Numerical simulation of microwave distillation reactor with multi-physical field coupling: heating and boiling processes

doi:10.11949/0438-1157.20210153

Abstract

Abstract:

A coupled multi-physical field model of electromagnetic field, fluid flow, heat transfer and species transport is developed in COMSOL Multiphysics to simulate the microwave distillation reactor. The thermal evolution, phase change, temperature distribution of water load under microwave irradiation are investigated. The whole process includes the water load heated from 293 K to boiling. The simulation results show that the temperature of the water sample is hierarchically distributed during the microwave heating stage, with the temperature of the upper section of the reactor being significantly higher than that of the lower section. At the same time, the generation of natural convection improves the temperature uniformity. During the microwave boiling stage, the boiling will not start immediately due to that the lower region of the sample with nucleation sites does not reach the saturation temperature. The accumulation of heat within the reactor leads to overheating in the water load. However, it is worth noting that the free surface of the water load maintains at the saturation temperature due to the high evaporation rate. After the temperature of the lower region of the reactor reaches the saturation temperature, the turbulence caused by the boiling bubbles improves the temperature uniformity, while the boiling eliminates this overheating phenomenon to a certain extend. Furthermore, the surface evaporation plays a major role in the dissipation of superheat compared to internal boiling evaporation. The final temperature depends on the relative value of the heat energy converted by microwave and the evaporative dissipation energy. This study can provide theoretical guidance for microwave-assisted separation, reaction and other chemical processes.

Key words: computational fluid dynamics, two-phase flow, evaporation, microwave, heat transfer

摘要：

使用COMSOL Multiphysics软件建立了耦合电磁场、流体流动、传热以及物质传输的多物理场模型用于模拟蒸馏型反应器的微波能量利用过程，探究了蒸馏反应器中水负载在微波能辐射作用下从升温至沸腾过程，阐明了在升温阶段，样品温度呈上下层分布，上层温度较高，最大温差达20 K，自然对流的产生改善了温度分布的不均匀性；在沸腾阶段，由于下层温度较低，沸腾现象有延迟，气泡的产生消除了部分过热，其中表面蒸发量更大，最大时约为内部蒸发量的3倍，与此同时湍流现象明显改善了温度均匀性。探究了馈入功率对全沸腾状态的影响，揭示了全沸腾状态的最终温度取决于馈入功率和蒸发损耗功率的相对大小。研究结果可为微波辅助分离、反应等化工过程及装备设计提供理论基础与借鉴。

关键词: 计算流体力学, 两相流, 蒸发, 微波, 传热

CLC Number:

TQ 02

ZHAO Haifeng, LI Hong, LI Xingang, GAO Xin. Numerical simulation of microwave distillation reactor with multi-physical field coupling: heating and boiling processes[J]. CIESC Journal, 2021, 72(S1): 266-277.

赵海峰, 李洪, 李鑫钢, 高鑫. 多物理场耦合模拟微波蒸馏反应器：升温和沸腾过程[J]. 化工学报, 2021, 72(S1): 266-277.

Figures/Tables 19

Fig.1 Illustration of heating process of microwave distillation reactor

Fig.2 Three-dimensional physical model

Table 1 Grid irrelevance test in cavity and waveguide

最大网格尺寸	网格单元质量	温度/K
最大网格尺寸	网格单元质量	最高温度	最低温度
h/10	0.6618	294.61	352.60
h/8	0.6611	294.62	352.71
h/6	0.6598	294.63	352.29
h/5	0.6593	294.66	352.00
h/4	0.6588	294.77	349.07

Fig.3 Grid irrelevance test in sample

Fig.4 Mesh element quality (MEQ) distribution of model

Fig.5 Comparison of simulated and experimental temperatures

Fig.6 Variation of gas phase volume fraction with pipe length

Fig.7 Electric field redistribution in cavity and reactor

Fig.8 Temperature and resistive loss power density with time

Fig.9 Temperature and electric field uniformity of water load in the reactor

Fig.10 Evaporation rate distribution

Table 2 Variation of total surface loss power with time

时间/s	蒸发损耗功率/W
5	2.10
20	3.15
35	4.05
50	5.37
75	7.88

Fig.11 Temperature distribution at initial boiling stage

Fig.12 Temperature distribution at totally boiling stage

Fig.13 Gas phase volume fraction distribution

Fig.14 Power loss with time

Fig.15 Average temperature at totally boiling at 700 W with time

Fig.16 Average temperature at totally boiling at 500 W with time

Fig.17 Average temperature at totally boiling at 300 W with time

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