方形微通道内超临界CO2流动换热特性研究

doi:10.11949/0438-1157.20211466

摘要/Abstract

摘要：

采用SST k-ω湍流模型对加热条件下超临界CO₂在方形微通道内的流动换热特性进行了数值模拟。通过对比三种壁面平均传热系数、浮升力参数和二次流强度的沿程变化研究了管型、热通量、质量流量和倾斜角度对微通道内流动换热性能的影响。结果表明：水平方形微通道的整体换热性能优于相同水力直径的半圆形微通道。流体域典型截面的温度分布、速度分布和湍动能分布等信息可以很好地解释水平方向流动时上、下壁面传热差异的现象。减小热通量、增大质量流量或减小流体流动方向与重力方向之间的夹角，可提高方形微通道的整体换热水平。该模拟结果对以超临界CO₂为工质的微通道换热器的设计和优化具有一定的理论指导意义。

关键词: 超临界二氧化碳, 数值模拟, 传热, 浮升力效应, 二次流强度

Abstract:

The heat transfer characteristic of supercritical CO₂ in tubes is the key factor affecting the overall performance of heat exchangers. In recent years, square microchannels have shown broad application prospects in compact and efficient printed circuit heat exchangers. The SST k-ω turbulent model was used to simulate the heat transfer characteristics of supercritical CO₂ in square microchannels under uniform heating conditions. The effects of flow channel shape, heat flux, mass flow rate and inclination angle on heat transfer performance in microchannels were studied by comparing three average heat transfer coefficients, buoyancy parameter and secondary flow intensity. The results show that the overall heat transfer performance of horizontal square microchannels is better than that of semicircular microchannels with the same hydraulic diameter. The temperature distribution, velocity distribution and turbulent kinetic energy distribution of typical sections in the fluid domain can well explain the non-uniform heat transfer phenomenon in horizontal microchannels. The overall heat transfer level of square microchannels can be improved by decreasing the heat flux, increasing the mass flow rate or decreasing the angle between the flow direction and the gravity direction. The simulation results have certain theoretical significance for the design and optimization of microchannel heat exchangers using supercritical CO₂ as working medium.

Key words: supercritical CO₂, numerical simulation, heat transfer, buoyancy effect, second flow intensity

中图分类号:

TK 124

许婉婷, 许波, 王鑫, 陈振乾. 方形微通道内超临界CO₂流动换热特性研究[J]. 化工学报, 2022, 73(4): 1534-1545.

Wanting XU, Bo XU, Xin WANG, Zhenqian CHEN. Heat transfer characteristics of supercritical CO₂ in square microchannels[J]. CIESC Journal, 2022, 73(4): 1534-1545.

图/表 16

图1 CO2在压力为8.0 MPa时的热物性曲线

Fig.1 Thermal properties of CO2 at P = 8.0 MPa

图2 物理模型示意图

Fig.2 Schematic diagram of physical model

表1 不同模拟工况的边界条件设置

Table 1 Boundary conditions under different simulated conditions

入口温度T_in/K

热通量q /

(kW/m²)

质量流量G /

(kg/(m²·s))

0,30,60,90,-30,-60,-90

表2 不同尺寸网格下的流体温度和平均传热系数偏差

Table 2 Deviations of bulk temperature and average heat transfer coefficient under different grid sizes

序号	网格数量（径向）×横向	网格总数/万个	流体温度最大相对误差/%	平均传热系数最大相对误差/%
Grid1	(48×48)×500	115.2	0.10	3.69
Grid2	(48×48)×1000	230.4	0.03	3.11
Grid3	(48×48)×2000	460.8	0.01	2.29
Grid4	(54×54)×500	145.8	0.09	1.98
Grid5	(54×54)×1000	291.6	0.02	0.86
Grid6	(54×54)×2000	583.2	—	—

图3 模拟结果与传热关联式对比

Fig.3 Comparisons of simulated results with predicted data from heat transfer correlations

图4 方管和半圆管的壁面平均传热系数和流体温度（P=8.0 MPa, Tin=303.15 K,G=400 kg/(m2·s),q=60 kW/m2）

Fig.4 Heat transfer coefficient and local bulk temperature in square and semicircular tubes

图5 重力对壁面温度的影响(P=0.8 MPa, Tin=303.15 K, q=60 kW/m2, G=400 kg/(m2·s))

Fig.5 Effect of gravity on wall temperature

图6 热通量对传热系数的影响（P=8.0 MPa, Tin=303.15 K,G=400 kg/(m2·s)）

Fig.6 Effect of heat flux on heat transfer coefficient

图7 质量流量对传热系数的影响（P=8.0 MPa, Tin=303.15 K,q=60 kW/m2）

Fig.7 Effect of mass flow rate on heat transfer coefficient

图8 倾斜角度对传热系数的影响（P=8.0 MPa, Tin=303.15 K,q=60 kW/m2, G=400 kg/(m2·s)）

Fig.8 Effect of inclination angle on heat transfer coefficient

图9 特征截面处的典型参数分布

Fig.9 Distribution of typical parameters at characteristic cross-sections

图10 特征截面处的轴向速度和湍动能径向分布曲线

Fig.10 Radial distribution curves of axial velocity and turbulent kinetic energy at characteristic cross-sections

图11 沿程浮升力参数和二次流强度变化曲线（P=8.0 MPa, Tin=303.15 K,q=60 kW/m2,G=400 kg/(m2·s)）

Fig.11 Variation curves of buoyancy parameter and secondary flow intensity

图12 热通量对浮升力参数和二次流强度的影响（P=8.0 MPa, Tin=303.15 K,G=400 kg/(m2·s)）

Fig.12 Effect of heat flux on buoyancy parameter and secondary flow intensity

图13 质量流量对浮升力参数和二次流强度的影响（P=8.0 MPa, Tin=303.15 K,q=60 kW/m2）

Fig.13 Effect of mass flow rate on buoyancy parameter and secondary flow intensity

图14 倾斜角度对二次流强度和壁面温度的影响（P=8.0 MPa, Tin=303.15 K,q=60 kW/m2,G=400 kg/(m2·s)）

Fig.14 Effect of inclination angle on secondary flow intensity and wall temperature

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方形微通道内超临界CO₂流动换热特性研究

Heat transfer characteristics of supercritical CO₂ in square microchannels

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