Study on mechanism and key parameters of sessile droplets flash evaporation under reduced pressure

低压下固着液滴闪蒸的机制和关键影响参数研究

¹ Huaneng Yingkou Thermal Power Co., Ltd., Yingkou 115004, China
² School of Low-Carbon Energy and Power Engineering, China University of Mining and Technology, Xuzhou 221116, China
³ Huaneng Clean Energy Research Institute Co., Ltd., Beijing 102209, China
⁴ School of Engineering Science, University of Science and Technology of China, Hefei 230027, China

Received: 9 September 2024

Abstract

Droplet flash evaporation is widely used in waste heat recovery, artificial ice making and other industrial fields. The vigorous flash evaporation leads to the flash explosion, which might cause damage to the internal structure of the flash evaporation chamber. Therefore, how to suppress flash explosions for industrial production safety is imperatively studied. With a high-speed camera system, the flash evaporation process of sessile droplets (deionized water and desulfurization slurry) is experimentally investigated and the duration of the flash explosion is recorded. The finite element method establishes the droplet flash evaporation model, and the internal flow and heat transfer process of single/multiple droplets are numerically simulated. The results show that the initial diameter of the droplet can directly affect its internal temperature change rate, thus controlling the internal phase transition process and the occurrence of droplet flash explosion. In this paper, the critical diameter of droplet flash explosion under different conditions and effective methods to suppress flash explosion are pinpointed, including increasing ambient air pressure, reducing the initial temperature of the droplet, reducing droplet size, and using a low thermal conductivity substrate. In addition, the numerical results show that the internal flow of droplets is dominated by thermal convection in the flash evaporation. The three-dimensional simulation of a multi-droplet array shows that the decrease in flash evaporation rate is the low vapor concentration gradient near droplets.

摘要

液滴闪蒸广泛应用于余热回收、人工制冰等工业领域。其中剧烈闪蒸会引发液滴沸腾爆炸从而造成闪蒸腔内部结构的损伤, 因此研究如何抑制闪爆发生对工业生产安全至关重要。基于高速摄像技术, 实验观察了固着液滴(去离子水和脱硫浆液)发生闪爆的过程并记录闪爆发生时间; 利用有限元计算方法建立了液滴闪蒸模型, 并对单个/多个液滴发生闪蒸时内部流动和传热过程进行了数值模拟。结果表明, 液滴初始直径能够直接影响其内部温度变化率, 从而控制内部相变过程和液滴闪爆的发生。给出了不同条件下液滴发生闪爆的临界直径, 并指出有效抑制闪爆发生的方法, 包括提高环境气压、降低液滴初始温度、减小液滴尺寸和使用低导热系数基底。此外, 数值结果发现闪蒸过程中液滴内部由热对流主导, 并通过对多液滴阵列的三维计算发现闪蒸速率下降是由液滴附近较低的蒸汽浓度梯度造成的。