)energy or heat transport ,(ii)ma transport or diffusion ,and(iii)momentum transport or fluid dynamics.传递现象是工程科学三个典型领域系统性和综合性研究的总称：能量或热量传递，质量传递或扩散，以及动量传递或流体力学。Ofcourse , heat and ma transport occur frequently in fluids , and for this reason some engineering educators prefer to includes these procees in their treatment of fluid mechanics.当然，热量和质量传递在流体中经常发生，正因如此一些工程教育家喜欢把这些过程包含在流体力学的范畴内。Since transport phenomena also includes heat conduction and diffusion in solids , however , the subjectis actually ofwider scope than fluid mechanics.由于传递现象也包括固体中的热传导和扩散，因此，传递现象实际上比流体力学的领域更广。It is also distinguished from fluid mechanics in that the study of transport phenomena make use of the similarities between the equations used to describe the procees of heat,ma,and momentum transport.传递现象的研究充分利用描述传热，传质，动量传递过程的方程间的相似性，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.这些类推(通常被这么叫)常常可以与传递现象发生的物理机制间的相似性关联起来。As a consequence,an understanding of one transport proce can readily lead to an understanding of other procees.因此，一个传递过程的理解能够容易促使其他过程的理解。Moreover,ifthe differential equations and boundary conditions are the same,a solution need be obtained for only one of the procees since by changing the nomenclature that solution can be used to obtain the solution for any other transport proce.而且，如果微分方程和边界条件是一样的，只需获得一个传递过程的解决方案即可，因为通过改变名称就可以用来获得其他任何传递过程的解决方案。

It must be emphasized , however, that while there are similarities between the transport procees, there are also important differences , especially between the transport of momentum(a vector)and that of heat or ma(scalars).必须强调,虽然有相似之处,也有传递过程之间的差异,尤其重要的是运输动量(矢量)和热或质量(标量).Neverthele , a systematic study of the similarities between the transport procees makes it easier to identify and understand the differences between them.然而,系统地研究了相似性传递过程之间的相似性,使它更容易识别和理解它们之间的差别。

1．How We Approach the Subject怎么研究传递过程？

In order to demonstrate the analogies between the transport procees , we will study each of the proce in parallel-instead of studying momentum transport first , then energy transport , and finally ma transport.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 reason for not using the serial approach that is used in other textbooks : of the three procees, the concepts and equations involved in the study of momentum transport are the most difficult for the beginner to understand and to use.除了促进理解之外，对于不使用在其他教科书里用到的顺序法还有另一个教学的原因：在三个过程中，包含在动量传递研究中的概念和方程对初学者来说是最难以理解并使用。Because it is impoible to cover heat and ma transport thoroughly without prior knowledge of momentum transport ,one is forced under the serial approach to take up the most difficult subject(momentum transport)first.因为在不具有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，在顺序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 to the familiar subject of heat transport.另一方面，如果课程同时被研究，通过参照有关传热的熟悉课程动量传递就变得更好理解。Furthermore ,the parallel treatment makes it poible to study the simpler the physical procees that are occurring rather than the mathematical procedures and representations.而且，平行研究法可以先研究较为简单的概念，再深入到较难和较抽象的概念。我们可以先强调所发生的物理过程而不是数学性步骤和描述。For example，we will study one-dimensional transport phenomena first because equations instead of partial requiring vector notation and we can often use ordinary differential equations instead of partial differential equations ,which are harder to solve.例如，我们将先研究一维传递现象，因为它在不要求矢量标注下就可以被解决，并且我们常常可以使用普通的微分方程代替难以解决的偏微分方程。This procedure is also justified by the fact that many of the practical problems of transport phenomena can be solved by one-dimensional models.加上传递现象的许多实际问题可以通过一维模型解决的这样一个事实，这种处理做法也是合理的。

