文丘里效应下蒸汽在不凝气体中的接触冷凝和界面波动

Venturi tower is the main equipment used to recover coal–tar vapor through condensation in Yulin semi-coke industry. The presence of non-condensable gas reduces the condensation rate. However, the effect of non-condensable gas on the direct contact condensation in the venturi tube is poorly understood. This project aims to achieve 88%–95% condensation rate by enhancing the condensation through interface wave, which was induced by the strong convection of non-condensable gas due to a venturi effect. On the basis of experimental and numerical simulation methods, this project focuses on three aspects, such as interfacial wave features and their evolution mechanism, convection and diffusion laws of non-condensable gas during direct contact condensation in the venturi tube, and regulation of the effect of interfacial wave and non-condensable gas on direct contact condensation in the venturi tower. So, the aims of this project include three parts, such as the obtainment of the correlation between interfacial wave parameters and multiple factors groups, the determination of turbulent transition model of non-condensable gas under the venturi effect and the proposal of the synergy and linkage mechanism that the venturi effect improves the direct contact condensation. Pressure transmission, high-speed video camera technologies, and the numerical simulation technology of Eulerian-VOF model were applied in this project. In addition, the regulation of pressure drop gradient to interfacial wave factor and the action of non-condensable gas turbulivity on droplet formation were explored. The enhancemnt of the droplet dispersion in the venturi effect on the condensation phase-change interface effect and the temporal and spatial scale of interface instability during condensation phase-change in the multi-field coupling were clarified. Principle components analysis method was used to optimize the direct contact condensation heat transfer in the venturi tube, and the structural and working parameters of venturi tube for high efficient condensation were proposed. The obtained results provide a theoretical reference for energy conservation and consumption reduction.

文丘里塔是榆林兰炭产业冷凝回收煤焦油的主要设备，冷凝率仅为60%~80%，存在不凝气体是冷凝率下降的主要原因之一，文丘里管内接触冷凝中不凝气体作用机理的研究尚属薄弱领域。本项目围绕文丘里效应形成不凝气体强对流诱发界面波动实现88%~95%冷凝率，采用实验和数值模拟方法，系统研究文丘里管内接触冷凝中界面波动特征及演化机理、不凝气体对流扩散规律、不凝气体和界面波动对接触冷凝的调控效应，来获得界面波动参数与多因素团间的关联，确定文丘里效应下不凝气体湍流转捩模式，提出文丘里效应增强接触冷凝的协同联动机制。基于压力变送、高速摄像技术和Eulerian-VOF模型，探索压降梯度对界面波动因子的调控和不凝气体湍流度对液滴形变的作用，阐明文丘里效应中液滴分散对冷凝相变界面效应的强化和多场耦合中冷凝相变界面失稳的空间和时间尺度，采用主成分分析法优化文丘里管内接触冷凝换热参数，为文丘里塔节能降耗提供理论依据。

文丘里塔是榆林兰炭产业冷凝回收煤焦油的主要设备，冷凝率仅为60%~80%，存在不凝气是冷凝率下降的主要原因之一，文丘里管内接触冷凝中不凝气作用机理的研究尚属薄弱领域。本项目基于实验研究、两相边界层法的数值模拟，围绕文丘里管内接触冷凝中不凝气的对流扩散规律、界面波动特征及其演化机理、文丘里管内不凝气和界面波动对接触冷凝的调控效应展开研究。结果发现，蒸汽温度沿着垂直水柱流动方向下降，下降幅度随垂直水柱质量流量增大而增加；收缩段对蒸汽温降贡献为49.76%~58.14%，收缩段与喉段交汇区对蒸汽温降贡献为38.3%~49%，每米管长温降贡献提高了约4倍。纯蒸汽冷凝中，入口蒸汽流动冷凝和下游空气逆向扩散形成动态平衡，收缩段和喉段交汇处出现真空，真空度随垂直水柱质量流量升高而增大。蒸汽中空气含量增加会提高气相压力，真空度消失；蒸汽沿垂直水柱表面冷凝引起空气分压升高，在扩散段高达0.834。界面波动因子在文丘里管入口与收缩段和喉段交汇处大于1，截面收缩极大提高了界面波动因子，15倍的截面收缩比会将界面波动因子提高2.4~10.9倍，文丘里管中的不凝气会抑制截面收缩下的真空度，降低蒸汽冷凝换热，削弱界面波动；而截面收缩会提高界面波动性，强化蒸汽冷凝换热。当收缩角从7.2º增大到39º时，Nu、St、凝结热流、凝结换热系数均增大。蒸汽在垂直水柱表面的凝结长度随冷凝压力、水柱流速、喷嘴直径的增加而增加，随不凝气质量分数和收缩角的增加而减小。使蒸汽在较短的管段达到100%凝结。实验结果和数值模拟结果与多个文献实验数据相互比较，误差在-0.5%~16%。项目成果可以应用于脱除烟气中CO2气体和生物油高效冷凝中。