燃烧学的讲义_00006.doc

表 31 Basic definitions and relations 13 表 32 Notation for Mass and Molar Fluxes 14 表 33 Equivalent Forms of Fick’s (first) Law of Binary Diffusion 15 绪论 燃料 燃料的燃烧计算 输运现象[5][7][10] 输运什么呢？质量、动量、能量。The subject of transport phenomena includes three closely related topics: fluid dynamics, heat transfer, and mass transfer. Fluid dynamics involves the transport of momentum, heat transfer deals with the transport of energy, and mass transfer is concerned with the transport of mass of various chemical species. 具体的输运现象是什么呢？Conseder the molecules in any region of a fluid medium possess an excess of energy, concentration, or momentum, such that gradients of these properties exist in the neighborhood of this region, the system will attempt to restore spatial uniformity by transporting the relevant property in the direction of the deficient region. The transport occurs even in the absence of any bulk motion in this direction. As an illustration, consider a body of stagnant gas situated between two parallel plates as shown in Figure 4.1.1a. If plate Ais suddenly raised to a temperature TA TB, then the region around plate B also will be heated soon. Therefore there can be a net transfer of heat from Ato B despite the lack of any bulk fluid movement. Similarly if there is initially a higher concentration of a species at Arelative to its concentration at B, then its molecules will slowly migrate from A to B, as shown in Figure 4.1.1b. Finally, if plate A is impulsively started to move in a direction parallel to itself (Figure 4.1.1c), then plane B will soon feel the motion and, if unrestrained, will tend to be dragged along. Note that in the last case even though there is a bulk flow in the direction parallel to the plates, the transport occurs in the direction normal to the plates in which there is no motion. 图 31Schematic showing the diffusion of (a) heat, (b) mass, and (c) momentum. The phenomena we have described are known respectively as heat conduction, mass diffusion