【作者】 吴中华；

【导师】 刘相东；

【作者基本信息】 中国农业大学 ， 农产品加工及贮藏工程， 2002， 硕士

【摘要】 脉动燃烧干燥技术是近年来出现的一种新型干燥技术，它利用脉动燃烧器产生的高温，高频脉动尾气流直接雾化和干燥液态物料。与传统喷雾干燥相比，脉动燃烧喷雾干燥具有能耗低，热，质传递速度高，环境污染小，以及投资成本低等很多特点。长期以来，对喷雾干燥过程进行了各种试验和模拟研究，但这些研究并不能揭示喷雾干燥室内气体运动状态，颗粒群的运动轨迹和各种热力学参数分布信息，常规的测试手段又很难测得，而这些参数分布信息对干燥器没计和过程优化具有重要指导作用。为解决这一难题，本文利用气体一颗粒两相流理论和计算流体力学(CFD)技术，建立了更符合实际喷雾干燥过程的数学模型即喷雾干燥的CFD模型，并进行了脉动燃烧喷雾干燥过程模拟，其主要内容如下： (1)建立了脉动燃烧喷雾干燥的CFD模型 该数学模型建立在气体一颗粒两相流基础之上，用标准k-ε模型预测干燥室内的气体湍流运动过程，颗粒轨道模型追踪干燥室内颗粒群的运动轨迹，热质传递模型描述空气和液滴的热质传递过程。通过模型的求解，得到了干燥室内气体运动状态，气体温度、湿度分布，颗粒运动轨迹，颗粒沿运动轨迹质量变化，颗粒沿运动轨迹的温度变化等各种动力学和热力学参数分布信息。加深了对该干燥过程的了解。模拟结果还解释喷雾干燥过程中出现的涡流和颗粒粘壁现象，脉动燃烧喷雾干燥过程出现的回流现象等以往了解不够透彻的问题。模拟得到的干燥过程中各种热力学参数分布信息为脉动燃烧喷雾干燥器的设计，选型，干燥过程优化等捉供了参考。 (2)进行了模型的求解 本文详细论述了该类数学模型的具体求解过程，包括模拟区域的确定和非结构化网格的划分、边界条件的选择、微分方程的控制容积离散方法、代数方程组的SIMPLER解法以及两相流方程的LEGAMI算法。 (3)进行了脉动燃烧喷雾干燥试验和数学模型验证 建立了脉动燃烧喷雾干燥试验装置，并利用质量分数10％的NaCl溶液进行脉动气流直接雾化和干燥的试验。测量了实际干燥过程中干燥室内各点的气体温度，并进行了实验结果与模拟结果比较、分析。试验观察证实了模拟揭示的脉动燃烧喷雾干燥过程的粘壁现象。实验与模拟结果比较表明CFD模型能够比较精确地模拟脉动燃烧喷雾干燥过程。 (4)揭示了气流脉动对干燥过程的影响 本文模拟脉动频率为83Hz的脉动气流和非脉动气流的两种干燥过程，比较两种干燥过程的颗粒蒸发速率和湿含量变化。模拟结果表明气流脉动对喷雾干燥过程有一定增强作用。更多还原

【Abstract】 Pulsating spray drying technique makes use of the unstable hot gas stream generated by a pulse combustor to atomi/e and dry a liquid material. The unstable gas stream is characterized by oscillations with transient velocities of about +100 m/s and frequencies from 50-200H/. The enhanced mixing and transport processes lead to a highly efficient moisture evaporation rate during pulsating spray drying. The computational fluid dynamics (CFD) method was used for the analysis and investigation of flow,temperature and humidity fields in a pulsating flow spray-drying chamber.(1) The CFD model for the pulse combustion spray drying process.The Computational Fluid Dynamics (CFD) based on the two-phase flow theory was employed in this paper,which used the Standard k- e model and the Particle trajectory Model to simulate the gas-particle flow in the drying chamber. The momentum,heat and mass transfer regarding both gaseous and particulate phases during the spray drying inside the drying chamber was also revealed. The simulated profiles of flow field,temperature and humidity of the air phase,as well as particulate phase,in the drying chamber were showed. The simulation also showed that a large-scale vortex was observed in upper part of the drying chamber because of the unstable process of flow field and particle trajectories.(2) The explicit simulating process for the pulse combustion spray drying processIn this project,both the gas-phase equations and particle-phase equations were integrated in the computational cell using up-wild scheme to obtain the finite difference equations. The gas-phase equations are solved using the SIMPLER algorithm and multiple iterations have been made based on a two-way coupling (LEGAML algorithm).(3) Comparison of the experimental data with the simulation resultsA pulse combustion spray drying system was constructed and the spray drying of NaCl solution was carried out under a high oscillating flow field generated by the pulse combustor. The particle deposition which was observed in the simulation results and verified by the experiment and The comparison of the air experimental data with simulation data showed that the CFD model is a feasible way to simulate the spray drying process.(4) The influence of pulsation frequency on the drying processTwo special cases,in which the selected pulsation frequencies were 0 H/,and 83 H/ respectively,were simulated and their simulation results indicated that the pulsating frequency of gas stream could improve the intensities of the heat and mass transfer during a pulse combustion spray drying process.更多还原