2.Why Should Engineers Study Transport Phenomena? 为什么工程师要研究传递现象？

Since the discipline of transport phenomena deals with certain laws of nature , some people claify it as a branch of engineering.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 the economical design and operation of plants and equipment , quite properly should ask how transport phenomena will be of value in practice.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.对于那些问题有两种通用型答案。The first requires one to recognize that heat ,ma ,and momentum transport occur in many kinds of engineering , e.g., heat exchangers ,compreors ,nuclear and chemical reactors, humidifiers, air coolers ,driers , fractionaters , and absorbers.第一种要求大家认识到传热，传质和动量传递发生在许多工程设备中，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 procees are also involved in the human body as well as in the complex procees whereby pollutants react and diffuse in the atmosphere.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 procees if they are to understand what is taking place in engineering equipment and to make wise decisions with regard to its economical operation.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 proce equipment in which these procees are occurring.第二种答案是工程师需要能够运用自然定律的知识设计包含这些过程的工艺设备。To do so they must be able to predict rates of heat ,ma , or momentum transport.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.例如，考虑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 paes from the wall of the pipe to the fluid depends upon a parameter , etc.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 dimensionle empirical equations.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 heen taken.经验方程是适合一定数据范围的方程，它们不是建立在理论基础上而且在应用数据的范围外不能被精确使用。

The le 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.使用在传递现象中比较不耗费和通常较为可靠的方法是从以自然定律为基础的方程中预测传热系数。The predicted result would be obtained by a research engineer by solving some equations(often on a computer).预测的结果将由一个研究工程师通过解一些方程获得（常常在电脑上）A design engineer would then use the equation for the heat-transfer coefficient obtained by the research engineer.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 eentially the same no matter how the heat-transfer coefficients were originally obtained.要记住无论传热系数是怎么得来的设计热交换器的工作将基本上是一样的。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.正因如此，传递现象的一些课程只强调传热系数的决定而把真正的设计步骤留给单元操作中的一个课程。It is of cource 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.当然，能获得参数也就是设计中使用的传热系数是事实，并正因此，一个传递现象课程可被视为一个工程课程或一个科学课程。

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.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 reactor ,which might be illustrated as a pipe ,e.g., the heat exchanger described earlier, with a homogeneous chemical reaction occurring in the fluid within.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，在它里面的液相中发生着一个均相化学反应。The fluid enters with a certain concentration ofreactant and leaves the tube with a decreased concentration of reactant and an increased concentration of product.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 , the reactor wall will usually be maintained at a low temperature in order to remove the heat generated by the chemical reaction.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.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。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.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.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。Hence , concentration as well as temperature will vary both with radial position and with length.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.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。Since this mean concentration is obtained from the point values averaged over the cro section , we actually need to obtain the concentration at every point in the reactor , i.e., at every radial position and at every length.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!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.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.我们将不得到所包含的方程，但显然有一组必须由精细繁琐的步骤解决的复杂偏微分方程（通常在电脑上）。It should be apparent that we could not handle such a problem by the empirical design procedures used in unit operations courses for a heat exchanger.我们不能通过用于单元操作课程中关于热交换器的经验设计步骤来解决这样一个问题，应该是明显的。Instead the theory and mathematical procedures of transport phenomena are eential ,unle one wishes to go go the expense and take the time to build pilot plants of increasing size and measure the conersion in each.然而传递现象的理论和数学步骤是必不可少的，除非一个人愿意花金钱和时间去建立规模不断扩大的模范工厂并测出每一个工厂的产率。Even then the final scale-up is precarious and uncertain.即便最后的扩大规模是靠不住和不确定的。

Of course ,not all problems today can be solved by the methods of transport phenomena.当然，并非今天所有的问题都能通过传递现象的方法解决。However, with the development of the computer ,more and more problems are being solved by these methods.然而，随着电脑科技的发展，越来越多的问题通过这些方法正被解决。If engineering students are to have an education that is not become obsolete , they must be prepared, through an understanding of the methods oftransport phenomena , to make use of the computations that will be made in the future.如果工程学学生要得到一个不过时的教育，他们必须通过理解传递现象的方法准备好去充分利用将在未来形成的计算机计算。Because of its great potential as well as its current usefulne , a course in transport phenomena may ultimately prove to be the most practical and useful course on a student’s undergraduate career.由于其极大的潜能及当前的实用性，在一个大学生的在校学习生涯中，传递现象这门课程或许最终证明是最实用和有用的课程。