What do we mean by transport phenomena

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What do we mean by transport phenomena? Since the discipline of transport phenomena deals with certain laws of nature, some people classify it as a branch of engineering. 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. 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 equipment, e.g., heat exchangers, compressors, nuclear and chemical reactors, humidifiers, air coolers, driers, fractionaters, and absorbers. 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. 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 its economical operation. 既然传递现象这一学科涉及到(deal with)自然界的某些定律,因此,有人将它划分为工程学的一分支。因为这种原因,那些关心工厂和设备的经济性设计和操作的工程师,应该要好好的问一问:在实践中传递现象怎样才能有价值。回答该问题有两种普遍的答案。其一,需要我们意识到热量、质量和动量传递发生于多种工程设备中,如热交换器、压缩机、核反应堆、化学反应器、增湿器、空气冷凝器、干燥器、分馏器以及吸收器。这些传递过程也发生于人体和一些复杂的过程中,借此污染物发生反应,扩散于大气中。如果工程们要去理解工厂设计中发生什么以及关于其经济性操作作出英明的决策,那么理解支配这些传递过程的物理定律是至关重要的。 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. 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. 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. 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. Empirical equations are equations that fit the data over a certain range; they are not based upon theory and connot be used accurately outside the range for which the data have been taken. 第二个答案是,工程师们必须要能够运用他们对自然界定律的理解去设计这些过程发生的过程设备。要实现这一目标,他们必须能够预测热量、质量和动量传递速率。例如,要设计一个简单的热交换器,即用于加热流体的管道,必须要考虑到维持管壁的温度高于在其中流动的流体的温度。从管壁传给流体的热量的速率取决于一个叫做传热系数(heat-transfer coefficient)的参数,而传热系数取决于管道尺寸、流速、流体性质,等等。传统上(一般地),传热系数在费钱和耗时的实验室或中试工厂的测量之后得到,同时通过无数次经验式(dimensionless empirical equations)加以关联。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. The predicted results 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. 用于传递现象中的费用低而且通常更可靠的方法,是从以自然界定律为基础的等式去预测传热系数。预测的结果可以由研究工程师解方程(通常计算上)得到。设计工程师能利用(运用)研究工程师所得到的传热系数的等式。 Keep in mind that the job of designing the heat exchanger would be essentially the same no matter how the heat-transfer coefficients were originally obtained. 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. 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. 要牢记的是,不管传热系数是怎么样得到的,热交换器的设计工作基本上相同的。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。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. 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. 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. 事实上,在有些情况中,设计工程师在设计设备时,可能直接利用传递现象的方法和等式。管式反应器就是这样一个例子,在反应器里面的流体中发生均相反应,管式反应器可以由管道解释(阐述),如前所述的热交换器。含有一定溶度的反应物的流体流进管道,离开管道时,反应物的浓度减少,而产物的浓度增加。 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. Therefore the temperature will decrease with radial positions, 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. 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. To design the reactor we would need to know, at any given length, the mean concentration of the 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. 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! Furthermore, to calculate the temperature we also need to know the rate and the velocity of the fluid at every point. We will not go into the equations involved, but obviously we have a complicated set of partial differential