Numerical simulation of microwave-enhanced spiral liquid-bridge falling film evaporator

doi:10.11949/0438-1157.20210154

Abstract

Abstract:

Microwave-enhanced thin film evaporation promoting the separation of mixture with the difference in polarity still faces the challenges of low energy efficiency and uneven heating, where electric field distribution is generally seen as a significantly related factor. However, considering that the factors affecting the electric field distribution are very complicated and uncontrollable, it is advisable to solve the bottleneck of efficient utilization of microwave energy from the perspective of evaporator structure and fluid flow pattern design. For this reason, a liquid-bridge spiral falling film evaporator is proposed in this study. The three-dimensional model is established first by COMSOL and then to simulate the process of water spiral falling film flow and evaporation on the designed evaporator. Evaporation efficiency and coefficient of temperature variation (COV) are used as evaluation indicators to explore the influence of liquid film thickness, the width of the spiral channel, evaporator diameter, flowrate and time on microwave energy utilization efficiency. The results show that at a certain microwave input power, the evaporation efficiency of the liquid film finally reaches 29.26% and the corresponding COV is reduced to 0.0867, which will provide foundation for the design of microwave-enhanced evaporation and separation devices.

Key words: microwave, process intensification, spiral falling film, evaporation separation, numerical simulation, optimal design

摘要：

微波加热薄膜蒸发技术在促进极性/非极性混合物分离领域潜力巨大，但仍面临着能源利用效率低和加热不均的挑战，而电场分布不均是其根本原因，但影响电场分布的因素十分复杂且不可控，因此，从蒸发器结构及流体流动形式视角出发可为解决微波能高效利用的瓶颈提供新思路。为此本文提出了液桥式螺旋降膜蒸发器，通过COMSOL建立三维模型并模拟计算了微波能强化蒸发器上的螺旋降膜流动与蒸发过程，以蒸发率和温度变异系数作为评价指标，探究液膜厚度、螺距、蒸发器直径、流量以及时间对微波能利用效率的影响规律，研究结果表明该种结构在一定微波入射功率下，液膜蒸发率可达29.26%，温度变异系数降至0.0867，为微波能强化蒸发分离装置的设计提供了依据。

关键词: 微波, 过程强化, 螺旋降膜, 蒸发分离, 数值模拟, 优化设计

CLC Number:

TQ 025.1

ZHANG Yashuang, LI Hong, CONG Haifeng, HAN Hongming, LI Xingang, GAO Xin. Numerical simulation of microwave-enhanced spiral liquid-bridge falling film evaporator[J]. CIESC Journal, 2021, 72(S1): 227-235.

张亚爽, 李洪, 从海峰, 韩红明, 李鑫钢, 高鑫. 微波强化液桥式螺旋降膜蒸发器数值模拟[J]. 化工学报, 2021, 72(S1): 227-235.

Figures/Tables 14

Fig.1 Schematic illustration and geometric parameters of spiral falling film evaporator

Fig.2 Schematic diagram of microwave-assisted spiral falling film evaporation equipment

Table 1 Basic parameters setting

参数	数值
水的介电常数^[17]，ε′_water	88.15-0.414T+0.131e^-2T²-0.046e^-4T³
水的介电损耗^[17]，ε″_water	28.472-0.971T+1.555e^-2T²-1.205e^-4T³+4.638e^-7T⁴-1.387e^-9T⁵+2.82e^-12T⁶
初始温度，T₀	25 K
弹簧截面固定宽度，B	0.12 mm
蒸发潜热，?H_water	2257.6 kJ/kg
玻璃的相对介电常数	4.2
空气的相对介电常数	1
蒸发率测量时间	30 min

Fig.3 Meshing scheme (a) and quality evaluation of mesh (b) for microwave-assisted spiral falling film evaporation equipment

Fig.4 Schematic diagram of microwave heating equipment

Fig.5 Comparison between experimental and simulation result for characteristic of microwave heating in different flux

Fig.6 Effect of water film thickness on α and COV

Fig.7 Effect of width of spiral channel on α and COV

Fig.8 Effect of width of spiral channel on electric field distribution in fluid

Fig.9 Effect of evaporator outer diameter on α and COV

Fig.10 Effect of feed flowrate on α and COV

Fig.11 Effect of heating time on α and COV

Fig.12 Schematic diagrams of different field distribution

Fig.13 COV at different horizontal cross sections of spiral falling film evaporator

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