干燥物系的特征函数及其理论解

Characteristic functions of drying material system and its theoretical solution

干燥现象是物系对应外部约束条件的自发过程,是各种因素同时作用的结果。多种传递行为并存,得不到严格意义的传递系数的数学解。基于扩散动力学建立的干燥机理函数,又存在活化能、指前因子和过程指数难以定量等问题。如何获得干燥过程的理论解是长期以来干燥研究领域的重要命题。该研究以水分活度统一特征,以自由能传递和转换为统一尺度,建立干燥特征函数,说明基于物料的最大汽化速率、初始态水分活度、相平衡条件下的水分活度和介质的相对湿度,揭示实际物系状态变化规律和获得理论解的方法;给出自由含水比、干燥速率比和干燥速率随时间变化的数学解。该理论及方法不仅可以基于物系中的一个状态点,揭示出任意外部条件下的理论解,同时,按照水汽化、混合过程压力变化特征及其发生的位置,能够完整地解析出干燥过程物料内部的含水率偏差和水分活度分布;摆脱了基于单一行为描述的传递定率中的传递系数,对揭示物系传递机理,评价工艺系统能效,制定科学评价标准具有较高的理论价值,对指导工程实践具有重要的现实意义。

Drying system can normally be subject to momentum,heat,and mass transfer,three of which describe a single behavior.In drying process,there are no strictly mathematical solutions of models based on the laws,due to multiple transfer behaviors concurrently occur in a drying,particularly confined by various conditions of geometry,physics,time,and boundary.If some comprehensive factors of process were reduce to constant coefficients,mathematical models can obtain an approximate solution with some simplified actual conditions.However,diffusion-based drying function is difficult to quantify,due to activation energy,pre-finger factor and process index.It is necessary to explore theoretical solution of drying process since ancient times.Since drying can be defined as a process of spontaneous vaporization,and moisture migration,energy transfer and conversion can be measured by the changes in the drying rate and moisture content.Drying capacity of a system usually depends on water activity and external constraints,which can be characterized by free energy transfer and conversion.Taking free energy transfer and conversion as a unified scale,the present work aims to establish a characteristic function for a universal dry system and its theoretical solution under energy balance and external constraints,further to reveal drying mechanism.The results can be introduced:Theoretical expression of drying function was established using water activity in an initial state,water activity under phase equilibrium,and the relative humidity of medium,and the drying rate of the actual process.When time coordinates were introduced,the theoretical expressions were modified by the ratio of free water,and drying rate change with time.The characteristic function was also used to reveal water vaporization and pressure changes in the mixing process and their locations.An accurate solution can be achieved,particularly on the moisture content,and the distribution of water activity inside a material in the drying process.An analytical solution on a state point of a material can also be obtained from the established function for the actual drying process under various external conditions.Therefore,the theoretical solution can be gained at a state change process for a material under any external conditions,indicating breaking the description of a single behavior,such as the transfer coefficient in the transfer rate.The present work can provide a high theoretical basis for the transfer mechanism heat,mass,and momentum,in order to evaluate the energy efficiency of process system,and thereby formulate scientific evaluation standards.