Abstract:

Flash and mixing evaporation (FME) was one of the effective methods to achieve complete desalination of saline wastewater. In this paper, an experimental system of flash and cross-flow mixing evaporation was built, and the motion and evaporation characteristics of droplets in FME were investigated by combining PIV and Malvern laser particle size analyzer. In the experiment, on the wind side, air temperature ranged from 104.7℃ to 145.3℃, air speed from 10 m∙s^-1 to 17 m∙s^-1. On liquid side, the initial mass fraction of aqueous NaCl solution ranged from 0 to 0.15, its temperature from 20.0℃ to 132.0℃, and spray pressure ranged from 0.5 MPa to 1.2 MPa. Flash evaporation in FME mainly affected the atomization and fragmentation zone, while mixing evaporation mainly affected the evaporation zone. The initial particle size of droplets tended to be uniform with the increase of spraying pressure or mass fraction. While with the increase of droplet temperature, it tended to be uniform first and then became worse. The momentum/energy exchange between air and liquid mainly occurred in the horizontal direction in the evaporation zone. The average velocity of droplet cross-section along the horizontal direction was defined as the characteristic velocity of FME. This characteristic velocity first increases sharply and then increases slowly as the mixing distance increases. While at the same mixing distance, the characteristic velocity increased with the increase of mixing air temperature, air speed or spraying pressure. The Sauter diameter of droplets decreased along the mixing direction. The results pointed out that increasing the mixing air temperature, mixing air velocity or spray pressure was an effective means to enhance the FME process. The average surface heat transfer coefficient of the droplet population was calculated based on the experimental results and the experimental correlation equation for this heat transfer coefficient was set up and its relative error between the calculated and experimental values fell mainly in ±20%.

Key words: spray flash, evaporation, hydrodynamics, particle size distribution, average surface heat transfer coefficient, multiphase flow

摘要：

喷射闪蒸与热空气掺混蒸发（FME）结合是实现含盐废水深度脱盐的有效方法之一。本文搭建了喷射闪蒸-横流掺混蒸发实验系统，结合PIV和Malvern激光粒度仪对FME流场中液滴群的运动、蒸发特性开展了实验研究。实验中掺混风温为104.7~145.3℃、风速为10~17 m∙s^-1；液侧液滴初始盐质量分数为0~0.15，温度为20.0~132.0℃，喷射压力为0.5~1.2 MPa。FME中喷射闪蒸主要影响雾化破碎区，而掺混蒸发主要影响蒸发区。液滴群初始粒径随喷射压力或质量分数的提高趋于均匀，而随液滴温度的升高先趋于均匀而后均匀性变差。气液间的动量和能量交换主要发生在蒸发区内的水平方向；定义液滴沿水平方向截面平均速度为FME特征速度，该特征速度随掺混距离的增大先陡增后缓增，而在相同掺混距离处，该特征速度随掺混风温、风速或喷射压力的增大而增大；液滴群的Sauter平均直径沿掺混方向不断减小；增加掺混风温、提高掺混风速、增大喷射压力是强化FME蒸发的有效手段。根据实验结果计算了液滴群表面平均传热系数，并给出了该传热系数的实验关联式。在本文研究范围内，其计算值与实验值的主体误差在±20%之内。

关键词: 喷射闪蒸, 蒸发, 流体动力学, 粒径分布, 表面平均传热系数, 多相流