Experimental study on evaporation of aqueous NaCl solution droplet heating by thermal irradiation

doi:10.11949/0438-1157.20240337

Abstract

Abstract:

In order to achieve efficient and low-cost desalination of saline wastewater using solar energy, this study designed and constructed a single droplet experimental system with thermal radiation heating. NaCl water solution was used as the working fluid and experimental research on the evaporation process of single droplets under thermal radiation heating was conducted. A laser with wavelengths of 1450 and 1930 nm was used as the thermal radiation source, with a power density between 1.4×10⁵ and 2.1×10⁵ W·m^-2, the initial mass fraction of the droplets ranged from 0.13 to 0.26 and the initial volume ranged from 1.0 to 5.0 µl. The results indicate: (1) The evaporation process of NaCl water solution droplets can be divided into four stages: heating, crystallization, thermal expansion, and cooling stage, with crystallization and thermal expansion being the main stages of droplet evaporation. (2) Increasing external radiation power or using high absorption coefficient bands can simultaneously shorten the duration of the heating, crystallization, and thermal expansion stages; increase the average surface evaporation rate. (3) Increasing the initial mass fraction or initial volume of the droplets can simultaneously increase the crystallization temperature, the maximum temperature during the thermal expansion stage and the average surface evaporation rate; the duration of the crystallization stage decreases with an increase in the initial mass fraction, but it is independent of the initial volume of the droplets. (4) Within the scope of this study, the average surface evaporation rate of NaCl water solution droplets ranged from 0.7×10^-8 to 7.4×10^-8 kg∙s^-1 and the droplet temperature remains the main factor affecting the surface evaporation rate. Based on the experimental results, an experimental correlation formula for the average surface evaporation rate of NaCl water solution droplets under thermal radiation heating was provided, with the main error between calculated and experimental values within ±20%. The results of this study can provide reference for the design and operation of desalination systems for saline wastewater under thermal radiation drive.

Key words: brine droplet, radiation, absorption, evaporation, crystallization, surface evaporation rate

摘要：

为利用太阳能实现含盐废水的高效、低成本的深度脱盐，设计搭建了热辐射加热单液滴实验系统，以NaCl水溶液为工质，针对热辐射加热下单液滴蒸发过程开展了实验研究。采用波长为1450、1930 nm的激光器为热辐射源，功率密度为（1.4~2.1）×10⁵ W·m^-2；液滴初始质量分数为0.13~0.26、初始体积为1.0~5.0 μl。结果表明：（1）NaCl水溶液液滴蒸发过程可分为升温、析晶、热胀和降温四个阶段，其中析晶和热胀段是液滴蒸发的主要阶段；（2）提高外部辐射功率或采用大吸收系数波段可同时缩短升温、析晶及热胀段时长，提高表面平均蒸发速率；（3）提高液滴初始质量分数或初始体积可同时提高析晶温度、热胀段最高温度及表面平均蒸发速率，析晶段时长随初始质量分数的增大而缩短，但与液滴初始体积无关；（4）本文研究范围内NaCl水溶液液滴表面平均蒸发速率介于（0.7~7.4）×10^-8kg∙s^-1之间，液滴温度依然是影响表面平均蒸发速率的主要因素。根据实验结果给出了热辐射加热下NaCl水溶液液滴表面平均蒸发速率的实验关联式，其计算值与实验值的主体误差在±20%之内。本文的研究结果可为热辐射驱动下含盐废水深度脱盐系统的设计和运行提供参考。

关键词: 盐水液滴, 辐射, 吸收, 蒸发, 析晶, 表面蒸发速率

CLC Number:

TK 124

Wenbo ZHOU, Jiangwei YIN, Dan ZHANG, Yue YANG, Jiahao YU, Bingchao ZHAO. Experimental study on evaporation of aqueous NaCl solution droplet heating by thermal irradiation[J]. CIESC Journal, 2024, 75(S1): 85-94.

周文博, 殷姜维, 张丹, 杨越, 于佳豪, 赵冰超. 热辐射加热下NaCl水溶液液滴蒸发过程的实验研究[J]. 化工学报, 2024, 75(S1): 85-94.

Figures/Tables 14

Fig.1 The absorption coefficient of aqueous solution of NaCl and NaCl crystal

Fig.2 Experimental system of droplet evaporation

Table 1 Test equipment main experimental parameters and range

实验设备	型号	参数范围
NaCl晶体	DINGSHENGXIN	纯度：0.995
温湿度表	VICTOR231	温度：-25.0~75.0℃(±0.3℃) 环境温度：21.0~23.0℃
温湿度表	VICTOR231	湿度：0~99.9％ RH(±2％ RH) 环境湿度：18.0%~20.0％ RH
激光器	NMDL-1450-2.0型	波长：1450 nm 功率：0~1.8 W 光斑直径：3 mm
激光器	NMDL-1930-0.8型	波长：1930 nm 功率：0~0.5 W 光斑直径：3 mm
移液枪	UCHEN	体积：0.5~10 µl(0.1 µl)
浓度滴定仪	WDDY-3000	浓度：0~26 g/L(±0.01 g/L)
热电偶	T型	温度：-200~400℃(±0.5℃)
相机	Canon eos 550D	像素：3250×782
镜头	EF 50 mm f/1.0 L USM	焦距：50 mm

Fig.3 Particle size reading

Table 2 Uncertainty analyses of experimental parameters

实验参数	实验范围	绝对误差	最小测量值	最大相对不确定度
初始质量分数f_m,0	0.13~0.26	0.005	0.13	0.038
初始体积V₀/μl	1.0~5.0	0.05	1.0	0.050
温度t/℃	18.0~110.0	0.2	18.0	0.011
等效直径d/mm	1.20~4.80	4.8×10^-3	1.20	0.004
升温速率K/(K·s^-1)	1.27~5.41	0.03	1.27	0.023
功率密度为q_b/(W·m^-2)	(1.4~2.1)×10⁵	0.6×10⁴	1.4×10⁵	0.043
表面平均蒸发速率 $\dot{m}$ /(kg·s^-1)	(0.7~7.4)×10^-8	0.3×10^-9	0.7×10^-8	0.042

Table 2 Uncertainty analyses of experimental parameters

实验参数	实验范围	绝对误差	最小测量值	最大相对不确定度
初始质量分数f_m,0	0.13~0.26	0.005	0.13	0.038
初始体积V₀/μl	1.0~5.0	0.05	1.0	0.050
温度t/℃	18.0~110.0	0.2	18.0	0.011
等效直径d/mm	1.20~4.80	4.8×10^-3	1.20	0.004
升温速率K/(K·s^-1)	1.27~5.41	0.03	1.27	0.023
功率密度为q_b/(W·m^-2)	(1.4~2.1)×10⁵	0.6×10⁴	1.4×10⁵	0.043
表面平均蒸发速率 $\dot{m}$ /(kg·s^-1)	(0.7~7.4)×10^-8	0.3×10^-9	0.7×10^-8	0.042

Fig.4 Evolution of droplet evaporation morphology under different initial mass fractions

Fig.5 Evaporation temperature, particle size curve and stage division of droplets

Fig.6 Heating rate variation with initial mass fraction

Fig.7 Crystallization temperature variation with initial mass fraction of droplet

Fig.8 Crystallization time variation with initial mass fraction of droplet

Fig.9 Evaporation time variation with initial mass fraction of droplet

Fig.10 Variation of maximum temperature of droplet with initial mass fraction

Fig.11 Surface evaporation rate of droplet with initial mass fraction

Fig.12 Comparison of droplet surface evaporation rate error

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