传热和传质基本原理第三章传质理论

传热传质理论,（Heat and Mass Transfer Theory）,主讲：左然教授 徐谦博士,第三章 传质理论,3-1 概述,在材料加工、化工、冶金、低温工程、空间技术等领域中，质量传输是很重要的过程。许多加工工艺的单元操作，如加热、溶解、焊接、表面热处理等，也都涉及传质过程。,3,4,5,6,(1) 浓度梯度引起的分子扩散 (2) 温度梯度引起的热扩散 (3) 压力梯度引起的压力扩散 (4) 除重力外的其它外力引起的强迫扩散 (5) 强迫对流传质 (6) 自然对流传质 (7) 紊流传质 (8) 相际传质,传质现象的八种形式:,R.B. Bird, Theory of Diffusion, Adv. Chem. Eng., 1956 (1) 170.,7,3-2 质量传输方式、浓度、物质流,8,9,10,11,12,13,14,15,16,3-3 Fick第一定律,17,18,19,20,21,22,3-4 Fick第二定律,23,24,3-5 固体中的扩散和扩散系数,25,26,27,28,29,30,31,32,（4） 化学势驱动下的扩散系数,33,34,35,36,（5） 温度对扩散系数的影响,37,38,3-6 流体和多孔介质中的扩散和扩散系数,39,40,41,42,43,44,The origin of dispersion（弥散）,Physically, a non-constant advecting velocity occurs in porous media with disturbance near pore wall. Parts of the material are carried faster than others, so that the distribution gets stretched and distorted. This results in sharp gradients perpendicular to the flow, which diffusion can then iron out. The combination of a shearing advection velocity with weak diffusion can lead to quite effective mixing.,多孔介质中的弥散传质,45,Volume average,Intrinsic average,Microscopic governing equations,Theoretical analyses,Schematic of a porous media,46,Volume-averaged macroscopic GEs,47,Subtracting the volume-averaged equations from the microscopic equations reads the spatial deviation:,where,48,Note the relationship,(*),49,Thus the last equation can be simplified as:,Equation (*) now can be written as:,can be written as a function of Ddis,where Ddis can be determined.,50,3-7 对流传质,51,52,53,3-8 相际传质,54,The solubility of gas in the liquid,55,自由度 F 是当系统为平衡状态时，在不改变相的 数目情况下，可独立改变的因素（如温度和压力）。,56,57,58,59,60,61,62,63,64,65,8.1. 平衡溶解度,66,67,这三个表达式的本质是一致的,68,69,亨利定律的适用范围：,(4) 溶质遵守亨利定律；而溶剂遵守拉乌尔定律。,70,8.2. 相平衡与吸收过程的关系,71,8.2. 相平衡与吸收过程的关系,72,8.2. 相平衡与吸收过程的关系,73,8.2. 相平